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Title: Steam, Its Generation and Use

Author: Babcock & Wilcox Company

Release date: September 18, 2007 [eBook #22657]
Most recently updated: June 29, 2021

Language: English

Credits: Juliet Sutherland, Tony Browne, and the Online Distributed Proofreading Team

*** START OF THE PROJECT GUTENBERG EBOOK STEAM, ITS GENERATION AND USE ***

Steam: its Generation and Use


TRANSCRIBER'S TABLE OF CONTENTS—CHAPTERS
Chapter
LIST OF ILLUSTRATIONS
TITLE PAGE
The Early History of the Generation and Use of Steam
Brief History of Water-tube Boilers
Requirements of Steam Boilers
Evolution of the Babcock & Wilcox Water-tube Boiler
The Babcock & Wilcox Boiler
Advantages of the Babcock & Wilcox Boiler
Heat and its Measurement
The Theory of Steam Making
Properties of Water
Boiler Feed Water
Feed Water Heating and Methods of Feeding
Steam
Moisture in Steam
Superheated Steam
Properties of Air
Combustion
Analysis of Flue Gases
Classification of Fuels
The Determination of Heating Values of Fuels
Combustion of Coal
Solid Fuels other than Coal and their Combustion
Liquid Fuels and their Combustion
Gaseous Fuels and their Combustion
Utilization of Waste Heat
Chimneys and Draft
Efficiency and Capacity of Boilers
The Selection of Boilers with a Consideration of the Factors Determining such Selection
Operation and Care of Boilers
Brickwork Boiler Settings
Boiler Room Piping
Flow of Steam through Pipes and Orifices
Heat Transfer
INDEX

TRANSCRIBER'S LIST OF ILLUSTRATIONS
Illustration
Works at Bayonne
Works at Barberton
Works at Renfrew
Fonderies et Ateliers
Longitudinal Drum Boiler
Boiler at Ampere
Boilers and Superheaters in Chicago
Erie County Electric Co.
Boilers and Superheaters at Blue Island
Woolworth Building
Boilers in Philadelphia
Boilers in Boston
Boilers and Superheaters in South Boston
Boilers in Johnstown
Boilers and Superheaters in South Boston
Boilers in Pittsburgh
Longitudinal Drum Boiler
Cross Drum Boiler
Longitudinal Drum Boiler
Longitudinal Drum Boiler
Longitudinal Drum Boiler
Cross Drum Boiler
Boilers in Boston
McAlpin Hotel
Northwest Station Chicago
Vertical Header Boiler
Boilers in Raritan
Boilers in Pittsburgh
Ninety-sixth Street Station in New York
Boiler in Boston
Boilers and Superheaters in Chicago
Boilers and Superheaters in Washington
Boilers and Superheaters in Passaic
Boilers and Superheaters in Washington
Boilers in Newark
Boilers in Louisville
Superheater
Boilers and Superheaters in Atlanta
Boilers and Superheaters in Chicago
Boilers and Superheaters in New York City
Boilers and Superheaters in Pittsburgh
Boilers and Superheaters in Washington
Chain Grate Stoker
Boilers in Portland
Boiler and Superheater in Quincy
Boilers in San Francisco
Boiler with Oil Furnace
Boilers in Bethlehem
Boiler and Superheaters in Underwood
Boilers and Superheaters in Chicago
Boilers in Washington
Boilers in Chicago
Boilers in Northumberland
Boilers in Chicago
Boilers in Chicago
Boilers in Chicago
Boilers in Minneapolis
Boilers in Billings
Boilers and Superheaters in East Pittsburgh
Boilers and Superheaters in Long Island City
Boilers in Chicago
Boiler Casing
Boilers and Superheaters in Louisville
Bankers Trust Building
Vesta Coal Co.
Iron City Brewery

[Pg 1]

STEAM
ITS GENERATION AND USE

Aeolipile

THE BABCOCK & WILCOX CO.

NEW YORK

Pg 2]

Thirty-fifth Edition
4th Issue
Copyright, 1919, by The Babcock & Wilcox Co.

Bartlett Orr Press
New York

[Pg 3]

THE BABCOCK & WILCOX CO.

85 LIBERTY STREET, NEW YORK, U. S. A.

Works

BAYONNENEW JERSEY
BARBERTONOHIO

Officers

W. D. HOXIE,President
E. H. WELLS,Chairman of the Board
A. G. PRATT,Vice-President

Branch Offices

ATLANTACandler Building
BOSTON35 Federal Street
CHICAGOMarquette Building
CINCINNATITraction Building
CLEVELANDNew Guardian Building
DENVER435 Seventeenth Street
HAVANA, CUBA104 Calle de Aguiar
HOUSTONSouthern Pacific Building
LOS ANGELESI. N. Van Nuy’s Building
NEW ORLEANSShubert Arcade
PHILADELPHIANorth American Building
PITTSBURGHFarmers’ Deposit Bank Building
SALT LAKE CITYKearns Building
SAN FRANCISCOSheldon Building
SEATTLEL. C. Smith Building
TUCSON, ARIZ.Santa Rita Hotel Building
SAN JUAN, PORTO RICORoyal Bank Building

Export Department, New York: Alberto de Verastegni, Director

TELEGRAPHIC ADDRESS: FOR NEW YORK, “GLOVEBOXES

FOR HAVANA, “BABCOCK

[Pg 4]

Illustration:

Works of The Babcock & Wilcox Co., at Bayonne, New Jersey

[Pg 5]

[Pg 6]

Illustration:

Works of The Babcock & Wilcox Co., at Barberton, Ohio

[Pg 7]

[Pg 8]

Illustration:

Works of Babcock & Wilcox, Limited, Renfrew, SCOTLAND

[Pg 9]

BABCOCK & WILCOX Limited

ORIEL HOUSE, FARRINGDON STREET, LONDON, E. C.

WORKS: RENFREW, SCOTLAND

Directors

JOHN DEWRANCE, Chairman     CHARLES A. KNIGHT
ARTHUR T. SIMPSON     J. H. R. KEMNAL
WILLIAM D. HOXIE      Managing Director
E. H. WELLS     WALTER COLLS,Secretary

Branch Offices in Great Britain

GLASGOW: 29 St. Vincent PlaceMANCHESTER: 30 Cross Street
BIRMINGHAM: Winchester HouseMIDDLESBROUGH: The Exchange
CARDIFF: 129 Bute StreetNEWCASTLE: 42 Westgate Road
BELFAST: Ocean Buildings, Donegal Square, E.SHEFFIELD: 14 Bank Chambers, Fargate

Offices Abroad

BOMBAY: Wheeler’s Building, Hornby Road, FortMELBOURNE: 9 William Street
BRUSSELS: 187 Rue RoyalMEXICO: 22-23 Tiburcio
BILBAO: 1 Plaza de AlbiaMILAN: 22 Via Principe Umberto
CALCUTTA: Clive BuildingMONTREAL: College Street, St. Henry
JOHANNESBURG: Consolidated BuildingsNAPLES: 107 Via Santa Lucia
LIMA: PeruSHANGHAI: 1a Jinkee Road
LISBON: 84-86 Rua do CommercioSYDNEY: 427-429 Sussex Street
MADRID: Ventura de la VegaTOKYO: Japan
TORONTO: Traders’ Bank Building

Representatives and Licensees in

ADELAIDE, South AustraliaCAIRO, EgyptMOSCOW, Russia
ATHENS, GreeceCHILE, Valparaiso, So. AmericaPERTH, Western Australia
AUCKLAND, New ZealandCHRISTIANIA, NorwayPOLAND, Berlin
BAHIA, BrazilCOLOMBO, CeylonRANGOON, Burma
BANGKOK, SiamCOPENHAGEN, DenmarkRIO DE JANEIRO, Brazil
BARCELONA, SpainESKILSTUNA, SwedenSMYRNA, Asia Minor
BRUNN, AustriaGIJON, SpainSOURABAYA, Java
BUCHAREST, RoumaniaHELSINGFORS, FinlandST. PETERSBURG, Russia
BUDAPEST, HungaryHENGELO, HollandTAMMERFORS, Finland
BUENOS AYRES, Argentine Rep.KIMBERLEY, South AfricaTHE HAGUE, Holland

TELEGRAPHIC ADDRESS FOR ALL OFFICES EXCEPT BOMBAY AND CALCUTTA: “BABCOCK
FOR BOMBAY AND CALCUTTA: “BOILER

[Pg 10]

Illustration:

Fonderies et Ateliers de la Courneuve, Chaudières Babcock & Wilcox, Paris, France

[Pg 11]

FONDERIES ET ATELIERS DE LA COURNEUVE
CHAUDIÈRES

BABCOCK & WILCOX

6 RUE LAFERRIÈRE, PARIS

WORKS: SEINE—LA COURNEUVE

Directors

EDMOND DUPUIS     J. H. R. KEMNAL
ETIENNE BESSON     IRÉNÉE CHAVANNE
CHARLES A. KNIGHT     JULES LEMAIRE

Branch Offices

BORDEAUX: 30 Boulevard Antoine Gautier
LILLE: 23 Rue Faidherbe
LYON: 28 Quai de la Guillotier
MARSEILLE: 21 Cours Devilliers
MONTPELLIER: 1 Rue Boussairolles
NANCY: 2 Rue de Lorraine
ST. ETIENNE: 13 Rue de la Bourse

REPRESENTATIVE FOR SWITZERLAND: SPOERRI & CIE, ZURICH

TELEGRAPHIC ADDRESS: “BABCOCK-PARIS

[Pg 12]

Illustration:

Wrought-steel Vertical Header Longitudinal Drum Babcock & Wilcox Boiler, Equipped with Babcock & Wilcox Superheater and Babcock & Wilcox Chain Grate Stoker

[Pg 13]

THE EARLY HISTORY OF THE GENERATION AND USE OF STEAM

While the time of man’s first knowledge and use of the expansive force of the vapor of water is unknown, records show that such knowledge existed earlier than 150 B. C. In a treatise of about that time entitled “Pneumatica”, Hero, of Alexander, described not only existing devices of his predecessors and contemporaries but also an invention of his own which utilized the expansive force of steam for raising water above its natural level. He clearly describes three methods in which steam might be used directly as a motive of power; raising water by its elasticity, elevating a weight by its expansive power and producing a rotary motion by its reaction on the atmosphere. The third method, which is known as “Hero’s engine”, is described as a hollow sphere supported over a caldron or boiler by two trunnions, one of which was hollow, and connected the interior of the sphere with the steam space of the caldron. Two pipes, open at the ends and bent at right angles, were inserted at opposite poles of the sphere, forming a connection between the caldron and the atmosphere. Heat being applied to the caldron, the steam generated passed through the hollow trunnion to the sphere and thence into the atmosphere through the two pipes. By the reaction incidental to its escape through these pipes, the sphere was caused to rotate and here is the primitive steam reaction turbine.

Hero makes no suggestions as to application of any of the devices he describes to a useful purpose. From the time of Hero until the late sixteenth and early seventeenth centuries, there is no record of progress, though evidence is found that such devices as were described by Hero were sometimes used for trivial purposes, the blowing of an organ or the turning of a skillet.

Mathesius, the German author, in 1571; Besson, a philosopher and mathematician at Orleans; Ramelli, in 1588; Battista Delia Porta, a Neapolitan mathematician and philosopher, in 1601; Decause, the French engineer and architect, in 1615; and Branca, an Italian architect, in 1629, all published treatises bearing on the subject of the generation of steam.

To the next contributor, Edward Somerset, second Marquis of Worcester, is apparently due the credit of proposing, if not of making, the first useful steam engine. In the “Century of Scantlings and Inventions”, published in London in 1663, he describes devices showing that he had in mind the raising of water not only by forcing it from two receivers by direct steam pressure but also for some sort of reciprocating piston actuating one end of a lever, the other operating a pump. His descriptions are rather obscure and no drawings are extant so that it is difficult to say whether there were any distinctly novel features to his devices aside from the double action. While there is no direct authentic record that any of the devices he described were actually constructed, it is claimed by many that he really built and operated a steam engine containing pistons.

In 1675, Sir Samuel Moreland was decorated by King Charles II, for a demonstration of “a certain powerful machine to raise water.” Though there appears to be no record of the design of this machine, the mathematical dictionary, published in 1822, credits Moreland with the first account of a steam engine, on which subject he wrote a treatise that is still preserved in the British Museum.
[Pg 14]

Illustration:

397 Horse-power Babcock & Wilcox Boiler in Course of Erection at the Plant of the Crocker Wheeler Co., Ampere, N. J.

[Pg 15] Dr. Denys Papin, an ingenious Frenchman, invented in 1680 “a steam digester for extracting marrowy, nourishing juices from bones by enclosing them in a boiler under heavy pressure,” and finding danger from explosion, added a contrivance which is the first safety valve on record.

The steam engine first became commercially successful with Thomas Savery. In 1699, Savery exhibited before the Royal Society of England (Sir Isaac Newton was President at the time), a model engine which consisted of two copper receivers alternately connected by a three-way hand-operated valve, with a boiler and a source of water supply. When the water in one receiver had been driven out by the steam, cold water was poured over its outside surface, creating a vacuum through condensation and causing it to fill again while the water in the other reservoir was being forced out. A number of machines were built on this principle and placed in actual use as mine pumps.

The serious difficulty encountered in the use of Savery’s engine was the fact that the height to which it could lift water was limited by the pressure the boiler and vessels could bear. Before Savery’s engine was entirely displaced by its successor, Newcomen’s, it was considerably improved by Desaguliers, who applied the Papin safety valve to the boiler and substituted condensation by a jet within the vessel for Savery’s surface condensation.

In 1690, Papin suggested that the condensation of steam should be employed to make a vacuum beneath a cylinder which had previously been raised by the expansion of steam. This was the earliest cylinder and piston steam engine and his plan took practical shape in Newcomen’s atmospheric engine. Papin’s first engine was unworkable owing to the fact that he used the same vessel for both boiler and cylinder. A small quantity of water was placed in the bottom of the vessel and heat was applied. When steam formed and raised the piston, the heat was withdrawn and the piston did work on its down stroke under pressure of the atmosphere. After hearing of Savery’s engine, Papin developed an improved form. Papin’s engine of 1705 consisted of a displacement chamber in which a floating diaphragm or piston on top of the water kept the steam and water from direct contact. The water delivered by the downward movement of the piston under pressure, to a closed tank, flowed in a continuous stream against the vanes of a water wheel. When the steam in the displacement chamber had expanded, it was exhausted to the atmosphere through a valve instead of being condensed. The engine was, in fact, a non-condensing, single action steam pump with the steam and pump cylinders in one. A curious feature of this engine was a heater placed in the diaphragm. This was a mass of heated metal for the purpose of keeping the steam dry or preventing condensation during expansion. This device might be called the first superheater.

Among the various inventions attributed to Papin was a boiler with an internal fire box, the earliest record of such construction.

While Papin had neglected his earlier suggestion of a steam and piston engine to work on Savery’s ideas, Thomas Newcomen, with his assistant, John Cawley, put into practical form Papin’s suggestion of 1690. Steam admitted from the boiler to a cylinder raised a piston by its expansion, assisted by a counter-weight on the other end of a beam actuated by the piston. The steam valve was then shut and the steam condensed by a jet of cold water. The piston was then forced downward by atmospheric pressure and did work on the pump. The condensed water in the cylinder was expelled through an escapement valve by the next entry of steam. This engine used steam having pressure but little, if any, above that of the atmosphere.
[Pg 16]

Illustration:

Two Units of 8128 Horse Power of Babcock & Wilcox Boilers and Superheaters at the Fisk Street Station of the Commonwealth Edison Co., Chicago, Ill., 50,400 Horse Power being Installed in this Station. The Commonwealth Edison Co. Operates in its Various Stations a Total of 86,000 Horse Power of Babcock & Wilcox Boilers, all Fitted with Babcock & Wilcox Superheaters and Equipped with Babcock & Wilcox Chain Grate Stokers

[Pg 17] In 1711, this engine was introduced into mines for pumping purposes. Whether its action was originally automatic or whether dependent upon the hand operation of the valves is a question of doubt. The story commonly believed is that a boy, Humphrey Potter, in 1713, whose duty it was to open and shut such valves of an engine he attended, by suitable cords and catches attached to the beam, caused the engine to automatically manipulate these valves. This device was simplified in 1718 by Henry Beighton, who suspended from the bottom, a rod called the plug-tree, which actuated the valve by tappets. By 1725, this engine was in common use in the collieries and was changed but little for a matter of sixty or seventy years. Compared with Savery’s engine, from the aspect of a pumping engine, Newcomen’s was a distinct advance, in that the pressure in the pumps was in no manner dependent upon the steam pressure. In common with Savery’s engine, the losses from the alternate heating and cooling of the steam cylinder were enormous. Though obviously this engine might have been modified to serve many purposes, its use seems to have been limited almost entirely to the pumping of water.

The rivalry between Savery and Papin appears to have stimulated attention to the question of fuel saving. Dr. John Allen, in 1730, called attention to the fact that owing to the short length of time of the contact between the gases and the heating surfaces of the boiler, nearly half of the heat of the fire was lost. With a view to overcoming this loss at least partially, he used an internal furnace with a smoke flue winding through the water in the form of a worm in a still. In order that the length of passage of the gases might not act as a damper on the fire, Dr. Allen recommended the use of a pair of bellows for forcing the sluggish vapor through the flue. This is probably the first suggested use of forced draft. In forming an estimate of the quantity of fuel lost up the stack, Dr. Allen probably made the first boiler test.

Toward the end of the period of use of Newcomen’s atmospheric engine, John Smeaton, who, about 1770, built and installed a number of large engines of this type, greatly improved the design in its mechanical details.

The improvement in boiler and engine design of Smeaton, Newcomen and their contemporaries, were followed by those of the great engineer, James Watt, an instrument maker of Glasgow. In 1763, while repairing a model of Newcomen’s engine, he was impressed by the great waste of steam to which the alternating cooling and heating of the engine gave rise. His remedy was the maintaining of the cylinder as hot as the entering steam and with this in view he added a vessel separate from the cylinder, into which the steam should pass from the cylinder and be there condensed either by the application of cold water outside or by a jet from within. To preserve a vacuum in his condenser, he added an air pump which should serve to remove the water of condensation and air brought in with the injection water or due to leakage. As the cylinder no longer acted as a condenser, he could maintain it at a high temperature by covering it with non-conducting material and, in particular, by the use of a steam jacket. Further and with the same object in view, he covered the top of the cylinder and introduced steam above the piston to do the work previously accomplished by atmospheric pressure. After several trials with an experimental apparatus based on these ideas, Watt patented his improvements in 1769. Aside from their historical importance, Watt’s improvements, as described in his specification, are to this day a [Pg 18]
[Pg 19]
statement of the principles which guide the scientific development of the steam engine. His words are:

Illustration:

Erie County Electric Co., Erie, Pa., Operating 3082 Horse Power of Babcock & Wilcox Boilers and Superheaters, Equipped with Babcock & Wilcox Chain Grate Stokers

“My method of lessening the consumption of steam, and consequently fuel, in fire engines, consists of the following principles:

“First, That vessel in which the powers of steam are to be employed to work the engine, which is called the cylinder in common fire engines, and which I call the steam vessel, must, during the whole time the engine is at work, be kept as hot as the steam that enters it; first, by enclosing it in a case of wood, or any other materials that transmit heat slowly; secondly, by surrounding it with steam or other heated bodies; and, thirdly, by suffering neither water nor any other substance colder than the steam to enter or touch it during that time.

“Secondly, In engines that are to be worked wholly or partially by condensation of steam, the steam is to be condensed in vessels distinct from the steam vessels or cylinders, although occasionally communicating with them; these vessels I call condensers; and, whilst the engines are working, these condensers ought at least to be kept as cold as the air in the neighborhood of the engines, by application of water or other cold bodies.

“Thirdly, Whatever air or other elastic vapor is not condensed by the cold of the condenser, and may impede the working of the engine, is to be drawn out of the steam vessels or condensers by means of pumps, wrought by the engines themselves, or otherwise.

“Fourthly, I intend in many cases to employ the expansive force of steam to press on the pistons, or whatever may be used instead of them, in the same manner in which the pressure of the atmosphere is now employed in common fire engines. In cases where cold water cannot be had in plenty, the engines may be wrought by this force of steam only, by discharging the steam into the air after it has done its office….

“Sixthly, I intend in some cases to apply a degree of cold not capable of reducing the steam to water, but of contracting it considerably, so that the engines shall be worked by the alternate expansion and contraction of the steam.

“Lastly, Instead of using water to render the pistons and other parts of the engine air and steam tight, I employ oils, wax, resinous bodies, fat of animals, quick-silver and other metals in their fluid state.”

The fifth claim was for a rotary engine, and need not be quoted here.

The early efforts of Watt are typical of those of the poor inventor struggling with insufficient resources to gain recognition and it was not until he became associated with the wealthy manufacturer, Mattheu Boulton of Birmingham, that he met with the success upon which his present fame is based. In partnership with Boulton, the business of the manufacture and the sale of his engines were highly successful in spite of vigorous attacks on the validity of his patents.

Though the fourth claim of Watt’s patent describes a non-condensing engine which would require high pressures, his aversion to such practice was strong. Notwithstanding his entire knowledge of the advantages through added expansion under high pressure, he continued to use pressures not above 7 pounds per square inch above the atmosphere. To overcome such pressures, his boilers were fed through a stand-pipe of sufficient height to have the column of water offset the pressure within the boiler. Watt’s attitude toward high pressure made his influence felt long after his patents had expired.
[Pg 20]

Illustration:

Portion of 9600 Horse-power Installation of Babcock & Wilcox Boilers and Superheaters, Equipped with Babcock & Wilcox Chain Grate Stokers at the Blue Island, Ill., Plant of the Public Service Co. of Northern Illinois. This Company Operates 14,580 Horse Power of Babcock & Wilcox Boilers and Superheaters in its Various Stations

[Pg 21] In 1782, Watt patented two other features which he had invented as early as 1769. These were the double acting engine, that is, the use of steam on both sides of the piston and the use of steam expansively, that is, the shutting off of steam from the cylinder when the piston had made but a portion of its stroke, the power for the completion of the stroke being supplied by the expansive force of the steam already admitted.

He further added a throttle valve for the regulation of steam admission, invented the automatic governor and the steam indicator, a mercury steam gauge and a glass water column.

It has been the object of this brief history of the early developments in the use of steam to cover such developments only through the time of James Watt. The progress of the steam engine from this time through the stages of higher pressures, combining of cylinders, the application of steam vehicles and steamboats, the adding of third and fourth cylinders, to the invention of the turbine with its development and the accompanying development of the reciprocating engine to hold its place, is one long attribute to the inventive genius of man.

While little is said in the biographies of Watt as to the improvement of steam boilers, all the evidence indicates that Boulton and Watt introduced the first “wagon boiler”, so called because of its shape. In 1785, Watt took out a number of patents for variations in furnace construction, many of which contain the basic principles of some of the modern smoke preventing furnaces. Until the early part of the nineteenth century, the low steam pressures used caused but little attention to be given to the form of the boiler operated in connection with the engines above described. About 1800, Richard Trevithick, in England, and Oliver Evans, in America, introduced non-condensing, and for that time, high pressure steam engines. To the initiative of Evans may be attributed the general use of high pressure steam in the United States, a feature which for many years distinguished American from European practice. The demand for light weight and economy of space following the beginning of steam navigation and the invention of the locomotive required boilers designed and constructed to withstand heavier pressures and forced the adoption of the cylindrical form of boiler. There are in use to-day many examples of every step in the development of steam boilers from the first plain cylindrical boiler to the most modern type of multi-tubular locomotive boiler, which stands as the highest type of fire-tube boiler construction.

The early attempts to utilize water-tube boilers were few. A brief history of the development of the boilers, in which this principle was employed, is given in the following chapter. From this history it will be clearly indicated that the first commercially successful utilization of water tubes in a steam generator is properly attributed to George H. Babcock and Stephen Wilcox.
[Pg 22]

Illustration:

Copyright by Underwood & Underwood

Woolworth Building, New York City, Operating 2454 Horse Power of Babcock & Wilcox Boilers

[Pg 23]

BRIEF HISTORY OF WATER-TUBE BOILERS[1]

Blakey Boiler

Blakey, 1766

As stated in the previous chapter, the first water-tube boiler was built by John Blakey and was patented by him in 1766. Several tubes alternately inclined at opposite angles were arranged in the furnaces, the adjacent tube ends being connected by small pipes. The first successful user of water-tube boilers, however, was James Rumsey, an American inventor, celebrated for his early experiments in steam navigation, and it is he who may be truly classed as the originator of the water-tube boiler. In 1788 he patented, in England, several forms of boilers, some of which were of the water-tube type. One had a fire box with flat top and sides, with horizontal tubes across the fire box connecting the water spaces. Another had a cylindrical fire box surrounded by an annular water space and a coiled tube was placed within the box connecting at its two ends with the water space. This was the first of the “coil boilers”. Another form in the same patent was the vertical tubular boiler, practically as made at the present time.

Stevens Boiler

John Stevens, 1804

The first boiler made of a combination of small tubes, connected at one end to a reservoir, was the invention of another American, John Stevens, in 1804. This boiler was actually employed to generate steam for running a steamboat on the Hudson River, but like all the “porcupine” boilers, of which type it was the first, it did not have the elements of a continued success.

Stevens Boiler

John Cox Stevens, 1805

Another form of water tube was patented in 1805 by John Cox Stevens, a son of John Stevens. This boiler consisted of twenty vertical tubes, 1¼ inches internal diameter and 40½ inches long, arranged in a circle, the outside diameter of which was approximately 12 inches, connecting a water chamber at the bottom with a steam chamber at the top. The steam and water chambers were annular spaces of small cross section and contained approximately 33 cubic inches. The illustration shows the cap of the steam chamber secured by bolts. The steam outlet pipe “A” is a pipe of one inch diameter, the water entering through a similar aperture at the bottom. One of these boilers was for a long time at the Stevens Institute of Technology at Hoboken, and is now in the Smithsonian Institute at Washington.

About the same time, Jacob Woolf built a boiler of large horizontal tubes, extending across the furnace and connected at the ends to a longitudinal drum above. The first purely sectional [Pg 24] water-tube boiler was built by Julius Griffith, in 1821. In this boiler, a number of horizontal water tubes were connected to vertical side pipes, the side pipes were connected to horizontal gathering pipes, and these latter in turn to a steam drum.

Eve Boiler

Joseph Eve, 1825

In 1822, Jacob Perkins constructed a flash boiler for carrying what was then considered a high pressure. A number of cast-iron bars having 1½ inches annular holes through them and connected at their outer ends by a series of bent pipes, outside of the furnace walls, were arranged in three tiers over the fire. The water was fed slowly to the upper tier by a force pump and steam in the superheated state was discharged to the lower tiers into a chamber from which it was taken to the engine.

The first sectional water-tube boiler, with a well-defined circulation, was built by Joseph Eve, in 1825. The sections were composed of small tubes with a slight double curve, but being practically vertical, fixed in horizontal headers, which headers were in turn connected to a steam space above and a water space below formed of larger pipes. The steam and water spaces were connected by outside pipes to secure a circulation of the water up through the sections and down through the external pipes. In the same year, John M’Curdy of New York, built a “Duplex Steam Generator” of “tubes of wrought or cast iron or other material” arranged in several horizontal rows, connected together alternately at the front and rear by return bends. In the tubes below the water line were placed interior circular vessels closed at the ends in order to expose a thin sheet of water to the action of the fire.

Gurney Boiler

Gurney, 1826

In 1826, Goldsworthy Gurney built a number of boilers, which he used on his steam carriages. A number of small tubes were bent into the shape of a “U” laid [Pg 25] sidewise and the ends were connected with larger horizontal pipes. These were connected by vertical pipes to permit of circulation and also to a vertical cylinder which served as a steam and water reservoir. In 1828, Paul Steenstrup made the first shell boiler with vertical water tubes in the large flues, similar to the boiler known as the “Martin” and suggesting the “Galloway”.

The first water-tube boiler having fire tubes within water tubes was built in 1830, by Summers & Ogle. Horizontal connections at the top and bottom were connected by a series of vertical water tubes, through which were fire tubes extending through the horizontal connections, the fire tubes being held in place by nuts, which also served to make the joint.

Wilcox Boiler

Stephen Wilcox, 1856

Stephen Wilcox, in 1856, was the first to use inclined water tubes connecting water spaces at the front and rear with a steam space above. The first to make such inclined tubes into a sectional form was Twibill, in 1865. He used wrought-iron tubes connected at the front and rear with standpipes through intermediate connections. These standpipes carried the system to a horizontal cross drum at the top, the entrained water being carried to the rear.

Clarke, Moore, McDowell, Alban and others worked on the problem of constructing water-tube boilers, but because of difficulties of construction involved, met with no practical success.

Twibill Boiler

Twibill, 1865

It may be asked why water-tube boilers did not come into more general use at an early date, that is, why the number of water-tube boilers built was so small in comparison to the number of shell boilers. The reason for this is found in the difficulties involved in the design and construction of water-tube boilers, which design and construction required a high class of engineering and workmanship, while the plain cylindrical boiler is comparatively easy to build. The greater skill required to make a water-tube boiler successful is readily shown in the great number of failures in the attempts to make them.

 [Pg 26]

Illustration:

Partial View of 7000 Horse-power Installation of Babcock & Wilcox Boilers at the Philadelphia, Pa., Plant of the Baldwin Locomotive Works. This Company Operates in its Various Plants a Total of 9280 Horse Power of Babcock & Wilcox Boilers

FOOTNOTES

[1] See discussion by George H. Babcock, of Stirling’s paper on “Water-tube and Shell Boilers”, in Transactions, American Society of Mechanical Engineers, Volume VI., Page 601.

[Pg 27]

REQUIREMENTS OF STEAM BOILERS

Since the first appearance in “Steam” of the following “Requirements of a Perfect Steam Boiler”, the list has been copied many times either word for word or clothed in different language and applied to some specific type of boiler design or construction. In most cases, although full compliance with one or more of the requirements was structurally impossible, the reader was left to infer that the boiler under consideration possessed all the desirable features. It is noteworthy that this list of requirements, as prepared by George H. Babcock and Stephen Wilcox, in 1875, represents the best practice of to-day. Moreover, coupled with the boiler itself, which is used in the largest and most important steam generating plants throughout the world, the list forms a fitting monument to the foresight and genius of the inventors.

REQUIREMENTS OF A PERFECT STEAM BOILER

1st. Proper workmanship and simple construction, using materials which experience has shown to be the best, thus avoiding the necessity of early repairs.

2nd. A mud drum to receive all impurities deposited from the water, and so placed as to be removed from the action of the fire.

3rd. A steam and water capacity sufficient to prevent any fluctuation in steam pressure or water level.

4th. A water surface for the disengagement of the steam from the water, of sufficient extent to prevent foaming.

5th. A constant and thorough circulation of water throughout the boiler, so as to maintain all parts at the same temperature.

6th. The water space divided into sections so arranged that, should any section fail, no general explosion can occur and the destructive effects will be confined to the escape of the contents. Large and free passages between the different sections to equalize the water line and pressure in all.

7th. A great excess of strength over any legitimate strain, the boiler being so constructed as to be free from strains due to unequal expansion, and, if possible, to avoid joints exposed to the direct action of the fire.

8th. A combustion chamber so arranged that the combustion of the gases started in the furnace may be completed before the gases escape to the chimney.

9th. The heating surface as nearly as possible at right angles to the currents of heated gases, so as to break up the currents and extract the entire available heat from the gases.

10th. All parts readily accessible for cleaning and repairs. This is a point of the greatest importance as regards safety and economy.

11th. Proportioned for the work to be done, and capable of working to its full rated capacity with the highest economy.

12th. Equipped with the very best gauges, safety valves and other fixtures.

[Pg 28]

The exhaustive study made of each one of these requirements is shown by the following extract from a lecture delivered by Mr. Geo. H. Babcock at Cornell University in 1890 upon the subject:

THE CIRCULATION OF WATER IN STEAM BOILERS

You have all noticed a kettle of water boiling over the fire, the fluid rising somewhat tumultuously around the edges of the vessel, and tumbling toward the center, where it descends. Similar currents are in action while the water is simply being heated, but they are not perceptible unless there are floating particles in the liquid. These currents are caused by the joint action of the added temperature and two or more qualities which the water possesses.

1st. Water, in common with most other substances, expands when heated; a statement, however, strictly true only when referred to a temperature above 39 degrees F. or 4 degrees C., but as in the making of steam we rarely have to do with temperatures so low as that, we may, for our present purposes, ignore that exception.

2nd. Water is practically a non-conductor of heat, though not entirely so. If ice-cold water was kept boiling at the surface the heat would not penetrate sufficiently to begin melting ice at a depth of 3 inches in less than about two hours. As, therefore, the heated water cannot impart its heat to its neighboring particles, it remains expanded and rises by its levity, while colder portions come to be heated in turn, thus setting up currents in the fluid.

Fig. 1

Fig. 1

Now, when all the water has been heated to the boiling point corresponding to the pressure to which it is subjected, each added unit of heat converts a portion, about 7 grains in weight, into vapor, greatly increasing its volume; and the mingled steam and water rises more rapidly still, producing ebullition such as we have noticed in the kettle. So long as the quantity of heat added to the contents of the kettle continues practically constant, the conditions remain similar to those we noticed at first, a tumultuous lifting of the water around the edges, flowing toward the center and thence downward; if, however, the fire be quickened, the upward currents interfere with the downward and the kettle boils over (Fig. 1).

Fig. 2

Fig. 2

If now we put in the kettle a vessel somewhat smaller (Fig. 2) with a hole in the bottom and supported at a proper distance from the side so as to separate the upward from the downward currents, we can force the fires to a very much greater extent without causing the kettle to boil over, and when we place a deflecting plate so as to guide the rising column toward the center it will be almost impossible to produce that effect. This is the invention of Perkins in 1831 and forms the basis of very many of the arrangements for producing free circulation of the water in boilers which have been made since that time. It consists in dividing the currents so that they will not interfere each with the other.

[Pg 29]

Fig. 3

Fig. 3

But what is the object of facilitating the circulation of water in boilers? Why may we not safely leave this to the unassisted action of nature as we do in culinary operations? We may, if we do not care for the three most important aims in steam-boiler construction, namely, efficiency, durability, and safety, each of which is more or less dependent upon a proper circulation of the water. As for efficiency, we have seen one proof in our kettle. When we provided means to preserve the circulation, we found that we could carry a hotter fire and boil away the water much more rapidly than before. It is the same in a steam boiler. And we also noticed that when there was nothing but the unassisted circulation, the rising steam carried away so much water in the form of foam that the kettle boiled over, but when the currents were separated and an unimpeded circuit was established, this ceased, and a much larger supply of steam was delivered in a comparatively dry state. Thus, circulation increases the efficiency in two ways: it adds to the ability to take up the heat, and decreases the liability to waste that heat by what is technically known as priming. There is yet another way in which, incidentally, circulation increases efficiency of surface, and that is by preventing in a greater or less degree the formation of deposits thereon. Most waters contain some impurity which, when the water is evaporated, remains to incrust the surface of the vessel. This incrustation becomes very serious sometimes, so much so as to almost entirely prevent the transmission of heat from the metal to the water. It is said that an incrustation of only one-eighth inch will cause a loss of 25 per cent in efficiency, and this is probably within the truth in many cases. Circulation of water will not prevent incrustation altogether, but it lessens the amount in all waters, and almost entirely so in some, thus adding greatly to the efficiency of the surface.

Fig. 4

Fig. 4

A second advantage to be obtained through circulation is durability of the boiler. This it secures mainly by keeping all parts at a nearly uniform temperature. The way to secure the greatest freedom from unequal strains in a boiler is to provide for such a circulation of the water as will insure the same temperature in all parts.

3rd. Safety follows in the wake of durability, because a boiler which is not subject to unequal strains of expansion and contraction is not only less liable to ordinary repairs, but also to rupture and disastrous explosion. By far the most prolific cause of explosions is this same strain from unequal expansions.

Having thus briefly looked at the advantages of circulation of water in steam boilers, let us see what are the best means of securing it under the most efficient conditions We have seen in our kettle that one essential point was that the currents should be kept from interfering with each other. If we could look into an ordinary return tubular boiler when steaming, we should see a curious commotion of currents rushing hither and thither, and shifting continually as one or the other contending force gained a momentary mastery. The principal upward currents would be found at the two ends, one over the fire and the other over the first foot or so of the tubes. Between these, the downward currents struggle [Pg 30]
[Pg 31]
against the rising currents of steam and water. At a sudden demand for steam, or on the lifting of the safety valve, the pressure being slightly reduced, the water jumps up in jets at every portion of the surface, being lifted by the sudden generation of steam throughout the body of water. You have seen the effect of this sudden generation of steam in the well-known experiment with a Florence flask, to which a cold application is made while boiling water under pressure is within. You have also witnessed the geyser-like action when water is boiled in a test tube held vertically over a lamp (Fig. 3).

Illustration:

386 Horse-power Installation of Babcock & Wilcox Boilers at B. F. Keith’s Theatre, Boston, Mass.

Fig. 5

Fig. 5

If now we take a U-tube depending from a vessel of water (Fig. 4) and apply the lamp to one leg a circulation is at once set up within it, and no such spasmodic action can be produced. Thus U-tube is the representative of the true method of circulation within a water-tube boiler properly constructed. We can, for the purpose of securing more heating surface, extend the heated leg into a long incline (Fig. 5), when we have the well-known inclined-tube generator. Now, by adding other tubes, we may further increase the heating surface (Fig. 6), while it will still be the U-tube in effect and action. In such a construction the circulation is a function of the difference in density of the two columns. Its velocity is measured by the well-known Torricellian formula, V = (2gh)½, or, approximately V = 8(h)½, h being measured in terms of the lighter fluid. This velocity will increase until the rising column becomes all steam, but the quantity or weight circulated will attain a maximum when the density of the mingled steam and water in the rising column becomes one-half that of the solid water in the descending column which is nearly coincident with the condition of half steam and half water, the weight of the steam being very slight compared to that of the water.

It becomes easy by this rule to determine the circulation in any given boiler built on this principle, provided the construction is such as to permit a free flow of the water. Of course, every bend detracts a little and something is lost in getting up the velocity, but when the boiler is well arranged and proportioned these retardations are slight.

Fig. 6

Fig. 6

Let us take for example one of the 240 horse-power Babcock & Wilcox boilers here in the University. The height of the columns may be taken as 4½ feet, measuring from the surface of the water to about the center of the bundle of tubes over the fire, and the head would be equal to this height at the maximum of circulation. We should, therefore, have a velocity of 8(4½)½ = 16.97, say 17 feet per second. There are in this boiler fourteen sections, each having a 4-inch tube opening into the drum, the area of which (inside) is 11 square inches, the fourteen aggregating 154 square inches, or 1.07 square feet. This multiplied by the velocity, 16.97 feet, gives 18.16 cubic feet mingled steam and water discharged per second, one-half of which, or 9.08 cubic feet, is steam. Assuming this steam to be at 100 pounds gauge pressure, it will weigh 0.258 pound per cubic foot. Hence, 2.34 pounds of steam will be [Pg 32]
[Pg 33] discharged per second, and 8,433 pounds per hour. Dividing this by 30, the number of pounds representing a boiler horse power, we get 281.1 horse power, about 17 per cent, in excess of the rated power of the boiler. The water at the temperature of steam at 100 pounds pressure weighs 56 pounds per cubic foot, and the steam 0.258 pound, so that the steam forms but 1218 part of the mixture by weight, and consequently each particle of water will make 218 circuits before being evaporated when working at this capacity, and circulating the maximum weight of water through the tubes.

Illustration:

A Portion of 9600 Horse-power Installation of Babcock & Wilcox Boilers and Superheaters Being Erected at the South Boston, Mass., Station of the Boston Elevated Railway Co. This Company Operates in its Various Stations a Total of 46,400 Horse Power of Babcock & Wilcox Boilers

Fig. 7

Fig. 7

It is evident that at the highest possible velocity of exit from the generating tubes, nothing but steam will be delivered and there will be no circulation of water except to supply the place of that evaporated. Let us see at what rate of steaming this would occur with the boiler under consideration. We shall have a column of steam, say 4 feet high on one side and an equal column of water on the other. Assuming, as before, the steam at 100 pounds and the water at same temperature, we will have a head of 866 feet of steam and an issuing velocity of 235.5 feet per second. This multiplied by 1.07 square feet of opening by 3,600 seconds in an hour, and by 0.258 gives 234,043 pounds of steam, which, though only one-eighth the weight of mingled steam and water delivered at the maximum, gives us 7,801 horse power, or 32 times the rated power of the boiler. Of course, this is far beyond any possibility of attainment, so that it may be set down as certain that this boiler cannot be forced to a point where there will not be an efficient circulation of the water. By the same method of calculation it may be shown that when forced to double its rated power, a point rarely expected to be reached in practice, about two-thirds the volume of mixture of steam and water delivered into the drum will be steam, and that the water will make 110 circuits while being evaporated. Also that when worked at only about one-quarter its rated capacity, one-fifth of the volume will be steam and the water will make the rounds 870 times before it becomes steam. You will thus see that in the proportions adopted in this boiler there is provision for perfect circulation under all the possible conditions of practice.

Fig. 8

Fig. 8 [Developed to show Circulation]

In designing boilers of this style it is necessary to guard against having the uptake at the upper end of the tubes too large, for if sufficiently large to allow downward currents therein, the whole effect of the rising column in increasing the circulation in the tubes is nullified (Fig. 7). This will readily be seen if we consider the uptake very large when the only head producing circulation in the tubes will be that due to the inclination of each tube taken by itself. This objection is only overcome when the uptake is so small as to be entirely filled with the ascending current of mingled steam and water. It is also necessary that this uptake should be practically direct, and it should not be composed of frequent enlargements and [Pg 34]
[Pg 35]
contractions. Take, for instance, a boiler well known in Europe, copied and sold here under another name. It is made up of inclined tubes secured by pairs into boxes at the ends, which boxes are made to communicate with each other by return bends opposite the ends of the tubes. These boxes and return bends form an irregular uptake, whereby the steam is expected to rise to a reservoir above. You will notice (Fig. 8) that the upward current of steam and water in the return bend meets and directly antagonizes the upward current in the adjoining tube. Only one result can follow. If their velocities are equal, the momentum of both will be neutralized and all circulation stopped, or, if one be stronger, it will cause a back flow in the other by the amount of difference in force, with practically the same result.

Illustration:

4880 Horse-power Installation of Babcock & Wilcox Boilers at the Open Hearth Plant of the Cambria Steel Co., Johnstown, Pa. This Company Operates a Total of 52,000 Horse Power of Babcock & Wilcox Boilers

Fig. 9

Fig. 9

In a well-known boiler, many of which were sold, but of which none are now made and a very few are still in use, the inventor claimed that the return bends and small openings against the tubes were for the purpose of “restricting the circulation” and no doubt they performed well that office; but excepting for the smallness of the openings they were not as efficient for that purpose as the arrangement shown in Fig. 8.

Another form of boiler, first invented by Clarke or Crawford, and lately revived, has the uptake made of boxes into which a number, generally from two to four tubes, are expanded, the boxes being connected together by nipples (Fig. 9). It is a well-known fact that where a fluid flows through a conduit which enlarges and then contracts, the velocity is lost to a greater or less extent at the enlargements, and has to be gotten up again at the contractions each time, with a corresponding loss of head. The same thing occurs in the construction shown in Fig. 9. The enlargements and contractions quite destroy the head and practically overcome the tendency of the water to circulate.

Fig. 10

Fig. 10

A horizontal tube stopped at one end, as shown in Fig. 10, can have no proper circulation within it. If moderately driven, the water may struggle in against the issuing steam sufficiently to keep the surface covered, but a slight degree of forcing will cause it to act like the test tube in Fig. 3, and the more there are of them in a given boiler the more spasmodic will be its working.

The experiment with our kettle (Fig. 2) gives the clue to the best means of promoting circulation in ordinary shell boilers. Steenstrup or “Martin” and “Galloway” water tubes placed in such boilers also assist in directing the circulation therein, but it is almost impossible to produce in shell boilers, by any means the circulation of all the water in one continuous round, such as marks the well-constructed water-tube boiler.

As I have before remarked, provision for a proper circulation of water has been almost universally ignored in designing steam boilers, sometimes to the great damage of the owner, but oftener to the jeopardy of the lives of those who are employed to run them. The noted case of the Montana and her sister ship, where some $300,000 [Pg 36] was thrown away in trying an experiment which a proper consideration of this subject would have avoided, is a case in point; but who shall count the cost of life and treasure not, perhaps, directly traceable to, but, nevertheless, due entirely to such neglect in design and construction of the thousands of boilers in which this necessary element has been ignored?

In the light of the performance of the exacting conditions of present day power-plant practice, a review of this lecture and of the foregoing list of requirements reveals the insight of the inventors of the Babcock & Wilcox boiler into the fundamental principles of steam generator design and construction.

Since the Babcock & Wilcox boiler became thoroughly established as a durable and efficient steam generator, many types of water-tube boilers have appeared on the market. Most of them, failing to meet enough of the requirements of a perfect boiler, have fallen by the wayside, while a few failing to meet all of the requirements, have only a limited field of usefulness. None have been superior, and in the most cases the most ardent admirers of other boilers have been satisfied in looking up to the Babcock & Wilcox boiler as a standard and in claiming that the newer boilers were “just as good.”

Records of recent performances under the most severe conditions of services on land and sea, show that the Babcock & Wilcox boiler can be run continually and regularly at higher overloads, with higher efficiency, and lower upkeep cost than any other boiler on the market. It is especially adapted for power-plant work where it is necessary to use a boiler in which steam can be raised quickly and the boiler placed on the line either from a cold state or from a banked fire in the shortest possible time, and with which the capacity, with clean feed water, will be largely limited by the amount of coal that can be burned in the furnace.

The distribution of the circulation through the separate headers and sections and the action of the headers in forcing a maximum and continuous circulation in the lower tubes, permit the operation of the Babcock & Wilcox boiler without objectionable priming, with a higher degree of concentration of salts in the water than is possible in any other type of boiler.

Repeated daily performances at overloads have demonstrated beyond a doubt the correctness of Mr. Babcock’s computation regarding the circulating tube and header area required for most efficient circulation. They also have proved that enlargement of the area of headers and circulating tubes beyond a certain point diminishes the head available for causing circulation and consequently limits the ability of the boiler to respond to demands for overloads.

In this lecture Mr. Babcock made the prediction that with the circulating tube area proportioned in accordance with the principles laid down, the Babcock & Wilcox boiler could be continuously run at double its nominal rating, which at that time was based on 12 square feet of heating surface per horse power. This prediction is being fulfilled daily in all the large and prominent power plants in this country and abroad, and it has been repeatedly demonstrated that with clean water and clean tube surfaces it is possible to safely operate at over 300 per cent of the nominal rating.

In the development of electrical power stations it becomes more and more apparent that it is economical to run a boiler at high ratings during the times of peak loads, as by so doing the lay-over losses are diminished and the economy of the plant as a whole is increased.

[Pg 37]

The number and importance of the large electric lighting and power stations constructed during the last ten years that are equipped with Babcock & Wilcox boilers, is a most gratifying demonstration of the merit of the apparatus, especially in view of their satisfactory operation under conditions which are perhaps more exacting than those of any other service.

Time, the test of all, results with boilers as with other things, in the survival of the fittest. When judged on this basis the Babcock & Wilcox boiler stands pre-eminent in its ability to cover the whole field of steam generation with the highest commercial efficiency obtainable. Year after year the Babcock & Wilcox boiler has become more firmly established as the standard of excellence in the boiler making art.

Illustration:

South Boston Station of the Boston Elevated Ry. Co., Boston, Mass. 9600 Horse Power of Babcock & Wilcox Boilers and Superheaters Installed in this Station

[Pg 38]

Illustration:

3600 Horse-power Installation of Babcock & Wilcox Boilers at the Phipps Power House of the Duquesne Light Company, Pittsburgh, Pa.

[Pg 39]

EVOLUTION OF THE BABCOCK & WILCOX WATER-TUBE BOILER

Quite as much may be learned from the records of failures as from those of success. Where a device has been once fairly tried and found to be imperfect or impracticable, the knowledge of that trial is of advantage in further investigation. Regardless of the lesson taught by failure, however, it is an almost every-day occurrence that some device or construction which has been tried and found wanting, if not worthless, is again introduced as a great improvement upon a device which has shown by its survival to be the fittest.

The success of the Babcock & Wilcox boiler is due to many years of constant adherence to one line of research, in which an endeavor has been made to introduce improvements with the view to producing a boiler which would most effectively meet the demands of the times. During the periods that this boiler has been built, other companies have placed on the market more than thirty water-tube or sectional water-tube boilers, most of which, though they may have attained some distinction and sale, have now entirely disappeared. The following incomplete list will serve to recall the names of some of the boilers that have had a vogue at various times, but which are now practically unknown: Dimpfel, Howard, Griffith & Wundrum, Dinsmore, Miller “Fire Box”, Miller “American”, Miller “Internal Tube”, Miller “Inclined Tube”, Phleger, Weigant, the Lady Verner, the Allen, the Kelly, the Anderson, the Rogers & Black, the Eclipse or Kilgore, the Moore, the Baker & Smith, the Renshaw, the Shackleton, the “Duplex”, the Pond & Bradford, the Whittingham, the Bee, the Hazleton or “Common Sense”, the Reynolds, the Suplee or Luder, the Babbit, the Reed, the Smith, the Standard, etc., etc.

It is with the object of protecting our customers and friends from loss through purchasing discarded ideas that there is given on the following pages a brief history of the development of the Babcock & Wilcox boiler as it is built to-day. The illustrations and brief descriptions indicate clearly the various designs and constructions that have been used and that have been replaced, as experience has shown in what way improvement might be made. They serve as a history of the experimental steps in the development of the present Babcock & Wilcox boiler, the value and success of which, as a steam generator, is evidenced by the fact that the largest and most discriminating users continue to purchase them after years of experience in their operation.

No. 1

No. 1

No. 1. The original Babcock & Wilcox boiler was patented in 1867. The main idea in its design was safety, to which all other features were sacrificed wherever they conflicted. The boiler consisted of a nest of horizontal tubes, serving as a steam and water reservoir, placed above and connected at each end by bolted [Pg 40] joints to a second nest of inclined heating tubes filled with water. The tubes were placed one above the other in vertical rows, each row and its connecting end forming a single casting. Hand-holes were placed at each end for cleaning. Internal tubes were placed within the inclined tubes with a view to aiding circulation.

No. 2. This boiler was the same as No. 1, except that the internal circulating tubes were omitted as they were found to hinder rather than help the circulation.

Nos. 1 and 2 were found to be faulty in both material and design, cast metal proving unfit for heating surfaces placed directly over the fire, as it cracked as soon as any scale formed.

No. 3. Wrought-iron tubes were substituted for the cast-iron heating tubes, the ends being brightened, laid in moulds, and the headers cast on.

The steam and water capacity in this design were insufficient to secure regularity of action, there being no reserve upon which to draw during firing or when the water was fed intermittently. The attempt to dry the steam by superheating it in the nest of tubes forming the steam space was found to be impracticable. The steam delivered was either wet, dry or superheated, according to the rate at which it was being drawn from the boiler. Sediment was found to lodge in the lowermost point of the boiler at the rear end and the exposed portions cracked off at this point when subjected to the furnace heat.

No. 4

No. 4

No. 4. A plain cylinder, carrying the water line at its center and leaving the upper half for steam space, was substituted for the nest of tubes forming the steam and water space in Nos. 1, 2 and 3. The sections were made as in No. 3 and a mud drum added to the rear end of the sections at the point that was lowest and farthest removed from the fire. The gases were made to pass off at one side and did not come into contact with the mud drum. Dry steam was obtained through the increase of separating surface and steam space and the added water capacity furnished a storage for heat to tide over irregularities of firing and feeding. By the addition of the drum, the boiler became a serviceable and practical design, retaining all of the features of safety. As the drum was removed from the direct action of the fire, it was not subjected to excessive strain due to unequal expansion, and its diameter, if large in comparison with that of the tubes formerly used, was small when compared with that of cylindrical boilers. Difficulties were encountered in this boiler in securing reliable joints between the wrought-iron tubes and the cast-iron headers.

No. 5

No. 5

No. 5. In this design, wrought-iron water legs were substituted for the cast-iron headers, the tubes being expanded into the inside sheets and a large [Pg 41] cover placed opposite the front end of the tubes for cleaning. The tubes were staggered one above the other, an arrangement found to be more efficient in the absorption of heat than where they were placed in vertical rows. In other respects, the boiler was similar to No. 4, except that it had lost the important element of safety through the introduction of the very objectionable feature of flat stayed surfaces. The large doors for access to the tubes were also a cause of weakness.

An installation of these boilers was made at the plant of the Calvert Sugar Refinery in Baltimore, and while they were satisfactory in their operation, were never duplicated.

No. 6

No. 6

No. 6. This was a modification of No. 5 in which longer tubes were used and over which the gases were caused to make three passes with a view of better economy. In addition, some of the stayed surfaces were omitted and handholes substituted for the large access doors. A number of boilers of this design were built but their excessive first cost, the lack of adjustability of the structure under varying temperatures, and the inconvenience of transportation, led to No. 7.

No. 7

No. 7

No. 7. In this boiler, the headers and water legs were replaced by T-heads screwed to the ends of the inclined tubes. The faces of these Ts were milled and the tubes placed one above the other with the milled faces metal to metal. Long bolts passed through each vertical section of the T-heads and through connecting boxes on the heads of the drums holding the whole together. A large number of boilers of this design were built and many were in successful operation for over twenty years. In most instances, however, they were altered to later types.

No. 8

No. 8

       No. 9

No. 9

[Pg 42]

Nos. 8 and 9. These boilers were known as the Griffith & Wundrum type, the concern which built them being later merged in The Babcock & Wilcox Co. Experiments were made with this design with four passages of the gases across the tubes and the downward circulation of the water at the rear of the boiler was carried to the bottom row of tubes. In No. 9 an attempt was made to increase the safety and reduce the cost by reducing the amount of steam and water capacity. A drum at right angles to the line of tubes was used but as there was no provision made to secure dry steam, the results were not satisfactory. The next move in the direction of safety was the employment of several drums of small diameter instead of a single drum.

No. 10

No. 10

This is shown in No. 10. A nest of small horizontal drums, 15 inches in diameter, was used in place of the single drum of larger diameter. A set of circulation tubes was placed at an intermediate angle between the main bank of heating tubes and the horizontal drums forming the steam reservoir. These circulators were to return to the rear end of the circulating tubes the water carried up by the circulation, and in this way were to allow only steam to be delivered to the small drums above. There was no improvement in the action of this boiler over that of No. 9.

The four passages of the gas over the tubes tried in Nos. 8, 9 and 10 were not found to add to the economy of the boiler.

No. 11

No. 11

No. 11. A trial was next made of a box coil system, in which the water was made to transverse the furnace several times before being delivered to the drum above. The tendency here, as in all similar boilers, was to form steam in the middle of the coil and blow the water from each end, leaving the tubes practically dry until the steam found an outlet and the water returned. This boiler had, in addition to a defective circulation, a decidedly geyser-like action and produced wet steam.

No. 12

No. 12

All of the types mentioned, with the exception of Nos. 5 and 6, had between their several parts a large number of bolted joints which were subjected to the action [Pg 43] of the fire. When these boilers were placed in operation it was demonstrated that as soon as any scale formed on the heating surfaces, leaks were caused due to unequal expansion.

No. 12. With this boiler, an attempt was made to remove the joints from the fire and to increase the heating surface in a given space. Water tubes were expanded into both sides of wrought-iron boxes, openings being made for the admission of water and the exit of steam. Fire tubes were placed inside the water tubes to increase the heating surface. This design was abandoned because of the rapid stopping up of the tubes by scale and the impossibility of cleaning them.

No. 13

No. 13

No. 13. Vertical straight line headers of cast iron, each containing two rows of tubes, were bolted to a connection leading to the steam and water drum above.

No. 14

No. 14

No. 14. A wrought-iron box was substituted for the double cast-iron headers. In this design, stays were necessary and were found, as always, to be an element to be avoided wherever possible. The boiler was an improvement on No. 6, however. A slanting bridge wall was introduced underneath the drum to throw a larger portion of its heating surface into the combustion chamber under the bank of tubes.

This bridge wall was found to be difficult to keep in repair and was of no particular benefit.

No. 15

No. 15

No. 15. Each row of tubes was expanded at each end into a continuous header, cast of car wheel metal. The headers had a sinuous form so that they would lie close together and admit of a staggered position of the tubes when assembled. While other designs of header form were tried later, experience with Nos. 14 and 15 showed that the style here adopted was the best for all [Pg 44] purposes and it has not been changed materially since. The drum in this design was supported by girders resting on the brickwork. Bolted joints were discarded, with the exception of those connecting the headers to the front and rear ends of the drums and the bottom of the rear headers to the mud drum. Even such joints, however, were found objectionable and were superseded in subsequent construction by short lengths of tubes expanded into bored holes.

No. 16

No. 16

No. 16. In this design, headers were tried which were made in the form of triangular boxes, in each of which there were three tubes expanded. These boxes were alternately reversed and connected by short lengths of expanded tubes, being connected to the drum by tubes bent in a manner to allow them to enter the shell normally. The joints between headers introduced an element of weakness and the connections to the drum were insufficient to give adequate circulation.

No. 17

No. 17

No. 17. Straight horizontal headers were next tried, alternately shifted right and left to allow a staggering of tubes. These headers were connected to each other [Pg 45] and to the drums by expanded nipples. The objections to this boiler were almost the same as those to No. 16.

No. 18

No. 18

      
No. 19

No. 19

Nos. 18 and 19. These boilers were designed primarily for fire protection purposes, the requirements demanding a small, compact boiler with ability to raise steam quickly. These both served the purpose admirably but, as in No. 9, the only provision made for the securing of dry steam was the use of the steam dome, shown in the illustration. This dome was found inadequate and has since been abandoned in nearly all forms of boiler construction. No other remedy being suggested at the time, these boilers were not considered as desirable for general use as Nos. 21 and 22. In Europe, however, where small size units were more in demand, No. 18 was modified somewhat and used largely with excellent results. These experiments, as they may now be called, although many boilers of some of the designs were built, clearly demonstrated that the best construction and efficiency required adherence to the following elements of design:

1st. Sinuous headers for each vertical row of tubes.

2nd. A separate and independent connection with the drum, both front and rear, for each vertical row of tubes.

[Pg 46]

3rd. All joints between parts of the boiler proper to be made without bolts or screw plates.

4th. No surfaces to be used which necessitate the use of stays.

5th. The boiler supported independently of the brickwork so as to allow freedom for expansion and contraction as it is heated or cooled.

6th. Ample diameter of steam and water drums, these not to be less than 30 inches except for small size units.

7th. Every part accessible for cleaning and repairs.

No. 20A

No. 20A

      
No. 20B

No. 20B

With these points having been determined, No. 20 was designed. This boiler had all the desirable features just enumerated, together with a number of improvements as to detail of construction. The general form of No. 15 was adhered to but the bolted connections between sections and drum and sections and mud drum were discarded in favor of connections made by short lengths of boiler tubes expanded into the adjacent parts. This boiler was suspended from girders, like No. 15, but these in turn were carried on vertical supports, leaving the pressure parts entirely free from the brickwork, the mutually deteriorating strains present where one was supported by the other being in this way overcome. Hundreds of thousands of horse power of this design were built, giving great satisfaction. The boiler was known as the “C. I. F.” (cast-iron front) style, an ornamental cast-iron front having been usually furnished.

No. 21

No. 21

The next step, and the one which connects the boilers as described above to the boiler as it is built to-day, was the design illustrated in No. 21. These boilers were known as the “W. I. F.” style, the fronts furnished as part of the equipment being constructed largely of wrought iron. The cast-iron drumheads used in No. 20 were replaced by wrought-steel flanged and “bumped” heads. The drums were made longer and the sections connected to wrought-steel cross boxes riveted to the bottom of the drums. The boilers were supported by girders and columns as in No. 20.

No. 22

No. 22

[Pg 47]

No. 22. This boiler, which is designated as the “Vertical Header” type, has the same general features of construction as No. 21, except that the tube sheet side of the headers is “stepped” to allow the headers to be placed vertically and at right angles to the drum and still maintain the tubes at the angle used in Nos. 20 and 21.

No. 23

No. 23

No. 23, or the cross drum design of boiler, is a development of the Babcock & Wilcox marine boiler, in which the cross drum is used exclusively. The experience of the Glasgow Works of The Babcock & Wilcox, Ltd., with No. 18 proved that proper attention to details of construction would make it a most desirable form of boiler where headroom was limited. A large number of this design have been successfully installed and are giving satisfactory results under widely varying conditions. The cross drum boiler is also built in a vertical header design.

Boilers Nos. 21, 22 and 23, with a few modifications, are now the standard forms. These designs are illustrated, as they are constructed to-day, on pages 48, 52, 54, 58 and 60.

The last step in the development of the water-tube boiler, beyond which it seems almost impossible for science and skill to advance, consists in the making of all pressure parts of the boiler of wrought steel, including sinuous headers, cross boxes, nozzles, and the like. This construction was the result of the demands of certain Continental laws that are coming into general vogue in this country. The Babcock & Wilcox Co. have at the present time a plant producing steel forgings that have been pronounced by the London Engineer to be “a perfect triumph of the forgers’ art”.

The various designs of this all wrought-steel boiler are fully illustrated in the following pages.
[Pg 48]

Illustration:

Wrought-steel Vertical Header Longitudinal Drum Babcock & Wilcox Boiler, Equipped with Babcock & Wilcox Superheater and Babcock & Wilcox Chain Grate Stoker

[Pg 49]

THE BABCOCK & WILCOX BOILER

The following brief description of the Babcock & Wilcox boiler will clearly indicate the manner in which it fulfills the requirements of the perfect steam boiler already enumerated.

The Babcock & Wilcox boiler is built in two general classes, the longitudinal drum type and the cross drum type. Either of these designs may be constructed with vertical or inclined headers, and the headers in turn may be of wrought steel or cast iron dependent upon the working pressure for which the boiler is constructed. The headers may be of different lengths, that is, may connect different numbers of tubes, and it is by a change in the number of tubes in height per section and the number of sections in width that the size of the boiler is varied.

The longitudinal drum boiler is the generally accepted standard of Babcock & Wilcox construction. The cross drum boiler, though originally designed to meet certain conditions of headroom, has become popular for numerous classes of work where low headroom is not a requirement which must be met.

Drumhead

Forged-steel Drumhead
with Manhole Plate
in Position

LONGITUDINAL DRUM CONSTRUCTION—The heating surface of this type of boiler is made up of a drum or drums, depending upon the width of the boiler extending longitudinally over the other pressure parts. To the drum or drums there are connected through cross boxes at either end the sections, which are made up of headers and tubes. At the lower end of the sections there is a mud drum extending entirely across the setting and connected to all sections. The connections between all parts are by short lengths of tubes expanded into bored seats.

Drumhead Interior

Forged-steel Drumhead
Interior

The drums are of three sheets, of such thickness as to give the required factor of safety under the maximum pressure for which the boiler is constructed. The circular seams are ordinarily single lap riveted though these may be double lap riveted to meet certain requirements of pressure or of specifications. The longitudinal seams are properly proportioned butt and strap or lap riveted joints dependent upon the pressure for which the boilers are built. Where butt strap joints are used the straps are bent to the proper radius in an hydraulic press. The courses are built independently to template and are assembled by an hydraulic forcing press. All riveted holes are punched one-quarter inch smaller than the size of rivets as driven and are reamed to full size after the plates are assembled. All rivets are driven by hydraulic pressure and held until black.

The drumheads are hydraulic forged at a single heat, the manhole opening and stiffening ring being forged in position. Flat raised seats for water column and feed connections are formed in the forging.

[Pg 50]

All heads are provided with manholes, the edges of which are turned true. The manhole plates are of forged steel and turned to fit manhole opening. These plates are held in position by forged-steel guards and bolts.

Drum Nozzle

Forged-steel Drum Nozzle

The drum nozzles are of forged steel, faced, and fitted with taper thread stud bolts.

Cross boxes by means of which the sections are attached to the drums, are of forged steel, made from a single sheet.

Cross Box

Forged-steel Cross Box

Where two or more drums are used in one boiler they are connected by a cross pipe having a flanged outlet for the steam connection.

The sections are built of 4-inch hot finished seamless open-hearth steel tubes of No. 10 B. W. G. where the boilers are built for working pressures up to 210 pounds. Where the working pressure is to be above this and below 260 pounds, No. 9 B. W. G. tubes are supplied.

Vertical Header Fittings

Inside Handhole Fittings
Wrought-steel
Vertical Header

Vertical Header

Wrought-steel
Vertical Header

The tubes are expanded into headers of serpentine or sinuous form, which dispose the tubes in a staggered position when assembled as a complete boiler. These headers are of wrought steel or of cast iron, the latter being ordinarily supplied where the working pressure is not to exceed 160 pounds. The headers may be either vertical or inclined as shown in the various illustrations of assembled boilers.

Opposite each tube end in the headers there is placed a handhole of sufficient size to permit the cleaning, removal or renewal of a tube. These openings in the wrought steel vertical headers are elliptical in shape, machine faced, and milled to a true plane back from the edge a sufficient distance to make a seat. The openings are closed by inside fitting forged plates, shouldered to center in the opening, their flanged seats milled to a true plane. These plates are held in position by studs and forged-steel [Pg 51] binders and nuts. The joints between plates and headers are made with a thin gasket.

Inclined Header

Wrought-steel
Inclined Header

Inclined Header Fitting

Inside Handhole Fitting
Wrought-steel
Inclined Header

In the wrought-steel inclined headers the handhole openings are either circular or elliptical, the former being ordinarily supplied. The circular openings have a raised seat milled to a true plane. The openings are closed on the outside by forged-steel caps, milled and ground true, held in position by forged-steel safety clamps and secured by ball-headed bolts to assure correct alignment. With this style of fitting, joints are made tight, metal to metal, without packing of any kind.

Where elliptical handholes are furnished they are faced inside, closed by inside fitting forged-steel plates, held to their seats by studs and secured by forged-steel binders and nuts.

The joints between plates and header are made with a thin gasket.

Vertical Header

Cast-iron Vertical Header

The vertical cast-iron headers have elliptical handholes with raised seats milled to a true plane. These are closed on the outside by cast-iron caps milled true, held in position by forged-steel safety clamps, which close the openings from the inside and which are secured by ball-headed bolts to assure proper alignment. All joints are made tight, metal to metal, without packing of any kind.

The mud drum to which the sections are attached at the lower end of the rear headers, is a forged-steel box 7¼ inches square, and of such length as to be connected to all headers by means of wrought nipples expanded into counterbored seats. The mud drum is furnished with handholes for cleaning, these being closed from the inside by forged-steel plates with studs, and secured on a faced seat in the mud drum by forged-steel binders and nuts. The joints between the plates and the drum are made with thin gaskets. The mud drum is tapped for blow-off connection.

All connections between drums and sections and between sections and mud drum are of hot finished seamless open-hearth steel tubes of No. 9 B. W. G.

Boilers of the longitudinal drum type are suspended front and rear from wrought-steel supporting frames entirely independent of the brickwork. This allows for [Pg 52]
[Pg 53]
expansion and contraction of the pressure parts without straining either the boiler or the brickwork, and also allows of brickwork repair or renewal without in any way disturbing the boiler or its connections.

Illustration:

Babcock & Wilcox Wrought-steel Vertical Header Cross Drum Boiler

CROSS DRUM CONSTRUCTION—The cross drum type of boilers differs from the longitudinal only in drum construction and method of support. The drum in this type is placed transversely across the rear of the boiler and is connected to the sections by means of circulating tubes expanded into bored seats.

The drums for all pressures are of two sheets of sufficient thickness to give the required factor of safety. The longitudinal seams are double riveted butt strapped, the straps being bent to the proper radius in an hydraulic press. The circulating tubes are expanded into the drums at the seams, the butt straps serving as tube seats.

The drumheads, drum fittings and features of riveting are the same in the cross drum as in the longitudinal types. The sections and mud drum are also the same for the two types.

Cross drum boilers are supported at the rear on the mud drum which rests on cast-iron foundation plates. They are suspended at the front from a wrought-iron supporting frame, each section being suspended independently from the cross members by hook suspension bolts. This method of support is such as to allow for expansion and contraction without straining either the boiler or the brickwork and permits of repair or renewal of the latter without in any way disturbing the boiler or its connections.

Cross Drum Boiler Front

Cross Drum Boiler Front

The following features of design and of attachments supplied are the same for all types.

FRONTS—Ornamental fronts are fitted to the front supporting frame. These have large doors for access to the front headers and panels above the fire fronts. The fire fronts where furnished have independent frames for fire doors which are bolted on, and ashpit doors fitted with blast catches. The lugs on door frames and on doors are cast solid. The faces of doors and of frames are planed and the lugs milled. The doors and frames are placed in their final relative position, clamped, and the holes for hinge pins drilled while thus held. A perfect alignment of door and frame is thus assured and the method is representative of the care taken in small details of manufacture.

The front as a whole is so arranged that any stoker may be applied with but slight modification wherever boilers are set with sufficient furnace height.

In the vertical header boilers large wrought-iron doors, which give access to the rear headers, are attached to the rear supporting frame. [Pg 54]

Illustration:

Wrought-steel Inclined Header Longitudinal Drum Babcock & Wilcox Boiler, Equipped with Babcock & Wilcox Superheater

[Pg 55] FITTINGS—Each boiler is provided with the following fittings as part of the standard equipment:

Blow-off connections and valves attached to the mud drum.

Safety valves placed on nozzles on the steam drums.

A water column connected to the front of the drum.

A steam gauge attached to the boiler front.

Drumhead Stop and Check Valve

Automatic Drumhead Stop and Check Valve

Feed water connection and valves. A flanged stop and check valve of heavy pattern is attached directly to each drumhead, closing automatically in case of a rupture in the feed line.

All valves and fittings are substantially built and are of designs which by their successful service for many years have become standard with The Babcock & Wilcox Co.

The fixtures that are supplied with the boilers consist of:

Dead plates and supports, the plates arranged for a fire brick lining.

A full set of grate bars and bearers, the latter fitted with expansion sockets for side walls.

Flame bridge plates with necessary fastenings, and special fire brick for lining same.

Bridge wall girder for hanging bridge wall with expansion sockets for side walls.

A full set of access and cleaning doors through which all portions of the pressure parts may be reached.

A swing damper and frame with damper operating rig.

There are also supplied with each boiler a wrench for handhole nuts, a water-driven turbine tube cleaner, a set of fire tools and a metal steam hose and cleaning pipe equipped with a special nozzle for blowing dust and soot from the tubes.

Aside from the details of design and construction as covered in the foregoing description, a study of the illustrations will make clear the features of the boiler as a whole which have led to its success.

The method of supporting the boiler has been described. This allows it to be hung at any height that may be necessary to properly handle the fuel to be burned or to accommodate the stoker to be installed. The height of the nest of tubes which forms the roof of the furnace is thus the controlling feature in determining the furnace height, or the distance from the front headers to the floor line. The sides and front of the furnace are formed by the side and front boiler walls. The rear wall of the furnace consists of a bridge wall built from the bottom of the ashpit to the lower row of tubes. The location of this wall may be adjusted within limits to give the depth of furnace demanded by the fuel used. Ordinarily the bridge wall is the determining feature in the locating of the front baffle. Where a great depth of furnace is necessary, in which case, if the front baffle were placed at the bridge wall the front pass of the boiler would be relatively too long, a patented construction is used which maintains the baffle in what may be considered its normal position, and a connection made between the baffle and the bridge wall by means of a tile roof. Such furnace construction is known as a “Webster” furnace. [Pg 56]

[Pg 57] A consideration of this furnace will clearly indicate its adaptability, by reason of its flexibility, for any fuel and any design of stoker. The boiler lends itself readily to installation with an extension or Dutch oven furnace if this be demanded by the fuel to be used, and in general it may be stated that a satisfactory furnace arrangement may be made in connection with a Babcock & Wilcox boiler for burning any fuel, solid, liquid or gaseous.

Illustration:

Longitudinal Drum Boiler—Front View

The gases of combustion evolved in the furnace above described are led over the heating surfaces by two baffles. These are formed of cast-iron baffle plates lined with special fire brick and held in position by tube clamps. The front baffle leads the gases through the forward portion of the tubes to a chamber beneath the drum or drums. It is in this chamber that a superheater is installed where such an apparatus is desired. The gases make a turn over the front baffle, are led downward through the central portion of the tubes, called the second pass, by means of a hanging bridge wall of brick and the second baffle, around which they make a second turn upward, pass through the rear portion of the tubes and are led to the stack or flue through a damper box in the rear wall, or around the drums to a damper box placed overhead.

Method of Introducing Feed Water

Partial Vertical Section
Showing Method of
Introducing Feed Water

The space beneath the tubes between the bridge wall and the rear boiler wall forms a pocket into which much of the soot from the gases in their downward passage through the second pass will be deposited and from which it may be readily cleaned through doors furnished for the purpose.

The gas passages are ample and are so proportioned that the resistance offered to the gases is only such as will assure the proper abstraction of heat from the gases without causing undue friction, requiring excessive draft.

The method in which the feed water is introduced through the front drumhead of the boiler is clearly seen by reference to the illustration. From this point of introduction the water passes to the rear of the drum, downward through the rear circulating tubes to the sections, upward through the tubes of the sections to the front headers and through these headers and front circulating tubes again to the drum where such water as has not been formed into steam retraces its course. The steam formed in the passage through the tubes is liberated as the water reaches the front of the drum. The steam so formed is stored in the steam space above the water line, from which it is drawn through a so-called “dry pipe.” The dry pipe in the Babcock & Wilcox boiler is misnamed, as in reality it fulfills none of the functions ordinarily attributed to such a device. This function is [Pg 58]
[Pg 59]
usually to restrict the flow of steam from a boiler with a view to avoid priming. In the Babcock & Wilcox boiler its function is simply that of a collecting pipe, and as the aggregate area of the holes in it is greatly in excess of the area of the steam outlet from the drum, it is plain that there can be no restriction through this collecting pipe. It extends nearly the length of the drum, and draws steam evenly from the whole length of the steam space.

Illustration:

Cast-iron Vertical Header Longitudinal Drum Babcock & Wilcox Boiler

Side Dusting Doors—ClosedSide Dusting Doors—Open

Closed

Open

Patented Side Dusting Doors

The large tube doors through which access is had to the front headers and the doors giving such access to the rear headers in boilers of the vertical header type have already been described and are shown clearly by the illustrations on pages 56 and 74. In boilers of the inclined header type, access to the rear headers is secured through the chamber formed by the headers and the rear boiler wall. Large doors in the sides of the setting give full access to all parts for inspection and for removal of accumulations of soot. Small dusting doors are supplied for the side walls through which all of the heating surfaces may be cleaned by means of a steam dusting lance. These side dusting doors are a patented feature and the shutters are self closing. In wide boilers additional cleaning doors are supplied at the top of the setting to insure ease in reaching all portions of the heating surface.

The drums are accessible for inspection through the manhole openings. The removal of the handhole plates makes possible the inspection of each tube for its full length and gives the assurance that no defect can exist that cannot be actually seen. This is particularly advantageous when inspecting for the presence of scale.

The materials entering into the construction of the Babcock & Wilcox boiler are the best obtainable for the special purpose for which they are used and are subjected to rigid inspection and tests.

The boilers are manufactured by means of the most modern shop equipment and appliances in the hands of an old and well-tried organization of skilled mechanics under the supervision of experienced engineers. [Pg 60]

Illustration:

Cast-iron Vertical Header Cross Drum Babcock & Wilcox Boiler

[Pg 61]

ADVANTAGES OF THE BABCOCK & WILCOX BOILER

The advantages of the Babcock & Wilcox boiler may perhaps be most clearly set forth by a consideration, 1st, of water-tube boilers as a class as compared with shell and fire-tube boilers; and 2nd, of the Babcock & Wilcox boiler specifically as compared with other designs of water-tube boilers.

WATER-TUBE VERSUS FIRE-TUBE BOILERS

Safety—The most important requirement of a steam boiler is that it shall be safe in so far as danger from explosion is concerned. If the energy in a large shell boiler under pressure is considered, the thought of the destruction possible in the case of an explosion is appalling. The late Dr. Robert H. Thurston, Dean of Sibley College, Cornell University, and past president of the American Society of Mechanical Engineers, estimated that there is sufficient energy stored in a plain cylinder boiler under 100 pounds steam pressure to project it in case of an explosion to a height of over 3½ miles; a locomotive boiler at 125 pounds pressure from one-half to one-third of a mile; and a 60 horse-power return tubular boiler under 75 pounds pressure somewhat over a mile. To quote: “A cubic foot of heated water under a pressure of from 60 to 70 pounds per square inch has about the same energy as one pound of gunpowder.” From such a consideration, it may be readily appreciated how the advent of high pressure steam was one of the strongest factors in forcing the adoption of water-tube boilers. A consideration of the thickness of material necessary for cylinders of various diameters under a steam pressure of 200 pounds and assuming an allowable stress of 12,000 pounds per square inch, will perhaps best illustrate this point. Table 1 gives such thicknesses for various diameters of cylinders not taking into consideration the weakening effect of any joints which may be necessary. The rapidity with which the plate thickness increases with the diameter is apparent and in practice, due to the fact that riveted joints must be used, the thicknesses as given in the table, with the exception of the first, must be increased from 30 to 40 per cent.

In a water-tube boiler the drums seldom exceed 48 inches in diameter and the thickness of plate required, therefore, is never excessive. The thinner metal can be rolled to a more uniform quality, the seams admit of better proportioning, and the joints can be more easily and perfectly fitted than is the case where thicker plates are necessary. All of these points contribute toward making the drums of water-tube boilers better able to withstand the stress which they will be called upon to endure.

The essential constructive difference between water-tube and fire-tube boilers lies in the fact that the former is composed of parts of relatively small diameter as against the large diameters necessary in the latter.

The factor of safety of the boiler parts which come in contact with the most intense heat in water-tube boilers can be made much higher than would be practicable in a shell boiler. Under the assumptions considered above in connection with the thickness of plates required, a number 10 gauge tube (0.134 inch), which is standard in Babcock & Wilcox boilers for pressures up to 210 pounds under the same allowable stress as was used in computing Table 1, the safe working pressure for the tubes is 870 pounds per square inch, indicating the very large margin of safety of such tubes as compared with that possible with the shell of a boiler.

[Pg 62]

TABLE 1

PLATE THICKNESS REQUIRED FOR
VARIOUS CYLINDER DIAMETERS

ALLOWABLE STRESS, 12000 POUNDS
PER SQUARE INCH, 200 POUNDS
GAUGE PRESSURE, NO JOINTS
Diameter
Inches
Thickness
Inches
Diameter
Inches
Thickness
Inches
  40.033  720.600
360.3001080.900
480.4001201.000
600.5001441.200

A further advantage in the water-tube boiler as a class is the elimination of all compressive stresses. Cylinders subjected to external pressures, such as fire tubes or the internally fired furnaces of certain types of boilers, will collapse under a pressure much lower than that which they could withstand if it were applied internally. This is due to the fact that if there exists any initial distortion from its true shape, the external pressure will tend to increase such distortion and collapse the cylinder, while an internal pressure tends to restore the cylinder to its original shape.

Stresses due to unequal expansion have been a fruitful source of trouble in fire-tube boilers.

In boilers of the shell type, the riveted joints of the shell, with their consequent double thickness of metal exposed to the fire, gives rise to serious difficulties. Upon these points are concentrated all strains of unequal expansion, giving rise to frequent leaks and oftentimes to actual ruptures. Moreover, in the case of such rupture, the whole body of contained water is liberated instantaneously and a disastrous and usually fatal explosion results.

Further, unequal strains result in shell or fire-tube boilers due to the difference in temperature of the various parts. This difference in temperature results from the lack of positive well defined circulation. While such a circulation does not necessarily accompany all water-tube designs, in general, the circulation in water-tube boilers is much more defined than in fire-tube or shell boilers.

A positive and efficient circulation assures that all portions of the pressure parts will be at approximately the same temperature and in this way strains resulting from unequal temperatures are obviated.

If a shell or fire-tubular boiler explodes, the apparatus as a whole is destroyed. In the case of water-tube boilers, the drums are ordinarily so located that they are protected from intense heat and any rupture is usually in the case of a tube. Tube failures, resulting from blisters or burning, are not serious in their nature. Where a tube ruptures because of a flaw in the metal, the result may be more severe, but there cannot be the disastrous explosion such as would occur in the case of the explosion of a shell boiler.

To quote Dr. Thurston, relative to the greater safety of the water-tube boiler: “The stored available energy is usually less than that of any of the other stationary boilers and not very far from the amount stored, pound for pound, in the plain tubular boiler. It is evident that their admitted safety from destructive explosion does not come from this relation, however, but from the division of the contents into small portions and especially from those details of construction which make it tolerably certain that any rupture shall be local. A violent explosion can only come from the general disruption of a boiler and the liberation at once of large masses of steam and water.”

Economy—The requirement probably next in importance to safety in a steam boiler is economy in the use of fuel. To fulfill such a requirement, the three items, of [Pg 63] proper grate for the class of fuel to be burned, a combustion chamber permitting complete combustion of gases before their escape to the stack, and the heating surface of such a character and arrangement that the maximum amount of available heat may be extracted, must be co-ordinated.

Fire-tube boilers from the nature of their design do not permit the variety of combinations of grate surface, heating surface, and combustion space possible in practically any water-tube boiler.

In securing the best results in fuel economy, the draft area in a boiler is an important consideration. In fire-tube boilers this area is limited to the cross sectional area of the fire tubes, a condition further aggravated in a horizontal boiler by the tendency of the hot gases to pass through the upper rows of tubes instead of through all of the tubes alike. In water-tube boilers the draft area is that of the space outside of the tubes and is hence much greater than the cross sectional area of the tubes.

Capacity—Due to the generally more efficient circulation found in water-tube than in fire-tube boilers, rates of evaporation are possible with water-tube boilers that cannot be approached where fire-tube boilers are employed.

Quick Steaming—Another important result of the better circulation ordinarily found in water-tube boilers is in their ability to raise steam rapidly in starting and to meet the sudden demands that may be thrown on them.

In a properly designed water-tube boiler steam may be raised from a cold boiler to 200 pounds pressure in less than one-half hour.

For the sake of comparison with the figure above, it may be stated that in the U. S. Government Service the shortest time allowed for getting up steam in Scotch marine boilers is 6 hours and the time ordinarily allowed is 12 hours. In large double-ended Scotch boilers, such as are generally used in Trans-Atlantic service, the fires are usually started 24 hours before the time set for getting under way. This length of time is necessary for such boilers in order to eliminate as far as possible excessive strains resulting from the sudden application of heat to the surfaces.

Accessibility—In the “Requirements of a Perfect Steam Boiler”, as stated by Mr. Babcock, he demonstrates the necessity for complete accessibility to all portions of the boiler for cleaning, inspection and repair.

Cleaning—When the great difference is realized in performance, both as to economy and capacity of a clean boiler and one in which the heating surfaces have been allowed to become fouled, it may be appreciated that the ability to keep heating surfaces clean internally and externally is a factor of the highest importance.

Such results can be accomplished only by the use of a design in boiler construction which gives complete accessibility to all portions. In fire-tube boilers the tubes are frequently nested together with a space between them often less than 1¼ inches and, as a consequence, nearly the entire tube surface is inaccessible. When scale forms upon such tubes it is impossible to remove it completely from the inside of the boiler and if it is removed by a turbine hammer, there is no way of knowing how thorough a job has been done. With the formation of such scale there is danger through overheating and frequent tube renewals are necessary.

In Scotch marine boilers, even with the engines operating condensing, complete tube renewals at intervals of six or seven years are required, while large replacements are often necessary in less than one year. In return tubular boilers operated with bad feed water, complete tube renewals annually are not uncommon. In this type of boiler [Pg 64]
[Pg 65]
much sediment falls on the bottom sheets where the intense heat to which they are subjected bakes it to such an excessive hardness that the only method of removing it is to chisel it out. This can be done only by omitting tubes enough to leave a space into which a man can crawl and the discomforts under which he must work are apparent. Unless such a deposit is removed, a burned and buckled plate will invariably result, and if neglected too long an explosion will follow.

Illustration:

Portion of 29,000 Horse-power Installation of Babcock & Wilcox Boilers in the L Street Station of the Edison Electric Illuminating Co. of Boston, Mass. This Company Operates in its Various Stations a Total of 39,000 Horse Power of Babcock & Wilcox Boilers

In vertical fire-tube boilers using a water leg construction, a deposit of mud in such legs is an active agent in causing corrosion and the difficulty of removing such deposit through handholes is well known. A complete removal is practically impossible and as a last resort to obviate corrosion in certain designs, the bottom of the water legs in some cases have been made of copper. A thick layer of mud and scale is also liable to accumulate on the crown sheet of such boilers and may cause the sheet to crack and lead to an explosion.

The soot and fine coal swept along with the gases by the draft will settle in fire tubes and unless removed promptly, must be cut out with a special form of scraper. It is not unusual where soft coal is used to find tubes half filled with soot, which renders useless a large portion of the heating surface and so restricts the draft as to make it difficult to burn sufficient coal to develop the required power from such heating surface as is not covered by soot.

Water-tube boilers in general are from the nature of their design more readily accessible for cleaning than are fire-tube boilers.

Inspection—The objections given above in the consideration of the inability to properly clean fire-tube boilers hold as well for the inspection of such boilers.

Repairs—The lack of accessibility in fire-tube boilers further leads to difficulties where repairs are required.

In fire-tube boilers tube renewals are a serious undertaking. The accumulation of hard deposit on the exterior of the surfaces so enlarges the tubes that it is oftentimes difficult, if not impossible, to draw them through the tube sheets and it is usually necessary to cut out such tubes as will allow access to the one which has failed and remove them through the manhole.

When a tube sheet blisters, the defective part must be cut out by hand-tapped holes drilled by ratchets and as it is frequently impossible to get space in which to drive rivets, a “soft patch” is necessary. This is but a makeshift at best and usually results in either a reduction of the safe working pressure or in the necessity for a new plate. If the latter course is followed, the old plate must be cut out, a new one scribed to place to locate rivet holes and in order to obtain room for driving rivets, the boiler will have to be re-tubed.

The setting must, of course, be at least partially torn out and replaced.

In case of repairs, of such nature in fire-tube boilers, the working pressure of such repaired boilers will frequently be lowered by the insurance companies when the boiler is again placed in service.

In the case of a rupture in a water-tube boiler, the loss will ordinarily be limited to one or two tubes which can be readily replaced. The fire-tube boiler will be so completely demolished that the question of repairs will be shifted from the boiler to the surrounding property, the damage to which will usually exceed many times the cost of a boiler of a type which would have eliminated the possibility of a disastrous explosion. In considering the proper repair cost of the two types of boilers, the fact [Pg 66] should not be overlooked that it is poor economy to invest large sums in equipment that, through a possible accident to the boiler may be wholly destroyed or so damaged that the cost of repairs, together with the loss of time while such repairs are being made, would purchase boilers of absolute safety and leave a large margin beside. The possibility of loss of human life should also be considered, though this may seem a far cry from the question of repair costs.

TABLE 2

COMPARATIVE APPROXIMATE FLOOR
SPACE OCCUPIED BY BABCOCK & WILCOX
AND H. R. T. BOILERS
Size of unit
Horse Power
Babcock & Wilcox
Feet and Inches
H. R. T.
Feet and Inches
100  7   3 × 19 910 0 × 20   0
150  7 10 × 19 910 0 × 22   6
200  9   0 × 19 911 6 × 23 10
250  9   0 × 19 911 6 × 23 10
30010   2 × 19 912 0 × 25   0

Space Occupied—The space required for the boilers in a plant often exceeds the requirements for the remainder of the plant equipment. Any saving of space in a boiler room will be a large factor in reducing the cost of real estate and of the building. Even when the boiler plant is comparatively small, the saving in space frequently will amount to a considerable percentage of the cost of the boilers. Table 2 shows the difference in floor space occupied by fire-tube boilers and Babcock & Wilcox boilers of the same capacity, the latter being taken as representing the water-tube class. This saving in space will increase with the size of the plant for the reason that large size boiler units while common in water-tube practice are impracticable in fire-tube practice.

BABCOCK & WILCOX BOILERS AS COMPARED WITH OTHER WATER-TUBE DESIGNS

It must be borne in mind that the simple fact that a boiler is of the water-tube design does not as a necessity indicate that it is a good or safe boiler.

Safety—Many of the water-tube boilers on the market are as lacking as are fire-tube boilers in the positive circulation which, as has been demonstrated by Mr. Babcock’s lecture, is so necessary in the requirements of the perfect steam boiler. In boilers using water-leg construction, there is danger of defective circulation, leaks are common, and unsuspected corrosion may be going on in portions of the boiler that cannot be inspected. Stresses due to unequal expansion of the metal cannot be well avoided but they may be minimized by maintaining at the same temperature all pressure parts of the boiler. The result is to be secured only by means of a well defined circulation.

The main feature to which the Babcock & Wilcox boiler owes its safety is the construction made possible by the use of headers, by which the water in each vertical row of tubes is separated from that in the adjacent rows. This construction results in the very efficient circulation produced through the breaking up of the steam and water in the front headers, the effect of these headers in producing such a positive circulation having been clearly demonstrated in Mr. Babcock’s lecture. The use of a number of sections, thus composed of headers and tubes, has a distinct advantage over the use of a common chamber at the outlet ends of the tubes. In the former case the circulation of water in one vertical row of tubes cannot interfere with that in the other rows, [Pg 67] while in the latter construction there will be downward as well as upward currents and such downward currents tend to neutralize any good effect there might be through the diminution of the density of the water column by the steam.

Further, the circulation results directly from the design of the boiler and requires no assistance from “retarders”, check valves and the like, within the boiler. All such mechanical devices in the interior of a boiler serve only to complicate the design and should not be used.

This positive and efficient circulation assures that all portions of the pressure parts of the Babcock & Wilcox boiler will be at approximately the same temperature and in this way strains resulting from unequal temperatures are obviated.

Where the water throughout the boiler is at the temperature of the steam contained, a condition to be secured only by proper circulation, danger from internal pitting is minimized, or at least limited only to effects of the water fed the boiler. Where the water in any portion of the boiler is lower than the temperature of the steam corresponding to the pressure carried, whether the fact that such lower temperatures exist as a result of lack of circulation, or because of intentional design, internal pitting or corrosion will almost invariably result.

Dr. Thurston has already been quoted to the effect that the admitted safety of a water-tube boiler is the result of the division of its contents into small portions. In boilers using a water-leg construction, while the danger from explosion will be largely limited to the tubes, there is the danger, however, that such legs may explode due to the deterioration of their stays, and such an explosion might be almost as disastrous as that of a shell boiler. The headers in a Babcock & Wilcox boiler are practically free from any danger of explosion. Were such an explosion to occur, it would still be localized to a much larger extent than in the case of a water-leg boiler and the header construction thus almost absolutely localizes any danger from such a cause.

Staybolts are admittedly an undesirable element of construction in any boiler. They are wholly objectionable and the only reason for the presence of staybolts in a boiler is to enable a cheaper form of construction to be used than if they were eliminated.

In boilers utilizing in their design flat-stayed surfaces, or staybolt construction under pressure, corrosion and wear and tear in service tends to weaken some single part subject to continual strain, the result being an increased strain on other parts greatly in excess of that for which an allowance can be made by any reasonable factor of safety. Where the construction is such that the weakening of a single part will produce a marked decrease in the safety and reliability of the whole, it follows of necessity, that there will be a corresponding decrease in the working pressure which may be safely carried.

In water-leg boilers, the use of such flat-stayed surfaces under pressure presents difficulties that are practically unsurmountable. Such surfaces exposed to the heat of the fire are subject to unequal expansion, distortion, leakage and corrosion, or in general, to many of the objections that have already been advanced against the fire-tube boilers in the consideration of water-tube boilers as a class in comparison with fire-tube boilers.

Aside from the difficulties that may arise in actual service due to the failure of staybolts, or in general, due to the use of flat-stayed surfaces, constructional features are encountered in the actual manufacture of such boilers that make it difficult if not [Pg 68]
[Pg 69]
impossible to produce a first-class mechanical job. It is practically impossible in the building of such a boiler to so design and place the staybolts that all will be under equal strain. Such unequal strains, resulting from constructional difficulties, will be greatly multiplied when such a boiler is placed in service. Much of the riveting in boilers of this design must of necessity be hand work, which is never the equal of machine riveting. The use of water-leg construction ordinarily requires the flanging of large plates, which is difficult, and because of the number of heats necessary and the continual working of the material, may lead to the weakening of such plates.

Illustration:

McAlpin Hotel, New York City, Operating 2360 Horse Power of Babcock & Wilcox Boilers

In vertical or semi-vertical water-tube boilers utilizing flat-stayed surfaces under pressure, these surfaces are ordinarily so located as to offer a convenient lodging place for flue dust, which fuses into a hard mass, is difficult of removal and under which corrosion may be going on with no possibility of detection.

Where stayed surfaces or water legs are features in the design of a water-tube boiler, the factor of safety of such parts must be most carefully considered. In such parts too, is the determination of the factor most difficult, and because of the “rule-of-thumb” determination frequently necessary, the factor of safety becomes in reality a factor of ignorance. As opposed to such indeterminate factors of safety, in the Babcock & Wilcox boiler, when the factor of safety for the drum or drums has been determined, and such a factor may be determined accurately, the factors for all other portions of the pressure parts are greatly in excess of that of the drum. All Babcock & Wilcox boilers are built with a factor of safety of at least five, and inasmuch as the factor of the safety of the tubes and headers is greatly in excess of this figure, it applies specifically to the drum or drums. This factor represents a greater degree of safety than a considerably higher factor applied to a boiler in which the shell or any riveted portion is acted upon directly by the fire, or the same factor applied to a boiler utilizing flat-stayed surface construction, where the accurate determination of the limiting factor of safety is difficult, if not impossible.

That the factor of safety of stayed surfaces is questionable may perhaps be best realized from a consideration of the severe requirements as to such factor called for by the rules and regulations of the Board of Supervising Inspectors, U. S. Government.

In view of the above, the absence of any stayed surfaces in the Babcock & Wilcox boiler is obviously a distinguishing advantage where safety is a factor. It is of interest to note, in the article on the evolution of the Babcock & Wilcox boiler, that staybolt construction was used in several designs, found unsatisfactory and unsafe, and discarded.

Another feature in the design of the Babcock & Wilcox boiler tending toward added safety is its manner of suspension. This has been indicated in the previous chapter and is of such nature that all of the pressure parts are free to expand or contract under variations of temperature without in any way interfering with any part of the boiler setting. The sectional nature of the boiler allows a flexibility under varying temperature changes that practically obviates internal strain.

In boilers utilizing water-leg construction, on the other hand, the construction is rigid, giving rise to serious internal strains and the method of support ordinarily made necessary by the boiler design is not only unmechanical but frequently dangerous, due to the fact that proper provision is not made for expansion and contraction under temperature variations.

[Pg 70]

Boilers utilizing water-leg construction are not ordinarily provided with mud drums. This is a serious defect in that it allows impurities and sediment to collect in a portion of the boiler not easily inspected, and corrosion may result.

Economy—That the water-tube boiler as a class lends itself more readily than does the fire-tube boiler to a variation in the relation of grate surface, heating surface and combustion space has been already pointed out. In economy again, the construction made possible by the use of headers in Babcock & Wilcox boilers appears as a distinct advantage. Because of this construction, there is a flexibility possible, in an unlimited variety of heights and widths that will satisfactorily meet the special requirements of the fuel to be burned in individual cases.

An extended experience in the design of furnaces best suited for a wide variety of fuels has made The Babcock & Wilcox Co. leaders in the field of economy. Furnaces have been built and are in successful operation for burning anthracite and bituminous coals, lignite, crude oil, gas-house tar, wood, sawdust and shavings, bagasse, tan bark, natural gas, blast furnace gas, by-product coke oven gas and for the utilization of waste heat from commercial processes. The great number of Babcock & Wilcox boilers now in satisfactory operation under such a wide range of fuel conditions constitutes an unimpeachable testimonial to the ability to meet all of the many conditions of service.

The limitations in the draft area of fire-tube boilers as affecting economy have been pointed out. That a greater draft area is possible in water-tube boilers does not of necessity indicate that proper advantage of this fact is taken in all boilers of the water-tube class. In the Babcock & Wilcox boiler, the large draft area taken in connection with the effective baffling allows the gases to be brought into intimate contact with all portions of the heating surfaces and renders such surfaces highly efficient.

In certain designs of water-tube boilers the baffling is such as to render ineffective certain portions of the heating surface, due to the tendency of soot and dirt to collect on or behind baffles, in this way causing the interposition of a layer of non-conducting material between the hot gases and the heating surfaces.

In Babcock & Wilcox boilers the standard baffle arrangement is such as to allow the installation of a superheater without in any way altering the path of the gases from furnace to stack, or requiring a change in the boiler design. In certain water-tube boilers the baffle arrangement is such that if a superheater is to be installed a complete change in the ordinary baffle design is necessary. Frequently to insure sufficiently hot gas striking the heating surfaces, a portion is by-passed directly from the furnace to the superheater chamber without passing over any of the boiler heating surfaces. Any such arrangement will lead to a decrease in economy and the use of boilers requiring it should be avoided.

Capacity—Babcock & Wilcox boilers are run successfully in every-day practice at higher ratings than any other boilers in practical service. The capacities thus obtainable are due directly to the efficient circulation already pointed out. Inasmuch as the construction utilizing headers has a direct bearing in producing such circulation, it is also connected with the high capacities obtainable with this apparatus.

Where intelligently handled and kept properly cleaned, Babcock & Wilcox boilers are operated in many plants at from 200 to 225 per cent of their rated evaporative capacity and it is not unusual for them to be operated at 300 per cent of such rated capacity during periods of peak load.

[Pg 71]

Dry Steam—In the list of the requirements of the perfect steam boiler, the necessity that dry steam be generated has been pointed out. The Babcock & Wilcox boiler will deliver dry steam under higher capacities and poorer conditions of feed water than any other boiler now manufactured. Certain boilers will, when operated at ordinary ratings, handle poor feed water and deliver steam in which the moisture content is not objectionable. When these same boilers are driven at high overloads, there will be a direct tendency to prime and the percentage of moisture in the steam delivered will be high. This tendency is the result of the lack of proper circulation and once more there is seen the advantage of the headers of the Babcock & Wilcox boiler, resulting as it does in the securing of a positive circulation.

In the design of the Babcock & Wilcox boiler sufficient space is provided between the steam outlet and the disengaging point to insure the steam passing from the boiler in a dry state without entraining or again picking up any particles of water in its passage even at high rates of evaporation. Ample time is given for a complete separation of steam from the water at the disengaging surface before the steam is carried from the boiler. These two features, which are additional causes for the ability of the Babcock & Wilcox boiler to deliver dry steam, result from the proper proportioning of the steam and water space of the boiler. From the history of the development of the boiler, it is evident that the cubical capacity per horse power of the steam and water space has been adopted after numerous experiments.

That the “dry pipe” serves in no way the generally understood function of such device has been pointed out. As stated, the function of the “dry pipe” in a Babcock & Wilcox boiler is simply that of a collecting pipe and this statement holds true regardless of the rate of operation of the boiler.

In certain boilers, “superheating surface” is provided to “dry the steam,” or to remove the moisture due to priming or foaming. Such surface is invariably a source of trouble unless the steam is initially dry and a boiler which will deliver dry steam is obviously to be preferred to one in which surface must be supplied especially for such purpose. Where superheaters are installed with Babcock & Wilcox boilers, they are in every sense of the word superheaters and not driers, the steam being delivered to them in a dry state.

The question has been raised in connection with the cross drum design of the Babcock & Wilcox boiler as to its ability to deliver dry steam. Experience has shown the absolute lack of basis for any such objection. The Babcock & Wilcox Company at its Bayonne Works some time ago made a series of experiments to see in what manner the steam generated was separated from the water either in the drum or in its passage to the drum. Glass peepholes were installed in each end of a drum in a boiler of the marine design, at the point midway between that at which the horizontal circulating tubes entered the drum and the drum baffle plate. By holding a light at one of these peepholes the action in the drum was clearly seen through the other. It was found that with the boiler operated under three-quarter inch ashpit pressure, which, with the fuel used would be equivalent to approximately 185 per cent of rating for stationary boiler practice, that each tube was delivering with great velocity a stream of solid water, which filled the tube for half its cross sectional area. There was no spray or mist accompanying such delivery, clearly indicating that the steam had entirely separated from the water in its passage through the horizontal circulating tubes, which in the boiler in question were but 50 inches long. [Pg 72]

Illustration:

Northwest Station of the Commonwealth Edison Co., Chicago, Ill. This Installation Consists of 11,360 Horse Power of Babcock & Wilcox Boilers and Superheaters, Equipped with Babcock & Wilcox Chain Grate Stokers

[Pg 73] These experiments proved conclusively that the size of the steam drums in the cross drum design has no appreciable effect in determining the amount of liberating surface, and that sufficient liberating surface is provided in the circulating tubes alone. If further proof of the ability of this design of boiler to deliver dry steam is required, such proof is perhaps best seen in the continued use of the Babcock & Wilcox marine boiler, in which the cross drum is used exclusively, and with which rates of evaporation are obtained far in excess of those secured in ordinary practice.

Quick Steaming—The advantages of water-tube boilers as a class over fire-tube boilers in ability to raise steam quickly have been indicated.

Due to the constant and thorough circulation resulting from the sectional nature of the Babcock & Wilcox boiler, steam may be raised more rapidly than in practically any other water-tube design.

In starting up a cold Babcock & Wilcox boiler with either coal or oil fuel, where a proper furnace arrangement is supplied, steam may be raised to a pressure of 200 pounds in less than half an hour. With a Babcock & Wilcox boiler in a test where forced draft was available, steam was raised from an initial temperature of the boiler and its contained water of 72 degrees to a pressure of 200 pounds, in 12½ minutes after lighting the fire. The boiler also responds quickly in starting from banked fires, especially where forced draft is available.

In Babcock & Wilcox boilers the water is divided into many small streams which circulate without undue frictional resistance in thin envelopes passing through the hottest part of the furnace, the steam being carried rapidly to the disengaging surface. There is no part of the boiler exposed to the heat of the fire that is not in contact with water internally, and as a result there is no danger of overheating on starting up quickly nor can leaks occur from unequal expansion such as might be the case where an attempt is made to raise steam rapidly in boilers using water leg construction.

Storage Capacity for Steam and Water—Where sufficient steam and water capacity are not provided in a boiler, its action will be irregular, the steam pressure varying over wide limits and the water level being subject to frequent and rapid fluctuation.

Owing to the small relative weight of steam, water capacity is of greater importance in this respect than steam space. With a gauge pressure of 180 pounds per square inch, 8 cubic feet of steam, which is equivalent to one-half cubic foot of water space, are required to supply one boiler horse power for one minute and if no heat be supplied to the boiler during such an interval, the pressure will drop to 150 pounds per square inch. The volume of steam space, therefore, may be over rated, but if this be too small, the steam passing off will carry water with it in the form of spray. Too great a water space results in slow steaming and waste of fuel in starting up; while too much steam space adds to the radiating surface and increases the losses from that cause.

That the steam and water space of the Babcock & Wilcox boiler are the result of numerous experiments has previously been pointed out.

Accessibility—Cleaning. That water-tube boilers are more accessible as a class than are fire-tube boilers has been indicated. All water-tube boilers, however, are not equally accessible. In certain designs, due to the arrangement of baffling used it is practically impossible to remove all deposits of soot and dirt. Frequently, in order to cheapen the product, sufficient cleaning and access doors are not supplied as part [Pg 74]
[Pg 75]
of the boiler equipment. The tendency of soot to collect on the crown sheets of certain vertical water-tube boilers has been noted. Such deposits are difficult to remove and if corrosion goes on beneath such a covering the sheet may crack and an explosion result.

Illustration:

Rear View—Longitudinal Drum Vertical Header Boiler, Showing Access Doors to Rear Headers

It is almost impossible to thoroughly clean water legs internally, and in such places also is there a tendency to unsuspected corrosion under deposits that cannot be removed.

In Babcock & Wilcox boilers every portion of the interior of the heating surfaces can be reached and kept clean, while any soot deposited on the exterior surfaces can be blown off while the boiler is under pressure.

Inspection—The accessibility which makes possible the thorough cleaning of all portions of the Babcock & Wilcox boiler also provides a means for a thorough inspection.

Drums are accessible for internal inspection by the removal of the manhole plates. Front headers may be inspected through large doors furnished for the purpose. Rear headers in the inclined header designs may be inspected from the chamber formed by such headers and the rear wall of the boiler. In the vertical header designs rear tube doors are furnished, as has been stated. In certain designs of water-tube boilers in order to assure accessibility for inspection of the rear ends of the tubes, the rear portion of the boiler is exposed to the atmosphere with resulting excessive radiation losses. In other designs the means of access to the rear ends of the tubes are of a makeshift and unworkmanlike character.

By the removal of handhole plates, all tubes in a Babcock & Wilcox boiler may be inspected for their full length either for the presence of scale or for suspected corrosion.

Repairs—In Babcock & Wilcox boilers the possession of great strength, the elimination of stresses due to uneven temperatures and of the resulting danger of leaks and corrosion, the protection of the drums from the intense heat of the fire, and the decreased liability of the scale forming matter to lodge on the hottest tube surfaces, all tend to minimize the necessity for repairs. The tubes of the Babcock & Wilcox boiler are practically the only part which may need renewal and these only at infrequent intervals When necessary, such renewals may be made cheaply and quickly. A small stock of tubes, 4 inches in diameter, of sufficient length for the boiler used, is all that need be carried to make renewals.

Repairs in water-leg boilers are difficult at best and frequently unsatisfactory when completed. When staybolt replacements are necessary, in order to get at the inner sheet of the water leg, several tubes must in some cases be cut out. Not infrequently a replacement of an entire water leg is necessary and this is difficult and requires a lengthy shutdown. With the Babcock & Wilcox boiler, on the other hand, even if it is necessary to replace a section, this may be done in a few hours after the boiler is cool.

In the case of certain staybolt failures the working pressure of a repaired boiler utilizing such construction will frequently be lowered by the insurance companies when the boiler is again placed in service. The sectional nature of the Babcock & Wilcox boiler enables it to maintain its original working pressure over long periods of time, almost regardless of the nature of any repair that may be required.

Durability—Babcock & Wilcox boilers are being operated in every-day service with entirely satisfactory results and under the same steam pressure as that for which [Pg 76]
[Pg 77]
they were originally sold that have been operated from thirty to thirty-five years. It is interesting to note in considering the life of a boiler that the length of life of a Babcock & Wilcox boiler must be taken as the criterion of what length of life is possible. This is due to the fact that there are Babcock & Wilcox boilers in operation to-day that have been in service from a time that antedates by a considerable margin that at which the manufacturer of any other water-tube boiler now on the market was started.

Illustration:

1456 Horse-power Installation of Babcock & Wilcox Boilers at the Raritan Woolen Mills, Raritan, N. J. The First of These Boilers were Installed in 1878 and 1881 and are still Operated at 80 Pounds Pressure

Probably the very best evidence of the value of the Babcock & Wilcox boiler as a steam generator and of the reliability of the apparatus, is seen in the sales of the company. Since the company was formed, there have been sold throughout the world over 9,900,000 horse power.

A feature that cannot be overlooked in the consideration of the advantages of the Babcock & Wilcox boiler is the fact that as a part of the organization back of the boiler, there is a body of engineers of recognized ability, ready at all times to assist its customers in every possible way.
[Pg 78]

Illustration:

2400 Horse-power Installation of Babcock & Wilcox Boilers in the Union Station Power House of the Pennsylvania Railroad Co., Pittsburgh, Pa. This Company has a Total of 28,500 Horse Power of Babcock & Wilcox Boilers Installed

[Pg 79]

HEAT AND ITS MEASUREMENT

The usual conception of heat is that it is a form of energy produced by the vibratory motion of the minute particles or molecules of a body. All bodies are assumed to be composed of these molecules, which are held together by mutual cohesion and yet are in a state of continual vibration. The hotter a body or the more heat added to it, the more vigorous will be the vibrations of the molecules.

As is well known, the effect of heat on a body may be to change its temperature, its volume, or its state, that is, from solid to liquid or from liquid to gaseous. Where water is melted from ice and evaporated into steam, the various changes are admirably described in the lecture by Mr. Babcock on “The Theory of Steam Making”, given in the next chapter.

Fig. 11

Fig. 11

The change in temperature of a body is ordinarily measured by thermometers, though for very high temperatures so-called pyrometers are used. The latter are dealt with under the heading “High Temperature Measurements” at the end of this chapter.

By reason of the uniform expansion of mercury and its great sensitiveness to heat, it is the fluid most commonly used in the construction of thermometers. In all thermometers the freezing point and the boiling point of water, under mean or average atmospheric pressure at sea level, are assumed as two fixed points, but the division of the scale between these two points varies in different countries. The freezing point is determined by the use of melting ice and for this reason is often called the melting point. There are in use three thermometer scales known as the Fahrenheit, the Centigrade or Celsius, and the Réaumur. As shown in Fig. 11, in the Fahrenheit scale, the space between the two fixed points is divided into 180 parts; the boiling point is marked 212, and the freezing point is marked 32, and zero is a temperature which, at the time this thermometer was invented, was incorrectly imagined to be the lowest temperature attainable. In the centigrade and the Réaumur scales, the distance between the two fixed points is divided into 100 and 80 parts, respectively. In each of these two scales the freezing point is marked zero, and the boiling point is marked 100 in the centigrade and 80 in the Réaumur. Each of the 180, 100 or 80 divisions in the respective thermometers is called a degree.

Table 3 and appended formulae are useful for converting from one scale to another.

In the United States the bulbs of high-grade thermometers are usually made of either Jena 58 III borosilicate thermometer glass or Jena 16 III glass, the stems being made of ordinary glass. The Jena 16 III glass is not suitable for use at temperatures much above 850 degrees Fahrenheit and the harder Jena 59 III should be used in thermometers for temperatures higher than this.

Below the boiling point, the hydrogen-gas thermometer is the almost universal standard with which mercurial thermometers may be compared, while above this point [Pg 80] the nitrogen-gas thermometer is used. In both of these standards the change in temperature is measured by the change in pressure of a constant volume of the gas.

In graduating a mercurial thermometer for the Fahrenheit scale, ordinarily a degree is represented as 1180 part of the volume of the stem between the readings at the melting point of ice and the boiling point of water. For temperatures above the latter, the scale is extended in degrees of the same volume. For very accurate work, however, the thermometer may be graduated to read true-gas-scale temperatures by comparing it with the gas thermometer and marking the temperatures at 25 or 50 degree intervals. Each degree is then 125 or 150 of the volume of the stem in each interval.

Every thermometer, especially if intended for use above the boiling point, should be suitably annealed before it is used. If this is not done, the true melting point and also the “fundamental interval”, that is, the interval between the melting and the boiling points, may change considerably. After continued use at the higher temperatures also, the melting point will change, so that the thermometer must be calibrated occasionally to insure accurate readings.

TABLE 3

COMPARISON OF THERMOMETER SCALES
  Fahrenheit Centigrade Réaumur   Fahrenheit Centigrade Réaumur
Absolute Zero -459.64 -273.13 -218.50     50   10.00     8.00
        0.00   -17.78   -14.22     75   23.89   19.11
      10.00   -12.22     -9.78   100   37.78   30.22
      20.00     -6.67     -5.33   200   93.33   74.67
      30.00     -1.11     -0.89 Boiling Point 212 100.00   80.00
Freezing Point     32.00       0.00       0.00   250 121.11   96.89
Maximum Density
of Water
    39.10       3.94       3.15   300 148.89 119.11
  350 176.67 141.33
F = 95 C+32° = 94 R+32°     C = 59 (F-32°) = 54 R     R = 49 (F-32°) = 45 C

As a general rule thermometers are graduated to read correctly for total immersion, that is, with bulb and stem of the thermometer at the same temperature, and they should be used in this way when compared with a standard thermometer. If the stem emerges into space either hotter or colder than that in which the bulb is placed, a “stem correction” must be applied to the observed temperature in addition to any correction that may be found in the comparison with the standard. For instance, for a particular thermometer, comparison with the standard with both fully immersed made necessary the following corrections:

Temperature Correction Temperature Correction
  40°F 0.0 300°F +2.5
100°F 0.0 400°F -0.5
200°F 0.0 500°F -2.5

When the sign of the correction is positive (+) it must be added to the observed reading, and when the sign is a negative (-) the correction must be subtracted. The formula for the stem correction is as follows:

Stem correction = 0.000085 × n (T- t )

[Pg 81] in which T is the observed temperature, t is the mean temperature of the emergent column, n is the number of degrees of mercury column emergent, and 0.000085 is the difference between the coefficient of expansion of the mercury and that in the glass in the stem.

Suppose the observed temperature is 400 degrees and the thermometer is immersed to the 200 degrees mark, so that 200 degrees of the mercury column project into the air. The mean temperature of the emergent column may be found by tying another thermometer on the stem with the bulb at the middle of the emergent mercury column as in Fig. 12. Suppose this mean temperature is 85 degrees, then

Fig. 12

Fig. 12

Fig. 13

Fig. 13

Stem correction = 0.000085 × 200 × (400 - 85) = 5.3 degrees.

As the stem is at a lower temperature than the bulb, the thermometer will evidently read too low, so that this correction must be added to the observed reading to find the reading corresponding to total immersion. The corrected reading will therefore be 405.3 degrees. If this thermometer is to be corrected in accordance with the calibrated corrections given above, we note that a further correction of 0.5 must be applied to the observed reading at this temperature, so that the correct temperature is 405.3 - 0.5 = 404.8 degrees or 405 degrees.

Fig. 12 shows how a stem correction can be obtained for the case just described.

Fig. 13 affords an opportunity for comparing the scale of a thermometer correct for total immersion with one which will read correctly when submerged to the 300 degrees mark, the stem being exposed at a mean temperature of 110 degrees Fahrenheit, a temperature often prevailing when thermometers are used for measuring temperatures in steam mains.

Absolute Zero —Experiments show that at 32 degrees Fahrenheit a perfect gas expands 1491.64 part of its volume if its pressure remains constant and its temperature is increased one degree. Thus if gas at 32 degrees Fahrenheit occupies 100 cubic feet and its temperature is increased one degree, its volume will be increased to 100 + 100491.64 = 100.203 cubic feet. For a rise of two degrees the volume would be 100 + (100 × 2) / 491.64 = 100.406 cubic feet. If this rate of expansion per one degree held good at all temperatures, and experiment shows that it does above the freezing point, the gas, if its pressure remained the same, would double its volume, if raised to a temperature of 32 + 491.64 = 523.64 degrees Fahrenheit, while under a diminution of temperature it would shrink and finally disappear at a temperature of 491.64 - 32 = 459.64 degrees below zero Fahrenheit. While undoubtedly some change in the law would take place before the lower temperature could be reached, there is no reason why the law may not be used within the range of temperature where it is known to hold good. From this explanation it is evident that under a constant pressure the volume of a gas will vary as the number of degrees between its temperature and the temperature of -459.64 degrees Fahrenheit. To simplify the [Pg 82]
[Pg 83]
application of the law, a new thermometric scale is constructed as follows: the point corresponding to -460 degrees Fahrenheit, is taken as the zero point on the new scale, and the degrees are identical in magnitude with those on the Fahrenheit scale. Temperatures referred to this new scale are called absolute temperatures and the point -460 degrees Fahrenheit (= -273 degrees centigrade) is called the absolute zero. To convert any temperature Fahrenheit to absolute temperature, add 460 degrees to the temperature on the Fahrenheit scale: thus 54 degrees Fahrenheit will be 54 + 460 = 514 degrees absolute temperature; 113 degrees Fahrenheit will likewise be equal to 113 + 460 = 573 degrees absolute temperature. If one pound of gas is at a temperature of 54 degrees Fahrenheit and another pound is at a temperature of 114 degrees Fahrenheit the respective volumes at a given pressure would be in the ratio of 514 to 573.

Illustration:

Ninety-sixth Street Station of the New York Railways Co., New York City, Operating 20,000 Horse Power of Babcock & Wilcox Boilers. This Company and its Allied Companies Operate a Total of 100,000 Horse Power of Babcock & Wilcox Boilers

British Thermal Unit —The quantitative measure of heat is the British thermal unit, ordinarily written B. t. u. This is the quantity of heat required to raise the temperature of one pound of pure water one degree at 62 degrees Fahrenheit; that is, from 62 degrees to 63 degrees. In the metric system this unit is the calorie and is the heat necessary to raise the temperature of one kilogram of pure water from 15 degrees to 16 degrees centigrade. These two definitions lead to a discrepancy of 0.03 of 1 per cent, which is insignificant for engineering purposes, and in the following the B. t. u. is taken with this discrepancy ignored. The discrepancy is due to the fact that there is a slight difference in the specific heat of water at 15 degrees centigrade and 62 degrees Fahrenheit. The two units may be compared thus:

1 Calorie = 3.968 B. t. u.        1 B. t. u. = 0.252 Calories.
Unit   Water   Temperature Rise
1 B. t. u.     1 Pound     1 Degree Fahrenheit
1 Calorie     1 Kilogram     1 Degree centigrade

But 1 kilogram = 2.2046 pounds and 1 degree centigrade = 95 degree Fahrenheit.

Hence 1 calorie = (2.2046 × 95 ) = 3.968 B. t. u.

The heat values in B. t. u. are ordinarily given per pound, and the heat values in calories per kilogram, in which case the B. t. u. per pound are approximately equivalent to 95 the calories per kilogram.

As determined by Joule, heat energy has a certain definite relation to work, one British thermal unit being equivalent from his determinations to 772 foot pounds. Rowland, a later investigator, found that 778 foot pounds were a more exact equivalent. Still later investigations indicate that the correct value for a B. t. u. is 777.52 foot pounds or approximately 778. The relation of heat energy to work as determined is a demonstration of the first law of thermo-dynamics, namely, that heat and mechanical energy are mutually convertible in the ratio of 778 foot pounds for one British thermal unit. This law, algebraically expressed, is W = JH; W being the work done in foot pounds, H being the heat in B. t. u., and J being Joules equivalent. Thus 1000 B. t. u.’s would be capable of doing 1000 × 778 = 778000 foot pounds of work.

Specific Heat —The specific heat of a substance is the quantity of heat expressed in thermal units required to raise or lower the temperature of a unit weight of any substance at a given temperature one degree. This quantity will vary for different substances For example, it requires about 16 B. t. u. to raise the temperature of one [Pg 84] pound of ice 32 degrees or 0.5 B. t. u. to raise it one degree, while it requires approximately 180 B. t. u. to raise the temperature of one pound of water 180 degrees or one B. t. u. for one degree.

If then, a pound of water be considered as a standard, the ratio of the amount of heat required to raise a similar unit of any other substance one degree, to the amount required to raise a pound of water one degree is known as the specific heat of that substance. Thus since one pound of water required one B. t. u. to raise its temperature one degree, and one pound of ice requires about 0.5 degrees to raise its temperature one degree, the ratio is 0.5 which is the specific heat of ice. To be exact, the specific heat of ice is 0.504, hence 32 degrees × 0.504 = 16.128 B. t. u. would be required to raise the temperature of one pound of ice from 0 to 32 degrees. For solids, at ordinary temperatures, the specific heat may be considered a constant for each individual substance, although it is variable for high temperatures. In the case of gases a distinction must be made between specific heat at constant volume, and at constant pressure.

Where specific heat is stated alone, specific heat at ordinary temperature is implied, and mean specific heat refers to the average value of this quantity between the temperatures named.

The specific heat of a mixture of gases is obtained by multiplying the specific heat of each constituent gas by the percentage by weight of that gas in the mixture, and dividing the sum of the products by 100. The specific heat of a gas whose composition by weight is CO 2 , 13 per cent; CO, 0.4 per cent; O, 8 per cent; N, 78.6 per cent, is found as follows:

CO 2   13.0 × 0.2170 =   2.82100
CO     0.4 × 0.2479 =   0.09916
O     8.0 × 0.2175 =   1.74000
N   78.6 × 0.2438 = 19.16268
  ––––       –––––––
  100.0     = 23.82284

and 23.8228 ÷ 100 = 0.238 = specific heat of the gas.

The specific heats of various solids, liquids and gases are given in Table 4 .

Sensible Heat —The heat utilized in raising the temperature of a body, as that in raising the temperature of water from 32 degrees up to the boiling point, is termed sensible heat. In the case of water, the sensible heat required to raise its temperature from the freezing point to the boiling point corresponding to the pressure under which ebullition occurs, is termed the heat of the liquid.

Latent Heat —Latent heat is the heat which apparently disappears in producing some change in the condition of a body without increasing its temperature If heat be added to ice at freezing temperature, the ice will melt but its temperature will not be raised. The heat so utilized in changing the condition of the ice is the latent heat and in this particular case is known as the latent heat of fusion. If heat be added to water at 212 degrees under atmospheric pressure, the water will not become hotter but will be evaporated into steam, the temperature of which will also be 212 degrees. The heat so utilized is called the latent heat of evaporation and is the heat which apparently disappears in causing the substance to pass from a liquid to a gaseous state.

[Pg 85]

TABLE 4

SPECIFIC HEATS OF VARIOUS SUBSTANCES
SOLIDS
  Temperature [2]
Degrees
Fahrenheit
Specific Heat   Temperature [2]
Degrees
Fahrenheit
Specific Heat
Copper      59 460      .0951 Glass (normal ther. 16 III )      66 212      .1988
Gold      32 212      .0316 Lead      59     .0299
Wrought Iron      59 212      .1152 Platinum      32 212      .0323
Cast Iron      68 212      .1189 Silver      32 212      .0559
Steel (soft)      68 208      .1175 Tin      -105 64      .0518
Steel (hard)      68 208      .1165 Ice       .5040
Zinc      32 212      .0935 Sulphur (newly fused)       .2025
Brass (yellow)      32     .0883  
LIQUIDS
  Temperature [2]
Degrees
Fahrenheit
Specific Heat   Temperature [2]
Degrees
Fahrenheit
Specific Heat
Water [3]      59     1.00000   Sulphur (melted)      246 297      .23500
Alcohol      32     .54750 Tin (melted)       .06370
     176     .76940 Sea Water (sp. gr. 1.0043)      64     .98000
Mercury      32     .03346 Sea Water (sp. gr. 1.0463)      64     .90300
Benzol      50     .40660 Oil of Turpentine      32     .41100
       122     .45020 Petroleum      64 210      .49800
Glycerine      59 102      .57600 Sulphuric Acid      68 133      .33630
Lead (Melted)      to   360      .04100  
GASES
  Temperature [2]
Degrees
Fahrenheit
Specific
Heat at
Constant
Pressure
Specific
Heat at
Constant
Volume
  Temperature [2]
Degrees
Fahrenheit
Specific
Heat at
Constant
Pressure
Specific
Heat at
Constant
Volume
Air      32 392      .2375 .1693 Carbon Monoxide      41 208      .2425 .1728
Oxygen      44 405      .2175 .1553 Carbon Dioxide      52 417      .2169 .1535
Nitrogen      32 392      .2438 .1729 Methane      64 406      .5929 .4505
Hydrogen      54 388      3.4090 2.4141 Blast Fur. Gas (approx.)      . . . .2277
Superheated Steam See table 25 Flue gas (approx.)      . . . .2400

Latent heat is not lost, but reappears whenever the substances pass through a reverse cycle, from a gaseous to a liquid, or from a liquid to a solid state. It may, therefore, be defined as stated, as the heat which apparently disappears, or is lost to thermometric measurement, when the molecular constitution of a body is being changed. Latent heat is expended in performing the work of overcoming the molecular cohesion of the particles of the substance and in overcoming the resistance of external pressure to change of volume of the heated body. Latent heat of evaporation, therefore, may be said to consist of internal and external heat, the former being [Pg 86] utilized in overcoming the molecular resistance of the water in changing to steam, while the latter is expended in overcoming any resistance to the increase of its volume during formation. In evaporating a pound of water at 212 degrees to steam at 212 degrees, 897.6 B. t. u. are expended as internal latent heat and 72.8 B. t. u. as external latent heat. For a more detailed description of the changes brought about in water by sensible and latent heat, the reader is again referred to the chapter on “The Theory of Steam Making”.

TABLE 5

BOILING POINTS AT ATMOSPHERIC PRESSURE
  Degrees
Fahrenheit
  Degrees
Fahrenheit
Ammonia 140 Water 212    
Bromine 145 Average Sea Water 213.2
Alcohol 173 Saturated Brine 226    
Benzine 212 Mercury 680    

Ebullition —The temperature of ebullition of any liquid, or its boiling point, may be defined as the temperature which exists where the addition of heat to the liquid no longer increases its temperature, the heat added being absorbed or utilized in converting the liquid into vapor. This temperature is dependent upon the pressure under which the liquid is evaporated, being higher as the pressure is greater.

Total Heat of Evaporation —The quantity of heat required to raise a unit of any liquid from the freezing point to any given temperature, and to entirely evaporate it at that temperature, is the total heat of evaporation of the liquid for that temperature. It is the sum of the heat of the liquid and the latent heat of evaporation.

To recapitulate, the heat added to a body is divided as follows:

Total heat = Heat to change the temperature + heat to overcome the molecular cohesion + heat to overcome the external pressure resisting an increase of volume of the body.

Where water is converted into steam, this total heat is divided as follows:

Total heat = Heat to change the temperature of the water + heat to separate the molecules of the water + heat to overcome resistance to increase in volume of the steam,
  = Heat of the liquid + internal latent heat + external latent heat,
  = Heat of the liquid + total latent heat of steam,
  = Total heat of evaporation.

The steam tables given on pages 122 to 127 give the heat of the liquid and the total latent heat through a wide range of temperatures.

Gases —When heat is added to gases there is no internal work done; hence the total heat is that required to change the temperature plus that required to do the external work. If the gas is not allowed to expand but is preserved at constant volume, the entire heat added is that required to change the temperature only.

Linear Expansion of Substances by Heat —To find the increase in the length of a bar of any material due to an increase of temperature, multiply the number of degrees of increase in temperature by the coefficient of expansion for one degree and by the length of the bar. Where the coefficient of expansion is given for 100 degrees, as in Table 6 , the result should be divided by 100. The expansion of metals [Pg 87] per one degree rise of temperature increases slightly as high temperatures are reached, but for all practical purposes it may be assumed to be constant for a given metal.

TABLE 6

LINEAL EXPANSION OF SOLIDS AT ORDINARY TEMPERATURES

(Tabular values represent increase per foot per 100 degrees increase in temperature, Fahrenheit or centigrade)
Substance Temperature Conditions [4]
Degrees Fahrenheit
Coefficient per 100
Degrees Fahrenheit
Coefficient per 100
Degrees Centigrade
Brass (cast)               32 to 212                           .001042             .001875
Brass (wire)               32 to 212                           .001072             .001930
Copper               32 to 212                           .000926             .001666
Glass (English flint)               32 to 212                           .000451             .000812
Glass (French flint)               32 to 212                           .000484             .000872
Gold               32 to 212                           .000816             .001470
Granite (average)               32 to 212                           .000482             .000868
Iron (cast)   104               .000589             .001061
Iron (soft forged)               0 to 212                           .000634             .001141
Iron (wire)               32 to 212                           .000800             .001440
Lead               32 to 212                           .001505             .002709
Mercury               32 to 212                           .009984 [5]             .017971
Platinum   104               .000499             .000899
Limestone               32 to 212                           .000139             .000251
Silver   104               .001067             .001921
Steel (Bessemer rolled, hard)               0 to 212                           .00056             .00101
Steel (Bessemer rolled, soft)               0 to 212                           .00063             .00117
Steel (cast, French)   104               .000734             .001322
Steel (cast annealed, English)   104               .000608             .001095

High Temperature Measurements —The temperatures to be dealt with in steam-boiler practice range from those of ordinary air and steam to the temperatures of burning fuel. The gases of combustion, originally at the temperature of the furnace, cool as they pass through each successive bank of tubes in the boiler, to nearly the temperature of the steam, resulting in a wide range of temperatures through which definite measurements are sometimes required.

Of the different methods devised for ascertaining these temperatures, some of the most important are as follows:

1st. Mercurial pyrometers for temperatures up to 1000 degrees Fahrenheit.

2nd. Expansion pyrometers for temperatures up to 1500 degrees Fahrenheit.

3rd. Calorimetry for temperatures up to 2000 degrees Fahrenheit.

4th. Thermo-electric pyrometers for temperatures up to 2900 degrees Fahrenheit.

5th. Melting points of metal which flow at various temperatures up to the melting point of platinum 3227 degrees Fahrenheit.

6th. Radiation pyrometers for temperatures up to 3600 degrees Fahrenheit.

7th. Optical pyrometers capable of measuring temperatures up to 12,600 degrees Fahrenheit. [6] For ordinary boiler practice however, their range is 1600 to 3600 degrees Fahrenheit. [Pg 88]

Illustration:

228 Horse-power Babcock & Wilcox Boiler, Installed at the Wentworth Institute, Boston, Mass.

[Pg 89] Table 7 gives the degree of accuracy of high temperature measurements.

TABLE 7

ACCURACY OF HIGH TEMPERATURE MEASUREMENTS [7]
Centigrade Fahrenheit
Temperature
Range
Accuracy
Plus or
Minus
Degrees
Temperature
Range
Accuracy
Plus or
Minus
Degrees
  200 500     0.5   392 932     0.9
  500 800     2.0   932 1472     3.6
  800 1100     3.0 1472 2012     5.4
1100 1600   15.0 2012 2912   27.0
1600 2000   25.0 2912 3632   45.0

Mercurial Pyrometers —At atmospheric pressure mercury boils at 676 degrees Fahrenheit and even at lower temperatures the mercury in thermometers will be distilled and will collect in the upper part of the stem. Therefore, for temperatures much above 400 degrees Fahrenheit, some inert gas, such as nitrogen or carbon dioxide, must be forced under pressure into the upper part of the thermometer stem. The pressure at 600 degrees Fahrenheit is about 15 pounds, or slightly above that of the atmosphere, at 850 degrees about 70 pounds, and at 1000 degrees about 300 pounds.

Flue-gas temperatures are nearly always taken with mercurial thermometers as they are the most accurate and are easy to read and manipulate. Care must be taken that the bulb of the instrument projects into the path of the moving gases in order that the temperature may truly represent the flue gas temperature. No readings should be considered until the thermometer has been in place long enough to heat it up to the full temperature of the gases.

Expansion Pyrometers —Brass expands about 50 per cent more than iron and in both brass and iron the expansion is nearly proportional to the increase in temperature. This phenomenon is utilized in expansion pyrometers by enclosing a brass rod in an iron pipe, one end of the rod being rigidly attached to a cap at the end of the pipe, while the other is connected by a multiplying gear to a pointer moving around a graduated dial. The whole length of the expansion piece must be at a uniform temperature before a correct reading can be obtained. This fact, together with the lost motion which is likely to exist in the mechanism connected to the pointer, makes the expansion pyrometer unreliable; it should be used only when its limitations are thoroughly understood and it should be carefully calibrated. Unless the brass and iron are known to be of the same temperature, its action will be anomalous: for instance, if it be allowed to cool after being exposed to a high temperature, the needle will rise before it begins to fall. Similarly, a rise in temperature is first shown by the instrument as a fall. The explanation is that the iron, being on the outside, heats or cools more quickly than the brass.

Calorimetry —This method derives its name from the fact that the process is the same as the determination of the specific heat of a substance by the water calorimeter, except that in one case the temperature is known and the specific heat is required, while in the other the specific heat is known and the temperature is required. The temperature is found as follows:

A given weight of some substance such as iron, nickel or fire brick, is heated to the unknown temperature and then plunged into water and the rise in temperature noted.

[Pg 90]

If X = temperature to be measured, w = weight of heated body in pounds, W = weight of water in pounds, T = final temperature of water, t = difference between initial and final temperatures of water, s = known specific heat of body. Then X = T + W t ÷ w s

Any temperatures secured by this method are affected by so many sources of error that the results are very approximate.

Thermo-electric Pyrometers —When wires of two different metals are joined at one end and heated, an electromotive force will be set up between the free ends of the wires. Its amount will depend upon the composition of the wires and the difference in temperature between the two. If a delicate galvanometer of high resistance be connected to the “thermal couple”, as it is called, the deflection of the needle, after a careful calibration, will indicate the temperature very accurately.

In the thermo-electric pyrometer of Le Chatelier, the wires used are platinum and a 10 per cent alloy of platinum and rhodium, enclosed in porcelain tubes to protect them from the oxidizing influence of the furnace gases. The couple with its protecting tubes is called an “element”. The elements are made in different lengths to suit conditions.

It is not necessary for accuracy to expose the whole length of the element to the temperature to be measured, as the electromotive force depends only upon the temperature of the juncture at the closed end of the protecting tube and that of the cold end of the element. The galvanometer can be located at any convenient point, since the length of the wires leading to it simply alter the resistance of the circuit, for which allowance may be made.

The advantages of the thermo-electric pyrometer are accuracy over a wide range of temperatures, continuity of readings, and the ease with which observations can be taken. Its disadvantages are high first cost and, in some cases, extreme delicacy.

Melting Points of Metals —The approximate temperature of a furnace or flue may be determined, if so desired, by introducing certain metals of which the melting points are known. The more common metals form a series in which the respective melting points differ by 100 to 200 degrees Fahrenheit, and by using these in order, the temperature can be fixed between the melting points of some two of them. This method lacks accuracy, but it suffices for determinations where approximate readings are satisfactory.

The approximate melting points of certain metals that may be used for determinations of this nature are given in Table 8 .

Radiation Pyrometers —These are similar to thermo-electric pyrometers in that a thermo-couple is employed. The heat rays given out by the hot body fall on a concave mirror and are brought to a focus at a point at which is placed the junction of a thermo-couple. The temperature readings are obtained from an indicator similar to that used with thermo-electric pyrometers.

Optical Pyrometers —Of the optical pyrometers the Wanner is perhaps the most reliable. The principle on which this instrument is constructed is that of comparing the quantity of light emanating from the heated body with a constant source of light, in this case a two-volt osmium lamp. The lamp is placed at one end of an optical tube, while at the other an eyepiece is provided and a scale. A battery of cells furnishes the current for the lamp. On looking through the pyrometer, a circle [Pg 91] of red light appears, divided into distinct halves of different intensities. Adjustment may be made so that the two halves appear alike and a reading is then taken from the scale. The temperatures are obtained from a table of temperatures corresponding to scale readings. For standardizing the osmium lamp, an amylacetate lamp, is provided with a stand for holding the optical tube.

TABLE 8

APPROXIMATE MELTING POINTS OF METALS [8]
Metal Temperature
Degrees Fahrenheit
Metal Temperature
Degrees Fahrenheit
Wrought Iron 2737 Lead   621
Pig Iron (gray) 2190-2327 Bismuth   498
Cast Iron (white) 2075 Tin   449
Steel 2460-2550 Platinum 3191
Steel (cast) 2500 Gold 1946
Copper 1981 Silver 1762
Zinc   786 Aluminum 1216
Antimony 1166  

Determination of Temperature from Character of Emitted Light —As a further means of determining approximately the temperature of a furnace, Table 9 , compiled by Messrs. White & Taylor, may be of service. The color at a given temperature is approximately the same for all kinds of combustibles under similar conditions.

TABLE 9

CHARACTER OF EMITTED LIGHT AND CORRESPONDING
APPROXIMATE TEMPERATURE [9]
Character of Emitted Light Temperature
Degrees
Fahrenheit
Character of Emitted Light Temperature
Degrees
Fahrenheit
Dark red, blood red, low red 1050 Light orange 1725
Dark cherry red 1175 Yellow 1825
Cherry, full red 1375 Light yellow 1975
Light cherry, bright cherry, light red 1550 White 2200
Orange 1650  

FOOTNOTES

[2] When one temperature alone is given the “true” specific heat is given; otherwise the value is the “mean” specific heat for the range of temperature given.

[3] For variation, see Table 13 .

[4] Where range of temperature is given, coefficient is mean over range.

[5] Coefficient of cubical expansion.

[6] Le Chatelier’s Investigations.

[7] Burgess-Le Chatelier.

[8] For accuracy of high temperature measurements, see Table 7 .

[9] Messrs. White & Taylor Trans. A. S. M. E., Vol. XXI, 1900.

[Pg 92]

THE THEORY OF STEAM MAKING

[Extracts from a Lecture delivered by George H. Babcock, at Cornell University, 1887[10]]

The chemical compound known as H2O exists in three states or conditions—ice, water and steam; the only difference between these states or conditions is in the presence or absence of a quantity of energy exhibited partly in the form of heat and partly in molecular activity, which, for want of a better name, we are accustomed to call “latent heat”; and to transform it from one state to another we have only to supply or extract heat. For instance, if we take a quantity of ice, say one pound, at absolute zero[11] and supply heat, the first effect is to raise its temperature until it arrives at a point 492 Fahrenheit degrees above the starting point. Here it stops growing warmer, though we keep on adding heat. It, however, changes from ice to water, and when we have added sufficient heat to have made it, had it remained ice, 283 degrees hotter or a temperature of 315 degrees Fahrenheit’s thermometer, it has all become water, at the same temperature at which it commenced to change, namely, 492 degrees above absolute zero, or 32 degrees by Fahrenheit’s scale. Let us still continue to add heat, and it will now grow warmer again, though at a slower rate—that is, it now takes about double the quantity of heat to raise the pound one degree that it did before—until it reaches a temperature of 212 degrees Fahrenheit, or 672 degrees absolute (assuming that we are at the level of the sea). Here we find another critical point. However much more heat we may apply, the water, as water, at that pressure, cannot be heated any hotter, but changes on the addition of heat to steam; and it is not until we have added heat enough to have raised the temperature of the water 966 degrees, or to 1,178 degrees by Fahrenheit’s thermometer (presuming for the moment that its specific heat has not changed since it became water), that it has all become steam, which steam, nevertheless, is at the temperature of 212 degrees, at which the water began to change. Thus over four-fifths of the heat which has been added to the water has disappeared, or become insensible in the steam to any of our instruments.

It follows that if we could reduce steam at atmospheric pressure to water, without loss of heat, the heat stored within it would cause the water to be red hot; and if we could further change it to a solid, like ice, without loss of heat, the solid would be white hot, or hotter than melted steel—it being assumed, of course, that the specific heat of the water and ice remain normal, or the same as they respectively are at the freezing point.

After steam has been formed, a further addition of heat increases the temperature again at a much faster ratio to the quantity of heat added, which ratio also varies according as we maintain a constant pressure or a constant volume; and I am not aware that any other critical point exists where this will cease to be the fact until we arrive at that very high temperature, known as the point of dissociation, at which it becomes resolved into its original gases.

The heat which has been absorbed by one pound of water to convert it into a pound of steam at atmospheric pressure is sufficient to have melted 3 pounds of steel or 13 pounds of gold. This has been transformed into something besides heat; [Pg 93] stored up to reappear as heat when the process is reversed. That condition is what we are pleased to call latent heat, and in it resides mainly the ability of the steam to do work.

Graph of Temperature against Heat

The diagram shows graphically the relation of heat to temperature, the horizontal scale being quantity of heat in British thermal units, and the vertical temperature in Fahrenheit degrees, both reckoned from absolute zero and by the usual scale. The dotted lines for ice and water show the temperature which would have been obtained if the conditions had not changed. The lines marked “gold” and “steel” show the relation to heat and temperature and the melting points of these metals. All the inclined lines would be slightly curved if attention had been paid to the changing specific heat, but the curvature would be small. It is worth noting that, with one or two exceptions, the curves of all substances lie between the vertical and that for water. That is to say, that water has a greater capacity for heat than all other substances except two, hydrogen and bromine.

In order to generate steam, then, only two steps are required: 1st, procure the heat, and 2nd, transfer it to the water. Now, you have it laid down as an axiom that when a body has been transferred or transformed from one place or state into another, the same work has been done and the same energy expended, whatever may have been the intermediate steps or conditions, or whatever the apparatus. Therefore, when a given quantity of water at a given temperature has been made into steam at a given temperature, a certain definite work has been done, and a certain amount of energy expended, from whatever the heat may have been obtained, or whatever boiler may have been employed for the purpose.

A pound of coal or any other fuel has a definite heat producing capacity, and is capable of evaporating a definite quantity of water under given conditions. That is the limit beyond which even perfection cannot go, and yet I have known, and doubtless you have heard of, cases where inventors have claimed, and so-called engineers have certified to, much higher results.

The first step in generating steam is in burning the fuel to the best advantage. A pound of carbon will generate 14,500 British thermal units, during combustion into carbonic dioxide, and this will be the same, whatever the temperature or the rapidity at which the combustion may take place. If possible, we might oxidize it at as slow a rate as that with which iron rusts or wood rots in the open air, or we might burn it [Pg 94] with the rapidity of gunpowder, a ton in a second, yet the total heat generated would be precisely the same. Again, we may keep the temperature down to the lowest point at which combustion can take place, by bringing large bodies of air in contact with it, or otherwise, or we may supply it with just the right quantity of pure oxygen, and burn it at a temperature approaching that of dissociation, and still the heat units given off will be neither more nor less. It follows, therefore, that great latitude in the manner or rapidity of combustion may be taken without affecting the quantity of heat generated.

But in practice it is found that other considerations limit this latitude, and that there are certain conditions necessary in order to get the most available heat from a pound of coal. There are three ways, and only three, in which the heat developed by the combustion of coal in a steam boiler furnace may be expended.

1st, and principally. It should be conveyed to the water in the boiler, and be utilized in the production of steam. To be perfect, a boiler should so utilize all the heat of combustion, but there are no perfect boilers.

2nd. A portion of the heat of combustion is conveyed up the chimney in the waste gases. This is in proportion to the weight of the gases, and the difference between their temperature and that of the air and coal before they entered the fire.

3rd. Another portion is dissipated by radiation from the sides of the furnace. In a stove the heat is all used in these latter two ways, either it goes off through the chimney or is radiated into the surrounding space. It is one of the principal problems of boiler engineering to render the amount of heat thus lost as small as possible.

The loss from radiation is in proportion to the amount of surface, its nature, its temperature, and the time it is exposed. This loss can be almost entirely eliminated by thick walls and a smooth white or polished surface, but its amount is ordinarily so small that these extraordinary precautions do not pay in practice.

It is evident that the temperature of the escaping gases cannot be brought below that of the absorbing surfaces, while it may be much greater even to that of the fire. This is supposing that all of the escaping gases have passed through the fire. In case air is allowed to leak into the flues, and mingle with the gases after they have left the heating surfaces, the temperature may be brought down to almost any point above that of the atmosphere, but without any reduction in the amount of heat wasted. It is in this way that those low chimney temperatures are sometimes attained which pass for proof of economy with the unobserving. All surplus air admitted to the fire, or to the gases before they leave the heating surfaces, increases the losses.

We are now prepared to see why and how the temperature and the rapidity of combustion in the boiler furnace affect the economy, and that though the amount of heat developed may be the same, the heat available for the generation of steam may be much less with one rate or temperature of combustion than another.

Assuming that there is no air passing up the chimney other than that which has passed through the fire, the higher the temperature of the fire and the lower that of the escaping gases the better the economy, for the losses by the chimney gases will bear the same proportion to the heat generated by the combustion as the temperature of those gases bears to the temperature of the fire. That is to say, if the temperature of the fire is 2500 degrees and that of the chimney gases 500 degrees above that of the atmosphere, the loss by the chimney will be 5002500 = 20 per cent. Therefore, as the escaping gases cannot be brought below the temperature of the absorbing surface, [Pg 95] which is practically a fixed quantity, the temperature of the fire must be high in order to secure good economy.

The losses by radiation being practically proportioned to the time occupied, the more coal burned in a given furnace in a given time, the less will be the proportionate loss from that cause.

It therefore follows that we should burn our coal rapidly and at a high temperature to secure the best available economy.

Illustration:

Portion of 9880 Horse-power Installation of Babcock & Wilcox Boilers and Superheaters, Equipped with Babcock & Wilcox Chain Grate Stokers at the South Side Elevated Ry. Co., Chicago, Ill.

FOOTNOTES

[10] See Scientific American Supplement, 624, 625, December, 1887.

[11] 460 degrees below the zero of Fahrenheit. This is the nearest approximation in whole degrees to the latest determinations of the absolute zero of temperature

[Pg 96]

PROPERTIES OF WATER

Pure water is a chemical compound of one volume of oxygen and two volumes of hydrogen, its chemical symbol being H 2 O.

The weight of water depends upon its temperature. Its weight at four temperatures, much used in physical calculations, is given in Table 10 .

TABLE 10

WEIGHT OF WATER AT TEMPERATURES
USED IN PHYSICAL CALCULATIONS
Temperature Degrees
Fahrenheit
Weight per
Cubic Foot
Pounds
Weight per
Cubic Inch
Pounds
At 32 degrees or freezing
point at sea level
62.418 0.03612
At 39.2 degrees or point of
maximum density
62.427 0.03613
At 62 degrees or standard
temperature
62.355 0.03608
At 212 degrees or boiling
point at sea level
59.846 0.03469

While authorities differ as to the weight of water, the range of values given for 62 degrees Fahrenheit (the standard temperature ordinarily taken) being from 62.291 pounds to 62.360 pounds per cubic foot, the value 62.355 is generally accepted as the most accurate.

A United States standard gallon holds 231 cubic inches and weighs, at 62 degrees Fahrenheit, approximately 8 13 pounds.

A British Imperial gallon holds 277.42 cubic inches and weighs, at 62 degrees Fahrenheit, 10 pounds.

The above are the true weights corrected for the effect of the buoyancy of the air, or the weight in vacuo . If water is weighed in air in the ordinary way, there is a correction of about one-eighth of one per cent which is usually negligible.

TABLE 11

VOLUME AND WEIGHT OF DISTILLED WATER AT VARIOUS TEMPERATURES [12]
Temper-
ature
Degrees
Fahren-
heit
Relative
Volume
Water
at 39.2°
= 1.000
Weight
per
Cubic
Foot
Pounds
Temper-
ature
Degrees
Fahren-
heit
Relative
Volume
Water
at 39.2°
= 1.000
Weight
per
Cubic
Foot
Pounds
Temper-
ature
Degrees
Fahren-
heit
Relative
Volume
Water
at 39.2°
= 1.000
Weight
per
Cubic
Foot
Pounds
Temper-
ature
Degrees
Fahren-
heit
Relative
Volume
Water
at 39.2°
= 1.000
Weight
per
Cubic
Foot
Pounds
      32  1.000176   62.42     160  1.02337   61.00     290  1.0830   57.65     430  1.197   52.2
      39.2  1.000000   62.43     170  1.02682   60.80     300  1.0890   57.33     440  1.208   51.7
      40  1.000004   62.43     180  1.03047   60.58     310  1.0953   57.00     450  1.220   51.2
      50  1.00027   62.42     190  1.03431   60.36     320  1.1019   56.66     460  1.232   50.7
      60  1.00096   62.37     200  1.03835   60.12     330  1.1088   56.30     470  1.244   50.2
      70  1.00201   62.30     210  1.04256   59.88     340  1.1160   55.94     480  1.256   49.7
      80  1.00338   62.22     212  1.04343   59.83     350  1.1235   55.57     490  1.269   49.2
      90  1.00504   62.11     220  1.0469   59.63     360  1.1313   55.18     500  1.283   48.7
    100  1.00698   62.00     230  1.0515   59.37     370  1.1396   54.78     510  1.297   48.1
    110  1.00915   61.86     240  1.0562   59.11     380  1.1483   54.36     520  1.312   47.6
    120  1.01157   61.71     250  1.0611   58.83     390  1.1573   53.94     530  1.329   47.0
    130  1.01420   61.55     260  1.0662   58.55     400  1.167   53.5     540  1.35   46.3
    140  1.01705   61.38     270  1.0715   58.26     410  1.177   53.0     550  1.37   45.6
    150  1.02011   61.20     280  1.0771   57.96     420  1.187   52.6     560  1.39   44.9

[Pg 97]

Water is but slightly compressible and for all practical purposes may be considered non-compressible. The coefficient of compressibility ranges from 0.000040 to 0.000051 per atmosphere at ordinary temperatures, this coefficient decreasing as the temperature increases.

Table 11 gives the weight in vacuo and the relative volume of a cubic foot of distilled water at various temperatures.

The weight of water at the standard temperature being taken as 62.355 pounds per cubic foot, the pressure exerted by the column of water of any stated height, and conversely the height of any column required to produce a stated pressure, may be computed as follows:

The pressure in pounds per square foot = 62.355 × height of column in feet.

The pressure in pounds per square inch = 0.433 × height of column in feet.

Height of column in feet = pressure in pounds per square foot ÷ 62.355.

Height of column in feet = pressure in pounds per square inch ÷ 0.433.

Height of column in inches = pressure in pounds per square inch × 27.71.

Height of column in inches = pressure in ounces per square inch × 1.73.

By a change in the weights given above, the pressure exerted and height of column may be computed for temperatures other than 62 degrees.

A pressure of one pound per square inch is exerted by a column of water 2.3093 feet or 27.71 inches high at 62 degrees Fahrenheit.

Water in its natural state is never found absolutely pure. In solvent power water has a greater range than any other liquid. For common salt, this is approximately a constant at all temperatures, while with such impurities as magnesium and sodium sulphates, this solvent power increases with an increase in temperature.

TABLE 12

BOILING POINT OF WATER AT VARIOUS ALTITUDES
Boiling
Point
Degrees
Fahrenheit
Altitude
Above
Sea Level
Feet
Atmospheric
Pressure
Pounds per
Square Inch
Barometer
Reduced to
32 Degrees
Inches
Boiling
Point
Degrees
Fahrenheit
Altitude
Above
Sea Level
Feet
Atmospheric
Pressure
Pounds per
Square Inch
Barometer
Reduced to
32 Degrees
Inches
184       15221    8.20       16.70      199       6843    11.29       22.99     
185       14649    8.38       17.06      200       6304    11.52       23.47     
186       14075    8.57       17.45      201       5764    11.76       23.95     
187       13498    8.76       17.83      202       5225    12.01       24.45     
188       12934    8.95       18.22      203       4697    12.26       24.96     
189       12367    9.14       18.61      204       4169    12.51       25.48     
190       11799    9.34       19.02      205       3642    12.77       26.00     
191       11243    9.54       19.43      206       3115    13.03       26.53     
192       10685    9.74       19.85      207       2589    13.30       27.08     
193       10127    9.95       20.27      208       2063    13.57       27.63     
194       9579    10.17       20.71      209       1539    13.85       28.19     
195       9031    10.39       21.15      210       1025    14.13       28.76     
196       8481    10.61       21.60      211       512    14.41       29.33     
197       7932    10.83       22.05      212       Sea Level 14.70       29.92     
198       7381    11.06       22.52       

Sea water contains on an average approximately 3.125 per cent of its weight of solid matter or a thirty-second part of the weight of the water and salt held in solution. [Pg 98]
[Pg 99]
The approximate composition of this solid matter will be: sodium chloride 76 per cent, magnesium chloride 10 per cent, magnesium sulphate 6 per cent, calcium sulphate 5 per cent, calcium carbonate 0.5 per cent, other substances 2.5 per cent.

Illustration:

7200 Horse-power Installation of Babcock & Wilcox Boilers and Superheaters at the Capital Traction Co., Washington, D. C.

The boiling point of water decreases as the altitude above sea level increases. Table 12 gives the variation in the boiling point with the altitude.

Water has a greater specific heat or heat-absorbing capacity than any other known substance (bromine and hydrogen excepted) and its specific heat is the basis for measurement of the capacity of heat absorption of all other substances. From the definition, the specific heat of water is the number of British thermal units required to raise one pound of water one degree. This specific heat varies with the temperature of the water. The generally accepted values are given in Table 13 , which indicates the values as determined by Messrs. Marks and Davis and Mr. Peabody.

TABLE 13

SPECIFIC HEAT OF WATER AT VARIOUS TEMPERATURES
MARKS AND DAVIS
From Values of Barnes and Dieterici
PEABODY
From Values of Barnes and Regnault
Temperature Specific
Heat
Temperature Specific
Heat
Temperature Specific
Heat
Temperature Specific
Heat
Degrees
Fahrenheit
Degrees
Fahrenheit
Degrees
Centigrade
Degrees
Fahrenheit
Degrees
Centigrade
Degrees
Fahrenheit
30          1.0098 130          0.9979           0 32        1.0094         50 122        0.9980
40          1.0045 140          0.9986           5 41        1.0053         55 131        0.9985
50          1.0012 150          0.9994         10 50        1.0023         60 140        0.9994
55          1.0000 160          1.0002         15 59        1.0003         65 149        1.0004
60          0.9990 170          1.0010         16.11 61        1.0000         70 158        1.0015
70          0.9977 180          1.0019         20 68        0.9990         75 167        1.0028
80          0.9970 190          1.0029         25 77        0.9981         80 176        1.0042
90          0.9967 200          1.0039         30 86        0.9976         85 185        1.0056
100          0.9967 210          1.0052         35 95        0.9974         90 194        1.0071
110          0.9970 220          1.007         40 104        0.9974         95 203        1.0086
120          0.9974 230          1.009         45 113        0.9976       100 212        1.0101

In consequence of this variation in specific heat, the variation in the heat of the liquid of the water at different temperatures is not a constant. Table 22 [13] gives the heat of the liquid in a pound of water at temperatures ranging from 32 to 340 degrees Fahrenheit.

The specific heat of ice at 32 degrees is 0.463. The specific heat of saturated steam (ice and saturated steam representing the other forms in which water may exist), is something that is difficult to define in any way which will not be misleading. When no liquid is present the specific heat of saturated steam is negative. [14] The use of the value of the specific heat of steam is practically limited to instances where superheat is present, and the specific heat of superheated steam is covered later in the book.

FOOTNOTES

[12] Marks and Davis

[13] See page 120 .

[14] See Trans., A. S. M. E., Vol. XIV., Page 79.

[Pg 100]

BOILER FEED WATER

All natural waters contain some impurities which, when introduced into a boiler, may appear as solids. In view of the apparent present-day tendency toward large size boiler units and high overloads, the importance of the use of pure water for boiler feed purposes cannot be over-estimated.

Ordinarily, when water of sufficient purity for such use is not at hand, the supply available may be rendered suitable by some process of treatment. Against the cost of such treatment, there are many factors to be considered. With water in which there is a marked tendency toward scale formation, the interest and depreciation on the added boiler units necessary to allow for the systematic cleaning of certain units must be taken into consideration. Again there is a considerable loss in taking boilers off for cleaning and replacing them on the line. On the other hand, the decrease in capacity and efficiency accompanying an increased incrustation of boilers in use has been too generally discussed to need repetition here. Many experiments have been made and actual figures reported as to this decrease, but in general, such figures apply only to the particular set of conditions found in the plant where the boiler in question was tested. So many factors enter into the effect of scale on capacity and economy that it is impossible to give any accurate figures on such decrease that will serve all cases, but that it is large has been thoroughly proven.

While it is almost invariably true that practically any cost of treatment will pay a return on the investment of the apparatus, the fact must not be overlooked that there are certain waters which should never be used for boiler feed purposes and which no treatment can render suitable for such purpose. In such cases, the only remedy is the securing of other feed supply or the employment of evaporators for distilling the feed water as in marine service.

TABLE 14

APPROXIMATE CLASSIFICATION OF IMPURITIES FOUND IN FEED WATERS
THEIR EFFECT AND ORDINARY METHODS OF RELIEF
Difficulty Resulting
from Presence of
Nature of
Difficulty
Ordinary Method of
Overcoming or Relieving
Sediment, Mud, etc.IncrustationSettling tanks, filtration, blowing down.
Readily Soluble SaltsIncrustationBlowing down.
Bicarbonates of Lime, Magnesia, etc.IncrustationHeating feed.
Treatment by addition of lime or of lime and soda.
Barium carbonate.
Sulphate of LimeIncrustationTreatment by addition of soda. Barium carbonate.
Chloride and Sulphate of MagnesiumCorrosionTreatment by addition of carbonate of soda.
AcidCorrosionAlkali.
Dissolved Carbonic Acid and OxygenCorrosionHeating feed. Keeping air from feed.
Addition of caustic soda or slacked lime.
GreaseCorrosionFilter. Iron alum as coagulent.
Neutralization by carbonate of soda.
Use of best hydrocarbon oils.
Organic MatterCorrosionFilter. Use of coagulent.
Organic Matter (Sewage)PrimingSettling tanks. Filter in connection with coagulent.
Carbonate of Soda in large quantitiesPrimingBarium carbonate. New feed supply.
If from treatment, change.

[Pg 101]

It is evident that the whole subject of boiler feed waters and their treatment is one for the chemist rather than for the engineer. A brief outline of the difficulties that may be experienced from the use of poor feed water and a suggestion as to a method of overcoming certain of these difficulties is all that will be attempted here. Such a brief outline of the subject, however, will indicate the necessity for a chemical analysis of any water before a treatment is tried and the necessity of adapting the treatment in each case to the nature of the difficulties that may be experienced.

Table 14 gives a list of impurities which may be found in boiler feed water, grouped according to their effect on boiler operation and giving the customary method used for overcoming difficulty to which they lead.

Scale—Scale is formed on boiler heating surfaces by the depositing of impurities in the feed water in the form of a more or less hard adherent crust. Such deposits are due to the fact that water loses its soluble power at high temperatures or because the concentration becomes so high, due to evaporation, that the impurities crystallize and adhere to the boiler surfaces. The opportunity for formation of scale in a boiler will be apparent when it is realized that during a month’s operation of a 100 horse-power boiler, 300 pounds of solid matter may be deposited from water containing only 7 grains per gallon, while some spring and well waters contain sufficient to cause a deposit of as high as 2000 pounds.

TABLE 15

SOLUBILITY OF MINERAL SALTS IN WATER (SPARKS)
IN GRAINS PER U. S. GALLON (58,381 GRAINS),
EXCEPT AS NOTED
Temperature Degrees Fahrenheit60 Degrees212 Degrees
Calcium Carbonate2.51.5
Calcium Sulphate140.0125.0
Magnesium Carbonate1.01.8
Magnesium Sulphate3.0 pounds12.0 pounds
Sodium Chloride3.5 pounds4.0 pounds
Sodium Sulphate1.1 pounds5.0 pounds

The salts usually responsible for such incrustation are the carbonates and sulphates of lime and magnesia, and boiler feed treatment in general deals with the getting rid of these salts more or less completely.

Table 15 gives the solubility of these mineral salts in water at various temperatures in grains per U. S. gallon (58,381 grains). It will be seen from this table that the carbonates of lime and magnesium are not soluble above 212 degrees, and calcium sulphate while somewhat insoluble above 212 degrees becomes more greatly so as the temperature increases.

Scale is also formed by the settling of mud and sediment carried in suspension in water. This may bake or be cemented to a hard scale when mixed with other scale-forming ingredients.

CALCIUM SULPHATE AT TEMPERATURE ABOVE
212 DEGREES (CHRISTIE)
Temperature degrees Fahrenheit284329347-365464482
Corresponding gauge pressure3887115-149469561
Grains per gallon45.532.715.710.59.3

Corrosion—Corrosion, or a chemical action leading to the actual destruction of the boiler metal, is due to the solvent or oxidizing properties of the feed water. It [Pg 102] results from the presence of acid, either free or developed[15] in the feed, the admixture of air with the feed water, or as a result of a galvanic action. In boilers it takes several forms:

1st. Pitting, which consists of isolated spots of active corrosion which does not attack the boiler as a whole.

2nd. General corrosion, produced by naturally acid waters and where the amount is so even and continuous that no accurate estimate of the metal eaten away may be made.

3rd. Grooving, which, while largely a mechanical action which may occur in neutral waters, is intensified by acidity.

Foaming—This phenomenon, which ordinarily occurs with waters contaminated with sewage or organic growths, is due to the fact that the suspended particles collect on the surface of the water in the boiler and render difficult the liberation of steam bubbles arising to that surface. It sometimes occurs with water containing carbonates in solution in which a light flocculent precipitate will be formed on the surface of the water. Again, it is the result of an excess of sodium carbonate used in treatment for some other difficulty where animal or vegetable oil finds its way into the boiler.

Priming—Priming, or the passing off of steam from a boiler in belches, is caused by the concentration of sodium carbonate, sodium sulphate or sodium chloride in solution. Sodium sulphate is found in many southern waters and also where calcium or magnesium sulphate is precipitated with soda ash.

Treatment of Feed Water—For scale formation. The treatment of feed water, carrying scale-forming ingredients, is along two main lines: 1st, by chemical means by which such impurities as are carried by the water are caused to precipitate; and 2nd, by the means of heat, which results in the reduction of the power of water to hold certain salts in solution. The latter method alone is sufficient in the case of certain temporarily hard waters, but the heat treatment, in general, is used in connection with a chemical treatment to assist the latter.

Before going further into detail as to the treatment of water, it may be well to define certain terms used.

Hardness, which is the most widely known evidence of the presence in water of scale-forming matter, is that quality, the variation of which makes it more difficult to obtain a lather or suds from soap in one water than in another. This action is made use of in the soap test for hardness described later. Hardness is ordinarily classed as either temporary or permanent. Temporarily hard waters are those containing carbonates of lime and magnesium, which may be precipitated by boiling at 212 degrees and which, if they contain no other scale-forming ingredients, become “soft” under such treatment. Permanently hard waters are those containing mainly calcium sulphate, which is only precipitated at the high temperatures found in the boiler itself, 300 degrees Fahrenheit or more. The scale of hardness is an arbitrary one, based on the number of grains of solids per gallon and waters may be classed on such a basis as follows: 1-10 grain per gallon, soft water; 10-20 grain per gallon, moderately hard water; above 25 grains per gallon, very hard water.

[Pg 103]

Alkalinity is a general term used for waters containing compounds with the power of neutralizing acids.

Causticity, as used in water treatment, is a term coined by A. McGill, indicating the presence of an excess of lime added during treatment. Though such presence would also indicate alkalinity, the term is arbitrarily used to apply to those hydrates whose presence is indicated by phenolphthalein.

Of the chemical methods of water treatment, there are three general processes:

1st. Lime Process. The lime process is used for waters containing bicarbonates of lime and magnesia. Slacked lime in solution, as lime water, is the reagent used. This combines with the carbonic acid which is present, either free or as carbonates, to form an insoluble monocarbonate of lime. The soluble bicarbonates of lime and magnesia, losing their carbonic acid, thereby become insoluble and precipitate.

2nd. Soda Process. The soda process is used for waters containing sulphates of lime and magnesia. Carbonate of soda and hydrate of soda (caustic soda) are used either alone or together as the reagents. Carbonate of soda, added to water containing little or no carbonic acid or bicarbonates, decomposes the sulphates to form insoluble carbonate of lime or magnesia which precipitate, the neutral soda remaining in solution. If free carbonic acid or bicarbonates are present, bicarbonate of lime is formed and remains in solution, though under the action of heat, the carbon dioxide will be driven off and insoluble monocarbonates will be formed. Caustic soda used in this process causes a more energetic action, it being presumed that the caustic soda absorbs the carbonic acid, becomes carbonate of soda and acts as above.

3rd. Lime and Soda Process. This process, which is the combination of the first two, is by far the most generally used in water purification. Such a method is used where sulphates of lime and magnesia are contained in the water, together with such quantity of carbonic acid or bicarbonates as to impair the action of the soda. Sufficient soda is used to break down the sulphates of lime and magnesia and as much lime added as is required to absorb the carbonic acid not taken up in the soda reaction.

All of the apparatus for effecting such treatment of feed waters is approximately the same in its chemical action, the numerous systems differing in the methods of introduction and handling of the reagents.

The methods of testing water treated by an apparatus of this description follow.

When properly treated, alkalinity, hardness and causticity should be in the approximate relation of 6, 5 and 4. When too much lime is used in the treatment, the causticity in the purified water, as indicated by the acid test, will be nearly equal to the alkalinity. If too little lime is used, the causticity will fall to approximately half the alkalinity. The hardness should not be in excess of two points less than the alkalinity. Where too great a quantity of soda is used, the hardness is lowered and the alkalinity raised. If too little soda, the hardness is raised and the alkalinity lowered.

Alkalinity and causticity are tested with a standard solution of sulphuric acid. A standard soap solution is used for testing for hardness and a silver nitrate solution may also be used for determining whether an excess of lime has been used in the treatment.

Alkalinity: To 50 cubic centimeters of treated water, to which there has been added sufficient methylorange to color it, add the acid solution, drop by drop, until the mixture is on the point of turning red. As the acid solution is first added, the red color, which shows quickly, disappears on shaking the mixture, and this color [Pg 104]
[Pg 105]
disappears more slowly as the critical point is approached. One-tenth cubic centimeter of the standard acid solution corresponds to one degree of alkalinity.

Illustration:

2640 Horse-power Installation of Babcock & Wilcox Boilers at the Botany Worsted Mills, Passaic, N. J.

Causticity: To 50 cubic centimeters of treated water, to which there has been added one drop of phenolphthalein dissolved in alcohol to give the water a pinkish color, add the acid solution, drop by drop, shaking after each addition, until the color entirely disappears. One-tenth cubic centimeter of acid solution corresponds to one degree of causticity.

The alkalinity may be determined from the same sample tested for causticity by the coloring with methylorange and adding the acid until the sample is on the point of turning red. The total acid added in determining both causticity and alkalinity in this case is the measure of the alkalinity.

Hardness: 100 cubic centimeters of the treated water is used for this test, one cubic centimeter of the soap solution corresponding to one degree of hardness. The soap solution is added a very little at a time and the whole violently shaken. Enough of the solution must be added to make a permanent lather or foam, that is, the soap bubbles must not disappear after the shaking is stopped.

Excess of lime as determined by nitrate of silver: If there is an excess of lime used in the treatment, a sample will become a dark brown by the addition of a small quantity of silver nitrate, otherwise a milky white solution will be formed.

Combined Heat and Chemical Treatment: Heat is used in many systems of feed treatment apparatus as an adjunct to the chemical process. Heat alone will remove temporary hardness by the precipitation of carbonates of lime and magnesia and, when used in connection with the chemical process, leaves only the permanent hardness or the sulphates of lime to be taken care of by chemical treatment.

TABLE 16

REAGENTS REQUIRED IN LIME AND SODA PROCESS
FOR TREATING 1000 U. S. GALLONS OF WATER
PER GRAIN PER GALLON OF CONTAINED IMPURITIES[16]
 Lime[17]
Pounds
Soda[18]
Pounds
 Lime[17]
Pounds
Soda[18]
Pounds
Calcium Carbonate0.098Ferrous Carbonate0.169
Calcium Sulphate0.124Ferrous Sulphate0.0700.110
Calcium Chloride0.151Ferric Sulphate0.0740.126
Calcium Nitrate0.104Aluminum Sulphate0.0870.147
Magnesium Carbonate0.234Free Sulphuric Acid0.1000.171
Magnesium Sulphate0.0790.141Sodium Carbonate0.093
Magnesium Chloride0.1030.177Free Carbon Dioxide0.223
Magnesium Nitrate0.0670.115Hydrogen Sulphite0.288

The chemicals used in the ordinary lime and soda process of feed water treatment are common lime and soda. The efficiency of such apparatus will depend wholly upon the amount and character of the impurities in the water to be treated. Table 16 gives the amount of lime and soda required per 1000 gallons for each grain per gallon of the various impurities found in the water. This table is based on lime containing 90 per cent calcium oxide and soda containing 58 per cent sodium oxide, [Pg 106] which correspond to the commercial quality ordinarily purchasable. From this table and the cost of the lime and soda, the cost of treating any water per 1000 gallons may be readily computed.

Less Usual Reagents—Barium hydrate is sometimes used to reduce permanent hardness or the calcium sulphate component. Until recently, the high cost of barium hydrate has rendered its use prohibitive but at the present it is obtained as a by-product in cement manufacture and it may be purchased at a more reasonable figure than heretofore. It acts directly on the soluble sulphates to form barium sulphate which is insoluble and may be precipitated. Where this reagent is used, it is desirable that the reaction be allowed to take place outside of the boiler, though there are certain cases where its internal use is permissible.

Barium carbonate is sometimes used in removing calcium sulphate, the products of the reaction being barium sulphate and calcium carbonate, both of which are insoluble and may be precipitated. As barium carbonate in itself is insoluble, it cannot be added to water as a solution and its use should, therefore, be confined to treatment outside of the boiler.

Silicate of soda will precipitate calcium carbonate with the formation of a gelatinous silicate of lime and carbonate of soda. If calcium sulphate is also present, carbonate of soda is formed in the above reaction, which in turn will break down the sulphate.

Oxalate of soda is an expensive but efficient reagent which forms a precipitate of calcium oxalate of a particularly insoluble nature.

Alum and iron alum will act as efficient coagulents where organic matter is present in the water. Iron alum has not only this property but also that of reducing oil discharged from surface condensers to a condition in which it may be readily removed by filtration.

Corrosion—Where there is a corrosive action because of the presence of acid in the water or of oil containing fatty acids which will decompose and cause pitting wherever the sludge can find a resting place, it may be overcome by the neutralization of the water by carbonate of soda. Such neutralization should be carried to the point where the water will just turn red litmus paper blue. As a preventative of such action arising from the presence of the oil, only the highest grades of hydrocarbon oils should be used.

Acidity will occur where sea water is present in a boiler. There is the possibility of such an occurrence in marine practice and in stationary plants using sea water for condensing, due to leaky condenser tubes, priming in the evaporators, etc. Such acidity is caused through the dissociation of magnesium chloride into hydrochloride acid and magnesia under high temperatures. The acid in contact with the metal forms an iron salt which immediately upon its formation is neutralized by the free magnesia in the water, thereby precipitating iron oxide and reforming magnesium chloride. The preventive for corrosion arising from such acidity is the keeping tight of the condenser. Where it is unavoidable that some sea water should find its way into a boiler, the acidity resulting should be neutralized by soda ash. This will convert the magnesium chloride into magnesium carbonate and sodium chloride, neither of which is corrosive but both of which are scale-forming.

The presence of air in the feed water which is sucked in by the feed pump is a well recognized cause of corrosion. Air bubbles form below the water line and attack [Pg 107] the metal of the boiler, the oxygen of the air causing oxidization of the boiler metal and the formation of rust. The particle of rust thus formed is swept away by the circulation or is dislodged by expansion and the minute pit thus left forms an ideal resting place for other air bubbles and the continuation of the oxidization process. The prevention is, of course, the removing of the air from the feed water. In marine practice, where there has been experienced the most difficulty from this source, it has been found to be advantageous to pump the water from the hot well to a filter tank placed above the feed pump suction valves. In this way the air is liberated from the surface of the tank and a head is assured for the suction end of the pump. In this same class of work, the corrosive action of air is reduced by introducing the feed through a spray nozzle into the steam space above the water line.

Galvanic action, resulting in the eating away of the boiler metal through electrolysis was formerly considered practically the sole cause of corrosion. But little is known of such action aside from the fact that it does take place in certain instances. The means adopted as a remedy is usually the installation of zinc plates within the boiler, which must have positive metallic contact with the boiler metal. In this way, local electrolytic effects are overcome by a still greater electrolytic action at the expense of the more positive zinc. The positive contact necessary is difficult to maintain and it is questionable just what efficacy such plates have except for a short period after their installation when the contact is known to be positive. Aside from protection from such electrolytic action, however, the zinc plates have a distinct use where there is the liability of air in the feed, as they offer a substance much more readily oxidized by such air than the metal of the boiler.

Foaming—Where foaming is caused by organic matter in suspension, it may be largely overcome by filtration or by the use of a coagulent in connection with filtration, the latter combination having come recently into considerable favor. Alum, or potash alum, and iron alum, which in reality contains no alumina and should rather be called potassia-ferric, are the coagulents generally used in connection with filtration. Such matter as is not removed by filtration may, under certain conditions, be handled by surface blowing. In some instances, settling tanks are used for the removal of matter in suspension, but where large quantities of water are required, filtration is ordinarily substituted on account of the time element and the large area necessary in settling tanks.

Where foaming occurs as the result of overtreatment of the feed water, the obvious remedy is a change in such treatment.

Priming—Where priming is caused by excessive concentration of salts within a boiler, it may be overcome largely by frequent blowing down. The degree of concentration allowable before priming will take place varies widely with conditions of operation and may be definitely determined only by experience with each individual set of conditions. It is the presence of the salts that cause priming that may result in the absolute unfitness of water for boiler feed purposes. Where these salts exist in such quantities that the amount of blowing down necessary to keep the degree of concentration below the priming point results in excessive losses, the only remedy is the securing of another supply of feed, and the results will warrant the change almost regardless of the expense. In some few instances, the impurities may be taken care of by some method of water treatment but such water should be submitted to an authority on the subject before any treatment apparatus is installed.
[Pg 108]

[Pg 109] Boiler Compounds—The method of treatment of feed water by far the most generally used is by the use of some of the so-called boiler compounds. There are many reliable concerns handling such compounds who unquestionably secure the promised results, but there is a great tendency toward looking on the compound as a “cure all” for any water difficulties and care should be taken to deal only with reputable concerns.

Illustration:

3000 Horse-power Installation of Cross Drum Babcock & Wilcox Boilers and Superheaters Equipped with Babcock & Wilcox Chain Grate Stokers at the Washington Terminal Co., Washington, D. C.

The composition of these compounds is almost invariably based on soda with certain tannic substances and in some instances a gelatinous substance which is presumed to encircle scale particles and prevent their adhering to the boiler surfaces. The action of these compounds is ordinarily to reduce the calcium sulphate in the water by means of carbonate of soda and to precipitate it as a muddy form of calcium carbonate which may be blown off. The tannic compounds are used in connection with the soda with the idea of introducing organic matter into any scale already formed. When it has penetrated to the boiler metal, decomposition of the scale sets in, causing a disruptive effect which breaks the scale from the metal sometimes in large slabs. It is this effect of boiler compounds that is to be most carefully guarded against or inevitable trouble will result from the presence of loose scale with the consequent danger of tube losses through burning.

When proper care is taken to suit the compound to the water in use, the results secured are fairly effective. In general, however, the use of compounds may only be recommended for the prevention of scale rather than with the view to removing scale which has already formed, that is, the compounds should be introduced with the feed water only when the boiler has been thoroughly cleaned.

FOOTNOTES

[15] Some waters, not naturally acid, become so at high temperatures, as when chloride of magnesia decomposes with the formation of free hydrochloride acid; such phenomena become more serious with an increase in pressure and temperature.

[16] L. M. Booth Company.

[17] Based on lime containing 90 per cent calcium oxide.

[18] Based on soda containing 58 per cent sodium oxide.

[Pg 110]

FEED WATER HEATING AND METHODS OF FEEDING

Before water fed into a boiler can be converted into steam, it must be first heated to a temperature corresponding to the pressure within the boiler. Steam at 160 pounds gauge pressure has a temperature of approximately 371 degrees Fahrenheit. If water is fed to the boiler at 60 degrees Fahrenheit, each pound must have 311 B. t. u. added to it to increase its temperature 371 degrees, which increase must take place before the water can be converted into steam. As it requires 1167.8 B. t. u. to raise one pound of water from 60 to 371 degrees and to convert it into steam at 160 pounds gauge pressure, the 311 degrees required simply to raise the temperature of the water from 60 to 371 degrees will be approximately 27 per cent of the total. If, therefore, the temperature of the water can be increased from 60 to 371 degrees before it is introduced into a boiler by the utilization of heat from some source that would otherwise be wasted, there will be a saving in the fuel required of 311 ÷ 1167.8 = 27 per cent, and there will be a net saving, provided the cost of maintaining and operating the apparatus for securing this saving is less than the value of the heat thus saved.

The saving in the fuel due to the heating of feed water by means of heat that would otherwise be wasted may be computed from the formula:

Fuel saving per cent=
100 (tti)
––––––––––––––––––
H + 32 − ti
            (1)

where, t = temperature of feed water after heating, ti = temperature of feed water before heating, and H = total heat above 32 degrees per pound of steam at the boiler pressure. Values of H may be found in Table 23. Table 17 has been computed from this formula to show the fuel saving under the conditions assumed with the boiler operating at 180 pounds gauge pressure.

TABLE 17

SAVING IN FUEL, IN PER CENT, BY HEATING FEED WATER
GAUGE PRESSURE 180 POUNDS
Init’l
Temp.
° Fahr.
Final Temperature—Degrees FahrenheitInit’l
Temp.
° Fahr.
Final Temperature—Degrees Fahrenheit
120140160180200250300120140160180200250300
  327.359.0210.6912.3614.0418.2022.38  952.203.975.737.499.2513.6618.07
  357.128.7910.4612.1413.8218.0022.181001.773.545.317.088.8513.2817.70
  406.728.4110.0911.7713.4517.6521.86110.892.684.476.258.0412.5016.97
  456.338.029.7111.4013.0817.3021.52120.001.803.615.417.2111.7116.22
  505.937.639.3211.0212.7216.9521.19130 .912.734.556.3710.9115.46
  555.537.248.9410.6412.3416.6020.86140 .001.843.675.5110.0914.68
  605.136.848.5510.2711.9716.2420.52150  .932.784.639.2613.89
  654.726.448.169.8711.5915.8820.18160  .001.873.748.4113.09
  704.316.047.779.4811.2115.5219.83170   .942.837.5512.27
  753.905.647.369.0910.8215.1619.48180   .001.916.6711.43
  803.485.226.968.7010.4414.7919.13190    .965.7710.58
  853.064.806.558.3010.0514.4118.78200    .004.869.71
  902.634.396.147.899.6514.0418.43210     3.928.82

[Pg 111]

Besides the saving in fuel effected by the use of feed water heaters, other advantages are secured. The time required for the conversion of water into steam is diminished and the steam capacity of the boiler thereby increased. Further, the feeding of cold water into a boiler has a tendency toward the setting up of temperature strains, which are diminished in proportion as the temperature of the feed approaches that of the steam. An important additional advantage of heating feed water is that in certain types of heaters a large portion of the scale forming ingredients are precipitated before entering the boiler, with a consequent saving in cleaning and losses through decreased efficiency and capacity.

In general, feed water heaters may be divided into closed heaters, open heaters and economizers; the first two depend for their heat upon exhaust, or in some cases live steam, while the last class utilizes the heat of the waste flue gases to secure the same result. The question of the type of apparatus to be installed is dependent upon the conditions attached to each individual case.

In closed heaters the feed water and the exhaust steam do not come into actual contact with each other. Either the steam or the water passes through tubes surrounded by the other medium, as the heater is of the steam-tube or water-tube type. A closed heater is best suited for water free from scale-forming matter, as such matter soon clogs the passages. Cleaning such heaters is costly and the efficiency drops off rapidly as scale forms. A closed heater is not advisable where the engines work intermittently, as is the case with mine hoisting engines. In this class of work the frequent coolings between operating periods and the sudden heatings when operation commences will tend to loosen the tubes or even pull them apart. For this reason, an open heater, or economizer, will give more satisfactory service with intermittently operating apparatus.

Open heaters are best suited for waters containing scale-forming matter. Much of the temporary hardness may be precipitated in the heater and the sediment easily removed. Such heaters are frequently used with a reagent for precipitating permanent hardness in the combined heat and chemical treatment of feed water. The so-called live steam purifiers are open heaters, the water being raised to the boiling temperature and the carbonates and a portion of the sulphates being precipitated. The disadvantage of this class of apparatus is that some of the sulphates remain in solution to be precipitated as scale when concentrated in the boiler. Sufficient concentration to have such an effect, however, may often be prevented by frequent blowing down.

Economizers find their largest field where the design of the boiler is such that the maximum possible amount of heat is not extracted from the gases of combustion. The more wasteful the boiler, the greater the saving effected by the use of the economizer, and it is sometimes possible to raise the temperature of the feed water to that of high pressure steam by the installation of such an apparatus, the saving amounting in some cases to as much as 20 per cent. The fuel used bears directly on the question of the advisability of an economizer installation, for when oil is the fuel a boiler efficiency of 80 per cent or over is frequently realized, an efficiency which would leave a small opportunity for a commercial gain through the addition of an economizer.

From the standpoint of space requirements, economizers are at a disadvantage in that they are bulky and require a considerable increase over space occupied by a heater of the exhaust type. They also require additional brickwork or a metal casing, which [Pg 112] increases the cost. Sometimes, too, the frictional resistance of the gases through an economizer make its adaptability questionable because of the draft conditions. When figuring the net return on economizer investment, all of these factors must be considered.

When the feed water is such that scale will quickly encrust the economizer and throw it out of service for cleaning during an excessive portion of the time, it will be necessary to purify water before introducing it into an economizer to make it earn a profit on the investment.

From the foregoing, it is clearly indicated that it is impossible to make a definite statement as to the relative saving by heating feed water in any of the three types. Each case must be worked out independently and a decision can be reached only after an exhaustive study of all the conditions affecting the case, including the time the plant will be in service and probable growth of the plant. When, as a result of such study, the possible methods for handling the problem have been determined, the solution of the best apparatus can be made easily by the balancing of the saving possible by each method against its first cost, depreciation, maintenance and cost of operation.

Feeding of Water—The choice of methods to be used in introducing feed water into a boiler lies between an injector and a pump. In most plants, an injector would not be economical, as the water fed by such means must be cold, a fact which makes impossible the use of a heater before the water enters the injector. Such a heater might be installed between the injector and the boiler but as heat is added to the water in the injector, the heater could not properly fulfill its function.

TABLE 18

COMPARISON OF PUMPS AND INJECTORS
Method of Supplying
Feed-water to Boiler.
Temperature of feed-water
as delivered to the pump,
or to injector,
60 degrees Fahrenheit.
Rate of evaporation of
boiler, to pounds of water
per pound of coal from and
at 212 degrees Fahrenheit
Relative amount of
coal required per
unit of time, the
amount for a
direct-acting pump,
feeding water at
60 degrees
without a heater,
being taken as unity
Saving of fuel over
the amount required
when the boiler
is fed by a
direct-acting pump
without heater
Per Cent
Direct-acting Pump feeding water
at 60 degrees without a heater
1.000    .0
Injector feeding water at 150
degrees without a heater
  .985  1.5
Injector feeding through
a heater in which the water is
heated from 150 to 200 degrees
  .938  6.2
Direct-acting Pump feeding
water through a heater in which it
is heated from 60 to 200 degrees
  .87912.1
Geared Pump run from the
engine, feeding water through a
heater in which it is heated
from 60 to 200 degrees
  .86813.2

The injector, considered only in the light of a combined heater and pump, is claimed to have a thermal efficiency of 100 per cent, since all of the heat in the steam used is returned to the boiler with the water. This claim leads to an erroneous idea. If a pump is used in feeding the water to a boiler and the heat in the exhaust from the pump is imparted to the feed water, the pump has as high a thermal efficiency as the injector. The pump has the further advantage that it uses so much less steam for the forcing of a given quantity [Pg 113] of water into the boiler that it makes possible a greater saving through the use of the exhaust from other auxiliaries for heating the feed, which exhaust, if an injector were used, would be wasted, as has been pointed out.

In locomotive practice, injectors are used because there is no exhaust steam available for heating the feed, this being utilized in producing a forced draft, and because of space requirements. In power plant work, however, pumps are universally used for regular operation, though injectors are sometimes installed as an auxiliary method of feeding.

Table 18 shows the relative value of injectors, direct-acting steam pumps and pumps driven from the engine, the data having been obtained from actual experiment. It will be noted that when feeding cold water direct to the boilers, the injector has a slightly greater economy but when feeding through a heater, the pump is by far the more economical.

Auxiliaries—It is the general impression that auxiliaries will take less steam if the exhaust is turned into the condensers, in this way reducing the back pressure. As a matter of fact, vacuum is rarely registered on an indicator card taken from the cylinders of certain types of auxiliaries unless the exhaust connection is short and without bends, as long pipes and many angles offset the effect of the condenser. On the other hand, if the exhaust steam from the auxiliaries can be used for heating the feed water, all of the latent heat less only the loss due to radiation is returned to the boiler and is saved instead of being lost in the condensing water or wasted with the free exhaust. Taking into consideration the plant as a whole, it would appear that the auxiliary machinery, under such conditions, is more efficient than the main engines. [Pg 114]

Illustration:

Portion of 4160 Horse-power Installation of Babcock & Wilcox Boilers at the Prudential Life Insurance Co. Building, Newark, N. J.

[Pg 115]

STEAM

When a given weight of a perfect gas is compressed or expanded at a constant temperature, the product of the pressure and volume is a constant. Vapors, which are liquids in aeriform condition, on the other hand, can exist only at a definite pressure corresponding to each temperature if in the saturated state, that is, the pressure is a function of the temperature only. Steam is water vapor, and at a pressure of, say, 150 pounds absolute per square inch saturated steam can exist only at a temperature 358 degrees Fahrenheit. Hence if the pressure of saturated steam be fixed, its temperature is also fixed, and vice versa .

Saturated steam is water vapor in the condition in which it is generated from water with which it is in contact. Or it is steam which is at the maximum pressure and density possible at its temperature. If any change be made in the temperature or pressure of steam, there will be a corresponding change in its condition. If the pressure be increased or the temperature decreased, a portion of the steam will be condensed. If the temperature be increased or the pressure decreased, a portion of the water with which the steam is in contact will be evaporated into steam. Steam will remain saturated just so long as it is of the same pressure and temperature as the water with which it can remain in contact without a gain or loss of heat. Moreover, saturated steam cannot have its temperature lowered without a lowering of its pressure, any loss of heat being made up by the latent heat of such portion as will be condensed. Nor can the temperature of saturated steam be increased except when accompanied by a corresponding increase in pressure, any added heat being expended in the evaporation into steam of a portion of the water with which it is in contact.

Dry saturated steam contains no water. In some cases, saturated steam is accompanied by water which is carried along with it, either in the form of a spray or is blown along the surface of the piping, and the steam is then said to be wet. The percentage weight of the steam in a mixture of steam and water is called the quality of the steam. Thus, if in a mixture of 100 pounds of steam and water there is three-quarters of a pound of water, the quality of the steam will be 99.25.

Heat may be added to steam not in contact with water, such an addition of heat resulting in an increase of temperature and pressure if the volume be kept constant, or an increase in temperature and volume if the pressure remain constant. Steam whose temperature thus exceeds that of saturated steam at a corresponding pressure is said to be superheated and its properties approximate those of a perfect gas.

As pointed out in the chapter on heat, the heat necessary to raise one pound of water from 32 degrees Fahrenheit to the point of ebullition is called the heat of the liquid . The heat absorbed during ebullition consists of that necessary to dissociate the molecules, or the inner latent heat , and that necessary to overcome the resistance to the increase in volume, or the outer latent heat . These two make up the latent heat of evaporation and the sum of this latent heat of evaporation and the heat of the liquid make the total heat of the steam. These values for various pressures are given in the steam tables , pages 122 to 127 .

The specific volume of saturated steam at any pressure is the volume in cubic feet of one pound of steam at that pressure.

The density of saturated steam, that is, its weight per cubic foot, is obviously the reciprocal of the specific volume. This density varies as the 1617 power over the [Pg 116] ordinary range of pressures used in steam boiler work and may be found by the formula, D = .003027 p .941 , which is correct within 0.15 per cent up to 250 pounds pressure.

The relative volume of steam is the ratio of the volume of a given weight to the volume of the same weight of water at 39.2 degrees Fahrenheit and is equal to the specific volume times 62.427.

As vapors are liquids in their gaseous form and the boiling point is the point of change in this condition, it is clear that this point is dependent upon the pressure under which the liquid exists. This fact is of great practical importance in steam condenser work and in many operations involving boiling in an open vessel, since in the latter case its altitude will have considerable influence. The relation between altitude and boiling point of water is shown in Table 12 .

The conditions of feed temperature and steam pressure in boiler tests, fuel performances and the like, will be found to vary widely in different trials. In order to secure a means for comparison of different trials, it is necessary to reduce all results to some common basis. The method which has been adopted for the reduction to a comparable basis is to transform the evaporation under actual conditions of steam pressure and feed temperature which exist in the trial to an equivalent evaporation under a set of standard conditions. These standard conditions presuppose a feed water temperature of 212 degrees Fahrenheit and a steam pressure equal to the normal atmospheric pressure at sea level, 14.7 pounds absolute. Under such conditions steam would be generated at a temperature of 212 degrees, the temperature corresponding to atmospheric pressure at sea level, from water at 212 degrees. The weight of water which would be evaporated under the assumed standard conditions by exactly the amount of heat absorbed by the boiler under actual conditions existing in the trial, is, therefore, called the equivalent evaporation “from and at 212 degrees.”

The factor for reducing the weight of water actually converted into steam from the temperature of the feed, at the steam pressure existing in the trial, to the equivalent evaporation under standard conditions is called the factor of evaporation. This factor is the ratio of the total heat added to one pound of steam under the standard conditions to the heat added to each pound of steam in heating the water from the temperature of the feed in the trial to the temperature corresponding to the pressure existing in the trial. This heat added is obviously the difference between the total heat of evaporation of the steam at the pressure existing in the trial and the heat of the liquid in the water at the temperature at which it was fed in the trial. To illustrate by an example:

In a boiler trial the temperature of the feed water is 60 degrees Fahrenheit and the pressure under which steam is delivered is 160.3 pounds gauge pressure or 175 pounds absolute pressure. The total heat of one pound of steam at 175 pounds pressure is 1195.9 B. t. u. measured above the standard temperature of 32 degrees Fahrenheit. But the water fed to the boiler contained 28.08 B. t. u. as the heat of the liquid measured above 32 degrees Fahrenheit. Therefore, to each pound of steam there has been added 1167.82 B. t. u. To evaporate one pound of water under standard conditions would, on the other hand, have required but 970.4 B. t. u., which, as described, is the latent heat of evaporation at 212 degrees Fahrenheit. Expressed differently, the total heat of one pound of steam at the pressure corresponding to a temperature of 212 degrees is 1150.4 B. t. u. One pound of water at 212 degrees [Pg 117] contains 180 B. t. u. of sensible heat above 32 degrees Fahrenheit. Hence, under standard conditions, 1150.4 - 180 = 970.4 B. t. u. is added in the changing of one pound of water into steam at atmospheric pressure and a temperature of 212 degrees. This is in effect the definition of the latent heat of evaporation.

Hence, if conditions of the trial had been standard, only 970.4 B. t. u. would be required and the ratio of 1167.82 to 970.4 B. t. u. is the ratio determining the factor of evaporation. The factor in the assumed case is 1167.82 ÷ 970.4 = 1.2034 and if the same amount of heat had been absorbed under standard conditions as was absorbed in the trial condition, 1.2034 times the amount of steam would have been generated. Expressed as a formula for use with any set of conditions, the factor is,

F  = 
H − h
––––––––––
970.4
            ( 2 )
Where H = the total heat of steam above 32 degrees Fahrenheit from steam tables,
h = sensible heat of feed water above 32 degrees Fahrenheit from Table 22 .

In the form above, the factor may be determined with either saturated or superheated steam, provided that in the latter case values of H are available for varying degrees of superheat and pressures.

Where such values are not available, the form becomes,

F  = 
H − h + s ( t supt sat )
––––––––––––––––––––––––––––––––––
970.4
            ( 3 )
Where s = mean specific heat of superheated steam at the pressure existing in the trial from saturated steam to the temperature existing in the trial,
t sup = final temperature of steam,
t sat = temperature of saturated steam, corresponding to pressure existing,
( t sup  −  t sat ) = degrees of superheat.

The specific heat of superheated steam will be taken up later.

Table 19 gives factors of evaporation for saturated steam boiler trials to cover a large range of conditions. Except for the most refined work, intermediate values may be determined by interpolation.

Steam gauges indicate the pressure above the atmosphere. As has been pointed out, the atmospheric pressure changes according to the altitude and the variation in the barometer. Hence, calculations involving the properties of steam are based on absolute pressures, which are equal to the gauge pressure plus the atmospheric pressure in pounds to the square inch. This latter is generally assumed to be 14.7 pounds per square inch at sea level, but for other levels it must be determined from the barometric reading at that place.

Vacuum gauges indicate the difference, expressed in inches of mercury, between atmospheric pressure and the pressure within the vessel to which the gauge is attached. For approximate purposes, 2.04 inches height of mercury may be considered equal to a pressure of one pound per square inch at the ordinary temperatures at which mercury gauges are used. Hence for any reading of the vacuum gauge in inches, G, the absolute pressure for any barometer reading in inches, B, will be (B - G) ÷ 2.04. If the barometer is 30 inches measured at ordinary temperatures and not corrected to 32 degrees Fahrenheit and the vacuum gauge 24 inches, the absolute pressure will be (30 - 24) ÷ 2.04 = 2.9 pounds per square inch.

[Pg 118]

TABLE 19

FACTORS OF EVAPORATION
CALCULATED FROM MARKS AND DAVIS TABLES
Feed
Temp-
erature
Degrees
Fahren-
heit
Steam Pressure by Gauge
50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250
32 1.2143 1.2170 1.2194 1.2215 1.2233 1.2233 1.2265 1.2280 1.2292 1.2304 1.2314 1.2323 1.2333 1.2342 1.2350 1.2357 1.2364 1.2372 1.2378 1.2384 1.2390
40 1.2060 1.2087 1.2111 1.2131 1.2150 1.2168 1.2181 1.2196 1.2209 1.2221 1.2231 1.2241 1.2250 1.2259 1.2267 1.2274 1.2282 1.2289 1.2295 1.2301 1.2307
50 1.1957 1.1984 1.2008 1.2028 1.2047 1.2065 1.2079 1.2093 1.2106 1.2117 1.2128 1.2137 1.2147 1.2156 1.2164 1.2171 1.2178 1.2186 1.2192 1.2198 1.2204
60 1.1854 1.1881 1.1905 1.1925 1.1944 1.1961 1.1976 1.1990 1.2003 1.2014 1.2025 1.2034 1.2044 1.2053 1.2061 1.2068 1.2075 1.2083 1.2089 1.2095 1.2101
70 1.1750 1.1778 1.1802 1.1822 1.1841 1.1859 1.1873 1.1887 1.1900 1.1911 1.1922 1.1931 1.1941 1.1950 1.1958 1.1965 1.1972 1.1980 1.1986 1.1992 1.1998
80 1.1649 1.1675 1.1699 1.1720 1.1738 1.1756 1.1770 1.1785 1.1797 1.1809 1.1819 1.1828 1.1838 1.1847 1.1855 1.1863 1.1869 1.1877 1.1883 1.1889 1.1895
90 1.1545 1.1572 1.1596 1.1617 1.1636 1.1653 1.1668 1.1682 1.1695 1.1706 1.1717 1.1725 1.1735 1.1744 1.1750 1.1760 1.1766 1.1774 1.1780 1.1786 1.1792
100 1.1443 1.1470 1.1493 1.1514 1.1533 1.1550 1.1565 1.1579 1.1592 1.1603 1.1614 1.1623 1.1633 1.1642 1.1650 1.1657 1.1664 1.1671 1.1678 1.1684 1.1690
110 1.1340 1.1367 1.1391 1.1411 1.1430 1.1448 1.1462 1.1477 1.1489 1.1500 1.1511 1.1520 1.1530 1.1539 1.1547 1.1554 1.1562 1.1569 1.1575 1.1581 1.1587
120 1.1237 1.1264 1.1288 1.1309 1.1327 1.1345 1.1359 1.1374 1.1386 1.1398 1.1408 1.1418 1.1427 1.1436 1.1444 1.1452 1.1459 1.1466 1.1472 1.1478 1.1484
130 1.1134 1.1161 1.1185 1.1206 1.1225 1.1242 1.1257 1.1271 1.1284 1.1295 1.1305 1.1315 1.1324 1.1333 1.1341 1.1349 1.1356 1.1363 1.1369 1.1375 1.1381
140 1.1031 1.1058 1.1082 1.1103 1.1122 1.1139 1.1154 1.1168 1.1181 1.1192 1.1203 1.1212 1.1221 1.1230 1.1239 1.1246 1.1253 1.1260 1.1266 1.1272 1.1278
150 1.0928 1.0955 1.0979 1.1000 1.1019 1.1036 1.1051 1.1065 1.1078 1.1089 1.1099 1.1109 1.1118 1.1127 1.1136 1.1143 1.1150 1.1157 1.1163 1.1169 1.1176
160 1.0825 1.0852 1.0876 1.0897 1.0916 1.0933 1.0948 1.0962 1.0975 1.0986 1.0997 1.1006 1.1015 1.1024 1.1033 1.1040 1.1047 1.1054 1.1060 1.1066 1.1073
170 1.0722 1.0749 1.0773 1.0794 1.0813 1.0830 1.0845 1.0859 1.0872 1.0883 1.0893 1.0903 1.0912 1.0921 1.0930 1.0937 1.0944 1.0951 1.0957 1.0963 1.0969
180 1.0619 1.0646 1.0670 1.0691 1.0709 1.0727 1.0741 1.0756 1.0768 1.0780 1.0790 1.0800 1.0809 1.0818 1.0826 1.0834 1.0841 1.0848 1.0854 1.0860 1.0866
190 1.0516 1.0543 1.0567 1.0587 1.0606 1.0624 1.0638 1.0653 1.0665 1.0676 1.0687 1.0696 1.0706 1.0715 1.0723 1.0730 1.0737 1.0745 1.0751 1.0757 1.0763
200 1.0412 1.0439 1.0463 1.0484 1.0503 1.0520 1.0535 1.0549 1.0562 1.0573 1.0584 1.0593 1.0602 1.0611 1.0620 1.0627 1.0634 1.0641 1.0647 1.0653 1.0660
210 1.0309 1.0336 1.0360 1.0380 1.0399 1.0417 1.0432 1.0446 1.0458 1.0469 1.0480 1.0489 1.0499 1.0508 1.0516 1.0523 1.0530 1.0538 1.0544 1.0550 1.0556

[Pg 119]

The temperature, pressure and other properties of steam for varying amounts of vacuum and the pressure above vacuum corresponding to each inch of reading of the vacuum gauge are given in Table 20 .

TABLE 20

PROPERTIES OF SATURATED STEAM FOR VARYING AMOUNTS OF VACUUM
CALCULATED FROM MARKS AND DAVIS TABLE
Vacuum
Ins. Hg.
Absolute
Pressure
Pounds
Temperature
Degrees
Fahrenheit
Heat of the Liquid
Above 32 Degrees
B. t. u.
Latent Heat
Above 32 Degrees
B. t. u.
Total Heat
Above 32 Degrees
B. t. u.
Density or Weight
per Cubic Foot
Pounds
29.5   .207   54.1   22.18 1061.0 1083.2 0.000678
29       .452   76.6   44.64 1048.7 1093.3 0.001415
28.5   .698   90.1   58.09 1041.1 1099.2 0.002137
28       .944   99.9   67.87 1035.6 1103.5 0.002843
27     1.44   112.5   80.4   1028.6 1109.0 0.00421  
26     1.93   124.5   92.3   1022.0 1114.3 0.00577  
25     2.42   132.6 100.5   1017.3 1117.8 0.00689  
24     2.91   140.1 108.0   1013.1 1121.1 0.00821  
22     3.89   151.7 119.6   1006.4 1126.0 0.01078  
20     4.87   161.1 128.9   1001.0 1129.9 0.01331  
18     5.86   168.9 136.8     996.4 1133.2 0.01581  
16     6.84   175.8 143.6     992.4 1136.0 0.01827  
14     7.82   181.8 149.7     988.8 1138.5 0.02070  
12     8.80   187.2 155.1     985.6 1140.7 0.02312  
10     9.79   192.2 160.1     982.6 1142.7 0.02554  
  5     12.24   202.9 170.8     976.0 1146.8 0.03148  

From the steam tables , the condensed Table 21 of the properties of steam at different pressures may be constructed. From such a table there may be drawn the following conclusions.

TABLE 21

VARIATION IN PROPERTIES OF
SATURATED STEAM WITH PRESSURE
Pressure
Pounds
Absolute
Temperature
Degrees
Fahrenheit
Heat of
Liquid
B. t. u.
Latent
Heat
B. t. u.
Total
Heat
B. t. u.
  14.7 212.0 180.0 970.4 1150.4
  20.0 228.0 196.1 960.0 1156.2
100.0 327.8 298.3 888.0 1186.3
300.0 417.5 392.7 811.3 1204.1

As the pressure and temperature increase, the latent heat decreases. This decrease, however, is less rapid than the corresponding increase in the heat of the liquid and hence the total heat increases with an increase in the pressure and temperature. The percentage increase in the total heat is small, being 0.5, 3.1, and 4.7 per cent for 20, 100, and 300 pounds absolute pressure respectively above the total heat in one pound of steam at 14.7 pounds absolute. The temperatures, on the other hand, increase at the rates of 7.5, 54.6, and 96.9 per cent. The efficiency of a perfect steam engine is proportional to the expression ( t - t 1 )/ t in which t and t 1 are the absolute temperatures of the saturated steam at admission and exhaust respectively. While actual engines only approximate the ideal engine in efficiency, yet they follow the same general law. Since the exhaust temperature cannot be lowered beyond present practice, it follows that the only available method of increasing the efficiency is by an increase in the temperature of the steam at admission. How this may be [Pg 120] accomplished by an increase of pressure is clearly shown, for the increase of fuel necessary to increase the pressure is negligible, as shown by the total heat, while the increase in economy, due to the higher pressure, will result directly from the rapid increase of the corresponding temperature.

TABLE 22

HEAT UNITS PER POUND AND WEIGHT PER CUBIC FOOT OF WATER
BETWEEN 32 DEGREES FAHRENHEIT AND 340 DEGREES FAHRENHEIT
Temp-
erature
Degrees
Fahren-
heit
Heat
Units
per
Pound
Weight
per
Cubic
Foot
Temp-
erature
Degrees
Fahren-
heit
Heat
Units
per
Pound
Weight
per
Cubic
Foot
Temp-
erature
Degrees
Fahren-
heit
Heat
Units
per
Pound
Weight
per
Cubic
Foot
Temp-
erature
Degrees
Fahren-
heit
Heat
Units
per
Pound
Weight
per
Cubic
Foot
  32   0.00 62.42   89 57.00 62.13 146 113.86 61.27 203 170.95 60.05
  33   1.01 62.42   90 58.00 62.12 147 114.86 61.25 204 171.96 60.02
  34   2.01 62.42   91 59.00 62.11 148 115.86 61.24 205 172.96 60.00
  35   3.02 62.43   92 60.00 62.09 149 116.86 61.22 206 173.97 59.98
  36   4.03 62.43   93 60.99 62.08 150 117.86 61.20 207 174.97 59.95
  37   5.04 62.43   94 61.99 62.07 151 118.86 61.18 208 175.98 59.93
  38   6.04 62.43   95 62.99 62.06 152 119.86 61.16 209 176.98 59.90
  39   7.05 62.43   96 63.98 62.05 153 120.86 61.14 210 177.99 59.88
  40   8.05 62.43   97 64.98 62.04 154 121.86 61.12 211 178.99 59.85
  41   9.05 62.43   98 65.98 62.03 155 122.86 61.10 212 180.00 59.83
  42 10.06 62.43   99 66.97 62.02 156 123.86 61.08 213 181.0   59.80
  43 11.06 62.43 100 67.97 62.00 157 124.86 61.06 214 182.0   59.78
  44 12.06 62.43 101 68.97 61.99 158 125.86 61.04 215 183.0   59.75
  45 13.07 62.42 102 69.96 61.98 159 126.86 61.02 216 184.0   59.73
  46 14.07 62.42 103 70.96 61.97 160 127.86 61.00 217 185.0   59.70
  47 15.07 62.42 104 71.96 61.95 161 128.86 60.98 218 186.1   59.68
  48 16.07 62.42 105 72.95 61.94 162 129.86 60.96 219 187.1   59.65
  49 17.08 62.42 106 73.95 61.93 163 130.86 60.94 220 188.1   59.63
  50 18.08 62.42 107 74.95 61.91 164 131.86 60.92 221 189.1   59.60
Temp-
erature
Degrees
Fahren-
heit
Heat
Units
per
Pound
Weight
per
Cubic
Foot
Temp-
erature
Degrees
Fahren-
heit
Heat
Units
per
Pound
Weight
per
Cubic
Foot
Temp-
erature
Degrees
Fahren-
heit
Heat
Units
per
Pound
Weight
per
Cubic
Foot
Temp-
erature
Degrees
Fahren-
heit
Heat
Units
per
Pound
Weight
per
Cubic
Foot
  51 19.08 62.41 108 75.95 61.90 165 132.86 60.90 222 190.1   59.58
  52 20.08 62.41 109 76.94 61.88 166 133.86 60.88 223 191.1   59.55
  53 21.08 62.41 110 77.94 61.86 167 134.86 60.86 224 192.1   59.53
  54 22.08 62.40 111 78.94 61.85 168 135.86 60.84 225 193.1   59.50
  55 23.08 62.40 112 79.93 61.83 169 136.86 60.82 226 194.1   59.48
  56 24.08 62.39 113 80.93 61.82 170 137.87 60.80 227 195.2   59.45
  57 25.08 62.39 114 81.93 61.80 171 138.87 60.78 228 196.2   59.42
  58 26.08 62.38 115 82.92 61.79 172 139.87 60.76 229 197.2   59.40
  59 27.08 62.37 116 83.92 61.77 173 140.87 60.73 230 198.2   59.37
  60 28.08 62.37 117 84.92 61.75 174 141.87 60.71 231 199.2   59.34
  61 29.08 62.36 118 85.92 61.74 175 142.87 60.69 232 200.2   59.32
  62 30.08 62.36 119 86.91 61.72 176 143.87 60.67 233 201.2   59.29
  63 31.07 62.35 120 87.91 61.71 177 144.88 60.65 234 202.2   59.27
  64 32.07 62.35 121 88.91 61.69 178 145.88 60.62 235 203.2   59.24
  65 33.07 62.34 122 89.91 61.68 179 146.88 60.60 236 204.2   59.21
  66 34.07 62.33 123 90.90 61.66 180 147.88 60.58 237 205.3   59.19
  67 35.07 62.33 124 91.90 61.65 181 148.88 60.56 238 206.3   59.16
  68 36.07 62.32 125 92.90 61.63 182 149.89 60.53 239 207.3   59.14
  69 37.06 62.31 126 93.90 61.61 183 150.89 60.51 240 208.3   59.11
Temp-
erature
Degrees
Fahren-
heit
Heat
Units
per
Pound
Weight
per
Cubic
Foot
Temp-
erature
Degrees
Fahren-
heit
Heat
Units
per
Pound
Weight
per
Cubic
Foot
Temp-
erature
Degrees
Fahren-
heit
Heat
Units
per
Pound
Weight
per
Cubic
Foot
Temp-
erature
Degrees
Fahren-
heit
Heat
Units
per
Pound
Weight
per
Cubic
Foot
  70 38.06 62.30 127 94.89 61.59 184 151.89 60.49 241 209.3   59.08
  71 39.06 62.30 128 95.89 61.58 185 152.89 60.47 242 210.3   59.05
  72 40.05 62.29 129 96.89 61.56 186 153.89 60.45 243 211.4   59.03
  73 41.05 62.28 130 97.89 61.55 187 154.90 60.42 244 212.4   59.00
  74 42.05 62.27 131 98.89 61.53 188 155.90 60.40 245 213.4   58.97
  75 42.05 62.26 132 99.88 61.52 189 156.90 60.38 246 214.4   58.94
  76 44.04 62.26 133 100.88 61.50 190 157.91 60.36 247 215.4   58.91
  77 45.04 62.25 134 101.88 61.49 191 158.91 60.33 248 216.4   58.89
  78 46.04 62.24 135 102.88 61.47 192 159.91 60.31 249 217.4   58.86
  79 47.04 62.23 136 103.88 61.45 193 160.91 60.29 250 218.5   58.83
  80 48.03 62.22 137 104.87 61.43 194 161.92 60.27 260 228.6   58.55
  81 49.03 62.21 138 105.87 61.41 195 162.92 60.24 270 238.8   58.26
  82 50.03 62.20 139 106.87 61.40 196 163.92 60.22 280 249.0   57.96
  83 51.02 62.19 140 107.87 61.38 197 164.93 60.19 290 259.3   57.65
  84 52.02 62.18 141 108.87 61.36 198 165.93 60.17 300 269.6   57.33
  85 53.02 62.17 142 109.87 61.34 199 166.94 60.15 310 279.9   57.00
  86 54.01 62.16 143 110.87 61.33 200 167.94 60.12 320 290.2   56.66
  87 55.01 62.15 144 111.87 61.31 201 168.94 60.10 330 300.6   56.30
  88 56.01 62.14 145 112.86 61.29 202 169.95 60.07 340 311.0   55.94

[Pg 121]

The gain due to superheat cannot be predicted from the formula for the efficiency of a perfect steam engine given on page 119 . This formula is not applicable in cases where superheat is present since only a relatively small amount of the heat in the steam is imparted at the maximum or superheated temperature.

The advantage of the use of high pressure steam may be also indicated by considering the question from the aspect of volume. With an increase of pressure comes a decrease in volume, thus one pound of saturated steam at 100 pounds absolute pressure occupies 4.43 cubic feet, while at 200 pounds pressure it occupies 2.29 cubic feet. If then, in separate cylinders of the same dimensions, one pound of steam at 100 pounds absolute pressure and one pound at 200 pounds absolute pressure enter and are allowed to expand to the full volume of each cylinder, the high-pressure steam, having more room and a greater range for expansion than the low-pressure steam, will thus do more work. This increase in the amount of work, as was the increase in temperature, is large relative to the additional fuel required as indicated by the total heat. In general, it may be stated that the fuel required to impart a given amount of heat to a boiler is practically independent of the steam pressure, since the temperature of the fire is so high as compared with the steam temperature that a variation in the steam temperature does not produce an appreciable effect.

The formulae for the algebraic expression of the relation between saturated steam pressures, temperatures and steam volumes have been up to the present time empirical. These relations have, however, been determined by experiment and, from the experimental data, tables have been computed which render unnecessary the use of empirical formulae. Such formulae may be found in any standard work of thermo-dynamics. The following tables cover all practical cases.

Table 22 gives the heat units contained in water above 32 degrees Fahrenheit at different temperatures.

Table 23 gives the properties of saturated steam for various pressures.

Table 24 gives the properties of superheated steam at various pressures and temperatures.

These tables are based on those computed by Lionel S. Marks and Harvey N. Davis, these being generally accepted as being the most correct.

[Pg 122]

TABLE 23

PROPERTIES OF SATURATED STEAM REPRODUCED BY PERMISSION FROM
MARKS AND DAVIS “STEAM TABLES AND DIAGRAMS”
(Copyright, 1909, by Longmans, Green & Co.)
Pressure,
Pounds Absolute
Temperature
Degrees F.
Specific Volume
Cu. Ft. per Pound
Heat of the
Liquid, B. t. u.
Latent Heat of
Evap., B. t. u.
Total Heat of
Steam, B. t. u.
    1 101.83 333.0       69.8 1034.6 1104.4
    2 126.15 173.5       94.0 1021.0 1115.0
    3 141.52 118.5     109.4 1012.3 1121.6
    4 153.01   90.5     120.9 1005.7 1126.5
    5 162.28   73.33   130.1 1000.3 1130.5
    6 170.06   61.89   137.9   995.8 1133.7
    7 176.85   53.56   144.7   991.8 1136.5
    8 182.86   47.27   150.8   988.2 1139.0
    9 188.27   42.36   156.2   985.0 1141.1
  10 193.22   38.38   161.1   982.0 1143.1
  11 197.75   35.10   165.7   979.2 1144.9
  12 201.96   32.36   169.9   976.6 1146.5
  13 205.87   30.03   173.8   974.2 1148.0
  14 209.55   28.02   177.5   971.9 1149.4
  15 213.0     26.27   181.0   969.7 1150.7
  16 216.3     24.79   184.4   967.6 1152.0
  17 219.4     23.38   187.5   965.6 1153.1
  18 222.4     22.16   190.5   963.7 1154.2
  19 225.2     21.07   193.4   961.8 1155.2
  20 228.0     20.08   196.1   960.0 1156.2
  22 233.1     18.37   201.3   956.7 1158.0
  24 237.8     16.93   206.1   953.5 1159.6
  26 242.2     15.72   210.6   950.6 1161.2
  28 246.4     14.67   214.8   947.8 1162.6
  30 250.3     13.74   218.8   945.1 1163.9
  32 254.1     12.93   222.6   942.5 1165.1
  34 257.6     12.22   226.2   940.1 1166.3
  36 261.0     11.58   229.6   937.7 1167.3
  38 264.2     11.01   232.9   935.5 1168.4
  40 267.3     10.49   236.1   933.3 1169.4
  42 270.2     10.02   239.1   931.2 1170.3
  44 273.1       9.59   242.0   929.2 1171.2
  46 275.8       9.20   244.8   927.2 1172.0
  48 278.5       8.84   247.5   925.3 1172.8
  50 281.0       8.51   250.1   923.5 1173.6
  52 283.5       8.20   252.6   921.7 1174.3
Pressure,
Pounds Absolute
Temperature
Degrees F.
Specific Volume
Cu. Ft. per Pound
Heat of the
Liquid, B. t. u.
Latent Heat of
Evap., B. t. u.
Total Heat of
Steam, B. t. u.
  54 285.9       7.91   255.1   919.9 1175.0
  56 288.2       7.65   257.5   918.2 1175.7
  58 290.5       7.40   259.8   916.5 1176.4
  60 292.7       7.17   262.1   914.9 1177.0
  62 294.9       6.95   264.3   913.3 1177.6
  64 297.0       6.75   266.4   911.8 1178.2
  66 299.0       6.56   268.5   910.2 1178.8
  68 301.0       6.38   270.6   908.7 1179.3
  70 302.9       6.20   272.6   907.2 1179.8 [Pg 123]
  72 304.8       6.04   274.5   905.8 1180.4
  74 306.7       5.89   276.5   904.4 1180.9
  76 308.5       5.74   278.3   903.0 1181.4
  78 310.3       5.60   280.2   901.7 1181.8
  80 312.0       5.47   282.0   900.3 1182.3
  82 313.8       5.34   283.8   899.0 1182.8
  84 315.4       5.22   285.5   897.7 1183.2
  86 317.1       5.10   287.2   896.4 1183.6
  88 318.7       5.00   288.9   895.2 1184.0
  90 320.3       4.89   290.5   893.9 1184.4
  92 321.8       4.79   292.1   892.7 1184.8
  94 323.4       4.69   293.7   891.5 1185.2
  96 324.9       4.60   295.3   890.3 1185.6
  98 326.4       4.51   296.8   889.2 1186.0
100 327.8       4.429 298.3   888.0 1186.3
105 331.4       4.230 302.0   885.2 1187.2
110 334.8       4.047 305.5   882.5 1188.0
115 338.1       3.880 309.0   879.8 1188.8
120 341.3       3.726 312.3   877.2 1189.6
125 344.4       3.583 315.5   874.7 1190.3
130 347.4       3.452 318.6   872.3 1191.0
135 350.3       3.331 321.7   869.9 1191.6
140 353.1       3.219 324.6   867.6 1192.2
145 355.8       3.112 327.4   865.4 1192.8
150 358.5       3.012 330.2   863.2 1193.4
155 361.0       2.920 332.9   861.0 1194.0
160 363.6       2.834 335.6   858.8 1194.5
Pressure,
Pounds Absolute
Temperature
Degrees F.
Specific Volume
Cu. Ft. per Pound
Heat of the
Liquid, B. t. u.
Latent Heat of
Evap., B. t. u.
Total Heat of
Steam, B. t. u.
165 366.0       2.753 338.2   856.8 1195.0
170 368.5       2.675 340.7   854.7 1195.4
175 370.8       2.602 343.2   852.7 1195.9
180 373.1       2.533 345.6   850.8 1196.4
185 375.4       2.468 348.0   848.8 1196.8
190 377.6       2.406 350.4   846.9 1197.3
195 379.8       2.346 352.7   845.0 1197.7
200 381.9       2.290 354.9   843.2 1198.1
205 384.0       2.237 357.1   841.4 1198.5
210 386.0       2.187 359.2   839.6 1198.8
215 388.0       2.138 361.4   837.9 1199.2
220 389.9       2.091 363.4   836.2 1199.6
225 391.9       2.046 365.5   834.4 1199.9
230 393.8       2.004 367.5   832.8 1200.2
235 395.6       1.964 369.4   831.1 1200.6
240 397.4       1.924 371.4   829.5 1200.9
245 399.3       1.887 373.3   827.9 1201.2
250 401.1       1.850 375.2   826.3 1201.5 [Pg 124]
Illustration:

Portion of 6100 Horse-power Installation of Babcock & Wilcox Boilers Equipped with Babcock & Wilcox Chain Grate Stokers at the Campbell Street Plant of the Louisville Railway Co., Louisville, Ky. This Company Operates a Total of 14,000 Horse Power of Babcock & Wilcox Boilers

[Pg 125]

TABLE 24

PROPERTIES OF SATURATED STEAM REPRODUCED BY PERMISSION FROM
MARKS AND DAVIS “STEAM TABLES AND DIAGRAMS”
(Copyright, 1909, by Longmans, Green & Co.)
    Degrees of Superheat
Pressure
Pounds
Absolute
  Saturated
Steam
50 100 150 200 250 300
    5 t
v
h
162.3  
73.3  
1130.5  
212.3  
79.7  
1153.5  
262.3  
85.7  
1176.4  
312.3  
91.8  
1199.5  
362.3  
97.8  
1222.5  
412.3  
103.8  
1245.6  
462.3  
109.8  
1268.7  
  10 t
v
h
193.2  
38.4  
1143.1  
243.2  
41.5  
1166.3  
293.2  
44.6  
1189.5  
343.2  
47.7  
1212.7  
393.2  
50.7  
1236.0  
443.2  
53.7  
1259.3  
493.2  
56.7  
1282.5  
  15 t
v
h
213.0  
26.27
1150.7  
263.0  
28.40
1174.2  
313.0  
30.46
1197.6  
363.0  
32.50
1221.0  
413.0  
34.53
1244.4  
463.0  
36.56
1267.7  
513.0  
38.58
1291.1  
  20 t
v
h
228.0  
20.08
1156.2  
278.0  
21.69
1179.9  
328.0  
23.25
1203.5  
378.0  
24.80
1227.1  
428.0  
26.33
1250.6  
478.0  
27.85
1274.1  
528.0  
29.37
1297.6  
  25 t
v
h
240.1  
16.30
1160.4  
290.1  
17.60
1184.4  
340.1  
18.86
1208.2  
390.1  
20.10
1231.9  
440.1  
21.32
1255.6  
490.1  
22.55
1279.2  
540.1  
23.77
1302.8  
  30 t
v
h
250.4  
13.74
1163.9  
300.4  
14.83
1188.1  
350.4  
15.89
1212.1  
400.4  
16.93
1236.0  
450.4  
17.97
1259.7  
500.4  
18.99
1283.4  
550.4  
20.00
1307.1  
  35 t
v
h
259.3  
11.89
1166.8  
309.3  
12.85
1191.3  
359.3  
13.75
1215.4  
409.3  
14.65
1239.4  
459.3  
15.54
1263.3  
509.3  
16.42
1287.1  
559.3  
17.30
1310.8  
  40 t
v
h
267.3  
10.49
1169.4  
317.3  
11.33
1194.0  
367.3  
12.13
1218.4  
417.3  
12.93
1242.4  
467.3  
13.70
1266.4  
517.3  
14.48
1290.3  
567.3  
15.25
1314.1  
  45 t
v
h
274.5  
9.39
1171.6  
324.5  
10.14
1196.6  
374.5  
10.86
1221.0  
424.5  
11.57
1245.2  
474.5  
12.27
1269.3  
524.5  
12.96
1293.2  
574.5  
13.65
1317.0  
  50 t
v
h
281.0  
8.51
1173.6  
331.0  
9.19
1198.8  
381.0  
9.84
1223.4  
431.0  
10.48
1247.7  
481.0  
11.11
1271.8  
531.0  
11.74
1295.8  
581.0  
12.36
1319.7  
  60 t
v
h
287.1  
7.78
1175.4  
337.1  
8.40
1200.8  
387.1  
9.00
1225.6  
437.1  
9.59
1250.0  
487.1  
10.16
1274.2  
537.1  
10.73
1298.1  
587.1  
11.30
1322.0  
    Degrees of Superheat
Pressure
Pounds
Absolute
  Saturated
Steam
50 100 150 200 250 300
  60 t
v
h
292.7  
7.17
1177.0  
342.7  
7.75
1202.6  
392.7  
8.30
1227.6  
442.7  
8.84
1252.1  
492.7  
9.36
1276.4  
542.7  
9.89
1300.4  
592.7  
10.41
1324.3  
  65 t
v
h
298.0  
6.65
1178.5  
348.0  
7.20
1204.4  
398.0  
7.70
1229.5  
448.0  
8.20
1254.0  
498.0  
8.69
1278.4  
548.0  
9.17
1302.4  
598.0  
9.65
1326.4  
  70 t
v
h
302.9  
6.20
1179.8  
352.9  
6.71
1205.9  
402.9  
7.18
1231.2  
452.9  
7.65
1255.8  
502.9  
8.11
1280.2  
552.9  
8.56
1304.3  
602.9  
9.01
1328.3  
  75 t
v
h
307.6  
5.81
1181.1  
357.6  
6.28
1207.5  
407.6  
6.73
1232.8  
457.6  
7.17
1257.5  
507.6  
7.60
1282.0  
557.6  
8.02
1306.1  
607.6  
8.44
1330.1  
  80 t
v
h
312.0  
5.47
1182.3  
362.0  
5.92
1208.8  
412.0  
6.34
1234.3  
462.0  
6.75
1259.0  
512.0  
7.17
1283.6  
562.0  
7.56
1307.8  
612.0  
7.95
1331.9  
  85 t
v
h
316.3  
5.16
1183.4  
366.3  
5.59
1210.2  
416.3  
6.99
1235.8  
466.3  
6.38
1260.6  
516.3  
6.76
1285.2  
566.3  
7.14
1309.4  
616.3  
7.51
1333.5  
  90 t
v
h
320.3  
4.89
1184.4  
370.3  
5.29
1211.4  
420.3  
5.67
1237.2  
470.3  
6.04
1262.0  
520.3  
6.40
1286.6  
570.3  
6.76
1310.8  
620.3   [Pg 126]
7.11
1334.9  
  95 t
v
h
324.1  
4.65
1185.4  
374.1  
5.03
1212.6  
424.1  
5.39
1238.4  
474.1  
5.74
1263.4  
524.1  
6.09
1288.1  
574.1  
6.43
1312.3  
624.1  
6.76
1336.4  
100 t
v
h
327.8  
4.43
1186.3  
377.8  
4.79
1213.8  
427.8  
5.14
1239.7  
477.8  
5.47
1264.7  
527.8  
5.80
1289.4  
577.8  
6.12
1313.6  
627.8  
6.44
1337.8  
105 t
v
h
331.4  
4.23
1187.2  
381.4  
4.58
1214.9  
431.4  
4.91
1240.8  
481.4  
5.23
1265.9  
531.4  
5.54
1290.6  
581.4  
5.85
1314.9  
631.4  
6.15
1339.1  
110 t
v
h
334.8  
4.05
1188.0  
384.8  
4.38
1215.9  
434.8  
4.70
1242.0  
484.8  
5.01
1267.1  
534.8  
5.31
1291.9  
584.8  
5.61
1316.2  
634.8  
5.90
1340.4  
    Degrees of Superheat
Pressure
Pounds
Absolute
  Saturated
Steam
50 100 150 200 250 300
115 t
v
h
338.1  
3.88
1188.8  
388.1  
4.20
1216.9  
438.1  
4.51
1243.1  
488.1  
4.81
1268.2  
538.1  
5.09
1293.0  
588.1  
5.38
1317.3  
638.1  
5.66
1341.5  
120 t
v
h
341.3  
3.73
1189.6  
391.3  
4.04
1217.9  
441.3  
4.33
1244.1  
491.3  
4.62
1269.3  
541.3  
4.89
1294.1  
591.3  
5.17
1318.4  
641.3  
5.44
1342.7  
125 t
v
h
344.4  
3.58
1190.3  
394.4  
3.88
1218.8  
444.4  
4.17
1245.1  
494.4  
4.45
1270.4  
544.4  
4.71
1295.2  
594.4  
4.97
1319.5  
644.4  
5.23
1343.8  
130 t
v
h
347.4  
3.45
1191.0  
397.4  
3.74
1219.7  
447.4  
4.02
1246.1  
497.4  
4.28
1271.4  
547.4  
4.54
1296.2  
597.4  
4.80
1320.6  
647.4  
5.05
1344.9  
135 t
v
h
350.3  
3.33
1191.6  
400.3  
3.61
1220.6  
450.3  
3.88
1247.0  
500.3  
4.14
1272.3  
550.3  
4.38
1297.2  
600.3  
4.63
1321.6  
650.3  
4.87
1345.9  
140 t
v
h
353.1  
3.22
1192.2  
403.1  
3.49
1221.4  
453.1  
3.75
1248.0  
503.1  
4.00
1273.3  
553.1  
4.24
1298.2  
603.1  
4.48
1322.6  
653.1  
4.71
1346.9  
145 t
v
h
355.8  
3.12
1192.8  
405.8  
3.38
1222.2  
455.8  
3.63
1248.8  
505.8  
3.87
1274.2  
555.8  
4.10
1299.1  
605.8  
4.33
1323.6  
655.8  
4.56
1347.9  
150 t
v
h
358.5  
3.01
1193.4  
408.5  
3.27
1223.0  
458.5  
3.50
1249.6  
508.5  
3.75
1275.1  
558.5  
3.97
1300.0  
608.5  
4.19
1324.5  
658.5  
4.41
1348.8  
155 t
v
h
361.0  
2.92
1194.0  
411.0  
3.17
1223.6  
461.0  
3.41
1250.5  
511.0  
3.63
1276.0  
561.0  
3.85
1300.8  
611.0  
4.06
1325.3  
661.0  
4.28
1349.7  
160 t
v
h
363.6  
2.83
1194.5  
413.6  
3.07
1224.5  
463.6  
3.30
1251.3  
513.6  
3.53
1276.8  
563.6  
3.74
1301.7  
613.6  
3.95
1326.2  
663.6  
4.15
1350.6  
165 t
v
h
366.0  
2.75
1195.0  
416.0  
2.99
1225.2  
466.0  
3.21
1252.0  
516.0  
3.43
1277.6  
566.0  
3.64
1302.5  
616.0  
3.84
1327.1  
666.0  
4.04
1351.5  
    Degrees of Superheat
Pressure
Pounds
Absolute
  Saturated
Steam
50 100 150 200 250 300
170 t
v
h
368.5  
2.68
1195.4  
418.5  
2.91
1225.9  
468.5  
3.12
1252.8  
518.5  
3.34
1278.4  
568.5  
3.54
1303.3  
618.5  
3.73
1327.9  
668.5  
3.92
1352.3  
175 t
v
h
370.8  
2.60
1195.9  
420.8  
2.83
1226.6  
470.8  
3.04
1253.6  
520.8  
3.24
1279.1  
570.8  
3.44
1304.1  
620.8  
3.63
1328.7  
670.8   [Pg 127]
3.82
1353.2  
180 t
v
h
373.1  
2.53
1196.4  
423.1  
2.75
1227.2  
473.1  
2.96
1254.3  
523.1  
3.16
1279.9  
573.1  
3.35
1304.8  
623.1  
3.54
1329.5  
673.1  
3.72
1353.9  
185 t
v
h
375.4  
2.47
1196.8  
425.4  
2.68
1227.9  
475.4  
2.89
1255.0  
525.4  
3.08
1280.6  
575.4  
3.27
1305.6  
625.4  
3.45
1330.2  
675.4  
3.63
1354.7  
190 t
v
h
377.6  
2.41
1197.3  
427.6  
2.62
1228.6  
477.6  
2.81
1255.7  
527.6  
3.00
1281.3  
577.6  
3.19
1306.3  
627.6  
3.37
1330.9  
677.6  
3.55
1355.5  
195 t
v
h
379.8  
2.35
1197.7  
429.8  
2.55
1229.2  
479.8  
2.75
1256.4  
529.8  
2.93
1282.0  
579.8  
3.11
1307.0  
629.8  
3.29
1331.6  
679.8  
3.46
1356.2  
200 t
v
h
381.9  
2.29
1198.1  
431.9  
2.49
1229.8  
481.9  
2.68
1257.1  
531.9  
2.86
1282.6  
581.9  
3.04
1307.7  
631.9  
3.21
1332.4  
681.9  
3.38
1357.0  
205 t
v
h
384.0  
2.24
1198.5  
434.0  
2.44
1230.4  
484.0  
2.62
1257.7  
534.0  
2.80
1283.3  
584.0  
2.97
1308.3  
634.0  
3.14
1333.0  
684.0  
3.30
1357.7  
210 t
v
h
386.0  
2.19
1198.8  
436.0  
2.38
1231.0  
486.0  
2.56
1258.4  
536.0  
2.74
1284.0  
586.0  
2.91
1309.0  
636.0  
3.07
1333.7  
686.0  
3.23
1358.4  
215 t
v
h
388.0  
2.14
1199.2  
438.0  
2.33
1231.6  
488.0  
2.51
1259.0  
538.0  
2.68
1284.6  
588.0  
2.84
1309.7  
638.0  
3.00
1334.4  
688.0  
3.16
1359.1  
220 t
v
h
389.9  
2.09
1199.6  
439.9  
2.28
1232.2  
489.9  
2.45
1259.6  
539.9  
2.62
1285.2  
589.9  
2.78
1310.3  
639.9  
2.94
1335.1  
689.9  
3.10
1359.8  
    Degrees of Superheat
Pressure
Pounds
Absolute
  Saturated
Steam
50 100 150 200 250 300
225 t
v
h
391.9  
2.05
1199.9  
441.9  
2.23
1232.7  
491.9  
2.40
1260.2  
541.9  
2.57
1285.9  
591.9  
2.72
1310.9  
641.9  
2.88
1335.7  
691.9  
3.03
1360.3  
230 t
v
h
393.8  
2.00
1200.2  
443.8  
2.18
1233.2  
493.8  
2.35
1260.7  
543.8  
2.51
1286.5  
593.8  
2.67
1311.6  
643.8  
2.82
1336.3  
693.8  
2.97
1361.0  
235 t
v
h
395.6  
1.96
1200.6  
445.6  
2.14
1233.8  
495.6  
2.30
1261.4  
545.6  
2.46
1287.1  
595.6  
2.62
1312.2  
645.6  
2.77
1337.0  
695.6  
2.91
1361.7  
240 t
v
h
397.4  
1.92
1200.9  
447.4  
2.09
1234.3  
497.4  
2.26
1261.9  
547.4  
2.42
1287.6  
597.4  
2.57
1312.8  
647.4  
2.71
1337.6  
697.4  
2.85
1362.3  
245 t
v
h
399.3  
1.89
1201.2  
449.3  
2.05
1234.8  
499.3  
2.22
1262.5  
549.3  
2.37
1288.2  
599.3  
2.52
1313.3  
649.3  
2.66
1338.2  
699.3  
2.80
1362.9  
250 t
v
h
401.0  
1.85
1201.5  
451.0  
2.02
1235.4  
501.0  
2.17
1263.0  
551.0  
2.33
1288.8  
601.0  
2.47
1313.9  
651.0  
2.61
1338.8  
701.0  
2.75
1363.5  
255 t
v
h
402.8  
1.81
1201.8  
452.8  
1.98
1235.9  
502.8  
2.14
1263.6  
552.8  
2.28
1289.3  
602.8  
2.43
1314.5  
652.8  
2.56
1339.3  
702.8  
2.70
1364.1  
t = Temperature, degrees Fahrenheit.
v = Specific volume, in cubic feet, per pound.
h = Total heat from water at 32 degrees, B. t. u.

[Pg 128]

Graph of Moisture Content

Fig. 15. Graphic Method of Determining Moisture Contained in Steam from Calorimeter Readings

[Pg 129]

MOISTURE IN STEAM

The presence of moisture in steam causes a loss, not only in the practical waste of the heat utilized to raise this moisture from the temperature of the feed water to the temperature of the steam, but also through the increased initial condensation in an engine cylinder and through friction and other actions in a steam turbine. The presence of such moisture also interferes with proper cylinder lubrication, causes a knocking in the engine and a water hammer in the steam pipes. In steam turbines it will cause erosion of the blades.

The percentage by weight of steam in a mixture of steam and water is called the quality of the steam.

The apparatus used to determine the moisture content of steam is called a calorimeter though since it may not measure the heat in the steam, the name is not descriptive of the function of the apparatus. The first form used was the “barrel calorimeter”, but the liability of error was so great that its use was abandoned. Modern calorimeters are in general of either the throttling or separator type.

Throttling Calorimeter—Fig. 14 shows a typical form of throttling calorimeter. Steam is drawn from a vertical main through the sampling nipple, passes around the first thermometer cup, then through a one-eighth inch orifice in a disk between two flanges, and lastly around the second thermometer cup and to the atmosphere. Thermometers are inserted in the wells, which should be filled with mercury or heavy cylinder oil.

Throttling Calorimeter

Fig. 14. Throttling Calorimeter
and Sampling Nozzle

The instrument and all pipes and fittings leading to it should be thoroughly insulated to diminish radiation losses. Care must be taken to prevent the orifice from becoming choked with dirt and to see that no leaks occur. The exhaust pipe should be short to prevent back pressure below the disk.

When steam passes through an orifice from a higher to a lower pressure, as is the case with the throttling calorimeter, no external work has to be done in overcoming a resistance. Hence, if there is no loss from radiation, the quantity of heat in the steam will be exactly the same after passing the orifice as before passing. If the higher steam pressure is 160 pounds gauge and the lower pressure that of the atmosphere, the total heat in a pound of dry steam at the former pressure is 1195.9 B. t. u. and at the latter pressure 1150.4 B. t. u., a difference of 45.4 B. t. u. As this heat will still exist in the steam at the lower pressure, since there is no external work done, its effect must be to superheat the steam. Assuming the specific heat of superheated steam to be 0.47, each pound passing through will be superheated 45.40.47 = 96.6 degrees. If, however, the steam had contained one per cent of moisture, it would have contained less heat units per pound than if it were dry. Since the latent heat of steam at 160 [Pg 130] pounds gauge pressure is 852.8 B. t. u., it follows that the one per cent of moisture would have required 8.5 B. t. u. to evaporate it, leaving only 45.4 - 8.5 = 36.9 B. t. u. available for superheating; hence, the superheat would be 36.90.47 = 78.5 degrees, as against 96.6 degrees for dry steam. In a similar manner, the degree of superheat for other percentages of moisture may be determined. The action of the throttling calorimeter is based upon the foregoing facts, as shown below.

LetH=total heat of one pound of steam at boiler pressure,
L=latent heat of steam at boiler pressure,
h=total heat of steam at reduced pressure after passing orifice,
t1=temperature of saturated steam at the reduced pressure,
t2=temperature of steam after expanding through the orifice in the disc,
0.47=the specific heat of saturated steam at atmospheric pressure,
x=proportion by weight of moisture in steam.

The difference in B. t. u. in a pound of steam at the boiler pressure and after passing the orifice is the heat available for evaporating the moisture content and superheating the steam. Therefore,

H - hxL + 0.47(t2 - t1)
or x = 
H - h - 0.47(t2 - t1)
––––––––––––––––––––––––––––––––
L
            (4)

Almost invariably the lower pressure is taken as that of the atmosphere. Under such conditions, h = 1150.4 and t1 = 212 degrees. The formula thus becomes:

x = 
H - 1150.4 - 0.47(t2 - 212)
––––––––––––––––––––––––––––––––––––––––––––
L
            (5)

For practical work it is more convenient to dispense with the upper thermometer in the calorimeter and to measure the pressure in the steam main by an accurate steam pressure gauge.

A chart may be used for determining the value of x for approximate work without the necessity for computation. Such a chart is shown in Fig. 15 and its use is as follows: Assume a gauge pressure of 180 pounds and a thermometer reading of 295 degrees. The intersection of the vertical line from the scale of temperatures as shown by the calorimeter thermometer and the horizontal line from the scale of gauge pressures will indicate directly the per cent of moisture in the steam as read from the diagonal scale. In the present instance, this per cent is 1.0.

Sources of Error in the Apparatus—A slight error may arise from the value, 0.47, used as the specific heat of superheated steam at atmospheric pressure. This value, however is very nearly correct and any error resulting from its use will be negligible.

There is ordinarily a larger source of error due to the fact that the stem of the thermometer is not heated to its full length, to an initial error in the thermometer and to radiation losses.

With an ordinary thermometer immersed in the well to the 100 degrees mark, the error when registering 300 degrees would be about 3 degrees and the true temperature be 303 degrees.[19]

The steam is evidently losing heat through radiation from the moment it enters the sampling nipple. The heat available for evaporating moisture and superheating [Pg 131] steam after it has passed through the orifice into the lower pressure will be diminished by just the amount lost through radiation and the value of t2, as shown by the calorimeter thermometer, will, therefore, be lower than if there were no such loss. The method of correcting for the thermometer and radiation error recommended by the Power Test Committee of the American Society of Mechanical Engineers is by referring the readings as found on the boiler trial to a “normal” reading of the thermometer. This normal reading is the reading of the lower calorimeter thermometer for dry saturated steam, and should be determined by attaching the instrument to a horizontal steam pipe in such a way that the sampling nozzle projects upward to near the top of the pipe, there being no perforations in the nozzle and the steam taken only through its open upper end. The test should be made with the steam in a quiescent state and with the steam pressure maintained as nearly as possible at the pressure observed in the main trial, the calorimeter thermometer to be the same as was used on the trial or one exactly similar.

With a normal reading thus obtained for a pressure approximately the same as existed in the trial, the true percentage of moisture in the steam, that is, with the proper correction made for radiation, may be calculated as follows:

Let T denote the normal reading for the conditions existing in the trial. The effect of radiation from the instrument as pointed out will be to lower the temperature of the steam at the lower pressure. Let x1 represent the proportion of water in the steam which will lower its temperature an amount equal to the loss by radiation. Then,

x1 = 
H - h - 0.47(T - t1)
––––––––––––––––––––––––––––––
L

This amount of moisture, x1 was not in the steam originally but is the result of condensation in the instrument through radiation. Hence, the true amount of moisture in the steam represented by X is the difference between the amount as determined in the trial and that resulting from condensation, or,

X = x - x1
X = 
H - h - 0.47(t2 - t1)
–––––––––––––––––––––––––––––––
L
 - 
H - h - 0.47(T - t1)
–––––––––––––––––––––––––––––––
L
X = 
0.47(T - t2)
––––––––––––––––––––
L
            (6)

As T and t2 are taken with the same thermometer under the same set of conditions, any error in the reading of the thermometers will be approximately the same for the temperatures T and t2 and the above method therefore corrects for both the radiation and thermometer errors. The theoretical readings for dry steam, where there are no losses due to radiation, are obtainable from formula (5) by letting x = 0 and solving for t2. The difference between the theoretical reading and the normal reading for no moisture will be the thermometer and radiation correction to be applied in order that the correct reading of t2 may be obtained.

For any calorimeter within the range of its ordinary use, such a thermometer and radiation correction taken from one normal reading is approximately correct for any conditions with the same or a duplicate thermometer.

The percentage of moisture in the steam, corrected for thermometer error and radiation and the correction to be applied to the particular calorimeter used, would be [Pg 132] determined as follows: Assume a gauge pressure in the trial to be 180 pounds and the thermometer reading to be 295 degrees. A normal reading, taken in the manner described, gives a value of T = 303 degrees; then, the percentage of moisture corrected for thermometer error and radiation is,

x = 
0.47(303 - 295)
––––––––––––––––––––––––––
845.0
x = 0.45 per cent.

The theoretical reading for dry steam will be,

 0 = 
1197.7 - 1150.4 - 0.47(t2 - 212)
––––––––––––––––––––––––––––––––––––––––––––––––––––
845.0
t2 = 313 degrees.

The thermometer and radiation correction to be applied to the instrument used, therefore over the ordinary range of pressure is

Correction = 313 - 303 = 10 degrees

The chart may be used in the determination of the correct reading of moisture percentage and the permanent radiation correction for the instrument used without computation as follows: Assume the same trial pressure, feed temperature and normal reading as above. If the normal reading is found to be 303 degrees, the correction for thermometer and radiation will be the theoretical reading for dry steam as found from the chart, less this normal reading, or 10 degrees correction. The correct temperature for the trial in question is, therefore, 305 degrees. The moisture corresponding to this temperature and 180 pounds gauge pressure will be found from the chart to be 0.45 per cent.

Compact Throttling Calorimeter

Fig. 16. Compact Throttling
Calorimeter

There are many forms of throttling calorimeter, all of which work upon the same principle. The simplest one is probably that shown in Fig. 14. An extremely convenient and compact design is shown in Fig. 16. This calorimeter consists of two concentric metal cylinders screwed to a cap containing a thermometer well. The steam pressure is measured by a gauge placed in the supply pipe or other convenient location. Steam passes through the orifice A and expands to atmospheric pressure, its temperature at this pressure being measured by a thermometer placed in the cup C. To prevent as far as possible radiation losses, the annular space between the two cylinders is used as a jacket, steam being supplied to this space through the hole B.

The limits of moisture within which the throttling calorimeter will work are, at sea level, from 2.88 per cent at 50 pounds gauge pressure and 7.17 per cent moisture at 250 pounds pressure.

Separating Calorimeter—The separating calorimeter mechanically separates the entrained water from the steam and collects it in a reservoir, where its amount is [Pg 133] either indicated by a gauge glass or is drained off and weighed. Fig. 17 shows a calorimeter of this type. The steam passes out of the calorimeter through an orifice of known size so that its total amount can be calculated or it can be weighed. A gauge is ordinarily provided with this type of calorimeter, which shows the pressure in its inner chamber and the flow of steam for a given period, this latter scale being graduated by trial.

Separating Calorimeter

Fig. 17. Separating Calorimeter

The instrument, like a throttling calorimeter, should be well insulated to prevent losses from radiation.

While theoretically the separating calorimeter is not limited in capacity, it is well in cases where the percentage of moisture present in the steam is known to be high, to attach a throttling calorimeter to its exhaust. This, in effect, is the using of the separating calorimeter as a small separator between the sampling nozzle and the throttling instrument, and is necessary to insure the determination of the full percentage of moisture in the steam. The sum of the percentages shown by the two instruments is the moisture content of the steam.

The steam passing through a separating calorimeter may be calculated by Napier’s formula, the size of the orifice being known. There are objections to such a calculation, however, in that it is difficult to accurately determine the areas of such small orifices. Further, small orifices have a tendency to become partly closed by sediment that may be carried by the steam. The more accurate method of determining the amount of steam passing through the instrument is as follows:

A hose should be attached to the separator outlet leading to a vessel of water on a platform scale graduated to 1100 of a pound. The steam outlet should be connected to another vessel of water resting on a second scale. In each case, the weight of each vessel and its contents should be noted. When ready for an observation, the instrument should be blown out thoroughly so that there will be no water within the separator. The separator drip should then be closed and the steam hose inserted into the vessel of water at the same instant. When the separator has accumulated a sufficient quantity of water, the valve of the instrument should be closed and the hose removed from the vessel of water. The separator should be emptied into the vessel on its scale. The final weight of each vessel and its contents are to be noted and the differences between the final and original weights will represent the weight of moisture collected by the separator and the weight of steam from which the moisture has been taken. The proportion of moisture can then be calculated from the following formula:

x = 
100 w
–––––––––––
W - w
            (7)
Where [Pg 134]x=per cent moisture in steam,
W=weight of steam condensed,
w=weight of moisture as taken out by the separating calorimeter.

Sampling Nipple—The principle source of error in steam calorimeter determinations is the failure to obtain an average sample of the steam delivered by the boiler and it is extremely doubtful whether such a sample is ever obtained. The two governing features in the obtaining of such a sample are the type of sampling nozzle used and its location.

The American Society of Mechanical Engineers recommends a sampling nozzle made of one-half inch iron pipe closed at the inner end and the interior portion perforated with not less than twenty one-eighth inch holes equally distributed from end to end and preferably drilled in irregular or spiral rows, with the first hole not less than one-half inch from the wall of the pipe. Many engineers object to the use of a perforated sampling nipple because it ordinarily indicates a higher percentage of moisture than is actually present in the steam. This is due to the fact that if the perforations come close to the inner surface of the pipe, the moisture, which in many instances clings to this surface, will flow into the calorimeter and cause a large error. Where a perforated nipple is used, in general it may be said that the perforations should be at least one inch from the inner pipe surface.

A sampling nipple, open at the inner end and unperforated, undoubtedly gives as accurate a measure as can be obtained of the moisture in the steam passing that end. It would appear that a satisfactory method of obtaining an average sample of the steam would result from the use of an open end unperforated nipple passing through a stuffing box which would allow the end to be placed at any point across the diameter of the steam pipe.

Sampling Nozzle

Fig. 18. Stott and Pigott
Sampling Nozzle

Incidental to a test of a 15,000 K. W. steam engine turbine unit, Mr. H. G. Stott and Mr. R. G. S. Pigott, finding no experimental data bearing on the subject of low pressure steam quality determinations, made a investigation of the subject and the sampling nozzle illustrated in Fig. 18 was developed. In speaking of sampling nozzles in the determination of the moisture content of low pressure steam, Mr. Pigott says, “the ordinary standard perforated pipe sampler is absolutely worthless in giving a true sample and it is vital that the sample be abstracted from the main without changing its direction or velocity until it is safely within the sample pipe and entirely isolated from the rest of the steam.”

It would appear that the nozzle illustrated is undoubtedly the best that has been developed for use in the determination of the moisture content of steam, not only in the case of low, but also in high pressure steam.

Location of Sampling Nozzle—The calorimeter should be located as near as possible to the point from which the steam is taken and the sampling nipple should be placed in a section of the main pipe near the boiler and where there is no chance of moisture pocketing in the pipe. The American Society of Mechanical Engineers recommends that a sampling nipple, of which a description has been given, should be located in a vertical main, rising from the boiler with its closed end extending nearly [Pg 135] across the pipe. Where non-return valves are used, or where there are horizontal connections leading from the boiler to a vertical outlet, water may collect at the lower end of the uptake pipe and be blown upward in a spray which will not be carried away by the steam owing to a lack of velocity. A sample taken from the lower part of this pipe will show a greater amount of moisture than a true sample. With goose-neck connections a small amount of water may collect on the bottom of the pipe near the upper end where the inclination is such that the tendency to flow backward is ordinarily counterbalanced by the flow of steam forward over its surface; but when the velocity momentarily decreases the water flows back to the lower end of the goose-neck and increases the moisture at that point, making it an undesirable location for sampling. In any case, it should be borne in mind that with low velocities the tendency is for drops of entrained water to settle to the bottom of the pipe, and to be temporarily broken up into spray whenever an abrupt bend or other disturbance is met.

Sampling Nozzle Positions

Fig. 19. Illustrating the Manner in which Erroneous Calorimeter Readings may be Obtained
due to Improper Location of Sampling Nozzle

Case 1—Horizontal pipe. Water flows at bottom. If perforations in nozzle are too near bottom of pipe, water piles against nozzle, flows into calorimeter and gives false reading. Case 2—If nozzle located too near junction of two horizontal runs, as at a, condensation from vertical pipe which collects at this point will be thrown against the nozzle by the velocity of the steam, resulting in a false reading. Nozzle should be located far enough above junction to be removed from water kept in motion by the steam velocity, as at b. Case 3—Condensation in bend will be held by velocity of the steam as shown. When velocity is diminished during firing intervals and the like moisture flows back against nozzle, a, and false reading is obtained. A true reading will be obtained at b provided condensation is not blown over on nozzle. Case 4—Where non-return valve is placed before a bend, condensation will collect on steam line side and water will be swept by steam velocity against nozzle and false readings result.

Fig. 19 indicates certain locations of sampling nozzles from which erroneous results will be obtained, the reasons being obvious from a study of the cuts.

Before taking any calorimeter reading, steam should be allowed to flow through the instrument freely until it is thoroughly heated. The method of using a throttling calorimeter is evident from the description of the instrument given and the principle upon which it works.
[Pg 136]

Illustration:

Babcock & Wilcox Superheater

FOOTNOTES

[19] See Stem Correction, page 80.

[Pg 137]

SUPERHEATED STEAM

Superheated steam, as already stated, is steam the temperature of which exceeds that of saturated steam at the same pressure. It is produced by the addition of heat to saturated steam which has been removed from contact with the water from which it was generated. The properties of superheated steam approximate those of a perfect gas rather than of a vapor. Saturated steam cannot be superheated when it is in contact with water which is also heated, neither can superheated steam condense without first being reduced to the temperature of saturated steam. Just so long as its temperature is above that of saturated steam at a corresponding pressure it is superheated, and before condensation can take place that superheat must first be lost through radiation or some other means. Table 24[20] gives such properties of superheated steam for varying pressures as are necessary for use in ordinary engineering practice.

Specific Heat of Superheated Steam—The specific heat of superheated steam at atmospheric pressure and near saturation point was determined by Regnault, in 1862, who gives it the value of 0.48. Regnault’s value was based on four series of experiments, all at atmospheric pressure and with about the same temperature range, the maximum of which was 231.1 degrees centigrade. For fifty years after Regnault’s determination, this value was accepted and applied to higher pressures and temperatures as well as to the range of his experiments. More recent investigations have shown that the specific heat is not a constant and varies with both pressure and the temperature. A number of experiments have been made by various investigators and, up to the present, the most reliable appear to be those of Knoblauch and Jacob. Messrs. Marks and Davis have used the values as determined by Knoblauch and Jacob with slight modifications. The first consists in a varying of the curves at low pressures close to saturation because of thermodynamic evidence and in view of Regnault’s determination at atmospheric pressure. The second modification is at high degrees of superheat to follow Holborn’s and Henning’s curve, which is accepted as authentic.

For the sake of convenience, the mean specific heat of superheated steam at various pressures and temperatures is given in tabulated form in Table 25. These values have been calculated from Marks and Davis Steam Tables by deducting from the total heat of one pound of steam at any pressure for any degree of superheat the total heat of one pound of saturated steam at the same pressure and dividing the difference by the number of degrees of superheat and, therefore, represent the average specific heat starting from that at saturation to the value at the particular pressure and temperature.[21] Expressed as a formula this calculation is represented by

Sp. Ht. = 
Hsup - Hsat
––––––––––––––––––––
Ssup - Ssat
            (8)
WhereHsup=total heat of one pound of superheated steam at any pressure and temperature,
Hsat=total heat of one pound of saturated steam at same pressure, [Pg 138]
Ssup=temperature of superheated steam taken,
Ssat=temperature of saturated steam corresponding to the pressure taken.
TABLE 25

MEAN SPECIFIC HEAT OF SUPERHEATED STEAM
CALCULATED FROM MARKS AND DAVIS TABLES
Gauge
Pressure
Degree of Superheat
  50  60  70  80  90100110120130140150160170180190200225250
  50.518.517.514.513.511.510.508.507.505.504.503.502.501.500.500.499.497.496
  60.528.525.523.521.519.517.515.513.512.511.509.508.507.506.504.504.502.500
  70.536.534.531.529.527.524.522.520.518.516.515.513.512.511.510.509.506.504
  80.544.542.539.535.532.530.528.526.524.522.520.518.516.515.514.513.511.508
  90.553.550.546.543.539.536.534.532.529.527.525.523.521.519.518.517.514.510
100.562.557.553.549.544.542.539.536.533.531.529.527.525.523.522.521.517.513
110.570.565.560.556.552.548.545.542.539.536.534.532.529.528.526.525.520.517
120.578.573.567.561.557.554.550.546.543.540.537.535.533.531.529.528.523.519
130.586.580.574.569.564.560.555.552.548.545.542.539.537.535.533.531.527.523
140.594.588.581.575.570.565.561.557.553.550.547.544.541.539.536.534.530.526
150.604.595.587.581.576.570.566.561.557.554.550.547.544.542.539.537.533.529
160.612.603.596.589.582.576.571.566.562.558.554.551.548.545.543.541.536.531
170.620.612.603.595.588.582.576.571.566.562.558.555.552.549.546.544.538.533
180.628.618.610.601.593.587.581.575.570.566.561.558.555.552.549.546.540.536
190.638.627.617.608.599.592.585.579.574.569.565.562.558.555.552.549.543.538
200.648.635.624.614.605.597.590.584.578.574.569.566.562.558.555.552.546.541
210.656.643.631.620.611.602.595.588.583.578.573.569.565.561.558.555.549.543
220.664.650.637.626.616.607.600.592.586.581.577.572.568.564.561.558.551.545
230.672.658.644.633.622.613.605.597.591.585.580.575.572.567.564.561.554.548
240.684.668.653.640.629.619.610.602.595.589.584.579.575.571.567.564.556.550
250.692.675.659.645.633.623.614.606.599.593.587.582.577.574.570.567.559.553

Factor of Evaporation with Superheated Steam—When superheat is present in the steam during a boiler trial, where superheated steam tables are available, the formula for determining the factor of evaporation is that already given, (2),[22] namely,

Factor of evaporation = 
H - h
––––––––––
L

Here H = total heat in one pound of superheated steam from the table, h and L having the same values as in (2).

Where no such tables are available but the specific heat of superheat is known, the formula becomes:

Factor of evaporation = 
H - h + Sp. Ht.(T - t)
––––––––––––––––––––––––––––––––––
L
WhereH=total heat in one pound of saturated steam at pressure existing in trial,
h=sensible heat above 32 degrees in one pound of water at the temperature entering the boiler,
tsat=temperature of saturated steam, corresponding to pressure existing,
T=temperature of superheated steam as determined in the trial, [Pg 139]
t=temperature of saturated steam corresponding to the boiler pressure,
Sp. Ht.=mean specific heat of superheated steam at the pressure and temperature as found in the trial,
L=latent heat of one pound of saturated steam at atmospheric pressure.

Advantages of the Use of Superheated Steam—In considering the saving possible by the use of superheated steam, it is too often assumed that there is only a saving in the prime movers, a saving which is at least partially offset by an increase in the fuel consumption of the boilers generating steam. This misconception is due to the fact that the fuel consumption of the boiler is only considered in connection with a definite weight of steam. It is true that where such a definite weight is to be superheated, an added amount of fuel must be burned. With a properly designed superheater where the combined efficiency of the boiler and superheater will be at least as high as of a boiler alone, the approximate increase in coal consumption for producing a given weight of steam will be as follows:

Superheat
Degrees
Added Fuel
Per Cent
Superheat
Degrees
Added Fuel
Per Cent
  251.591005.69
  503.071508.19
  754.382000.58

These figures represent the added fuel necessary for superheating a definite weight of steam to the number of degrees as given. The standard basis, however, of boiler evaporation is one of heat units and, considered from such a standpoint, again providing the efficiency of the boiler and superheater is as high, as of a boiler alone, there is no additional fuel required to generate steam containing a definite number of heat units whether such units be due to superheat or saturation. That is, if 6 per cent more fuel is required to generate and superheat to 100 degrees, a definite weight of steam, over what would be required to produce the same weight of saturated steam, that steam when superheated, will contain 6 per cent more heat units above the fuel water temperature than if saturated. This holds true if the efficiency of the boiler and superheater combined is the same as of the boiler alone. As a matter of fact, the efficiency of a boiler and superheater, where the latter is properly designed and located, will be slightly higher for the same set of furnace conditions than would the efficiency of a boiler in which no superheater were installed. A superheater, properly placed within the boiler setting in such way that products of combustion for generating saturated steam are utilized as well for superheating that steam, will not in any way alter furnace conditions. With a given set of such furnace conditions for a given amount of coal burned, the fact that additional surface, whether as boiler heating or superheating surface, is placed in such a manner that the gases must sweep over it, will tend to lower the temperature of the exit gases. It is such a lowering of exit gas temperatures that is the ultimate indication of added efficiency. Though the amount of this added efficiency is difficult to determine by test, that there is an increase is unquestionable.

Where a properly designed superheater is installed in a boiler the heating surface of the boiler proper, in the generation of a definite number of heat units, is relieved of a portion of the work which would be required were these heat units delivered in saturated steam. Such a superheater needs practically no attention, is not subject to a large upkeep cost or depreciation, and performs its function without in any way [Pg 140] interfering with the operation of the boiler. Its use, therefore from the standpoint of the boiler room, results in a saving in wear and tear due to the lower ratings at which the boiler may be run, or its use will lead to the possibility of obtaining the same number of boiler horse power from a smaller number of boilers, with the boiler heating surface doing exactly the same amount of work as if the superheaters were not installed. The saving due to the added boiler efficiency that will be obtained is obvious.

Following the course of the steam in a plant, the next advantage of the use of superheated steam is the absence of water in the steam pipes. The thermal conductivity of superheated steam, that is, its power to give up its heat to surrounding bodies, is much lower than that of saturated steam and its heat, therefore, will not be transmitted so rapidly to the walls of the pipes as when saturated steam is flowing through the pipes. The loss of heat radiated from a steam pipe, assuming no loss in pressure, represents the equivalent condensation when the pipe is carrying saturated steam. In well-covered steam mains, the heat lost by radiation when carrying superheated steam is accompanied only by a reduction of the superheat which, if it be sufficiently high at the boiler, will enable a considerable amount of heat to be radiated and still deliver dry or superheated steam to the prime movers.

It is in the prime movers that the advantages of the use of superheated steam are most clearly seen.

In an engine, steam is admitted into a space that has been cooled by the steam exhausted during the previous stroke. The heat necessary to warm the cylinder walls from the temperature of the exhaust to that of the entering steam can be supplied only by the entering steam. If this steam be saturated, such an adding of heat to the walls at the expense of the heat of the entering steam results in the condensation of a portion. This initial condensation is seldom less than from 20 to 30 per cent of the total weight of steam entering the cylinder. It is obvious that if the steam entering be superheated, it must be reduced to the temperature of saturated steam at the corresponding pressure before any condensation can take place. If the steam be superheated sufficiently to allow a reduction in temperature equivalent to the quantity of heat that must be imparted to the cylinder walls and still remain superheated, it is clear that initial condensation is avoided. For example: assume one pound of saturated steam at 200 pounds gauge pressure to enter a cylinder which has been cooled by the exhaust. Assume the initial condensation to be 20 per cent. The latent heat of the steam is given up in condensation; hence, .20 × 838 = 167.6 B. t. u. are given up by the steam. If one pound of superheated steam enters the same cylinder, it would have to be superheated to a point where its total heat is 1199 + 168 = 1367 B. t. u. or, at 200 pounds gauge pressure, superheated approximately 325 degrees if the heat given up to the cylinder walls were the same as for the saturated steam. As superheated steam conducts heat less rapidly than saturated steam, the amount of heat imparted will be less than for the saturated steam and consequently the amount of superheat required to prevent condensation will be less than the above figure. This, of course, is the extreme case of a simple engine with the range of temperature change a maximum. As cylinders are added, the range in each is decreased and the condensation is proportionate.

The true economy of the use of superheated steam is best shown in a comparison of the “heat consumption” of an engine. This is the number of heat units required [Pg 141] in developing one indicated horse power and the measure of the relative performance of two engines is based on a comparison of their heat consumption as the measure of a boiler is based on its evaporation from and at 212 degrees. The water consumption of an engine in pounds per indicated horse power is in no sense a true indication of its efficiency. The initial pressures and corresponding temperatures may differ widely and thus make a difference in the temperature of the exhaust and hence in the temperature of the condensed steam returned to the boiler. For example: suppose a certain weight of steam at 150 pounds absolute pressure and 358 degrees be expanded to atmospheric pressure, the temperature then being 212 degrees. If the same weight of steam be expanded from an initial pressure of 125 pounds absolute and 344 degrees, to enable it to do the same amount of work, that is, to give up the same amount of heat, expansion then must be carried to a point below atmospheric pressure to, say, 13 pounds absolute, the final temperature of the steam then being 206 degrees. In actual practice, it has been observed that the water consumption of a compound piston engine running on 26-inch vacuum and returning the condensed steam at 140 degrees was approximately the same as when running on 28-inch vacuum and returning water at 90 degrees. With an equal water consumption for the two sets of conditions, the economy in the former case would be greater than in the latter, since it would be necessary to add less heat to the water returned to the boiler to raise it to the steam temperature.

The lower the heat consumption of an engine per indicated horse power, the higher its economy and the less the number of heat units must be imparted to the steam generated. This in turn leads to the lowering of the amount of fuel that must be burned per indicated horse power.

With the saving in fuel by the reduction of heat consumption of an engine indicated, it remains to be shown the effect of the use of superheated steam on such heat consumption. As already explained, the use of superheated steam reduces condensation not only in the mains but especially in the steam cylinder, leaving a greater quantity of steam available to do the work. Furthermore, a portion of the saturated steam introduced into a cylinder will condense during adiabatic expansion, this condensation increasing as expansion progresses. Since superheated steam cannot condense until it becomes saturated, not only is initial condensation prevented by its use but also such condensation as would occur during expansion. When superheated sufficiently, steam delivered by the exhaust will still be dry. In the avoidance of such condensation, there is a direct saving in the heat consumption of an engine, the heat given up being utilized in the developing of power and not in changing the condition of the working fluid. That is, while the number of heat units lost in overcoming condensation effects would be the same in either case, when saturated steam is condensed the water of condensation has no power to do work while the superheated steam, even after it has lost a like number of heat units, still has the power of expansion. The saving through the use of superheated steam in the heat consumption of an engine decreases demands on the boiler and hence the fuel consumption per unit of power.

Superheated Steam for Steam Turbines—Experience in using superheated steam in connection with steam turbines has shown that it leads to economy and that it undoubtedly pays to use superheated steam in place of saturated steam. This is so well established that it is standard practice to use superheated steam in connection [Pg 142] with steam turbines. Aside from the economy secured through using superheated steam, there is an important advantage arising through the fact that it materially reduces the erosion of the turbine blades by the action of water that would be carried by saturated steam. In using saturated steam in a steam turbine or piston engine, the work done on expanding the steam causes condensation of a portion of the steam, so that even were the steam dry on entering the turbine, it would contain water on leaving the turbine. By superheating the steam the water that exists in the low pressure stages of the turbine may be reduced to an amount that will not cause trouble.

Again, if saturated steam contains moisture, the effect of this moisture on the economy of a steam turbine is to reduce the economy to a greater extent than the proportion by weight of water, one per cent of water causing approximately a falling off of 2 per cent in the economy.

The water rate of a large economical steam turbine with superheated steam is reduced about one per cent, for every 12 degrees of superheat up to 200 degrees Fahrenheit of superheat. To superheat one pound of steam 12 degrees requires about 7 B. t. u. and if 1050 B. t. u. are required at the boiler to evaporate one pound of the saturated steam from the temperature of the feed water, the heat required for the superheated steam would be 1057 degrees. One per cent of saving, therefore, in the water consumption would correspond to a net saving of about one-third of one per cent in the coal consumption. On this basis 100 degrees of superheat with an economical steam turbine would result in somewhat over 3 per cent of saving in the coal for equal boiler efficiencies. As a boiler with a properly designed superheater placed within the setting is more economical for a given capacity than a boiler without a superheater, the minimum gain in the coal consumption would be, say, 4 or 5 per cent as compared to a plant with the same boilers without superheaters.

The above estimates are on the basis of a thoroughly dry saturated steam or steam just at the point of being superheated or containing a few degrees of superheat. If the saturated steam is moist, the saving due to superheat is more and ordinarily the gain in economy due to superheated steam, for equal boiler efficiencies, as compared with commercially dry steam is, say, 5 per cent for each 100 degrees of superheat. Aside from this gain, as already stated, superheated steam prevents erosion of the turbine buckets that would be caused by water in the steam, and for the reasons enumerated it is standard practice to use superheated steam for turbine work. The less economical the steam motor, the more the gain due to superheated steam, and where there are a number of auxiliaries that are run with superheated steam, the percentage of gain will be greater than the figures given above, which are the minimum and are for the most economical type of large steam turbines.

An example from actual practice will perhaps best illustrate and emphasize the foregoing facts. In October 1909, a series of comparable tests were conducted by The Babcock & Wilcox Co. on the steam yacht “Idalia” to determine the steam consumption both with saturated and superheated steam of the main engine on that yacht, including as well the feed pump, circulating pump and air pump. These tests are more representative than are most tests of like character in that the saving in the steam consumption of the auxiliaries, which were much more wasteful than the main engine, formed an important factor. A résumé of these tests was published in the Journal of the Society of Naval Engineers, November 1909.

[Pg 143]

The main engines of the “Idalia” are four cylinder, triple expansion, 1112 × 19 inches by 221116 × 18 inches stroke. Steam is supplied by a Babcock & Wilcox marine boiler having 2500 square feet of boiler heating surface, 340 square feet of superheating surface and 65 square feet of grate surface.

The auxiliaries consist of a feed pump 6 × 4 × 6 inches, an independent air pump 6 × 12 × 8 inches, and a centrifugal pump driven by a reciprocating engine 5716 × 5 inches. Under ordinary operating conditions the superheat existing is about 100 degrees Fahrenheit.

Tests were made with various degrees of superheat, the amount being varied by by-passing the gases and in the tests with the lower amounts of superheat by passing a portion of the steam from the boiler to the steam main without passing it through the superheater. Steam temperature readings were taken at the engine throttle. In the tests with saturated steam, the superheater was completely cut out of the system. Careful calorimeter measurements were taken, showing that the saturated steam delivered to the superheater was dry.

The weight of steam used was determined from the weight of the condensed steam discharge from the surface condenser, the water being pumped from the hot well into a tank mounted on platform scales. The same indicators, thermometers and gauges were used in all the tests, so that the results are directly comparable. The indicators used were of the outside spring type so that there was no effect of the temperature of the steam. All tests were of sufficient duration to show a uniformity of results by hours. A summary of the results secured is given in Table 26, which shows the water rate per indicated horse power and the heat consumption. The latter figures are computed on the basis of the heat imparted to the steam above the actual temperature of the feed water and, as stated, these are the results that are directly comparable.

TABLE 26

RESULTS OF “IDALIA” TESTS
Date1909Oct. 11Oct. 14Oct. 14Oct. 12Oct. 13
Degrees of superheat Fahrenheit      0          7        88        96      105    
Pressures, pounds per
square inch above
Atmospheric Pressure
Throttle  190      196      201      198      203    
First Receiver    68.4    66.0    64.3    61.9    63.0
Second Receiver      9.7      9.2      8.7      7.8      8.4
Vacuum, inches    25.5    25.9    25.9    25.4    25.2
Temperature, Degrees FahrenheitFeed  201      206      205      202      200    
Hot Well  116      109.5  115      111.5  111    
Revolutions per minuteAir Pump    57        56        53        54        45    
Circulating Pump  196      198      196      198      197    
Main Engine  194.3  191.5  195.1  191.5  193.1
Indicated Horse Power, Main Engine  512.3  495.2  521.1  498.3  502.2
Water per hour, total pounds9397    8430    8234    7902    7790    
Water per indicated Horse Power, pounds    18.3    17.0    15.8    15.8    15.5
B. t. u. per minute per indicated Horse Power  314      300      284      286      283    
Per cent Saving of Steam  …          7.1    13.7    13.7    15.3
Percent Saving of Fuel (computed)  …          4.4      9.5      8.9      9.9

The table shows that the saving in steam consumption with 105 degrees of superheat was 15.3 per cent and in heat consumption about 10 per cent. This may be [Pg 144] safely stated to be a conservative representation of the saving that may be accomplished by the use of superheated steam in a plant as a whole, where superheated steam is furnished not only to the main engine but also to the auxiliaries. The figures may be taken as conservative for the reason that in addition to the saving as shown in the table, there would be in an ordinary plant a saving much greater than is generally realized in the drips, where the loss with saturated steam is greatly in excess of that with superheated steam.

The most conclusive and most practical evidence that a saving is possible through the use of superheated steam is in the fact that in the largest and most economical plants it is used almost without exception. Regardless of any such evidence, however, there is a deep rooted conviction in the minds of certain engineers that the use of superheated steam will involve operating difficulties which, with additional first cost, will more than offset any fuel saving. There are, of course, conditions under which the installation of superheaters would in no way be advisable. With a poorly designed superheater, no gain would result. In general, it may be stated that in a new plant, properly designed, with a boiler and superheater which will have an efficiency at least as high as a boiler without a superheater, a gain is certain.

Such a gain is dependent upon the class of engine and the power plant equipment in general. In determining the advisability of making a superheater installation, all of the factors entering into each individual case should be considered and balanced, with a view to determining the saving in relation to cost, maintenance, depreciation etc.

In highly economical plants, where the water consumption for an indicated horse power is low, the gain will be less than would result from the use of superheated steam in less economical plants where the water consumption is higher. It is impossible to make an accurate statement as to the saving possible but, broadly, it may vary from 3 to 5 per cent for 100 degrees of superheat in the large and economical plants using turbines or steam engines, in which there is a large ratio of expansion, to from 10 to 25 per cent for 100 degrees of superheat for the less economical steam motors.

Though a properly designed superheater will tend to raise rather than to decrease the boiler efficiency, it does not follow that all superheaters are efficient, for if the gases in passing over the superheater do not follow the path they would ordinarily take in passing over the boiler heating surface, a loss may result. This is noticeably true where part of the gases are passed over the superheater and are allowed to pass over only a part or in some cases none of the boiler heating surface.

With moderate degrees of superheat, from 100 to 200 degrees, where the piping is properly installed, there will be no greater operating difficulties than with saturated steam. Engine and turbine builders guarantee satisfactory operation with superheated steam. With high degrees of superheat, say, over 250 degrees, apparatus of a special nature must be used and it is questionable whether the additional care and liability to operating difficulties will offset any fuel saving accomplished. It is well established, however, that the operating difficulties, with the degrees of superheat to which this article is limited, have been entirely overcome.

The use of cast-iron fittings with superheated steam has been widely discussed. It is an undoubted fact that while in some instances superheated steam has caused deterioration of such fittings, in others cast-iron fittings have been used with 150 degrees of superheat without the least difficulty. The quality of the cast iron used in [Pg 145] such fittings has doubtless a large bearing on the life of such fittings for this service. The difficulties that have been encountered are an increase in the size of the fittings and eventually a deterioration great enough to lead to serious breakage, the development of cracks, and when flanges are drawn up too tightly, the breaking of a flange from the body of the fitting. The latter difficulty is undoubtedly due, in certain instances, to the form of flange in which the strain of the connecting bolts tended to distort the metal.

The Babcock & Wilcox Co. have used steel castings in superheated steam work over a long period and experience has shown that this metal is suitable for the service. There seems to be a general tendency toward the use of steel fittings. In European practice, until recently, cast iron was used with apparently satisfactory results. The claim of European engineers was to the effect that their cast iron was of better quality than that found in this country and thus explained the results secured. Recently, however, certain difficulties have been encountered with such fittings and European engineers are leaning toward the use of steel for this work.

The degree of superheat produced by a superheater placed within the boiler setting will vary according to the class of fuel used, the form of furnace, the condition of the fire and the rate at which the boiler is being operated. This is necessarily true of any superheater swept by the main body of the products of combustion and is a fact that should be appreciated by the prospective user of superheated steam. With a properly designed superheater, however, such fluctuations would not be excessive, provided the boilers are properly operated. As a matter of fact the point to be guarded against in the use of superheated steam is that a maximum should not be exceeded. While, as stated, there may be a considerable fluctuation in the temperature of the steam as delivered from individual superheaters, where there are a number of boilers on a line the temperature of the combined flow of steam in the main will be found to be practically a constant, resulting from the offsetting of various furnace conditions of one boiler by another.
[Pg 146]

Illustration:

8400 Horse-power Installation of Babcock & Wilcox Boilers and Superheaters at the Butler Street Plant of the Georgia Railway and Power Co., Atlanta, Ga. This Company Operates a Total of 15,200 Horse Power of Babcock & Wilcox Boilers

FOOTNOTES

[20] See pages 125 to 127.

[21] The actual specific heat at a particular temperature and pressure is that corresponding to a change of one degree one way or the other and differs considerably from the average value for the particular temperature and pressure given in the table. The mean values given in the table give correct results when employed to determine the factor of evaporation whereas the actual values at the particular temperatures and pressures would not.

[22] See page 117.

[Pg 147]

PROPERTIES OF AIR

Pure air is a mechanical mixture of oxygen and nitrogen. While different authorities give slightly varying values for the proportion of oxygen and nitrogen contained, the generally accepted values are:

By volume,  oxygen 20.91 per cent,  nitrogen 79.09 per cent.
By weight,  oxygen 23.15 per cent,  nitrogen 76.85 per cent.

Air in nature always contains other constituents in varying amounts, such as dust, carbon dioxide, ozone and water vapor.

Being perfectly elastic, the density or weight per unit of volume decreases in geometric progression with the altitude. This fact has a direct bearing in the proportioning of furnaces, flues and stacks at high altitudes, as will be shown later in the discussion of these subjects. The atmospheric pressures corresponding to various altitudes are given in Table 12 .

The weight and volume of air depend upon the pressure and the temperature, as expressed by the formula:

P v  =  53.33 T             ( 9 )
Where P = the absolute pressure in pounds per square foot,
v = the volume in cubic feet of one pound of air,
T = the absolute temperature of the air in degrees Fahrenheit,
53.33 = a constant for air derived from the ratio of pressure, volume and temperature of a perfect gas.

The weight of one cubic foot of air will obviously be the reciprocal of its volume, that is, 1/ v pounds.

TABLE 27

VOLUME AND WEIGHT OF AIR
AT ATMOSPHERIC PRESSURE AT VARIOUS TEMPERATURES
Temperature
Degrees
Fahrenheit
Volume
One Pound
in
Cubic Feet
Weight One
Cubic Foot
in Pounds
Temperature
Degrees
Fahrenheit
Volume
One Pound
in
Cubic Feet
Weight One
Cubic Foot
in Pounds
Temperature
Degrees
Fahrenheit
Volume
One Pound
in
Cubic Feet
Weight One
Cubic Foot
in Pounds
  32 12.390 .080710 160 15.615 .064041 340 20.151 .049625
  50 12.843 .077863 170 15.867 .063024 360 20.655 .048414
  55 12.969 .077107 180 16.119 .062039 380 21.159 .047261
  60 13.095 .076365 190 16.371 .061084 400 21.663 .046162
  65 13.221 .075637 200 16.623 .060158 425 22.293 .044857
  70 13.347 .074923 210 16.875 .059259 450 22.923 .043624
  75 13.473 .074223 212 16.925 .059084 475 23.554 .042456
  80 13.599 .073535 220 17.127 .058388 500 24.184 .041350
  85 13.725 .072860 230 17.379 .057541 525 24.814 .040300
  90 13.851 .072197 240 17.631 .056718 550 25.444 .039302
  95 13.977 .071546 250 17.883 .055919 575 26.074 .038352
100 14.103 .070907 260 18.135 .055142 600 26.704 .037448
110 14.355 .069662 270 18.387 .054386 650 27.964 .035760
120 14.607 .068460 280 18.639 .053651 700 29.224 .034219
130 14.859 .067299 290 18.891 .052935 750 30.484 .032804
140 15.111 .066177 300 19.143 .052238 800 31.744 .031502
150 15.363 .065092 320 19.647 .050898 850 33.004 .030299

[Pg 148]

Example: Required the volume of air in cubic feet under 60.3 pounds gauge pressure per square inch at 115 degrees Fahrenheit.

P  =  144 (14.7 + 60.3)  =  10,800.
T  =  115 + 460  =  575 degrees.
Hence v  = 
53.33 × 575
––––––––––––––––––––
10,800
 =  2.84 cubic feet, and
Weight per cubic foot  = 
1
––––
v
 = 
1
––––––––
2.84
 =  0.352 pounds.

Table 27 gives the weights and volumes of air under atmospheric pressure at varying temperatures.

Formula ( 9 ) holds good for other gases with the change in the value of the constant as follows:

For oxygen 48.24, nitrogen 54.97, hydrogen 765.71.

The specific heat of air at constant pressure varies with its temperature. A number of determinations of this value have been made and certain of those ordinarily accepted as most authentic are given in Table 28 .

TABLE 28

SPECIFIC HEAT OF AIR
AT CONSTANT PRESSURE AND VARIOUS TEMPERATURES
Temperature Range Specific Heat Authority
Degrees Centigrade Degrees Fahrenheit
-30–  10 -22–    50 0.2377 Regnault
    0–100   32–  212 0.2374 Regnault
    0–200   32–  392 0.2375 Regnault
  20–440   68–  824 0.2366 Holborn and Curtis
  20–630   68–1166 0.2429 Holborn and Curtis
  20–800   68–1472 0.2430 Holborn and Curtis
    0–200   32–  392 0.2389 Wiedemann

This value is of particular importance in waste heat work and it is regrettable that there is such a variation in the different experiments. Mallard and Le Chatelier determined values considerably higher than any given in Table 28 . All things considered in view of the discrepancy of the values given, there appears to be as much ground for the use of a constant value for the specific heat of air at any temperature as for a variable value. Where this value is used throughout this book, it has been taken as 0.24.

Air may carry a considerable quantity of water vapor, which is frequently 3 per cent of the total weight. This fact is of importance in problems relating to heating drying and the compressing of air. Table 29 gives the amount of vapor required to saturate air at different temperatures, its weight, expansive force, etc., and contains sufficient information for solving practically all problems of this sort that may arise.

[Pg 149]

TABLE 29

WEIGHTS OF AIR, VAPOR OF WATER, AND SATURATED MIXTURES OF AIR AND VAPOR
AT DIFFERENT TEMPERATURES,
UNDER THE ORDINARY ATMOSPHERIC PRESSURE OF 29.921 INCHES OF MERCURY
Temper-
ature Degrees Fahr-
enheit
Volume of Dry Air at Different Temper-
atures, the Volume at 32 Degrees being 1.000
Weight of Cubic Foot of Dry Air at the Different Temper-
atures Pounds
Elastic Force of Vapor in Inches of Mercury (Regnault) Mixtures of Air Saturated with Vapor Cubic Feet of Vapor from One Pound of Water at its own Pressure in Column 4
Elastic Force of the Air in the Mixture of Air and Vapor in Inches of Mercury Weight of Cubic Foot of the Mixture of Air and Vapor Weight of Vapor Mixed with One Pound of Air, in Pounds Weight of Dry Air Mixed with One Pound of Vapor, in Pounds
Weight of the Air in Pounds Weight of the Vapor in Pounds Total Weight of Mixture in Pounds
  1   2   3   4   5   6   7   8   9 10 11
    0   .935 .0864     .044 29.877 .0863 .000079 .086379   .00092 1092.4    
  12   .960 .0842     .074 29.849 .0840 .000130 .084130   .00155   646.1    
  22   .980 .0824     .118 29.803 .0821 .000202 .082302   .00245   406.4    
  32 1.000 .0807     .181 29.740 .0802 .000304 .080504   .00379   263.81   3289    
  42 1.020 .0791     .267 29.654 .0784 .000440 .078840   .00561   178.18   2252    
  52 1.041 .0776     .388 29.533 .0766 .000627 .077227   .00810   122.17   1595    
  62 1.061 .0761     .556 29.365 .0747 .000881 .075581   .01179     84.79   1135    
  72 1.082 .0747     .785 29.136 .0727 .001221 .073921   .01680     59.54     819    
  82 1.102 .0733   1.092 28.829 .0706 .001667 .072267   .02361     42.35     600    
  92 1.122 .0720   1.501 28.420 .0684 .002250 .070717   .03289     30.40     444    
102 1.143 .0707   2.036 27.885 .0659 .002997 .068897   .04547     21.98     334    
112 1.163 .0694   2.731 27.190 .0631 .003946 .067046   .06253     15.99     253    
122 1.184 .0682   3.621 26.300 .0599 .005142 .065042   .08584     11.65     194    
132 1.204 .0671   4.752 25.169 .0564 .006639 .063039   .11771       8.49     151    
142 1.224 .0660   6.165 23.756 .0524 .008473 .060873   .16170       6.18     118    
152 1.245 .0649   7.930 21.991 .0477 .010716 .058416   .22465       4.45       93.3
162 1.265 .0638 10.099 19.822 .0423 .013415 .055715   .31713       3.15       74.5
172 1.285 .0628 12.758 17.163 .0360 .016682 .052682   .46338       2.16       59.2
182 1.306 .0618 15.960 13.961 .0288 .020536 .049336   .71300       1.402     48.6
192 1.326 .0609 19.828 10.093 .0205 .025142 .045642 1.22643         .815     39.8
202 1.347 .0600 24.450   5.471 .0109 .030545 .041445 2.80230         .357     32.7
212 1.367 .0591 29.921   0.000 .0000 .036820 .036820 Infinite         .000     27.1

Column 5 = barometer pressure of 29.921, minus the proportion of this due to vapor pressure from column 4.

[Pg 150]

COMBUSTION

Combustion may be defined as the rapid chemical combination of oxygen with carbon, hydrogen and sulphur, accompanied by the diffusion of heat and light. That portion of the substance thus combined with the oxygen is called combustible. As used in steam engineering practice, however, the term combustible is applied to that portion of the fuel which is dry and free from ash, thus including both oxygen and nitrogen which may be constituents of the fuel, though not in the true sense of the term combustible.

Combustion is perfect when the combustible unites with the greatest possible amount of oxygen, as when one atom of carbon unites with two atoms of oxygen to form carbon dioxide, CO2. The combustion is imperfect when complete oxidation of the combustible does not occur, or where the combustible does not unite with the maximum amount of oxygen, as when one atom of carbon unites with one atom of oxygen to form carbon monoxide, CO, which may be further burned to carbon dioxide.

Kindling Point—Before a combustible can unite with oxygen and combustion takes place, its temperature must first be raised to the ignition or kindling point, and a sufficient time must be allowed for the completion of the combustion before the temperature of the gases is lowered below that point. Table 30, by Stromeyer, gives the approximate kindling temperatures of different fuels.

TABLE 30

KINDLING TEMPERATURE OF VARIOUS FUELS
 Degrees
Fahrenheit
Lignite Dust 300
Dried Peat 435
Sulphur 470
Anthracite Dust570
Coal 600
Coke Red Heat
Anthracite Red Heat, 750
Carbon MonoxideRed Heat, 1211
Hydrogen 1030 or 1290

Combustibles—The principal combustibles in coal and other fuels are carbon, hydrogen and sulphur, occurring in varying proportions and combinations.

Carbon is by far the most abundant as is indicated in the chapters on fuels.

Hydrogen in a free state occurs in small quantities in some fuels, but is usually found in combination with carbon, in the form of hydrocarbons. The density of hydrogen is 0.0696 (Air = 1) and its weight per cubic foot, at 32 degrees Fahrenheit and under atmospheric pressure, is 0.005621 pounds.

Sulphur is found in most coals and some oils. It is usually present in combined form, either as sulphide of iron or sulphate of lime; in the latter form it has no heat value. Its presence in fuel is objectionable because of its tendency to aid in the formation of clinkers, and the gases from its combustion, when in the presence of moisture, may cause corrosion.

Nitrogen is drawn into the furnace with the air. Its density is 0.9673 (Air = 1); its weight, at 32 degrees Fahrenheit and under atmospheric pressure, is 0.07829 pounds per cubic foot; each pound of air at atmospheric pressure contains 0.7685 pounds of nitrogen, and one pound of nitrogen is contained in 1.301 pounds of air.

Nitrogen performs no useful office in combustion and passes through the furnace without change. It dilutes the air, absorbs heat, reduces the temperature of the products of combustion, and is the chief source of heat losses in furnaces.

[Pg 151]

Calorific Value—Each combustible element of gas will combine with oxygen in certain definite proportions and will generate a definite amount of heat, measured in B. t. u. This definite amount of heat per pound liberated by perfect combustion is termed the calorific value of that substance. Table 31, gives certain data on the reactions and results of combustion for elementary combustibles and several compounds.

TABLE 31

OXYGEN AND AIR REQUIRED FOR COMBUSTION
AT 32 DEGREES AND 29.92 INCHES
BY WEIGHT
  1  2  3  4  5  6  7  8  910
Oxidizable Substance or CombustibleChemical SymbolAtomic or Combining WeightChemical ReactionProduct of CombustionOxygen per Pound of Column 1 PoundsNitrogen per Pound of Column 1. 3.32[23] × O PoundsAir per Pound of Column 1. 4.32[24] × O PoundsGaseous Product per Pound of Column 1[25] + Column 8 PoundsHeat Value per Pound of Column 1 B. t. u.
CarbonC12C+2O = CO2Carbon Dioxide2.667  8.8511.5212.5214600
CarbonC12C+O = COCarbon Monoxide1.333  4.43  5.76  6.76  4450
Carbon MonoxideCO28CO+O = CO2Carbon Dioxide  .571  1.90  2.47  3.4710150
HydrogenH12H+O = H2OWater8        26.5634.5635.5662000
MethaneCH416CH4+4O = CO2+2H2OCarbon Dioxide and Water4        13.2817.2818.2823550
SulphurS32S+2O = SO2Sulphur Dioxide1          3.32  4.32  5.32  4050
BY VOLUME
  1  21112131415161718
Oxidizable Substance or CombustibleChemical SymbolVolumes of Column 1 Entering Combination VolumeVolumes of Oxygen Combining with Column 11 VolumeVolumes of Product Formed VolumeVolume per Pound of Column 1 in Gaseous Form Cubic FeetVolume of Oxygen per Pound of Column 1 Cubic FeetVolume of Products of Combustion per Pound of Column 1 Cubic FeetVolume of Nitrogen per Pound of Column 1. 3.782[26] × Column 15 Cubic FeetVolume of Gas per pound of Column 1 = Column 10 ÷ Column 17 Cubic Feet
CarbonC1C22CO2  14.9529.89  29.89112.98142.87
CarbonC1C12CO  14.9514.95  29.89  56.49  86.38
Carbon MonoxideCO2CO12CO2  12.80  6.40  12.80  24.20  37.00
HydrogenH2H12H2O179.3289.66179.32339.09518.41
MethaneCH41C4H41CO2 2H2O  22.4144.83  67.34169.55236.89
SulphurS1S21SO2    5.6011.21  11.21  42.39  53.60

It will be seen from this table that a pound of carbon will unite with 223 pounds of oxygen to form carbon dioxide, and will evolve 14,600 B. t. u. As an intermediate step, a pound of carbon may unite with 113 pounds of oxygen to form carbon monoxide and evolve 4450 B. t. u., but in its further conversion to CO2 it would unite with an additional 113 times its weight of oxygen and evolve the remaining 10,150 B. t. u. [Pg 152] When a pound of CO burns to CO2, however, only 4350 B. t. u. are evolved since the pound of CO contains but 37 pound carbon.

Air Required for Combustion—It has already been shown that each combustible element in fuel will unite with a definite amount of oxygen. With the ultimate analysis of the fuel known, in connection with Table 31, the theoretical amount of air required for combustion may be readily calculated.

Let the ultimate analysis be as follows:

 Per Cent
Carbon74.79
Hydrogen4.98
Oxygen6.42
Nitrogen1.20
Sulphur3.24
Water1.55
Ash7.82
–––––
100.00

When complete combustion takes place, as already pointed out, the carbon in the fuel unites with a definite amount of oxygen to form CO2. The hydrogen, either in a free or combined state, will unite with oxygen to form water vapor, H2O. Not all of the hydrogen shown in a fuel analysis, however, is available for the production of heat, as a portion of it is already united with the oxygen shown by the analysis in the form of water, H2O. Since the atomic weights of H and O are respectively 1 and 16, the weight of the combined hydrogen will be 18 of the weight of the oxygen, and the hydrogen available for combustion will be H - 18 O. In complete combustion of the sulphur, sulphur dioxide SO2 is formed, which in solution in water forms sulphuric acid.

Expressed numerically, the theoretical amount of air for the above analysis is as follows:

0.7479 C × 223 = 1.9944 O needed
(0.0498 - 
0.0642
––––––––––––
8
)H × 8
 = 0.3262 O needed
0.0324 S × 1 = 0.0324 O needed
  –––––––––––
Total = 2.3530 O needed

One pound of oxygen is contained in 4.32 pounds of air.

The total air needed per pound of coal, therefore, will be 2.353 × 4.32 = 10.165.

The weight of combustible per pound of fuel is .7479 + .0418[27] + .0324 + .012 = .83 pounds, and the air theoretically required per pound of combustible is 10.165 ÷ .83 = 12.2 pounds.

The above is equivalent to computing the theoretical amount of air required per pound of fuel by the formula:

Weight per pound = 11.52 C + 34.56
(H - 
O
––––
8
)
+ 4.32 S            (10)

where C, H, O and S are proportional parts by weight of carbon, hydrogen, oxygen and sulphur by ultimate analysis.

[Pg 153]

In practice it is impossible to obtain perfect combustion with the theoretical amount of air, and an excess may be required, amounting to sometimes double the theoretical supply, depending upon the nature of the fuel to be burned and the method of burning it. The reason for this is that it is impossible to bring each particle of oxygen in the air into intimate contact with the particles in the fuel that are to be oxidized, due not only to the dilution of the oxygen in the air by nitrogen, but because of such factors as the irregular thickness of the fire, the varying resistance to the passage of the air through the fire in separate parts on account of ash, clinker, etc. Where the difficulties of drawing air uniformly through a fuel bed are eliminated, as in the case of burning oil fuel or gas, the air supply may be materially less than would be required for coal. Experiment has shown that coal will usually require 50 per cent more than the theoretical net calculated amount of air, or about 18 pounds per pound of fuel either under natural or forced draft, though this amount may vary widely with the type of furnace, the nature of the coal, and the method of firing. If less than this amount of air is supplied, the carbon burns to monoxide instead of dioxide and its full heat value is not developed.

TABLE 32

CALCULATED THEORETICAL AMOUNT OF AIR
REQUIRED PER POUND OF VARIOUS FUELS
FuelWeight of Constituents in One
Pound Dry Fuel
Air Required
per Pound
of Fuel
Pounds
Carbon
Per Cent
Hydrogen
Per Cent
Oxygen
Per Cent
Coke94.010.8
Anthracite Coal91.5  3.5  2.611.7
Bituminous Coal87.0  5.0  4.011.6
Lignite70.0  5.020.0  8.9
Wood50.0  6.043.5  6.0
Oil85.0  3.0  1.014.3

An excess of air is also a source of waste, as the products of combustion will be diluted and carry off an excessive amount of heat in the chimney gases, or the air will so lower the temperature of the furnace gases as to delay the combustion to an extent that will cause carbon monoxide to pass off unburned from the furnace. A sufficient amount of carbon monoxide in the gases may cause the action known as secondary combustion, by igniting or mingling with air after leaving the furnace or in the flues or stack. Such secondary combustion which takes place either within the setting after leaving the furnace or in the flues or stack always leads to a loss of efficiency and, in some instances, leads to overheating of the flues and stack.

Table 32 gives the theoretical amount of air required for various fuels calculated from formula (10) assuming the analyses of the fuels given in the table.

The process of combustion of different fuels and the effect of variation in the air supply for their combustion is treated in detail in the chapters dealing with the various fuels.
[Pg 154]

Illustration:

4064 HORSE-POWER Installation of Babcock & Wilcox Boilers and Superheaters, Equipped with Babcock & Wilcox Chain Grate Stokers, at the Cosmopolitan Electric Co., Chicago, Ill.

FOOTNOTES

[23] Ratio by weight of O to N in air.

[24] 4.32 pounds of air contains one pound of O.

[25] Per pound of C in the CO.

[26] Ratio by volume of O to N in air.

[27] Available hydrogen.

[Pg 155]

ANALYSIS OF FLUE GASES

The object of a flue gas analysis is the determination of the completeness of the combustion of the carbon in the fuel, and the amount and distribution of the heat losses due to incomplete combustion. The quantities actually determined by an analysis are the relative proportions by volume, of carbon dioxide (CO 2 ), oxygen (O), and carbon monoxide (CO), the determinations being made in this order.

The variations of the percentages of these gases in an analysis is best illustrated in the consideration of the complete combustion of pure carbon, a pound of which requires 2.67 pounds of oxygen, [28] or 32 cubic feet at 60 degrees Fahrenheit. The gaseous product of such combustion will occupy, when cooled, the same volume as the oxygen, namely, 32 cubic feet. The air supplied for the combustion is made up of 20.91 per cent oxygen and 79.09 per cent nitrogen by volume. The carbon united with the oxygen in the form of carbon dioxide will have the same volume as the oxygen in the air originally supplied. The volume of the nitrogen when cooled will be the same as in the air supplied, as it undergoes no change. Hence for complete combustion of one pound of carbon, where no excess of air is supplied, an analysis of the products of combustion will show the following percentages by volume:

  Actual Volume
for One Pound Carbon
Cubic Feet
  Per Cent
by Volume
Carbon Dioxide   32 =   20.91
Oxygen     0 =     0.00
Nitrogen 121 =   79.09
  ––––––   –––––––––––
Air required for one pound Carbon 153 = 100.00

For 50 per cent excess air the volume will be as follows:

153 × 1½ = 229.5 cubic feet of air per pound of carbon.
  Actual Volume
for One Pound Carbon
Cubic Feet
  Per Cent
by Volume
 
Carbon Dioxide   32     =   13.91 } = 20.91 per cent
Oxygen   16     =     7.00
Nitrogen 181.5 =   79.09  
  ––––––––––   –––––––––––  
Air required for one pound Carbon 229.5 = 100.00  

For 100 per cent excess air the volume will be as follows:

153 × 2 = 306 cubic feet of air per pound of carbon.
  Actual Volume
for One Pound Carbon
Cubic Feet
  Per Cent
by Volume
 
Carbon Dioxide   32 =   10.45 } = 20.91 per cent
Oxygen   32 =   10.45
Nitrogen 242 =   79.09  
  ––––––   –––––––––––  
Air required for one pound Carbon 306 = 100.00  

In each case the volume of oxygen which combines with the carbon is equal to (cubic feet of air × 20.91 per cent)—32 cubic feet.

[Pg 156]

It will be seen that no matter what the excess of air supplied, the actual amount of carbon dioxide per pound of carbon remains the same, while the percentage by volume decreases as the excess of air increases. The actual volume of oxygen and the percentage by volume increases with the excess of air, and the percentage of oxygen is, therefore, an indication of the amount of excess air. In each case the sum of the percentages of CO 2 and O is the same, 20.9. Although the volume of nitrogen increases with the excess of air, its percentage by volume remains the same as it undergoes no change while combustion takes place; its percentage for any amount of air excess, therefore, will be the same after combustion as before, if cooled to the same temperature. It must be borne in mind that the above conditions hold only for the perfect combustion of a pound of pure carbon.

Carbon monoxide (CO) produced by the imperfect combustion of carbon, will occupy twice the volume of the oxygen entering into its composition and will increase the volume of the flue gases over that of the air supplied for combustion in the proportion of

1 to
100 + ½ the per cent CO
–––––––––––––––––––––––––––––––––––––––
100

When pure carbon is the fuel, the sum of the percentages by volume of carbon dioxide, oxygen and one-half of the carbon monoxide, must be in the same ratio to the nitrogen in the flue gases as is the oxygen to the nitrogen in the air supplied, that is, 20.91 to 79.09. When burning coal, however, the percentage of nitrogen is obtained by subtracting the sum of the percentages by volume of the other gases from 100. Thus if an analysis shows 12.5 per cent CO 2 , 6.5 per cent O, and 0.6 per cent CO, the percentage of nitrogen which ordinarily is the only other constituent of the gas which need be considered, is found as follows:

100 - (12.5 + 6.5 + 0.6) = 80.4 per cent.

The action of the hydrogen in the volatile constituents of the fuel is to increase the apparent percentage of the nitrogen in the flue gases. This is due to the fact that the water vapor formed by the combustion of the hydrogen will condense at a temperature at which the analysis is made, while the nitrogen which accompanied the oxygen with which the hydrogen originally combined maintains its gaseous form and passes into the sampling apparatus with the other gases. For this reason coals containing high percentages of volatile matter will produce a larger quantity of water vapor, and thus increase the apparent percentage of nitrogen.

Air Required and Supplied —When the ultimate analysis of a fuel is known, the air required for complete combustion with no excess can be found as shown in the chapter on combustion, or from the following approximate formula:

Pounds of air required per pound of fuel  =  34.56 (
C
––––
3
 +  ( H  - 
O
––––
8
)  + 
S
––––
8
) [29]             ( 11 )

where C, H and O equal the percentage by weight of carbon, hydrogen and oxygen in the fuel divided by 100.

[Pg 157]

When the flue gas analysis is known, the total, amount of air supplied is:

Pounds of air supplied per pound of fuel  =  3.036 (
N
–––––––––––––––––
CO 2 + CO
)  ×  C [30]             ( 12 )

where N, CO 2 and CO are the percentages by volume of nitrogen, carbon dioxide and carbon monoxide in the flue gases, and C the percentage by weight of carbon which is burned from the fuel and passes up the stack as flue gas. This percentage of C which is burned must be distinguished from the percentage of C as found by an ultimate analysis of the fuel. To find the percentage of C which is burned, deduct from the total percentage of carbon as found in the ultimate analysis, the percentage of unconsumed carbon found in the ash. This latter quantity is the difference between the percentage of ash found by an analysis and that as determined by a boiler test. It is usually assumed that the entire combustible element in the ash is carbon, which assumption is practically correct. Thus if the ash in a boiler test were 16 per cent and by an analysis contained 25 per cent of carbon, the percentage of unconsumed carbon would be 16 × .25 = 4 per cent of the total coal burned. If the coal contained by ultimate analysis 80 per cent of carbon the percentage burned, and of which the products of combustion pass up the chimney would be 80 - 4 = 76 per cent, which is the correct figure to use in calculating the total amount of air supplied by formula ( 12 ).

The weight of flue gases resulting from the combustion of a pound of dry coal will be the sum of the weights of the air per pound of coal and the combustible per pound of coal, the latter being equal to one minus the percentage of ash as found in the boiler test. The weight of flue gases per pound of dry fuel may, however, be computed directly from the analyses, as shown later, and the direct computation is that ordinarily used.

The ratio of the air actually supplied per pound of fuel to that theoretically required to burn it is:

3.036 (
N
–––––––––––––––––
CO 2 + CO
)  ×  C
            ( 13 )
–––––––––––––––––––––––––––––––––––––––––––––
34.56 (
C
–––
3
 +  H  - 
O
–––
8
)

in which the letters have the same significance as in formulae ( 11 ) and ( 12 ).

The ratio of the air supplied per pound of combustible to the amount theoretically required is:

N
––––––––––––––––––––––––––––––––––
N - 3.782(O - ½CO)
            ( 14 )

which is derived as follows:

The N in the flue gas is the content of nitrogen in the whole amount of air supplied. The oxygen in the flue gas is that contained in the air supplied and which was not utilized in combustion. This oxygen was accompanied by 3.782 times its volume of nitrogen. The total amount of excess oxygen in the flue gases is (O - ½CO); hence N - 3.782(O - ½CO) represents the nitrogen content in the air actually required for combustion and N ÷ (N - 3.782[O - ½CO]) is the [Pg 158] ratio of the air supplied to that required. This ratio minus one will be the proportion of excess air.

The heat lost in the flue gases is L = 0.24 W (T - t )            ( 15 )
 
Where L = B. t. u. lost per pound of fuel,
W = weight of flue gases in pounds per pound of dry coal,
T = temperature of flue gases,
t = temperature of atmosphere,
0.24 = specific heat of the flue gases.

The weight of flue gases, W, per pound of carbon can be computed directly from the flue gas analysis from the formula:

11 CO 2 + 8 O + 7 (CO + N)
–––––––––––––––––––––––––––––––––––––––––––––
3 (CO 2 + CO)
            ( 16 )

where CO 2 , O, CO, and N are the percentages by volume as determined by the flue gas analysis of carbon dioxide, oxygen, carbon monoxide and nitrogen.

The weight of flue gas per pound of dry coal will be the weight determined by this formula multiplied by the percentage of carbon in the coal from an ultimate analysis.

Footnote31 Graph of Heat Loss

Fig. 20. Loss Due to Heat Carried Away by Chimney Gases for Varying Percentages of Carbon Dioxide.
Based on Boiler Room Temperature = 80 Degrees Fahrenheit.
Nitrogen in Flue Gas = 80.5 Per Cent. Carbon Monoxide in Flue Gas = 0. Per Cent

Fig. 20 represents graphically the loss due to heat carried away by dry chimney gases for varying percentages of CO 2 , and different temperatures of exit gases.

[Pg 159]

The heat lost, due to the fact that the carbon in the fuel is not completely burned and carbon monoxide is present in the flue gases, in B. t. u. per pound of fuel burned is:

L'  =  10,150  ×  (
CO
–––––––––––––––––
CO + CO 2
)             ( 17 )

where, as before, CO and CO 2 are the percentages by volume in the flue gases and C is the proportion by weight of carbon which is burned and passes up the stack.

Fig. 21 represents graphically the loss due to such carbon in the fuel as is not completely burned but escapes up the stack in the form of carbon monoxide.

Footnote32 Graph of Heat Loss

Fig. 21. Loss Due to Unconsumed Carbon Contained in the CO in the Flue Gases

Apparatus for Flue Gas Analysis —The Orsat apparatus, illustrated in Fig. 22, is generally used for analyzing flue gases. The burette A is graduated in cubic centimeters up to 100, and is surrounded by a water jacket to prevent any change in temperature from affecting the density of the gas being analyzed.

For accurate work it is advisable to use four pipettes, B , C , D , E , the first containing a solution of caustic potash for the absorption of carbon dioxide, the second an alkaline solution of pyrogallol for the absorption of oxygen, and the remaining two an acid solution of cuprous chloride for absorbing the carbon monoxide. Each pipette contains a number of glass tubes, to which some of the solution clings, thus facilitating [Pg 160] the absorption of the gas. In the pipettes D and E , copper wire is placed in these tubes to re-energize the solution as it becomes weakened. The rear half of each pipette is fitted with a rubber bag, one of which is shown at K , to protect the solution from the action of the air. The solution in each pipette should be drawn up to the mark on the capillary tube.

Orsat Apparatus

Fig. 22. Orsat Apparatus

The gas is drawn into the burette through the U-tube H , which is filled with spun glass, or similar material, to clean the gas. To discharge any air or gas in the apparatus, the cock G is opened to the air and the bottle F is raised until the water in the burette reaches the 100 cubic centimeters mark. The cock G is then turned so as to close the air opening and allow gas to be drawn through H , the bottle F being lowered for this purpose. The gas is drawn into the burette to a point below the zero mark, the cock G then being opened to the air and the excess gas expelled until the level of the water in F and in A are at the zero mark. This operation is necessary in order to obtain the zero reading at atmospheric pressure.

The apparatus should be carefully tested for leakage as well as all connections leading thereto. Simple tests can be made; for example: If after the cock G is closed, the bottle F is placed on top of the frame for a short time and again brought to the zero mark, the level of the water in A is above the zero mark, a leak is indicated.

Before taking a final sample for analysis, the burette A should be filled with gas and emptied once or twice, to make sure that all the apparatus is filled with the new gas. The cock G is then closed and the cock I in the pipette B is opened and the gas driven over into B by raising the bottle F . The gas is drawn back into A by lowering F and when the solution in B has reached the mark in the capillary tube, the cock I is closed and a reading is taken on the burette, the level of the water in the bottle F being brought to the same level as the water in A . The operation is repeated until a constant reading is obtained, the number of cubic centimeters being the percentage of CO 2 in the flue gases.

The gas is then driven over into the pipette C and a similar operation is carried out. The difference between the resulting reading and the first reading gives the percentage of oxygen in the flue gases.

The next operation is to drive the gas into the pipette D , the gas being given a final wash in E , and then passed into the pipette C to neutralize any hydrochloric acid fumes which may have been given off by the cuprous chloride solution, which, especially if it be old, may give off such fumes, thus increasing the volume of the gases and making the reading on the burette less than the true amount.

The process must be carried out in the order named, as the pyrogallol solution will also absorb carbon dioxide, while the cuprous chloride solution will also absorb oxygen.

[Pg 161]

As the pressure of the gases in the flue is less than the atmospheric pressure, they will not of themselves flow through the pipe connecting the flue to the apparatus. The gas may be drawn into the pipe in the way already described for filling the apparatus, but this is a tedious method. For rapid work a rubber bulb aspirator connected to the air outlet of the cock G will enable a new supply of gas to be drawn into the pipe, the apparatus then being filled as already described. Another form of aspirator draws the gas from the flue in a constant stream, thus insuring a fresh supply for each sample.

The analysis made by the Orsat apparatus is volumetric; if the analysis by weight is required, it can be found from the volumetric analysis as follows:

Multiply the percentages by volume by either the densities or the molecular weight of each gas, and divide the products by the sum of all the products; the quotients will be the percentages by weight. For most work sufficient accuracy is secured by using the even values of the molecular weights.

The even values of the molecular weights of the gases appearing in an analysis by an Orsat are:

Carbon Dioxide      44
Carbon Monoxide      28
Oxygen      32
Nitrogen      28

Table 33 indicates the method of converting a volumetric flue gas analysis into an analysis by weight.

TABLE 33

CONVERSION OF A FLUE GAS ANALYSIS BY VOLUME TO ONE BY WEIGHT
Gas Analysis by Volume
Per Cent
Molecular Weight Volume times
Molecular Weight
Analysis by Weight
Per Cent
Carbon Dioxide CO 2   12.2 12+(2×16)   536.8
  536.8
–––––––––––
3022.8
 =    17.7    
Carbon Monoxide CO       .4 12+16     11.2
    11.2
–––––––––––
3022.8
 =        .4    
Oxygen O     6.9 2×16   220.8
  220.8
–––––––––––
3022.8
 =      7.3    
Nitrogen N   80.5 2×14 2254.0
2254.0
–––––––––––
3022.8
 =    74.6    
Total 100.0   3022.8
100.0    

Application of Formulae and Rules —Pocahontas coal is burned in the furnace, a partial ultimate analysis being:

  Per Cent
Carbon 82.1  
Hydrogen 4.25
Oxygen 2.6  
Sulphur 1.6  
Ash 6.0  
B. t. u., per pound dry 14500  

[Pg 162]

The flue gas analysis shows:

  Per Cent
CO 2 10.7  
O 9.0  
CO 0.0  
N (by difference) 80.3  

Determine: The flue gas analysis by weight (see Table 33 ), the amount of air required for perfect combustion, the actual weight of air per pound of fuel, the weight of flue gas per pound of coal, the heat lost in the chimney gases if the temperature of these is 500 degrees Fahrenheit, and the ratio of the air supplied to that theoretically required.

Solution: The theoretical weight of air required for perfect combustion, per pound of fuel, from formula ( 11 ) will be,

W  =  34.56
(
.821
–––––––
3
 +  ( .0425  - 
.026
–––––––
8
)  + 
.016
–––––––
8
)
 =  10.88 pounds.

If the amount of carbon which is burned and passes away as flue gas is 80 per cent, which would allow for 2.1 per cent of unburned carbon in terms of the total weight of dry fuel burned, the weight of dry gas per pound of carbon burned will be from formula ( 16 ):

W  = 
11 × 10.7 + 8 × 9.0 + 7(0 + 80.3)
–––––––––––––––––––––––––––––––––––––––––––––––––––––
3 (10.7 + 0)
 =  23.42 pounds

and the weight of flue gas per pound of coal burned will be .80 × 23.42 = 18.74 pounds.

The heat lost in the flue gases per pound of coal burned will be from formula ( 15 ) and the value 18.74 just determined.

Loss = .24 × 18.74 × (500 - 60) = 1979 B. t. u.

The percentage of heat lost in the flue gases will be 1979 ÷ 14500 = 13.6 per cent.

The ratio of air supplied per pound of coal to that theoretically required will be 18.74 ÷ 10.88 = 1.72 per cent.

The ratio of air supplied per pound of combustible to that required will be from formula ( 14 ):

.803
–––––––––––––––––––––––––––––––––––––––
.803 - 3.782(.09 - ½ × 0)
 =  1.73

The ratio based on combustible will be greater than the ratio based on fuel if there is unconsumed carbon in the ash.

Unreliability of CO 2 Readings Taken Alone —It is generally assumed that high CO 2 readings are indicative of good combustion and hence of high efficiency. This is true only in the sense that such high readings do indicate the small amount of excess air that usually accompanies good combustion, and for this reason high CO 2 readings alone are not considered entirely reliable. Wherever an automatic CO 2 recorder is used, it should be checked from time to time and the analysis carried further with a view to ascertaining whether there is CO present. As the percentage of CO 2 in these gases increases, there is a tendency toward the presence of CO, which, of course, cannot be shown by a CO 2 recorder, and which is often difficult to detect with an Orsat apparatus. The greatest care should be taken in preparing the cuprous chloride solution in making analyses and it must be known to be fresh and capable of absorbing CO. [Pg 163] In one instance that came to our attention, in using an Orsat apparatus where the cuprous chloride solution was believed to be fresh, no CO was indicated in the flue gases but on passing the same sample into a Hempel apparatus, a considerable percentage was found. It is not safe, therefore, to assume without question from a high CO 2 reading that the combustion is correspondingly good, and the question of excess air alone should be distinguished from that of good combustion. The effect of a small quantity of CO, say one per cent, present in the flue gases will have a negligible influence on the quantity of excess air, but the presence of such an amount would mean a loss due to the incomplete combustion of the carbon in the fuel of possibly 4.5 per cent of the total heat in the fuel burned. When this is considered, the importance of a complete flue gas analysis is apparent.

Table 34 gives the densities of various gases together with other data that will be of service in gas analysis work.

TABLE 34

DENSITY OF GASES AT 32 DEGREES FAHRENHEIT AND ATMOSPHERIC PRESSURE
ADAPTED FROM SMITHSONIAN TABLES
Gas Chemical
Symbol
Specific Gravity
Air=1
Weight of
One Cubic Foot
Pounds
Volume of
One Pound
Cubic Feet
Relative Density, Hydrogen = 1
Exact Approximate
Oxygen O 1.053   .08922     11.208 15.87 16
Nitrogen N 0.9673 .07829     12.773 13.92 14
Hydrogen H 0.0696 .005621 177.90     1.00   1
Carbon Dioxide CO 2 1.5291 .12269       8.151 21.83 22
Carbon Monoxide CO 0.9672 .07807     12.809 13.89 14
Methane CH 4 0.5576 .04470     22.371   7.95   8
Ethane C 2 H 6 1.075   .08379     11.935 14.91 15
Acetylene C 2 H 2 0.920   .07254     13.785 12.91 13
Sulphur Dioxide SO 2 2.2639 .17862       5.598 31.96 32
Air 1.0000 .08071     12.390

[Pg 164]
Illustration:

1942 Horse-power Installation of Babcock & Wilcox Boilers and Superheaters in the Singer Building, New York City

FOOTNOTES

[28] See Table 31 , page 151 .

[29] This formula is equivalent to ( 10 ) given in chapter on combustion . 34.56 = theoretical air required for combustion of one pound of H (see Table 31 ).

[30] For degree of accuracy of this formula, see Transactions, A. S. M. E., Volume XXI, 1900, page 94.

[31] For loss per pound of coal multiply by per cent of carbon in coal by ultimate analysis.

[32] For loss per pound of coal multiply by per cent of carbon in coal by ultimate analysis.

[Pg 165]

CLASSIFICATION OF FUELS

(WITH PARTICULAR REFERENCE TO COAL)

Fuels for steam boilers may be classified as solid, liquid or gaseous. Of the solid fuels, anthracite and bituminous coals are the most common, but in this class must also be included lignite, peat, wood, bagasse and the refuse from certain industrial processes such as sawdust, shavings, tan bark and the like. Straw, corn and coffee husks are utilized in isolated cases.

The class of liquid fuels is represented chiefly by petroleum, though coal tar and water-gas tar are used to a limited extent.

Gaseous fuels are limited to natural gas, blast furnace gas and coke oven gas, the first being a natural product and the two latter by-products from industrial processes. Though waste gases from certain processes may be considered as gaseous fuels, inasmuch as the question of combustion does not enter, the methods of utilizing them differ from that for combustible gaseous fuel, and the question will be dealt with separately.

Since coal is by far the most generally used of all fuels, this chapter will be devoted entirely to the formation, composition and distribution of the various grades, from anthracite to peat. The other fuels will be discussed in succeeding chapters and their combustion dealt with in connection with their composition.

TABLE 35

APPROXIMATE CHEMICAL CHANGES FROM WOOD
FIBER TO ANTHRACITE COAL
SubstanceCarbonHydrogenOxygen
Wood Fiber 52.655.25 42.10
Peat 59.575.96 34.47
Lignite 66.045.27 28.69
Earthy Brown Coal 73.185.68 21.14
Bituminous Coal 75.065.84 19.10
Semi-bituminous Coal89.295.05  5.66
Anthracite Coal 91.583.96  4.46

Formation of Coal—All coals are of vegetable origin and are the remains of prehistoric forests. Destructive distillation due to great pressures and temperatures, has resolved the organic matter into its invariable ultimate constituents, carbon, hydrogen, oxygen and other substances, in varying proportions. The factors of time, depth of beds, disturbance of beds and the intrusion of mineral matter resulting from such disturbances have produced the variation in the degree of evolution from vegetable fiber to hard coal. This variation is shown chiefly in the content of carbon, and Table 35 shows the steps of such variation.

Composition of Coal—The uncombined carbon in coal is known as fixed carbon. Some of the carbon constituent is combined with hydrogen and this, together with other gaseous substances driven off by the application of heat, form that portion of the coal known as volatile matter. The fixed carbon and the volatile matter constitute the combustible. The oxygen and nitrogen contained in the volatile matter are not combustible, but custom has applied this term to that portion of the coal which is dry and free from ash, thus including the oxygen and nitrogen.

The other important substances entering into the composition of coal are moisture and the refractory earths which form the ash. The ash varies in different coals from 3 to 30 per cent and the moisture from 0.75 to 45 per cent of the total weight of the [Pg 166] coal, depending upon the grade and the locality in which it is mined. A large percentage of ash is undesirable as it not only reduces the calorific value of the fuel, but chokes up the air passages in the furnace and through the fuel bed, thus preventing the rapid combustion necessary to high efficiency. If the coal contains an excessive quantity of sulphur, trouble will result from its harmful action on the metal of the boiler where moisture is present, and because it unites with the ash to form a fusible slag or clinker which will choke up the grate bars and form a solid mass in which large quantities of unconsumed carbon may be imbedded.

Moisture in coal may be more detrimental than ash in reducing the temperature of a furnace, as it is non-combustible, absorbs heat both in being evaporated and superheated to the temperature of the furnace gases. In some instances, however, a certain amount of moisture in a bituminous coal produces a mechanical action that assists in the combustion and makes it possible to develop higher capacities than with dry coal.

Classification of Coal—Custom has classified coals in accordance with the varying content of carbon and volatile matter in the combustible. Table 36 gives the approximate percentages of these constituents for the general classes of coals with the corresponding heat values per pound of combustible.

TABLE 36

APPROXIMATE COMPOSITION AND CALORIFIC VALUE
OF GENERAL GRADES OF COAL ON BASIS OF COMBUSTIBLE
Kind of CoalPer Cent of CombustibleB. t. u.
Per Pound of
Combustible
Fixed CarbonVolatile Matter
Anthracite97.0 to 92.5  3.0 to   7.514600 to 14800
Semi-anthracite92.5 to 87.5  7.5 to 12.514700 to 15500
Semi-bituminous87.5 to 75.012.5 to 25.015500 to 16000
Bituminous—Eastern75.0 to 60.025.0 to 40.014800 to 15300
Bituminous—Western65.0 to 50.035.0 to 50.013500 to 14800
LigniteUnder 50Over 5011000 to 13500

Anthracite—The name anthracite, or hard coal, is applied to those dry coals containing from 3 to 7 per cent volatile matter and which do not swell when burned. True anthracite is hard, compact, lustrous and sometimes iridescent, and is characterized by few joints and clefts. Its specific gravity varies from 1.4 to 1.8. In burning, it kindles slowly and with difficulty, is hard to keep alight, and burns with a short, almost colorless flame, without smoke.

Semi-anthracite coal has less density, hardness and luster than true anthracite, and can be distinguished from it by the fact that when newly fractured it will soot the hands. Its specific gravity is ordinarily about 1.4. It kindles quite readily and burns more freely than the true anthracites.

Semi-bituminous coal is softer than anthracite, contains more volatile hydrocarbons, kindles more easily and burns more rapidly. It is ordinarily free burning, has a high calorific value and is of the highest order for steam generating purposes.

[Pg 167]

Bituminous coals are still softer than those described and contain still more volatile hydrocarbons. The difference between the semi-bituminous and the bituminous coals is an important one, economically. The former have an average heating value per pound of combustible about 6 per cent higher than the latter, and they burn with much less smoke in ordinary furnaces. The distinctive characteristic of the bituminous coals is the emission of yellow flame and smoke when burning. In color they range from pitch black to dark brown, having a resinous luster in the most compact specimens, and a silky luster in such specimens as show traces of vegetable fiber. The specific gravity is ordinarily about 1.3.

Bituminous coals are either of the caking or non-caking class. The former, when heated, fuse and swell in size; the latter burn freely, do not fuse, and are commonly known as free burning coals. Caking coals are rich in volatile hydrocarbons and are valuable in gas manufacture.

Bituminous coals absorb moisture from the atmosphere. The surface moisture can be removed by ordinary drying, but a portion of the water can be removed only by heating the coal to a temperature of about 250 degrees Fahrenheit.

Cannel coal is a variety of bituminous coal, rich in hydrogen and hydrocarbons, and is exceedingly valuable as a gas coal. It has a dull resinous luster and burns with a bright flame without fusing. Cannel coal is seldom used for steam coal, though it is sometimes mixed with semi-bituminous coal where an increased economy at high rates of combustion is desired. The composition of cannel coal is approximately as follows: fixed carbon, 26 to 55 per cent; volatile matter, 42 to 64 per cent; earthy matter, 2 to 14 per cent. Its specific gravity is approximately 1.24.

Lignite is organic matter in the earlier stages of its conversion into coal, and includes all varieties which are intermediate between peat and coal of the older formation. Its specific gravity is low, being 1.2 to 1.23, and when freshly mined it may contain as high as 50 per cent of moisture. Its appearance varies from a light brown, showing a distinctly woody structure, in the poorer varieties, to a black, with a pitchy luster resembling hard coal, in the best varieties. It is non-caking and burns with a bright but slightly smoky flame with moderate heat. It is easily broken, will not stand much handling in transportation, and if exposed to the weather will rapidly disintegrate, which will increase the difficulty of burning it.

Its composition varies over wide limits. The ash may run as low as one per cent and as high as 50 per cent. Its high content of moisture and the large quantity of air necessary for its combustion cause large stack losses. It is distinctly a low-grade fuel and is used almost entirely in the districts where mined, due to its cheapness.

Peat is organic matter in the first stages of its conversion into coal and is found in bogs and similar places. Its moisture content when cut is extremely high, averaging 75 or 80 per cent. It is unsuitable for fuel until dried and even then will contain as much as 30 per cent moisture. Its ash content when dry varies from 3 to 12 per cent. In this country, though large deposits of peat have been found, it has not as yet been found practicable to utilize it for steam generating purposes in competition with coal. In some European countries, however, the peat industry is common.

Distribution—The anthracite coals are, with some unimportant exceptions, confined to five small fields in Eastern Pennsylvania, as shown in the following list. These fields are given in the order of their hardness.

Lehigh or Eastern Middle Field [Pg 168]      Wyoming or Northern Field
      Green Mountain District                  Continued
      Black Creek District            Pittston District
      Hazelton District            Wilkesbarre District
      Beaver Meadow District            Plymouth District
      Panther Creek District[33]      Schuylkill or Southern Field
Mahanoy or Western Field[34]            East Schuylkill District
      East Mahanoy District            West Schuylkill District
      West Mahanoy District            Louberry District
Wyoming or Northern Field      Lykens Valley or Southwestern Field
      Carbondale District            Lykens Valley District
      Scranton District            Shamokin District[35]

Anthracite is also found in Pulaski and Wythe Counties, Virginia; along the border of Little Walker Mountain, and in Gunnison County, Colorado. The areas in Virginia are limited, however, while in Colorado the quality varies greatly in neighboring beds and even in the same bed. An anthracite bed in New Mexico was described in 1870 by Dr. R. W. Raymond, formerly United States Mining Commissioner.

Semi-anthracite coals are found in a few small areas in the western part of the anthracite field. The largest of these beds is the Bernice in Sullivan County, Pennsylvania. Mr. William Kent, in his “Steam Boiler Economy”, describes this as follows: “The Bernice semi-anthracite coal basin lies between Beech Creek on the north and Loyalsock Creek on the south. It is six miles long, east and west, and hardly a third of a mile across. An 8-foot vein of coal lies in a bed of 12 feet of coal and slate. The coal of this bed is the dividing line between anthracite and semi-anthracite, and is similar to the coal of the Lykens Valley District. Mine analyses give a range as follows: moisture, 0.65 to 1.97; volatile matter, 3.56 to 9.40; fixed carbon, 82.52 to 89.39; ash, 3.27 to 9.34; sulphur, 0.24 to 1.04.”

Semi-bituminous coals are found on the eastern edge of the great Appalachian Field. Starting with Tioga and Bradford Counties of northern Pennsylvania, the bed runs southwest through Lycoming, Clearfield, Centre, Huntingdon, Cambria, Somerset and Fulton Counties, Pennsylvania; Allegheny County, Maryland; Buchannan, Dickinson, Lee, Russell, Scott, Tazewell and Wise Counties, Virginia; Mercer, McDowell, Fayette, Raleigh and Mineral Counties, West Virginia; and ending in northeastern Tennessee, where a small amount of semi-bituminous is mined.

The largest of the bituminous fields is the Appalachian. Beginning near the northern boundary of Pennsylvania, in the western portion of the State, it extends southwestward through West Virginia, touching Maryland and Virginia on their western borders, passing through southeastern Ohio, eastern Kentucky and central Tennessee, and ending in western Alabama, 900 miles from its northern extremity.

The next bituminous coal producing region to the west is the Northern Field, in north central Michigan. Still further to the west, and second in importance to the [Pg 169] Appalachian Field, is the Eastern Interior Field. This covers, with the exception of the upper northern portion, nearly the entire State of Illinois, southwest Indiana and the western portion of Kentucky.

The Western Field extends through central and southern Iowa, western Missouri, southwestern Kansas, eastern Oklahoma and the west central portion of Arkansas. The Southwestern Field is confined entirely to the north central portion of Texas, in which State there are also two small isolated fields along the Rio Grande River.

The remaining bituminous fields are scattered through what may be termed the Rocky Mountain Region, extending from Montana to New Orleans. A partial list of these fields and their location follows:

Judith Basin      Central Montana
Bull Mountain Field      Central Montana
Yellowstone Region      Southwestern Montana
Big Horn Basin Region      Southern Montana
Big Horn Basin Region      Northern Wyoming
Black Hills Region      Northeastern Wyoming
Hanna Field      Southern Wyoming
Green River Region      Southwestern Wyoming
Yampa Field      Northwestern Colorado
North Park Field      Northern Colorado
Denver Region      North Central Colorado
Uinta Region      Western Colorado
Uinta Region      Eastern Utah
Southwestern Region      Southwestern Utah
Raton Mountain Region      Southern Colorado
Raton Mountain Region      Northern New Mexico
San Juan River Region      Northwestern New Mexico
Capitan Field      Southern New Mexico

Along the Pacific Coast a few small fields are scattered in western California, southwestern Oregon, western and northwestern Washington.

Most of the coals in the above fields are on the border line between bituminous and lignite. They are really a low grade of bituminous coal and are known as sub-bituminous or black lignites.

Lignites—These resemble the brown coals of Europe and are found in the western states, Wyoming, New Mexico, Arizona, Utah, Montana, North Dakota, Nevada, California, Oregon and Washington. Many of the fields given as those containing bituminous coals in the western states also contain true lignite. Lignite is also found in the eastern part of Texas and in Oklahoma.

Alaska Coals—Coal has been found in Alaska and undoubtedly is of great value, though the extent and character of the fields have probably been exaggerated. Great quantities of lignite are known to exist, and in quality the coal ranges in character from lignite to anthracite. There are at present, however, only two fields of high-grade coals known, these being the Bering River Field, near Controllers Bay, and the Matanuska Field, at the head of Cooks Inlet. Both of these fields are known to contain both anthracite and high-grade bituminous coals, though as yet they cannot be said to have been opened up.

Weathering of Coal—The storage of coal has become within the last few years to a certain extent a necessity due to market conditions, danger of labor difficulties at the mines and in the railroads, and the crowding of transportation facilities. [Pg 170] The first cause is probably the most important, and this is particularly true of anthracite coals where a sliding scale of prices is used according to the season of the year. While market conditions serve as one of the principal reasons for coal storage, most power plants and manufacturing plants feel compelled to protect their coal supply from the danger of strikes, car shortages and the like, and it is customary for large power plants, railroads and coal companies themselves, to store bituminous coal. Naval coaling stations are also an example of what is done along these lines.

Anthracite is the nearest approach to the ideal coal for storing. It is not subject to spontaneous ignition, and for this reason is unlimited in the amount that may be stored in one pile. With bituminous coals, however, the case is different. Most bituminous coals will ignite if placed in large enough piles and all suffer more or less from disintegration. Coal producers only store such coals as are least liable to ignite, and which will stand rehandling for shipment.

The changes which take place in stored coal are of two kinds: 1st, the oxidization of the inorganic matter such as pyrites; and 2nd, the direct oxidization of the organic matter of the actual coal.

The first change will result in an increased volume of the coal, and sometimes in an increased weight, and a marked disintegration. The changes due to direct oxidization of the coal substances usually cannot be detected by the eye, but as they involve the oxidization of the carbon and available hydrogen and the absorption of the oxygen by unsaturated hydrocarbons, they are the chief cause of the weathering losses in heat value. Numerous experiments have led to the conclusion that this is also the cause for spontaneous combustion.

Experiments to show loss in calorific heat values due to weathering indicate that such loss may be as high as 10 per cent when the coal is stored in the air, and 8.75 per cent when stored under water. It would appear that the higher the volatile content of the coal, the greater will be the loss in calorific value and the more subject to spontaneous ignition.

Some experiments made by Messrs. S. W. Parr and W. F. Wheeler, published in 1909 by the Experiment Station of the University of Illinois, indicate that coals of the nature found in Illinois and neighboring states are not affected seriously during storage from the standpoint of weight and heating value, the latter loss averaging about 3½ per cent for the first year of storage. They found that the losses due to disintegration and to spontaneous ignition were of greater importance. Their conclusions agree with those deduced from the other experiments, viz., that the storing of a larger size coal than that which is to be used, will overcome to a certain extent the objection to disintegration, and that the larger sizes, besides being advantageous in respect to disintegration, are less liable to spontaneous ignition. Storage under water will, of course, entirely prevent any fire loss and, to a great extent, will stop disintegration and reduce the calorific losses to a minimum.

To minimize the danger of spontaneous ignition in storing coal, the piles should be thoroughly ventilated.

Pulverized Fuels—Considerable experimental work has been done with pulverized coal, utilizing either coal dust or pulverizing such coal as is too small to be burned in other ways. If satisfactorily fed to the furnace, it would appear to have several advantages. The dust burned in suspension would be more completely consumed than is the case with the solid coals, the production of smoke would be [Pg 171] minimized, and the process would admit of an adjustment of the air supply to a point very close to the amount theoretically required. This is due to the fact that in burning there is an intimate mixture of the air and fuel. The principal objections have been in the inability to introduce the pulverized fuel into the furnace uniformly, the difficulty of reducing the fuel to the same degree of fineness, liability of explosion in the furnace due to improper mixture with the air, and the decreased capacity and efficiency resulting from the difficulty of keeping tube surfaces clean.

Pressed Fuels—In this class are those composed of the dust of some suitable combustible, pressed and cemented together by a substance possessing binding and in most cases inflammable properties. Such fuels, known as briquettes, are extensively used in foreign countries and consist of carbon or soft coal, too small to be burned in the ordinary way, mixed usually with pitch or coal tar. Much experimenting has been done in this country in briquetting fuels, the government having taken an active interest in the question, but as yet this class of fuel has not come into common use as the cost and difficulty of manufacture and handling have made it impossible to place it in the market at a price to successfully compete with coal.

Coke is a porous product consisting almost entirely of carbon remaining after certain manufacturing processes have distilled off the hydrocarbon gases of the fuel used. It is produced, first, from gas coal distilled in gas retorts; second, from gas or ordinary bituminous coals burned in special furnaces called coke ovens; and third, from petroleum by carrying the distillation of the residuum to a red heat.

Coke is a smokeless fuel. It readily absorbs moisture from the atmosphere and if not kept under cover its moisture content may be as much as 20 per cent of its own weight.

Gas-house coke is generally softer and more porous than oven coke, ignites more readily, and requires less draft for its combustion.
[Pg 172]

Illustration:

16,000 Horse-power Installation of Babcock & Wilcox Boilers and Superheaters at the Brunot’s Island Plant of the Duquesne Light Co., Pittsburgh, Pa.

FOOTNOTES

[33] The Panther Creek District forms a part of what is known as the Southern Field; in the matter of hardness, however, these coals are more nearly akin to Lehigh coals.

[34] Sometimes called Western Middle or Northern Schuylkill Field.

[35] Geographically, the Shamokin District is part of the Western Middle Mahanoy Field, but the coals found in this section resemble more closely those of the Wyoming Field.

[Pg 173]

THE DETERMINATION OF HEATING VALUES OF FUELS

The heating value of a fuel may be determined either by a calculation from a chemical analysis or by burning a sample in a calorimeter.

In the former method the calculation should be based on an ultimate analysis, which reduces the fuel to its elementary constituents of carbon, hydrogen, oxygen, nitrogen, sulphur, ash and moisture, to secure a reasonable degree of accuracy. A proximate analysis, which determines only the percentage of moisture, fixed carbon, volatile matter and ash, without determining the ultimate composition of the volatile matter, cannot be used for computing the heat of combustion with the same degree of accuracy as an ultimate analysis, but estimates may be based on the ultimate analysis that are fairly correct.

An ultimate analysis requires the services of a competent chemist, and the methods to be employed in such a determination will be found in any standard book on engineering chemistry. An ultimate analysis, while resolving the fuel into its elementary constituents, does not reveal how these may have been combined in the fuel. The manner of their combination undoubtedly has a direct effect upon their calorific value, as fuels having almost identical ultimate analyses show a difference in heating value when tested in a calorimeter. Such a difference, however, is slight, and very close approximations may be computed from the ultimate analysis.

Ultimate analyses are given on both a moist and a dry fuel basis. Inasmuch as the latter is the basis generally accepted for the comparison of data, it would appear that it is the best basis on which to report such an analysis. When an analysis is given on a moist fuel basis it may be readily converted to a dry basis by dividing the percentages of the various constituents by one minus the percentage of moisture, reporting the moisture content separately.

       Moist Fuel      Dry Fuel
C   83.95   84.45
H     4.23     4.25
O     3.02     3.04
N     1.27     1.28
S       .91       .91
Ash     6.03     6.07
    –––––––––––
    100.00
Moisture       .59       .59
  –––––––––––  
  100.00  

Calculations from an Ultimate Analysis—The first formula for the calculation of heating values from the composition of a fuel as determined from an ultimate analysis is due to Dulong, and this formula, slightly modified, is the most commonly used to-day. Other formulae have been proposed, some of which are more accurate for certain specific classes of fuel, but all have their basis in Dulong’s formula, the accepted modified form of which is:

Heat units in B. t. u. per pound of dry fuel =

14,600 C + 62,000(H - 
O
––––
8
) + 4000 S            (18)

[Pg 174]

where C, H, O and S are the proportionate parts by weight of carbon, hydrogen, oxygen and sulphur.

Assume a coal of the composition given. Substituting in this formula (18),

Heating value per pound of dry coal

 = 14,600 × .8445 + 62,000(.0425 - 
.0304
–––––––––
8
) + 4000 × .0091 = 14,765 B. t. u.

This coal, by a calorimetric test, showed 14,843 B. t. u., and from a comparison the degree of accuracy of the formula will be noted.

The investigation of Lord and Haas in this country, Mabler in France, and Bunte in Germany, all show that Dulong’s formula gives results nearly identical with those obtained from calorimetric tests and may be safely applied to all solid fuels except cannel coal, lignite, turf and wood, provided the ultimate analysis is correct. This practically limits its use to coal. The limiting features are the presence of hydrogen and carbon united in the form of hydrocarbons. Such hydrocarbons are present in coals in small quantities, but they have positive and negative heats of combination, and in coals these appear to offset each other, certainly sufficiently to apply the formula to such fuels.

High and Low Heat Value of Fuels—In any fuel containing hydrogen the calorific value as found by the calorimeter is higher than that obtainable under most working conditions in boiler practice by an amount equal to the latent heat of the volatilization of water. This heat would reappear when the vapor was condensed, though in ordinary practice the vapor passes away uncondensed. This fact gives rise to a distinction in heat values into the so-called “higher” and “lower” calorific values. The higher value, i. e., the one determined by the calorimeter, is the only scientific unit, is the value which should be used in boiler testing work, and is the one recommended by the American Society of Mechanical Engineers.

There is no absolute measure of the lower heat of combustion, and in view of the wide difference in opinion among physicists as to the deductions to be made from the higher or absolute unit in this determination, the lower value must be considered an artificial unit. The lower value entails the use of an ultimate analysis and involves assumptions that would make the employment of such a unit impracticable for commercial work. The use of the low value may also lead to error and is in no way to be recommended for boiler practice.

An example of its illogical use may be shown by the consideration of a boiler operated in connection with a special economizer where the vapor produced by hydrogen is partially condensed by the economizer. If the low value were used in computing the boiler efficiency, it is obvious that the total efficiency of the combined boiler and economizer must be in error through crediting the combination with the heat imparted in condensing the vapor and not charging such heat to the heat value of the coal.

Heating Value of Gaseous Fuels—The method of computing calorific values from an ultimate analysis is particularly adapted to solid fuels, with the exceptions already noted. The heating value of gaseous fuels may be calculated by Dulong’s formula provided another term is added to provide for any carbon monoxide present. Such a method, however, involves the separating of the constituent gases into their elementary gases, which is oftentimes difficult and liable to simple arithmetical error. As the combustible portion of gaseous fuels is ordinarily composed of hydrogen, carbon [Pg 175] monoxide and certain hydrocarbons, a determination of the calorific value is much more readily obtained by a separation into their constituent gases and a computation of the calorific value from a table of such values of the constituents. Table 37 gives the calorific value of the more common combustible gases, together with the theoretical amount of air required for their combustion.

TABLE 37

WEIGHT AND CALORIFIC VALUE OF VARIOUS GASES
AT 32 DEGREES FAHRENHEIT AND ATMOSPHERIC PRESSURE
WITH THEORETICAL AMOUNT OF AIR REQUIRED FOR COMBUSTION
GasSymbolCubic Feet of
Gas per Pound
B. t. u. per
Pound
B. t. u. per
Cubic Foot
Cubic Feet of
Air Required
per Pound of
Gas
Cubic Feet of
Air Required
Per Cubic Foot
of Gas
HydrogenH177.9062000  349428.25  2.41
Carbon MonoxideCO    2.81  4450  347  30.60  2.39
MethaneCH4  22.37235501053214.00  9.57
AcetyleneC2H2  13.79214651556164.8711.93
Olefiant GasC2H4  12.80214401675183.6014.33
EthaneC2H6  11.94222301862199.8816.74

In applying this table, as gas analyses may be reported either by weight or volume, there is given in Table 33[36] a method of changing from volumetric analysis to analysis by weight.

Examples:

1st. Assume a blast furnace gas, the analysis of which in percentages by weight is, oxygen = 2.7, carbon monoxide = 19.5, carbon dioxide = 18.7, nitrogen = 59.1. Here the only combustible gas is the carbon monoxide, and the heat value will be,

0.195 × 4450 = 867.75 B. t. u. per pound.

The net volume of air required to burn one pound of this gas will be,

0.195 × 30.6 = 5.967 cubic feet.

2nd. Assume a natural gas, the analysis of which in percentages by volume is oxygen = 0.40, carbon monoxide = 0.95, carbon dioxide = 0.34, olefiant gas (C2H4) = 0.66, ethane (C2H6) = 3.55, marsh gas (CH4) = 72.15 and hydrogen = 21.95. All but the oxygen and the carbon dioxide are combustibles, and the heat per cubic foot will be,

From CO = 0.0095 × 347 = 3.30
 C2H4 = 0.0066 × 1675 = 11.05
 C2H6 = 0.0355 × 1862 = 66.10
 CH4 = 0.7215 × 1050 = 757.58
 H = 0.2195 × 349 = 76.61
–––––––––––
B. t. u. per cubic foot = 914.64

[Pg 176]

The net air required for combustion of one cubic foot of the gas will be,

CO = 0.0095 × 2.39 = 0.02
C2H4 = 0.0066 × 14.33 = 0.09
C2H6 = 0.0355 × 16.74 = 0.59
CH4 = 0.7215 × 9.57 = 6.90
H = 0.2195 × 2.41 = 0.53
–––––––
Total net air per cubic foot = 8.13

Proximate Analysis—The proximate analysis of a fuel gives its proportions by weight of fixed carbon, volatile combustible matter, moisture and ash. A method of making such an analysis which has been found to give eminently satisfactory results is described below.

From the coal sample obtained on the boiler trial, an average sample of approximately 40 grams is broken up and weighed. A good means of reducing such a sample is passing it through an ordinary coffee mill. This sample should be placed in a double-walled air bath, which should be kept at an approximately constant temperature of 105 degrees centigrade, the sample being weighed at intervals until a minimum is reached. The percentage of moisture can be calculated from the loss in such a drying.

For the determination of the remainder of the analysis, and the heating value of the fuel, a portion of this dried sample should be thoroughly pulverized, and if it is to be kept, should be placed in an air-tight receptacle. One gram of the pulverized sample should be weighed into a porcelain crucible equipped with a well fitting lid. This crucible should be supported on a platinum triangle and heated for seven minutes over the full flame of a Bunsen burner. At the end of such time the sample should be placed in a desiccator containing calcium chloride, and when cooled should be weighed. From the loss the percentage of volatile combustible matter may be readily calculated.

The same sample from which the volatile matter has been driven should be used in the determination of the percentage of ash. This percentage is obtained by burning the fixed carbon over a Bunsen burner or in a muffle furnace. The burning should be kept up until a constant weight is secured, and it may be assisted by stirring with a platinum rod. The weight of the residue determines the percentage of ash, and the percentage of fixed carbon is easily calculated from the loss during the determination of ash after the volatile matter has been driven off.

Proximate analyses may be made and reported on a moist or dry basis. The dry basis is that ordinarily accepted, and this is the basis adopted throughout this book. The method of converting from a moist to a dry basis is the same as described in the case of an ultimate analysis. A proximate analysis is easily made, gives information as to the general characteristics of a fuel and of its relative heating value.

Table 38 gives the proximate analysis and calorific value of a number of representative coals found in the United States.
[Pg 177]

TABLE 38

APPROXIMATE COMPOSITION AND CALORIFIC VALUE OF CERTAIN TYPICAL AMERICAN COALS
StateCountyField, Bed
or Vein
MineSizeProximate Analysis (Dry Coal)B. t. u.
Per
Pound
Dry
Coal
Authority
MoistureVolatile
Matter
Fixed
Carbon
Ash
ANTHRACITES 
Pa.CarbonLehighBeaver Meadow   1.50  2.4190.30  7.29 Gale
Pa.DauphinSchuylkill Buckwheat  2.1512.8878.23  8.8913137Whitham
Pa.LackawannaWyomingBelleviewNo. 2 Buck.  8.29  7.8177.1915.0012341Sadtler
Pa.LackawannaWyomingJohnsonCulm.13.9011.1665.9622.8810591B. & W. Co.
Pa.LuzerneWyomingPittstonNo. 2 Buck.  3.66  4.4078.9616.6412865B. & W. Co.
Pa.LuzerneWyomingMammothLarge  4.00  3.4490.59  5.9713720Carpenter
Pa.LuzerneWyomingExeterRice  0.25  8.1879.6112.2112400B. & W. Co.
Pa.NorthumberlandSchuylkillTreverton   0.84  6.7386.39  6.88 Isherwood
Pa.SchuylkillSchuylkillBuck Mountain    3.1792.41  4.4214220Carpenter
Pa.Schuylkill York FarmBuckwheat  0.81  5.5175.9018.5911430 
Pa.  VictoriaBuckwheat  4.30  0.5586.7312.7212642B. & W. Co.
Pa.CarbonLehighLehigh & Wilkes C. Co.Buck. & Pea  1.57  6.2766.5327.2012848B. & W. Co.
Pa.CarbonLehigh Buckwheat   5.0081.0014.0011800Carpenter
Pa.Lackawanna Del. & Hud. Co.No. 1 Buck.  6.20  11.6012100Denton
SEMI-ANTHRACITES 
Pa.LycomingLoyalsock    1.30  8.7284.44  6.84  
Pa.Sullivan Lopez   5.48  7.5381.0011.4713547B. & W. Co.
Pa.SullivanBernice    1.29  8.2184.43  7.36  
SEMI-BITUMINOUS 
Md.AlleghanyBig Vein, George's Crk.    3.5021.3372.47  6.2014682B. & W. Co.
Md.AlleghanyGeorge's Creek    3.6316.2776.93  6.8014695B. & W. Co.
Md.AlleghanyGeorge's Creek    2.2819.4377.44  6.1314793B. & W. Co.
Md.AlleghanyGeorge's CreekOcean No. 7Mine run  1.13   14451B. & W. Co.
Md.AlleghanyCumberland    1.5017.2676.65  6.0914700 
Md.Garrett Washington No. 3Mine run  2.3314.3874.9310.4914033U. S. Geo. S.[37]
Pa.Bradford Long Valley   1.5520.3368.3811.2912965 
Pa.Tioga Antrim   2.1918.4371.87  9.7013500 
Pa.Cambria"B" or MillerSoriman Shaft C. Co.   3.4020.7071.84  7.4614484N. Y. Ed. Co.
Pa.Cambria"B" or MillerHenrietta   1.2318.3775.28  6.4514770So. Eng. Co.
Pa.Cambria"B" or MillerPenker   3.6421.3470.48  8.1814401B. & W. Co.
Pa.Cambria"B" or MillerLancashire   4.3821.2070.27  8.5314453B. & W. Co.
Pa.CambriaLower KittanningPenn. C. & C. Co. No. 3Mine run  3.5117.4375.69  6.8814279U. S. Geo. S.
Pa.CambriaUpper KittanningValleyMine run  3.4014.8975.0310.0814152B. & W. Co.
Pa.ClearfieldLower KittanningEurekaMine run  5.9016.7177.22  6.0714843U. S. Geo. S.
Pa.Clearfield GhemMine run  3.4317.5369.6712.8013744B. & W. Co.
Pa.Clearfield Osceola   1.2425.4368.56  6.0113589B. & W. Co.
Pa.ClearfieldReynoldsville    2.9121.5569.03  9.4214685B. & W. Co.
Pa.ClearfieldAtlantic-Clearfield Mine run  1.5523.3671.15  5.9413963Whitham
Pa.HuntingtonBarnet & FultonCarbonMine run  4.5018.3473.06  8.6013770B. & W. Co.
Pa.Huntington Rock HillMine run  5.9117.5873.44  8.9914105B. & W. Co.
Pa.SomersetLower KittanningKimmeltonMine run  3.0917.8470.4711.6913424U. S. Geo. S.
Pa.Somerset"C" Prime VeinJennerMine run  9.3716.4775.76  7.7714507P. R. R.
W. Va.FayetteNew RiverRush RunMine run  2.1422.8771.56  5.5714959U. S. Geo. S.[Pg 178] 
W. Va.FayetteNew RiverLoup Creek   0.5519.3678.48  2.1614975Hill
W. Va.FayetteNew River Slack  6.6620.9473.16  5.9014412B. & W. Co.
W. Va.FayetteNew River Mine run  2.1617.8275.66  6.5214786B. & W. Co.
W. Va.FayetteNew RiverRush RunMine run  0.9422.1675.85  1.9915007B. & W. Co.
W. Va.McDowellPocahontas No. 3ZenithMine run  4.8517.1476.54  6.3214480U. S. Geo. S.
W. Va.McDowellTug RiverBig SandyMine run  1.5818.5576.44  4.9115170U. S. Geo. S.
W. Va.MercerPocahontasMoraLump  1.7418.5575.15  6.3015015U. S. Geo. S.
W. Va.MineralElk Garden    2.1015.7075.40  8.9014195B. & W. Co.
W. Va.McDowellPocahontasFlat TopMine run  0.5224.0274.59  1.3914490B. & W. Co.
W. Va.McDowellPocahontasFlat TopSlack  3.2415.3377.60  7.0714653B. & W. Co.
W. Va.McDowellPocahontasFlat TopLump  3.6316.0378.04  5.9314956B. & W. Co.
StateCountyField, Bed
or Vein
MineSizeProximate Analysis (Dry Coal)B. t. u.
Per
Pound
Dry
Coal
Authority
MoistureVolatile
Matter
Fixed
Carbon
Ash
BITUMINOUS 
Ala.BibbCahabaHill CreekMine run  6.1928.5855.6015.8212576B. & W. Co.
Ala.JeffersonPrattPratt No. 13   4.2925.7867.68  6.5414482B. & W. Co.
Ala.JeffersonPrattWarnerMine run  2.5127.8061.5010.7013628U. S. Geo. S.
Ala.Jefferson CoalburgMine run  0.9431.3465.65  3.0114513B. & W. Co.
Ala.WalkerHorse CreekIvy C. & I. Co. No. 8Nut  2.5631.8253.8914.2912937U. S. Geo. S.
Ala.WalkerJaggerGalloway C. Co. No. 5Mine run  4.8334.6551.1214.0312976U. S. Geo. S.
Ark.FranklinDenningWestern No. 4Nut  2.2212.8375.3511.82 U. S. Geo. S.
Ark.SebastianJenny LindMine No. 12Lump  1.0717.0474.45  8.5114252U. S. Geo. S.
Ark.SebastianHuntingtonCherokeeMine run  0.9719.8770.30  9.8314159U. S. Geo. S.
Col.BoulderSouth PlatteLafayetteMine run19.4838.8049.0012.2011939B. & W. Co.
Col.BoulderLaramieSimsonMine run19.7844.6948.62  6.6912577U. S. Geo. S.
Col.FremontCanon CityChandlerNut and Slack  9.3738.1051.7510.1511850B. & W. Co.
Col.Las AnimasTrinidadHastingsNut  2.1531.0753.4015.5312547B. & W. Co.
Col.Las AnimasTrinidadMoreleySlack  1.8828.4755.5815.9512703B. & W. Co.
Col.RouttYampaOak Creek   6.6742.9155.64  1.45 Hill
Ill.ChristianPanaPenwell Col.Lump  8.0543.6749.97  6.3610900Jones
Ill.FranklinNo. 6BentonEgg  8.3134.5254.0511.4311727U. S. Geo. S.
Ill.FranklinBig MuddyZeigler¾ inch13.2831.9757.3710.6612857U. S. Geo. S.
Ill.JacksonBig Muddy    4.8531.5562.19  6.2611466Breckenridge
Ill.La SalleStreator    8.4041.7651.42  6.8211727Breckenridge
Ill.La SalleStreatorMarseillesMine run12.9843.7349.13  7.1410899B. & W. Co.
Ill.MacoupinNilwoodMine No. 2Screenings13.3434.7544.5520.7010781B. & W. Co.
Ill.MacoupinMt. OliveMine No. 2Mine run13.5441.2846.3012.4210807U. S. Geo. S.
Ill.MadisonBellevilleDonk Bros.Lump13.4738.6948.0713.2412427U. S. Geo. S.
Ill.MadisonGlen Carbon Mine run  9.7838.1851.5210.3011672Bryan
Ill.Marion OdinLump  6.2042.9149.06  8.0311880Breckenridge
Ill.MercerGilchrist Screenings  8.5036.1741.6422.1910497Breckenridge
Ill.MontgomeryPana or No. 5CoffeenMine run11.9334.0549.8516.1010303U. S. Geo. S.
Ill.PeoriaNo. 5Empire 17.6431.9146.1721.9210705B. & W. Co.
Ill.PerryDu QuoinNumber 1Screenings  9.8133.6748.3617.9711229B. & W. Co.
Ill.PerryDu QuoinWillisMine run  7.2233.0653.9712.9711352U. S. Geo. S.[Pg 179] 
Ill.Sangamon PawneeSlack  4.8141.5339.6218.8510220Jones
Ill.St. GlairStandardNigger HollowMine run14.3932.9044.8422.2611059B. & W. Co.
Ill.St. ClairStandardMaryvilleMine run15.7138.1041.1020.8010999B. & W. Co.
Ill.WilliamsonBig MuddyDawsMine run  8.1734.3352.5013.1712643U. S. Geo. S.
Ill.WilliamsonCarterville or No. 7Carterville   4.6635.6556.86  7.4912286Univ. of Ill.
Ill.WilliamsonCarterville or No. 7BurrNut, Pea and Slack11.9133.7055.9010.4012932B. & W. Co.
Ind.BrazilBrazilGartsideBlock  2.8340.0351.97  8.0013375Stillman
Ind.Clay LouiseBlock  0.8339.7052.28  8.0213248Jones
Ind.GreenIsland City Mine run  6.1735.4253.5511.0311916Dearborn
Ind.KnoxVein No. 5TecumsehMine run10.7335.7554.46  9.7912911B. & W. Co.
Ind.ParkeVein No. 6Parke Coal Co.Lump10.7244.0246.33  9.6511767U. S. Geo. S.
Ind.SullivanSullivan No. 6MildredWashed16.5942.1748.44  9.5913377U. S. Geo. S.
Ind.VigoNumber 6FontanetMine run  2.2834.9550.5014.5511920Dearborn
StateCountyField, Bed
or Vein
MineSizeProximate Analysis (Dry Coal)B. t. u.
Per
Pound
Dry
Coal
Authority
MoistureVolatile
Matter
Fixed
Carbon
Ash
Ind.VigoNumber 7Red BirdMine run11.6241.1746.7612.0712740U. S. Geo. S.
IowaAppanooseMysticMine No. 3Lump13.4839.4043.0917.5111678U. S. Geo. S.
IowaLucasLucasInland No. 1Mine run16.0137.8246.2415.9411963U. S. Geo. S.
IowaMarionBig VeinLiberty No. 5Mine run14.8841.5339.6318.8411443U. S. Geo. S.
IowaPolkThird SeamAltoona No. 4Lump12.4441.2740.8617.8711671U. S. Geo. S.
IowaWapelloWapello Lump  8.6936.2343.6820.0911443U. S. Geo. S.
Kan.CherokeeWeir PittsburghSouthwestern Dev. Co.Lump  4.3133.8853.6712.4513144U. S. Geo. S.
Kan.CherokeeCherokee Screenings  6.1635.5646.9017.5410175Jones
Kan.CherokeeCherokee Lump  1.8134.7752.7712.4612557Jones
Kan.LinnBoicourt Lump  4.7436.5947.0716.3410392Jones
Ky.BellStraight CreekStr. Ck. C. & C. Co.Mine run  2.8936.6757.24  6.0914362U. S. Geo. S.
Ky.HopkinsBed No. 9EarlingtonLump  6.8940.3055.16  4.5413381St. Col. Ky.
Ky.HopkinsBed No. 9BarnsleyMine run  7.9240.5348.7010.7713036U. S. Geo. S.
Ky.HopkinsVein No. 14NeboPea and Slack  8.0231.9154.0214.0712448B. & W. Co.
Ky.JohnsonVein No. 1Miller's CreekMine run  5.1238.4658.63  2.9113743U. S. Geo. S.
Ky.MulenburgBed No. 9PiercePea and Slack  9.2233.9452.1813.8812229B. & W. Co.
Ky.Pulaski Greensburg   2.8026.5463.58  9.8814095N. Y. Ed. Co.
Ky.WebsterBed No. 9 Pea and Slack  7.3031.0860.72  8.2013600B. & W. Co.
Ky.Whitley JellicoNut and Slack  3.8231.8258.78  9.4013175B. & W. Co.
Mo.Adair DanforthMine run  9.0030.5546.2623.19  9889B. & W. Co.
Mo.BatesRich HillNew HomeMine run  7.2837.6243.8318.5512109U. S. Geo. S.
Mo.ClayLexingtonMo. City Coal Co. 12.4539.3948.4712.1412875Univ. of Mo.
Mo.LafayetteWaverlyBuckthorn   8.5841.7845.9912.2312735Univ. of Mo.
Mo.LafayetteWaverlyHigbee 10.8431.7255.2912.9912500Univ. of Mo.
Mo.LinnBevierMarceline   9.4536.7252.2011.0813180Univ. of Mo.
Mo.MaconBevierNorthwest Coal Co. 13.0937.8342.9519.2211500U. S. Geo. S.
Mo.MorganMorgan Co.Morgan Co. Coal Co.Mine run12.2445.6947.98  6.3314197U. S. Geo. S.
Mo.PutnamMendottaMendotta No. 8 20.7839.3650.0010.6412602U. S. Geo. S.
N.Mex.McKinleyGallupGibsonPea and Slack12.1736.3151.1712.5212126B. & W. Co.
OhioAthensHocking ValleySunday CreekSlack12.1634.6453.1012.2612214 [Pg 180] 
OhioBelmontPittsburgh No. 8Neff Coal Co.Mine run  5.3138.7852.22  9.0012843U. S. Geo. S.
OhioColumbianaMiddle KittanningPalestine   2.1537.5751.8010.6313370Lord & Haas
OhioCoshoctonMiddle KittanningMorgan RunMine run 41.7645.2413.0013239B. & W. Co.
OhioGuernseyVein No. 7Little Kate   6.1933.0259.96  7.0213634B. & W. Co.
OhioHockingHocking Valley Lump  6.4539.1250.0810.8012700Lord & Haas
OhioHockingHocking Valley    2.6040.8047.6011.6012175Jones
OhioJacksonBrookvilleSuperior Coal Co.Mine run  7.5938.4543.9917.5611704U. S. Geo. S.
OhioJacksonLower KittanningSuperior Coal Co.Mine run  8.9941.4350.06  8.5113113U. S. Geo. S.
OhioJacksonQuakertownWellston   3.3835.2654.18  7.5612506Hill
OhioJeffersonPittsburgh or No. 8Crow Hollow¾ inch  4.0440.0852.27  9.6513374U. S. Geo. S.
OhioJeffersonPittsburgh or No. 8Rush Run No. 1¾ inch  4.7436.0854.81  9.1113532U. S. Geo. S.
OhioPerryHockingCongo   6 4138.3346.7114.9612284B. & W. Co.
OhioStarkMassillon Slack  6.6740.0246.4613.5211860B. & W. Co.
OhioVintonBrookville or No. 4ClarionNut and Slack  2.4742.3850.39  6.2313421U. S. Geo. S.
StateCountyField, Bed
or Vein
MineSizeProximate Analysis (Dry Coal)B. t. u.
Per
Pound
Dry
Coal
Authority
MoistureVolatile
Matter
Fixed
Carbon
Ash
Okla.ChoctawMcAlesterEdwards No. 1Mine run  4.7939.1849.9710.8513005U. S. Geo. S.
Okla.ChoctawMcAlesterAdamsonSlack  4.7228.5458.1713.2912105B. & W. Co.
Okla.Creek HenriettaLump and Slack  7.6536.7750.1413.0912834U. S. Geo. S.
Pa.AlleghenyPittsburgh 3rd Pool Slack  1.7732.0657.1110.8313205Carpenter
Pa.AlleghenyMonongahelaTurtle Creek   1.7536.8553.94  9.2113480Lord & Haas
Pa.AlleghenyPittsburghBertha¾ inch  2.6135.8657.81  6.3313997U. S. Geo. S.
Pa.Cambria Beach CreekSlack  3.0132.8755.8611.2713755B. & W. Co.
Pa.CambriaMillerLincolnMine run  5.3930.8361.05  8.1213600B. & W. Co.
Pa.ClarionLower Freeport    0.5435.9357.66  6.4113547 
Pa.FayetteConnellsville Slack  1.8528.7363.22  7.9513775Whitham
Pa.GreeneYoughiogheny Lump  1.2532.6054.7012.7013100B. & W. Co.
Pa.GreeneWestmoreland Screenings11.1231.6755.6112.7213100P. R. R.
Pa.Indiana IselinMine run  2.7029.3363.56  7.1114220B. & W. Co.
Pa.Jefferson PunxsutawneyMine run  3.3829.3364.93  5.7314781B. & W. Co.
Pa.LawrenceMiddle Kittanning    0.7037.0656.24  6.7013840Lord & Haas
Pa.MercerTaylor    4.1832.1955.5512.2612820B. & W. Co.
Pa.WashingtonPittsburghEllsworth   2.4635.3558.46  6.1914013U. S. Geo. S.
Pa.WashingtonYoughioghenyAnderson¾ inch  1.0039.2954.80  5.9113729Jones
Pa.WestmorelandPittsburghScott HavenLump  4.0632.9159.78  7.3113934B. & W. Co.
Tenn.CampbellJellico    1.8037.7662.12  1.1213846U. S. Navy
Tenn.ClaiborneMingo    4.4034.3159.22  6.47 U. S. Geo. S.
Tenn.Marion Etna   3.1632.9856.5910.43  
Tenn.MorganBrushy Mt.    1.7733.4654.7311.8713824B. & W. Co.
Tenn.ScottGlen Mary No. 4Glen Mary   1.5340.8056.78  2.4214625Ky. State Col.
Tex.Maverick Eagle Pass   5.4233.7344.8921.3810945B. & W. Co.
Tex.Paolo Pinto ThurberMine run  1.9036.0149.0914.9012760B. & W. Co.
Tex.Paolo Pinto StrawnMine run  4.1935.4052.9811.6213202B. & W. Co.
Va.Henrico Gayton   0.8217.1474.92  7.9414363B. & W. Co.
Va.LeeDarbyDarby1½ inch  4.3538.4656.91  4.6313939U. S. Geo. S.[Pg 181] 
Va.LeeMcConnelWilsonMine run  3.3536.3557.88  5.7713931U. S. Geo. S.
Va.WiseUpper BannerCoburn3½ inch  3.0532.6562.73  4.6214470U. S. Geo. S.
Va.Rockingham Clover Hill  31.7757.9810.2513103 
Va.RusselClinchfield    2.0035.7256.12  8.1614200 
Va. MonongahelaBernmont  32.0059.90  8.1013424Carpenter
W. Va.HarrisonPittsburghOceanMine run  2.4739.3552.78  7.8714202U. S. Geo. S.
W. Va.Harrison GirardNut, Pea and Slack 36.6657.49  5.8514548B. & W. Co.
W. Va.KanawhaWinifredeWinifrede   1.0532.7464.38  2.8814111Hill
W. Va.KanawhaKeystoneKeystoneMine run  2.2133.2958.61  8.1014202U. S. Geo. S.
W. Va.LoganIsland Creek Nut and Slack  1.1238.6155.91  5.4814273Hill
W. Va.MarionFairmontKingmont   1.9035.3157.34  7.3514198U. S. Geo. S.
W. Va.MingoThackerMaritime   0.6831.8963.48  4.6314126Hill
W. Va.MingoGlen AlumGlen AlumMine run  3.0233.8159.45  6.7414414U. S. Geo. S.
W. Va.PrestonBakerstown    4.1429.0963.50  7.4114546U. S. Geo. S.
W. Va.PutnamPittsburghBlack BetsyBug dust  7.4132.8453.9613.2012568B. & W. Co.
W. Va.RandolphUpper FreeportCoaltonLump and Nut  2.1129.5759.9310.5013854U. S. Geo. S.
StateCountyField, Bed
or Vein
MineSizeProximate Analysis (Dry Coal)B. t. u.
Per
Pound
Dry
Coal
Authority
MoistureVolatile
Matter
Fixed
Carbon
Ash
LIGNITES AND LIGNITIC COALS 
Col.Boulder Rex 16.0542.1247.97  9.9110678B. & W. Co.
Col.El Paso Curtis 23.2542.1149.38  8.5111090B. & W. Co.
Col.El Paso Pike View 23.7748.7041.47  9.8310629B. & W. Co.
Col.GunnisonSouth PlatteMt. Carbon 20.3846.3847.50  6.12  
Col.Las Animas Acme 16.7447.9044.60  7.50 Col. Sc. of M.
Col. Lehigh  18.3045.2944.6710.04  
N. Dak.McLean EcklandMine run29.6545.5647.05  7.3910553Lord
N. Dak.McLean WiltonLump35.9649.8438.0512.1111036U. S. Geo. S.
N. Dak.McLean Casino 29.6546.5638.7014.74 Lord
N. Dak.StarkLehighLehighMine run35.8443.8439.5916.5710121U. S. Geo. S.
N. Dak.WilliamWilliston Mine run41.7639.3748.0912.5410121B. & W. Co.
N. Dak.WilliamWilliston Mine run42.7440.8347.7911.3810271B. & W. Co.
Tex.BastropBastropGlenham 32.7742.7636.8820.36  8958B. & W. Co.
Tex.HoustonCrockett  23.2740.9538.3720.6810886U. S. Geo. S.
Tex.Houston Houston C. & C. Co. 31.4846.9334.4018.8710176B. & W. Co.
Tex.MilamRockdaleWorley 32.4843.0441.1415.8210021B. & W. Co.
Tex.RobertsonCalvertCoaling No. 1 32.0143.7043.0813.2210753B. & W. Co.
Tex.WoodHoytConsumer's Lig. Co. 33.9846.9741.4011.6310600U. S. Geo. S.
Tex.WoodHoyt  30.2543.2741.4615.2710597 
Wash.King Black Diamond   3.7148.7246.56  4.72 Gale
Wyo.CarbonHanna Mine run  6.4451.3243.00  5.6811607B. & W. Co.
Wyo.CrookBlack HillsStilwell Coal Co. 19.0845.2146.42  8.3712641U. S. Geo. S.
Wyo.SheridanSheridanMonarch 21.1851.8740.43  7.7012316U. S. Geo. S.
Wyo.SweetwaterRock Spring Screenings  7.7038.5756.99  4.4412534B. & W. Co.
Wyo.UintaAdavilleLazeart 19.1545.5048.11  6.39  9868U. S. Geo. S.

[Pg 182]
Illustration:

Portion of 12,080 Horse-power Installation of Babcock & Wilcox Boilers and Superheaters at the Potomac Electric Co., Washington, D. C.

[Pg 183]

TABLE 39

SHOWING RELATION BETWEEN PROXIMATE AND ULTIMATE ANALYSES OF COAL
StateField or BedMineProximate AnalysisUltimate AnalysisCommon in
Proximate
& Ultimate
Analysis
Volatile
Matter
Fixed
Carbon
CarbonHydrogenOxygenNitrogenSulphurAshMoisture
Ala.Horse CreekIcy Coal & Iron Co., No. 831.8153.9072.02  4.78  6.45  1.66    .8014.29  2.56
Ark.HuntingtonCentral C. & C. Co., No. 318.9967.7176.37  3.90  3.71  1.49  1.2313.30  1.99
Ill.Pana or No. 5Clover Leaf, No. 137.2245.6463.04  4.4910.04  1.28  4.0117.1413.19
Ind.No. 5, Warrick Co.Electric41.8544.4568.08  4.78  7.56  1.35  4.5313.70  9.11
Ky.No. 11, Hopkins Co.St. Bernard, No. 1141.1049.6072.22  5.06  8.44  1.33  3.65  9.30  7.76
Pa."B" or Lower KittanningEureka, No. 3116.7177.2284.45  4.25  3.04  1.28    .91  6.07    .56
Pa.Indiana Co. 29.5562.6479.86  5.02  4.27  1.86  1.18  7.81  2.90
W. Va.Fire CreekRush Run22.8771.5683.71  4.64  3.67  1.70    .71  5.57  2.14

Table 39 gives for comparison the ultimate and proximate analyses of certain of the coals with which tests were made in the coal testing plant of the United States Geological Survey at the Louisiana Purchase Exposition at St. Louis.

The heating value of a fuel cannot be directly computed from a proximate analysis, due to the fact that the volatile content varies widely in different fuels in composition and in heating value.

Some methods have been advanced for estimating the calorific value of coals from the proximate analysis. William Kent[38] deducted from Mahler’s tests of European coals the approximate heating value dependent upon the content of fixed carbon in the combustible. The relation as deduced by Kent between the heat and value per pound of combustible and the per cent of fixed carbon referred to combustible is represented graphically by Fig. 23.

Goutal gives another method of determining the heat value from a proximate analysis, in which the carbon is given a fixed value and the heating value of the volatile matter is considered as a function of its percentage referred to combustible. Goutal’s method checks closely with Kent’s determinations.

All the formulae, however, for computing the calorific value of coals from a proximate analysis are ordinarily limited to certain classes of fuels. Mr. Kent, for instance, states that his deductions are correct within a close limit for fuels containing more than 60 per cent of fixed carbon in the combustible, while for those containing a lower percentage, the error may be as great as 4 per cent, either high or low.

While the use of such computations will serve where approximate results only are required, that they are approximate should be thoroughly understood.

Calorimetry—An ultimate or a proximate analysis of a fuel is useful in [Pg 184] determining its general characteristics, and as described on page 183, may be used in the calculation of the approximate heating value. Where the efficiency of a boiler is to be computed, however, this heating value should in all instances be determined accurately by means of a fuel calorimeter.

Graph of Heat Value

Fig. 23. Graphic Representation of Relation between
Heat Value Per Pound of Combustible and
Fixed Carbon in Combustible as Deduced by Wm. Kent.

In such an apparatus the fuel is completely burned and the heat generated by such combustion is absorbed by water, the amount of heat being calculated from the elevation in the temperature of the water. A calorimeter which has been accepted as the best for such work is one in which the fuel is burned in a steel bomb filled with compressed oxygen. The function of the oxygen, which is ordinarily under a pressure of about 25 atmospheres, is to cause the rapid and complete combustion of the fuel sample. The fuel is ignited by means of an electric current, allowance being made for the heat produced by such current, and by the burning of the fuse wire.

A calorimeter of this type which will be found to give satisfactory results is that of M. Pierre Mahler, illustrated in Fig. 24 and consisting of the following parts:

A water jacket A, which maintains constant conditions outside of the calorimeter proper, and thus makes possible a more accurate computation of radiation losses.

The porcelain lined steel bomb B, in which the combustion of the fuel takes place in compressed oxygen.

Mahler Bomb Calorimeter

Fig. 24. Mahler Bomb Calorimeter

[Pg 185]

The platinum pan C, for holding the fuel.

The calorimeter proper D, surrounding the bomb and containing a definite weighed amount of water.

An electrode E, connecting with the fuse wire F, for igniting the fuel placed in the pan C.

A support G, for a water agitator.

A thermometer I, for temperature determination of the water in the calorimeter. The thermometer is best supported by a stand independent of the calorimeter, so that it may not be moved by tremors in the parts of the calorimeter, which would render the making of readings difficult. To obtain accuracy of readings, they should be made through a telescope or eyeglass.

A spring and screw device for revolving the agitator.

A lever L, by the movement of which the agitator is revolved.

A pressure gauge M, for noting the amount of oxygen admitted to the bomb. Between 20 and 25 atmospheres are ordinarily employed.

An oxygen tank O.

A battery or batteries P, the current from which heats the fuse wire used to ignite the fuel.

This or a similar calorimeter is used in the determination of the heat of combustion of solid or liquid fuels. Whatever the fuel to be tested, too much importance cannot be given to the securing of an average sample. Where coal is to be tested, tests should be made from a portion of the dried and pulverized laboratory sample, the methods of obtaining which have been described. In considering the methods of calorimeter determination, the remarks applied to coal are equally applicable to any solid fuel, and such changes in methods as are necessary for liquid fuels will be self-evident from the same description.

Approximately one gram of the pulverized dried coal sample should be placed directly in the pan of the calorimeter. There is some danger in the using of a pulverized sample from the fact that some of it may be blown out of the pan when oxygen is admitted. This may be at least partially overcome by forming about two grams into a briquette by the use of a cylinder equipped with a plunger and a screw press. Such a briquette should be broken and approximately one gram used. If a pulverized sample is used, care should be taken to admit oxygen slowly to prevent blowing the coal out of the pan. The weight of the sample is limited to approximately one gram since the calorimeter is proportioned for the combustion of about this weight when under an oxygen pressure of about 25 atmospheres.

A piece of fine iron wire is connected to the lower end of the plunger to form a fuse for igniting the sample. The weight of iron wire used is determined, and if after combustion a portion has not been burned, the weight of such portion is determined. In placing the sample in the pan, and in adjusting the fuse, the top of the calorimeter is removed. It is then replaced and carefully screwed into place on the bomb by means of a long handled wrench furnished for the purpose.

The bomb is then placed in the calorimeter, which has been filled with a definite amount of water. This weight is the “water equivalent” of the apparatus, i. e., the weight of water, the temperature of which would be increased one degree for an equivalent increase in the temperature of the combined apparatus. It may be determined by calculation from the weights and specific heats of the various parts of [Pg 186] the apparatus. Such a determination is liable to error, however, as the weight of the bomb lining can only be approximated, and a considerable portion of the apparatus is not submerged. Another method of making such a determination is by the adding of definite weights of warm water to definite amounts of cooler water in the calorimeter and taking an average of a number of experiments. The best method for the making of such a determination is probably the burning of a definite amount of resublimed naphthaline whose heat of combustion is known.

The temperature of the water in the water jacket of the calorimeter should be approximately that of the surrounding atmosphere. The temperature of the weighed amount of water in the calorimeter is made by some experimenters slightly greater than that of the surrounding air in order that the initial correction for radiation will be in the same direction as the final correction. Other experimenters start from a temperature the same or slightly lower than the temperature of the room, on the basis that the temperature after combustion will be slightly higher than the room temperature and the radiation correction be either a minimum or entirely eliminated.

While no experiments have been made to show conclusively which of these methods is the better, the latter is generally used.

After the bomb has been placed in the calorimeter, it is filled with oxygen from a tank until the pressure reaches from 20 to 25 atmospheres. The lower pressure will be sufficient in all but exceptional cases. Connection is then made to a current from the dry batteries in series so arranged as to allow completion of the circuit with a switch. The current from a lighting system should not be used for ignition, as there is danger from sparking in burning the fuse, which may effect the results. The apparatus is then ready for the test.

Unquestionably the best method of taking data is by the use of co-ordinate paper and a plotting of the data with temperatures and time intervals as ordinates and abscissae. Such a graphic representation is shown in Fig. 25.

Graph of Calorimeter Results

Fig. 25. Graphic Method of Recording Bomb Calorimeter Results

After the bomb is placed in the calorimeter, and before the coal is ignited, readings of the temperature of the water should be taken at one minute intervals for a period long enough to insure a constant rate of change, and in this way determine the initial radiation. The coal is then ignited by completing the circuit, the temperature at the instant the circuit is closed being considered the temperature at the beginning of the combustion. After ignition the readings should be taken at one-half minute intervals, though because of the rapidity of the mercury’s rise approximate readings only may be possible for at least a minute after the firing, such readings, however, being sufficiently accurate for this period. The one-half minute readings should be taken [Pg 187] after ignition for five minutes, and for, say, five minutes longer at minute intervals to determine accurately the final rate of radiation.

Fig. 25 shows the results of such readings, plotted in accordance with the method suggested. It now remains to compute the results from this plotted data.

The radiation correction is first applied. Probably the most accurate manner of making such correction is by the use of Pfaundler’s method, which is a modification of that of Regnault. This assumes that in starting with an initial rate of radiation, as represented by the inclination of the line AB, Fig. 25, and ending with a final radiation represented by the inclination of the line CD, Fig. 25, that the rate of radiation for the intermediate temperatures between the points B and C are proportional to the initial and final rates. That is, the rate of radiation at a point midway between B and C will be the mean between the initial and final rates; the rate of radiation at a point three-quarters of the distance between B and C would be the rate at B plus three-quarters of the difference in rates at B and C, etc. This method differs from Regnault’s in that the radiation was assumed by Regnault to be in each case proportional to the difference in temperatures between the water of the calorimeter and the surrounding air plus a constant found for each experiment. Pfaundler’s method is more simple than that of Regnault, and the results by the two methods are in practical agreement.

Expressed as a formula, Pfaundler’s method is, though not in form given by him:

C = N
(R + 
R' - R
––––––––––
T' - T
(T" - T))
            (19)
WhereC=correction in degree centigrade,
N=number of intervals over which correction is made,
R=initial radiation in degrees per interval,
R'=final radiation in degrees per interval,
T=average temperature for period through which initial radiation is computed,
T"=average temperature over period of combustion[39],
T'=average temperature over period through which final radiation is computed.[39]

The application of this formula to Fig. 25 is as follows:

As already stated, the temperature at the beginning of combustion is the reading just before the current is turned on, or B in Fig. 25. The point C or the temperature at which combustion is presumably completed, should be taken at a point which falls well within the established final rate of radiation, and not at the maximum temperature that the thermometer indicates in the test, unless it lies on the straight line determining the final radiation. This is due to the fact that in certain instances local conditions will cause the thermometer to read higher than it should during the time that the bomb is transmitting heat to the water rapidly, and at other times the maximum temperature might be lower than that which would be indicated were readings to be taken at intervals of less than one-half minute, i. e., the point of maximum temperature will fall below the line determined by the final rate of radiation. With this understanding AB, Fig. 25, represents the time of initial radiation, BC the time of [Pg 188] combustion, and CD the time of final radiation. Therefore to apply Pfaundler’s correction, formula (19), to the data as represented by Fig. 25.

N = 6, R = 0, R' = .01, T = 20.29, T' = 22.83, 
T" = 
20.29 + 22.54 + 22.84 + 22.88 + 22.87 + 22.86
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
6
 = 22.36
C = 6
(0 + 
.01 - 0
–––––––––––––––––––––
22.85 - 20.29
(22.36 - 20.29))
 = 6 × .008 = .048

Pfaundler’s formula while simple is rather long. Mr. E. H. Peabody has devised a simpler formula with which, under proper conditions, the variation from correction as found by Pfaundler’s method is negligible.

It was noted throughout an extended series of calorimeter tests that the maximum temperature was reached by the thermometer slightly over one minute after the time of firing. If this period between the time of firing and the maximum temperature reported was exactly one minute, the radiation through this period would equal the radiation per one-half minute before firing plus the radiation per one-half minute after the maximum temperature is reached; or, the radiation through the one minute interval would be the average of the radiation per minute before firing and the radiation per minute after the maximum. A plotted chart of temperatures would take the form of a curve of three straight lines (B, C', D) in Fig. 25. Under such conditions, using the notation as in formula (19) the correction would become,

C = 
2R + 2R'
–––––––––––––––
2
 + (N - 2)R', or R + (N - 1)R'            (20)

This formula may be generalized for conditions where the maximum temperature is reached after a period of more than one minute as follows:

Let M = the number of intervals between the time of firing and the maximum temperature. Then the radiation through this period will be an average of the radiation for M intervals before firing and for M intervals after the maximum is recorded, or

C = 
MR + MR'
–––––––––––––––––
2
 + (N - M)R' = 
M
––––
2
R + (N - 
M
––––
2
)R'            (21)

In the case of Mr. Peabody’s deductions M was found to be approximately 2 and formula (21) becomes directly, C = R + (N - 1)R' or formula (20).

The corrections to be made, as secured by the use of this formula, are very close to those secured by Pfaundler’s method, where the point of maximum temperature is not more than five intervals later than the point of firing. Where a longer period than this is indicated in the chart of plotted temperatures, the approximate formula should not be used. As the period between firing and the maximum temperature is increased, the plotted results are further and further away from the theoretical straight line curve. Where this period is not over five intervals, or two and a half minutes, an approximation of the straight line curve may be plotted by eye, and ordinarily the radiation correction to be applied may be determined very closely from such an approximated curve.

Peabody’s approximate formula has been found from a number of tests to give results within .003 degrees Fahrenheit for the limits within which its application holds [Pg 189] good as described. The value of M, which is not necessarily a whole number, should be determined for each test, though in all probability such a value is a constant for any individual calorimeter which is properly operated.

The correction for radiation as found on page 188 is in all instances to be added to the range of temperature between the firing point and the point chosen from which the final radiation is calculated. This corrected range multiplied by the water equivalent of the calorimeter gives the heat of combustion in calories of the coal burned in the calorimeter together with that evolved by the burning of the fuse wire. The heat evolved by the burning of the fuse wire is found from the determination of the actual weight of wire burned and the heat of combustion of one milligram of the wire (1.7 calories), i. e., multiply the weight of wire used by 1.7, the result being in gram calories or the heat required to raise one gram of water one degree centigrade.

Other small corrections to be made are those for the formation of nitric acid and for the combustion of sulphur to sulphuric acid instead of sulphur dioxide, due to the more complete combustion in the presence of oxygen than would be possible in the atmosphere.

To make these corrections the bomb of the calorimeter is carefully washed out with water after each test and the amount of acid determined from titrating this water with a standard solution of ammonia or of caustic soda, all of the acid being assumed to be nitric acid. Each cubic centimeter of the ammonia titrating solution used is equivalent to a correction of 2.65 calories.

As part of acidity is due to the formation of sulphuric acid, a further correction is necessary. In burning sulphuric acid the heat evolved per gram of sulphur is 2230 calories in excess of the heat which would be evolved if the sulphur burned to sulphur dioxide, or 22.3 calories for each per cent of sulphur in the coal. One cubic centimeter of the ammonia solution is equivalent to 0.00286 grams of sulphur as sulphuric acid, or to 0.286 × 22.3 = 6.38 calories. It is evident therefore that after multiplying the number of cubic centimeters used in titrating by the heat factor for nitric acid (2.65) a further correction of 6.38 - 2.65 = 3.73 is necessary for each cubic centimeter used in titrating sulphuric instead of nitric acid. This correction will be 3.730.297 = 13 units for each 0.01 gram of sulphur in the coal.

The total correction therefore for the aqueous nitric and sulphuric acid is found by multiplying the ammonia by 2.65 and adding 13 calories for each 0.01 gram of sulphur in the coal. This total correction is to be deducted from the heat value as found from the corrected range and the amount equivalent to the calorimeter.

After each test the pan in which the coal has been burned must be carefully examined to make sure that all of the sample has undergone complete combustion. The presence of black specks ordinarily indicates unburned coal, and often will be found where the coal contains bone or slate. Where such specks are found the tests should be repeated. In testing any fuel where it is found difficult to completely consume a sample, a weighed amount of naphthaline may be added, the total weight of fuel and naphthaline being approximately one gram. The naphthaline has a known heat of combustion, samples for this purpose being obtainable from the United States Bureau of Standards, and from the combined heat of combustion of the fuel and naphthaline that of the former may be readily computed.

The heat evolved in burning of a definite weight of standard naphthaline may also be used as a means of calibrating the calorimeter as a whole.

FOOTNOTES

[36] See page 161.

[37] U. S. Geological Survey.

[38] See “Steam Boiler Economy”, page 47, First Edition.

[39] To agree with Pfaundler’s formula the end ordinates should be given half values in determining T", i. e., T" = ((Temp. at B + Temp. at C) ÷ 2 + Temp. all other ordinates) ÷ N

[Pg 190]

COMBUSTION OF COAL

The composition of coal varies over such a wide range, and the methods of firing have to be altered so greatly to suit the various coals and the innumerable types of furnaces in which they are burned, that any instructions given for the handling of different fuels must of necessity be of the most general character. For each kind of coal there is some method of firing which will give the best results for each individual set of conditions. General rules can be suggested, but the best results can be obtained only by following such methods as experience and practice show to be the best suited to the specific conditions.

The question of draft is an all important factor. If this be insufficient, proper combustion is impossible, as the suction in the furnace will not be great enough to draw the necessary amount of air through the fuel bed, and the gases may pass off only partially consumed. On the other hand, an excessive draft may cause losses due to the excess quantities of air drawn through holes in the fire. Where coal is burned however, there are rarely complaints from excessive draft, as this can be and should be regulated by the boiler damper to give only the draft necessary for the particular rate of combustion desired. The draft required for various kinds of fuel is treated in detail in the chapter on “Chimneys and Draft”. In this chapter it will be assumed that the draft is at all times ample and that it is regulated to give the best results for each kind of coal.

TABLE 40

ANTHRACITE COAL SIZES
Trade NameRound MeshTesting Segments
Standard Square
Mesh
Through
Inches
Over
Inches
Through
Inches
Over
Inches
Broken412  314  4     234  
Egg314  238  234  2     
Stove238  158  2     138  
Chestnut158    78  138    34  
Pea  78    58    34    12  
No. 1 Buckwheat  58    38    12    14  
No. 2 Buckwheat or Rice  38    316  14    18  
No. 3 Buckwheat or Barley  316  332  18    116

Anthracite—Anthracite coal is ordinarily marketed under the names and sizes given in Table 40.

The larger sizes of anthracite are rarely used for commercial steam generating purposes as the demand for domestic use now limits the supply. In commercial plants the sizes generally found are Nos. 1, 2 and 3 buckwheat. In some plants where the finer sizes are used, a small percentage of bituminous coal, say, 10 per cent, is sometimes mixed with the anthracite and beneficial results secured both in economy and capacity.

Anthracite coal should be fired evenly, in small quantities and at frequent intervals. If this method is not followed, dead spots will appear in the fire, and if the fire gets too irregular through burning in patches, nothing can be done to remedy it until the fire is cleaned as a whole. After this grade of fuel has been fired it should be left alone, and the fire tools used as little as possible. Owing to the difficulty of igniting this fuel, care must be taken in cleaning fires. The intervals of cleaning will, of course, depend upon the nature of the coal and the rate of combustion. With the small sizes and moderately high combustion rates, fires will have to be cleaned twice [Pg 191] on each eight-hour shift. As the fires become dirty the thickness of the fuel bed will increase, until this depth may be 12 or 14 inches just before a cleaning period. In cleaning, the following practice is usually followed: The good coal on the forward half of the grate is pushed to the rear half, and the refuse on the front portion either pulled out or dumped. The good coal is then pulled forward onto the front part of the grate and the refuse on the rear section dumped. The remaining good coal is then spread evenly over the whole grate surface and the fire built up with fresh coal.

A ratio of grate surface to heating surface of 1 to from 35 to 40 will under ordinary conditions develop the rated capacity of a boiler when burning anthracite buckwheat. Where the finer sizes are used, or where overloads are desirable, however, this ratio should preferably be 1 to 25 and a forced blast should be used. Grates 10 feet deep with a slope of 1½ inches to the foot can be handled comfortably with this class of fuel, and grates 12 feet deep with the same slope can be successfully handled. Where grates over 8 feet in depth are necessary, shaking grates or overlapping dumping grates should be used. Dumping grates may be applied either for the whole grate surface or to the rear section. Air openings in the grate bars should be made from 316 inch in width for No. 3 buckwheat to 516 inch for No. 1 buckwheat. It is important that these air openings be uniformly distributed over the whole surface to avoid blowing holes in the fire, and it is for this reason that overlapping grates are recommended.

No air should be admitted over the fire. Steam is sometimes introduced into the ashpit to soften any clinker that may form, but the quantity of steam should be limited to that required for this purpose. The steam that may be used in a steam jet blower for securing blast will in certain instances assist in softening the clinker, but a much greater quantity may be used by such an apparatus than is required for this purpose. Combustion arches sprung above the grates have proved of advantage in maintaining a high furnace temperature and in assisting in the ignition of fresh coal.

Stacks used with forced blast should be of such size as to insure a slight suction in the furnace under any conditions of operation. A blast up to 3 inches of water should be available for the finer sizes supplied by engine driven fans, automatically controlled by the boiler pressure. The blast required will increase as the depth of the fuel bed increases, and the slight suction should be maintained in the furnace by damper regulation.

The use of blast with the finer sizes causes rapid fouling of the heating surfaces of the boiler, the dust often amounting to over 10 per cent of the total fuel fired. Economical disposal of dust and ashes is of the utmost importance in burning fuel of this nature. Provision should be made in the baffling of the boiler to accommodate and dispose of this dust. Whenever conditions permit, the ashes can be economically disposed of by flushing them out with water.

Bituminous Coals—There is no classification of bituminous coal as to size that holds good in all localities. The American Society of Mechanical Engineers suggests the following grading:

Eastern Bituminous Coals
 (A)Run of mine coal; the unscreened coal taken from the mine.
 (B)Lump coal; that which passes over a bar-screen with openings 1¼ inches wide.
 (C) [Pg 192]Nut coal; that which passes through a bar-screen with 1¼-inch openings and over one with ¾-inch openings.
 (D)Slack coal; that which passes through a bar-screen with ¾-inch openings.
Western Bituminous Coals
 (E)Run of mine coal; the unscreened coal taken from the mine.
 (F)Lump coal; divided into 6-inch, 3-inch and 1¼-inch lump, according to the diameter of the circular openings over which the respective grades pass; also 6 × 3-inch lump and 3 × 1¼-inch lump, according as the coal passes through a circular opening having the diameter of the larger figure and over that of the smaller diameter.
 (G)Nut coal; divided into 3-inch steam nut, which passes through an opening 3 inches diameter and over 1¼ inches; 1¼ inch nut, which passes through a 1¼-inch diameter opening and over a ¾-inch diameter opening; ¾-inch nut, which passes through a ¾-inch diameter opening and over a 58-inch diameter opening.
 (H)Screenings; that which passes through a 1¼-inch diameter opening.

As the variation in character of bituminous coals is much greater than in the anthracites, any rules set down for their handling must be the more general. The difficulties in burning bituminous coals with economy and with little or no smoke increases as the content of fixed carbon in the coal decreases. It is their volatile content which causes the difficulties and it is essential that the furnaces be designed to properly handle this portion of the coal. The fixed carbon will take care of itself, provided the volatile matter is properly burned.

Mr. Kent, in his “Steam Boiler Economy”, described the action of bituminous coal after it is fired as follows: “The first thing that the fine fresh coal does is to choke the air spaces existing through the bed of coke, thus shutting off the air supply which is needed to burn the gases produced from the fresh coal. The next thing is a very rapid evaporation of moisture from the coal, a chilling process, which robs the furnace of heat. Next is the formation of water-gas by the chemical reaction, C + H2O = CO + 2H, the steam being decomposed, its oxygen burning the carbon of the coal to carbonic oxide, and the hydrogen being liberated. This reaction takes place when steam is brought in contact with highly heated carbon. This also is a chilling process, absorbing heat from the furnaces. The two valuable fuel gases thus generated would give back all the heat absorbed in their formation if they could be burned, but there is not enough air in the furnace to burn them. Admitting extra air through the fire door at this time will be of no service, for the gases being comparatively cool cannot be burned unless the air is highly heated. After all the moisture has been driven off from the coal, the distillation of hydrocarbons begins, and a considerable portion of them escapes unburned, owing to the deficiency of hot air, and to their being chilled by the relatively cool heating surfaces of the boiler. During all this time great volumes of smoke are escaping from the chimney, together with unburned hydrogen, hydrocarbons, and carbonic oxide, all fuel gases, while at the same time soot is being deposited on the heating surface, diminishing its efficiency in transmitting heat to the water.”

[Pg 193]

To burn these gases distilled from the coal, it is necessary that they be brought into contact with air sufficiently heated to cause them to ignite, that sufficient space be allowed for their mixture with the air, and that sufficient time be allowed for their complete combustion before they strike the boiler heating surfaces, since these surfaces are comparatively cool and will lower the temperature of the gases below their ignition point. The air drawn through the fire by the draft suction is heated in its passage and heat is added by radiation from the hot brick surfaces of the furnace, the air and volatile gases mixing as this increase in temperature is taking place. Thus in most instances is the first requirement fulfilled. The element of space for the proper mixture of the gases with the air, and of time in which combustion is to take place, should be taken care of by sufficiently large combustion chambers.

Certain bituminous coals, owing to their high volatile content, require that the air be heated to a higher temperature than it is possible for it to attain simply in its passage through the fire and by absorption from the side walls of the furnace. Such coals can be burned with the best results under fire brick arches. Such arches increase the temperature of the furnace and in this way maintain the heat that must be present for ignition and complete combustion of the fuels in question. These fuels too, sometimes require additional combustion space, and an extension furnace will give this in addition to the required arches.

As stated, the difficulty of burning bituminous coals successfully will increase with the increase in volatile matter. This percentage of volatile will affect directly the depth of coal bed to be carried and the intervals of firing for the most satisfactory results. The variation in the fuel over such wide ranges makes it impossible to definitely state the thickness of fires for all classes, and experiment with the class of fuel in use is the best method of determining how that particular fuel should be handled. The following suggestions, which are not to be considered in any sense hard and fast rules, may be of service for general operating conditions for hand firing:

Semi-bituminous coals, such as Pocahontas, New River, Clearfield, etc., require fires from 10 to 14 inches thick; fresh coal should be fired at intervals of 10 to 20 minutes and sufficient coal charged at each firing to maintain a uniform thickness. Bituminous coals from Pittsburgh Region require fires from 4 to 6 inches thick, and should be fired often in comparatively small charges. Kentucky, Tennessee, Ohio and Illinois coals require a thickness from 4 to 6 inches. Free burning coals from Rock Springs, Wyoming, require from 6 to 8 inches, while the poorer grades of Montana, Utah and Washington bituminous coals require a depth of about 4 inches.

In general as thin fires are found necessary, the intervals of firing should be made more frequent and the quantity of coal fired at each interval smaller. As thin fires become necessary due to the character of the coal, the tendency to clinker will increase if the thickness be increased over that found to give the best results.

There are two general methods of hand firing: 1st, the spreading method; and 2nd, the coking method.

In the spreading method but little fuel is fired at one time, and is spread evenly over the fuel bed from front to rear. Where there is more than one firing door the doors should be fired alternately. The advantage of alternate firing is the whole surface of the fire is not blanketed with green coal, and steam is generated more uniformly than if all doors were fired at one time. Again, a better combustion results [Pg 194]
[Pg 195]
due to the burning of more of the volatile matter directly after firing than where all doors are fired at one time.

Illustration:

Babcock & Wilcox Chain Grate Stoker

In the coking method, fresh coal is fired at considerable depth at the front of the grate and after it is partially coked it is pushed back into the furnace. The object of such a method is the preserving of a bed of carbon at the rear of the grate, in passing over which the volatile gases driven off from the green coal will be burned. This method is particularly adaptable to a grate in which the gases are made to pass horizontally over the fire. Modern practice for hand firing leans more and more toward the spread firing method. Again the tendency is to work bituminous coal fires less than formerly. A certain amount of slicing and raking may be necessary with either method of firing, but in general, the less the fire is worked the better the results.

Lignites—As the content of volatile matter and moisture in lignite is higher than in bituminous coal, the difficulties encountered in burning them are greater. A large combustion space is required and the best results are obtained where a furnace of the reverberatory type is used, giving the gases a long travel before meeting the tube surfaces. A fuel bed from 4 to 6 inches in depth can be maintained, and the coal should be fired in small quantities by the alternate method. Above certain rates of combustion clinker forms rapidly, and a steam jet in the ashpit for softening this clinker is often desirable. A considerable draft should be available, but it should be carefully regulated by the boiler damper to suit the condition of the fire. Smokelessness with hand firing with this class of fuel is a practical impossibility. It has a strong tendency to foul the heating surfaces rapidly and these surfaces should be cleaned frequently. Shaking grates, intelligently handled, aid in cleaning the fires, but their manipulation must be carefully watched to prevent good coal being lost in the ashpit.

Stokers—The term “automatic stoker” oftentimes conveys the erroneous impression that such an apparatus takes care of itself, and it must be thoroughly understood that any stoker requires expert attention to as high if not higher degree than do hand-fired furnaces.

Stoker-fired furnaces have many advantages over hand firing, but where a stoker installation is contemplated there are many factors to be considered. It is true that stokers feed coal to the fire automatically, but if the coal has first to be fed to the stoker hopper by hand, its automatic advantage is lost. This is as true of the removal of ash from a stoker. In a general way, it may be stated that a stoker installation is not advantageous except possibly for diminishing smoke, unless the automatic feature is carried to the handling of the coal and ash, as where coal and ash handling apparatus is not installed there is no saving in labor. In large plants, however, stokers used in conjunction with the modern methods of coal storage and coal and ash handling, make possible a large labor saving. In small plants the labor saving for stokers over hand-fired furnaces is negligible, and the expense of the installation no less proportionately than in large plants. Stokers are, therefore, advisable in small plants only where the saving in fuel will be large, or where the smoke question is important.

Interest on investment, repairs, depreciation and steam required for blast and stoker drive must all be considered. The upkeep cost will, in general, be higher than for hand-fired furnaces. Stokers, however, make possible the use of cheaper fuels with as high or higher economy than is obtainable under operating conditions in hand-fired furnaces with a better grade of fuel. The better efficiency obtainable with a [Pg 196] good stoker is due to more even and continuous firing as against the intermittent firing of hand-fired furnaces; constant air supply as against a variation in this supply to meet varying furnace conditions in hand-fired furnaces; and the doing away to a great extent with the necessity of working the fires.

Stokers under ordinary operating conditions will give more nearly smokeless combustion than will hand-fired furnaces and for this reason must often be installed regardless of other considerations. While a constant air supply for a given power is theoretically secured by the use of a stoker, and in many instances the draft is automatically governed, the air supply should, nevertheless, be as carefully watched and checked by flue gas analyses as in the case of hand-fired furnaces.

There is a tendency in all stokers to cause the loss of some good fuel or siftings in the ashpit, but suitable arrangements may be made to reclaim this.

In respect to efficiency of combustion, other conditions being equal, there will be no appreciable difference with the different types of stokers, provided that the proper type is used for the grade of fuel to be burned and the conditions of operation to be fulfilled. No stoker will satisfactorily handle all classes of fuel, and in making a selection, care should be taken that the type is suited to the fuel and the operating conditions. A cheap stoker is a poor investment. Only the best stoker suited to the conditions which are to be met should be adopted, for if there is to be a saving, it will more than cover the cost of the best over the cheaper stoker.

Mechanical Stokers are of three general types: 1st, overfeed; 2nd, underfeed; and 3rd, traveling grate. The traveling grate stokers are sometimes classed as overfeed but properly should be classed by themselves as under certain conditions they are of the underfeed rather than the overfeed type.

Overfeed Stokers in general may be divided into two classes, the distinction being in the direction in which the coal is fed relative to the furnaces. In one class the coal is fed into hoppers at the front end of the furnace onto grates with an inclination downward toward the rear of about 45 degrees. These grates are reciprocated, being made to take alternately level and inclined positions and this motion gradually carries the fuel as it is burned toward the rear and bottom of the furnace. At the bottom of the grates flat dumping sections are supplied for completing the combustion and for cleaning. The fuel is partly burned or coked on the upper portion of the grates, the volatile gases driven off in this process for a perfect action being ignited and burned in their passage over the bed of burning carbon lower on the grates, or on becoming mixed with the hot gases in the furnace chamber. In the second class the fuel is fed from the sides of the furnace for its full depth from front to rear onto grates inclined toward the center of the furnace. It is moved by rocking bars and is gradually carried to the bottom and center of the furnace as combustion advances. Here some type of a so-called clinker breaker removes the refuse.

Underfeed Stokers are either horizontal or inclined. The fuel is fed from underneath, either continuously by a screw, or intermittently by plungers. The principle upon which these stokers base their claims for efficiency and smokelessness is that the green fuel is fed under the coked and burning coal, the volatile gases from this fresh fuel being heated and ignited in their passage through the hottest portion of the fire on the top. In the horizontal classes of underfeed stokers, the action of a screw carries the fuel back through a retort from which it passes upward, as the fuel above is consumed, the ash being finally deposited on dead plates on either side of the [Pg 197] retort, from which it can be removed. In the inclined class, the refuse is carried downward to the rear of the furnace where there are dumping plates, as in some of the overfeed types.

Underfeed stokers are ordinarily operated with a forced blast, this in some cases being operated by the same mechanism as the stoker drive, thus automatically meeting the requirements of various combustion rates.

Traveling Grates are of the class best illustrated by chain grate stokers. As implied by the name these consist of endless grates composed of short sections of bars, passing over sprockets at the front and rear of the furnace. Coal is fed by gravity onto the forward end of the grates through suitable hoppers, is ignited under ignition arches and is carried with the grate toward the rear of the furnace as its combustion progresses. When operated properly, the combustion is completed as the fire reaches the end of the grate and the refuse is carried over this rear end by the grate in making the turn over the rear sprocket. In some cases auxiliary dumping grates at the rear of the chain grates are used with success.

Chain grate stokers in general produce less smoke than either overfeed or underfeed types, due to the fact that there are no cleaning periods necessary. Such periods occur with the latter types of stokers at intervals depending upon the character of the fuel used and the rate of combustion. With chain grate stokers the cleaning is continuous and automatic, and no periods occur when smoke will necessarily be produced.

In the earlier forms, chain grates had an objectionable feature in that the admission of large amounts of excess air at the rear of the furnace through the grates was possible. This objection has been largely overcome in recent models by the use of some such device as the bridge wall water box and suitable dampers. A distinct advantage of chain grates over other types is that they can be withdrawn from the furnace for inspection or repairs without interfering in any way with the boiler setting.

This class of stoker is particularly successful in burning low grades of coal running high in ash and volatile matter which can only be burned with difficulty on the other types. The cost of up-keep in a chain grate, properly constructed and operated, is low in comparison with the same cost for other stokers.

The Babcock & Wilcox chain grate is representative of this design of stoker.

Smoke—The question of smoke and smokelessness in burning fuels has recently become a very important factor of the problem of combustion. Cities and communities throughout the country have passed ordinances relative to the quantities of smoke that may be emitted from a stack, and the failure of operators to live up to the requirements of such ordinances, resulting as it does in fines and annoyance, has brought their attention forcibly to the matter.

The whole question of smoke and smokelessness is to a large extent a comparative one. There are any number of plants burning a wide variety of fuels in ordinary hand-fired furnaces, in extension furnaces and on automatic stokers that are operating under service conditions, practically without smoke. It is safe to say, however, that no plant will operate smokelessly under any and all conditions of service, nor is there a plant in which the degree of smokelessness does not depend largely upon the intelligence of the operating force.

[Pg 198]

Boiler and Superheater with Stoker

Fig. 26. Babcock & Wilcox Boiler and Superheater Equipped with Babcock & Wilcox Chain Grate Stoker.
This Setting has been Particularly Successful in Minimizing Smoke

When a condition arises in a boiler room requiring the fires to be brought up quickly, the operatives in handling certain types of stokers will use their slice bars freely to break up the green portion of the fire over the bed of partially burned coal. [Pg 199] In fact, when a load is suddenly thrown on a station the steam pressure can often be maintained only in this way, and such use of the slice bar will cause smoke with the very best type of stoker. In a certain plant using a highly volatile coal and operating boilers equipped with ordinary hand-fired furnaces, extension hand-fired furnaces and stokers, in which the boilers with the different types of furnaces were on separate stacks, a difference in smoke from the different types of furnaces was apparent at light loads, but when a heavy load was thrown on the plant, all three stacks would smoke to the same extent, and it was impossible to judge which type of furnace was on one or the other of the stacks.

In hand-fired furnaces much can be accomplished by proper firing. A combination of the alternate and spreading methods should be used, the coal being fired evenly, quickly, lightly and often, and the fires worked as little as possible. Smoke can be diminished by giving the gases a long travel under the action of heated brickwork before they strike the boiler heating surfaces. Air introduced over the fires and the use of heated arches, etc., for mingling the air with the gases distilled from the coal will also diminish smoke. Extension furnaces will undoubtedly lessen smoke where hand firing is used, due to the increase in length of gas travel and the fact that this travel is partially under heated brickwork. Where hand-fired grates are immediately under the boiler tubes, and a high volatile coal is used, if sufficient combustion space is not provided the volatile gases, distilled as soon as the coal is thrown on the fire, strike the tube surfaces and are cooled below the burning point before they are wholly consumed and pass through as smoke. With an extension furnace, these volatile gases are acted upon by the radiant heat from the extension furnace arch and this heat, together with the added length of travel causes their more complete combustion before striking the heating surfaces than in the former case.

Smoke may be diminished by employing a baffle arrangement which gives the gases a fairly long travel under heated brickwork and by introducing air above the fire. In many cases, however, special furnaces for smoke reduction are installed at the expense of capacity and economy.

From the standpoint of smokelessness, undoubtedly the best results are obtained with a good stoker, properly operated. As stated above, the best stoker will cause smoke under certain conditions. Intelligently handled, however, under ordinary operating conditions, stoker-fired furnaces are much more nearly smokeless than those which are hand fired, and are, to all intents and purposes, smokeless. In practically all stoker installations there enters the element of time for combustion, the volatile gases as they are distilled being acted upon by ignition or other arches before they strike the heating surfaces. In many instances too, stokers are installed with an extension beyond the boiler front, which gives an added length of travel during which, the gases are acted upon by the radiant heat from the ignition or supplementary arches, and here again we see the long travel giving time for the volatile gases to be properly consumed.

To repeat, it must be emphatically borne in mind that the question of smokelessness is largely one of degree, and dependent to an extent much greater than is ordinarily appreciated upon the handling of the fuel and the furnaces by the operators, be these furnaces hand fired or automatically fired.
[Pg 200]

Illustration:

3520 Horse-power Installation of Babcock & Wilcox Boilers at the Portland Railway, Light and Power Co., Portland, Ore. These Boilers are Equipped with Wood Refuse Extension Furnaces at the Front and Oil Burning Furnaces at the Mud Drum End

[Pg 201]

SOLID FUELS OTHER THAN COAL AND THEIR COMBUSTION

Wood —Wood is vegetable tissue which has undergone no geological change. Usually the term is used to designate those compact substances familiarly known as tree trunks and limbs. When newly cut, wood contains moisture varying from 30 per cent to 50 per cent. When dried for a period of about a year in the atmosphere, the moisture content will be reduced to 18 or 20 per cent.

TABLE 41

ULTIMATE ANALYSES AND
CALORIFIC VALUES OF DRY WOOD
(GOTTLIEB)
Kind
of
Wood
C H N O Ash B. t. u.
per
Pound
Oak 50.16 6.02 0.09 43.36 0.37    8316
Ash 49.18 6.27 0.07 43.91 0.57    8480
Elm 48.99 6.20 0.06 44.25 0.50    8510
Beech 49.06 6.11 0.09 44.17 0.57    8391
Birch 48.88 6.06 0.10 44.67 0.29    8586
Fir 50.36 5.92 0.05 43.39 0.28    9063
Pine 50.31 6.20 0.04 43.08 0.37    9153
Poplar 49.37 6.21 0.96 41.60 1.86    7834 [40]
Willow 49.96 5.96 0.96 39.56 3.37    7926 [40]

Wood is usually classified as hard wood, including oak, maple, hickory, birch, walnut and beech; and soft wood, including pine, fir, spruce, elm, chestnut, poplar and willow. Contrary to general opinion, the heat value per pound of soft wood is slightly greater than the same value per pound of hard wood. Table 41 gives the chemical composition and the heat values of the common woods. Ordinarily the heating value of wood is considered equivalent to 0.4 that of bituminous coal. In considering the calorific value of wood as given in this table , it is to be remembered that while this value is based on air-dried wood, the moisture content is still about 20 per cent of the whol