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CHAPTER XXIV.

COMBUSTION

IMPORTANCE OF COAL ECONOMY.
THE coal account of the locomotive department constitutes a very important element in railroad expenditures; it makes a heavy drain upon every railroad in the country. A saving of 15 per cent in the coal account of a railroad might often have been the means of keeping a company solvent that went into the hands of a receiver. A bad fireman generally wastes more than 15 per cent over the quantity of fuel used by a good fireman. We are told that the man who makes two blades of grass grow where one blade used to grow is a benefactor of the human race. As the quantity of coal provided for the use of mankind is limited, and the means of cultivating a fresh supply are not apparent, it would seem that the man who makes one pound of coal do the work that has generally called for the consumption of one and a half pounds is worthy of a share of the admiration accorded to the industrious agriculturist. There are locomotives in the country where the coal consumed, in the generation of steam, is used as economically as knowledge and skill combined can effect, but these cases are not so common as they ought to be. Much has been said and written of late years about proper methods of firing, founded on correct conceptions of the laws that regulate combustion, but a great many of our locomotives continue to be fired in a way that violates Nature's laws, and a senseless waste of coal is the result. The opportunities for firemen mending their ways and earning the distinction of being public benefactors, to say nothing of being better worthy of employment, are innumerable.

There are gratifying evidences that the modern engineer or fireman is striving to acquire the knowledge and the skill that make him thoroughly master of his business. For the help of such men the following chapter has been prepared.

MASTERING THE PRINCIPLES.
To properly comprehend what happens to keep a fire burning, we must understand something about the laws of Nature as they are explained under the science of chemistry. Practical men are generally easily repelled by the strange names which they meet with in reading anything where chemical terms are used. An engineer or fireman who is ambitious to learn the principles of his business ought to attack the hard words with a little courage and perseverance, when it will be found that the difficulties of understanding them will vanish.

SCIENTIFIC FIRING.
A man may become a good fireman without knowing anything about the laws of Nature that control combustion. This frequently happens. If he becomes skillful in making an engine steam freely, while using the least possible supply of fuel, he has learned by practice to put in the coal and to regulate the admission of air in a scientific manner. That is, he puts in the exact quantity of fuel to suit the amount of air that is passing into the fire-box, and in the shape that will cause it to produce the greatest possible amount of heat. When this degree of skill is attained by men, ignorant of Nature's laws, it is attained by groping in the dark to find out the right way. A man who has acquired his skill in this manner is not, however, perfectly master of the art of firing, for any change of furnace arrangement is likely to bewilder him, and he has to find out by repeated trying what method of firing suits best. He is also liable to waste fuel uselessly, or to cause delay by want of steam when anything unusual happens.

KNOWLEDGE IS POWER.
A knowledge of the laws of combustion teaches a man to go straight to the correct method, and the information possessed enables him to deal intelligently with the numerous difficulties which are constantly arising owing to inferior fuel, obstructed draft due to various causes, and to viciously designed fire-boxes and smoke-boxes. To illustrate: Engineer West was pulling a passenger train one day, and his grates got stuck. He ran as far as he could till he could do nothing more for want of steam, then he stopped and cleaned the fire; loss of time over one hour with an important train. Engineer Thomas, on the same road, had a similar experience with the grates; but he understood combustion, and knew that all the fire wanted was air put in so that it would strike the fire before it passed into the flues. He got an old scoop and rigged it in the fire-box door slanting towards the surface of the fire. He did not need to clean the fire, and he went in nearly on time. He could not get air to mix with the fire through the grates, so he devised a plan to inject it above the fire.

ELEMENTS THAT MAKE UP A FIRE.
The nature of fuel, the composition of the air that fans the fire, and the character of the gases formed by the burning fuel, and the proper proportions of air to fuel for producing the greatest degree of heat, are the principal things to be learned in the study of the laws relating to combustion.

All things are composed from about sixty-five elementary substances (this has changed considerably since the writing of this book in 1890), which have combined together to form the immense variety of substances found in and around the globe. A simple substance or element is something out of which nothing else can be got, no matter how finely it may be divided, or to what searching tests it may be subjected. Elements unite together to form compounds, or combine with compounds to form other compound substances. When elements or compounds combine to form new substances, they always do so in fixed proportions by weight; and if there is any excess of any substance present it does not combine, but remains unused. It is important to remember this, as it has a direct bearing upon the economy of fuel. A few of the principal elements are oxygen, hydrogen, nitrogen, carbon, sulphur, iron, copper, mercury, gold, and silver. We will have to deal principally with the four first mentioned.

The elements which perform the most important functions in the act of combustion are oxygen and carbon. Carbon is the fuel, and oxygen is the supporter of combustion. Combustion results from a strong natural tendency that oxygen and carbon have for each other, but they cannot unite freely till they reach a certain high temperature, when they combine very rapidly with violent evolution of light and heat.

FUEL AND ITS COMBINING ELEMENTS.
All the fuel used for steam-making is composed of carbon, or the compounds of carbon and hydrogen. Carbon is the principal element found in trees, and in all woody fiber; and is the fundamental ingredient of all kinds of coal. The ordinary run of American bituminous coal contains from 50 to 80 per cent of fixed carbon, which is the coke, and from 12 to 35 per cent of volatile substances, which burn with a lurid flame, and supply the ingredients of coal gas. These inflammable compounds are known as hydro-carbons, being combinations of hydrogen and carbon. Anthracite coal differs from other coals in the fact that it consists principally of fixed carbon, with but little volatile matter. Good anthracite contains as high as 90 per cent of pure carbon.

All the air required for furnace combustion is taken from the atmosphere, which consists of a mixture of 1 pound of oxygen to 3.35 pounds of nitrogen; or by volume, 1 cubic foot of oxygen to 3.76 cubic feet of nitrogen. Nitrogen is an inert, neutral gas that gives no aid in sustaining life or in promoting combustion; but it passes into the furnace with the oxygen, and has to be heated to the same temperature as the other gases.

SCIENTIFIC MEASUREMENTS.
In treating of combustion it is constantly necessary to speak of measuring gases by weight. How air and other gases can be weighed as if they were sugar or tea seems a puzzle to many men not acquainted with laboratory work; but they must take it for granted that these things are done.

Before dealing with the action of the air on the fuel resting on the grates, we might mention that scientists have devised a scale of measurement of heat, which is just as necessary for the comprehension of combustion as ordinary weights and measures are for mercantile purposes. The amount of heat necessary to raise the temperature of one pound of water, at its greatest density, one degree Fahrenheit is called a heat-unit, or sometimes a thermal unit. This is equivalent in mechanical energy to the power required for raising 772 pounds one foot high. The enormous amount of mechanical energy present in each pound of good coal will be understood from a small calculation. A pound of good coal, properly burned, generates about 14,500 heat-units. Then 14,500 multiplied by 772, the number of foot-pounds in each heat-unit, gives 11,194,000 foot-pounds, which is sufficient energy to raise the weight of one ton more than one mile high. Little more than 10 per cent of this energy is ever utilized by being converted into the work of driving machinery.

APPLYING THE PRINCIPLES OF COMBUSTION TO A FIRE-BOX.
Having mentioned the leading elements that take part in keeping a fire burning, we will now apply the operation to the work done in the fire-box of a locomotive. Let us take a common form of engine, such as that shown in Fig. 35, (chapter 23), with a fire-box 72 x 35 inches, which makes about 17 square feet of grate area. The engine starts with a fairly heavy train, and has to keep up a running speed of 40 miles an hour. To maintain steam for this work the engine burns 60 pounds of coal per mile, which is equal to 2,400 pounds per hour. This requires that about 141 pounds of coal must be burned on each square foot of grate surface every hour, a very rapid rate of combustion, but a rate common enough on many railroads. As shown in the cut referred to, the engine is of the kind most commonly found pulling our passenger trains, which have no other means of admitting air to the fire except through the ash-pan.

HEAT VALUE OF THE PROPER ADMIXTURE OF AIR.
When the air, drawn violently through the grates by the suction of the exhaust, strikes the glowing fuel, the oxygen in the air separates from the nitrogen and combines with the carbon of the coal. It has been mentioned that elements unite in certain fixed proportions. In some cases the same elements will combine in different proportions to form different kinds of products. If the supply of air is so liberal that there is abundance of oxygen for the burning fuel, the carbon will unite in the proportion of 12 parts by weight (one atom) with 32 parts by weight of oxygen (two atoms). This produces carbonic acid, an intensely hot gas, and therefore of great value in steam-making. If, however, the supply of air is restricted and the oxygen scarce, the atom of carbon is contented to grasp one atom of oxygen, and the combination is made at the rate of 12 parts by weight of carbon to 16 parts by weight of oxygen, producing carbonic oxide gas, which is not nearly so hot as carbonic acid gas. It makes a very important difference in the economical use of fuel which of these two gases is formed in the fire.

One pound of carbon uniting with oxygen to form carbonic acid gas generates 14,500 units of heat, or sufficient to raise 85 pounds of water from the tank temperature to the boiling-point. On the other hand, when one pound of carbon unites with oxygen to form carbonic oxide gas, only 4,500 heat-units are generated, or sufficient to raise 26½ pounds of water from the temperature of the tank to the boiling-point. The same quantity of fuel, it must be remembered, is used in both cases, the only difference being that less oxygen is in the fire mixture.

VOLUME OF AIR NEEDED TO FEED A FIRE.
Our engine using 2,400 pounds of coal per hour has to burn 23 pounds per minute on each square foot of grate. A very large volume of air has to pass through the grates to supply all the oxygen necessary to combine with the quantity of coal mentioned. The combining proportions of carbon and oxygen to form carbonic acid being 12 to 32, the combustion of each pound of carbon requires 2-and-two-thirds pounds of oxygen. It takes 4.35 pounds of atmospheric air to supply one pound of oxygen, therefore at the least calculation it will take more than 11½ pounds of air to provide the gas essential to the economical combustion of each pound of coal. But practice has demonstrated that where combustion is rapid, the fuel must be saturated with the air that contains the oxygen, bathed in it as it were; otherwise a large portion of the furnace gases will pass away uncombined with the element that gives them any heating value. So it is estimated that at least 20 pounds of air must be passed through the grates of a locomotive to supply the oxygen for each pound of coal burned. At this rate our engine must draw in 20 x 2-and-one-third = 46.66 pounds of air per minute through every foot of grate area. One pound of air, at ordinary temperature and atmospheric pressure, occupies about 13 cubic feet; so it takes over 600 cubic feet of air to pass every minute through each square foot of grate. This volume of air would be sufficient to fill a cylinder 18 x 24 inches nearly one hundred and seventy times. Or to put it another way, if there were no obstruction to the passage of air through each foot of grate, a trunk of air over 600 feet long has to pass into the fire every minute. As more than half the opening is obstructed by the iron and coal, a column at least 1,200 feet long has to be admitted each minute. With some forms of grates the openings are much more restricted, and consequently the inward rush of air must be faster in proportion.


VELOCITY OF THE FIRE GASES.
There are several practical objections to the air blowing through the grates like a hurricane. The high speed of the gases lifts the smaller particles of the fuel and starts them toward the entrance of the flues, helping to begin the action of spark throwing. Where they find a thin or dead part of the fire, the gases pass in below the igniting temperature, or tend in spots to reduce the heat below the igniting point, and go away unconsumed, at the same time making a cold streak in the fire-box, chilling the flues or other surface touched, and starting leaks and cracks. Then the great volume of air has, under ordinary circumstances, to be heated up to the temperature of the fire-box, and a considerable part of the heat produced from the coal has to be used up doing this before any of it can be utilized in steam making. When a large volume of gas is employed it must be passed through the furnace and tubes at a high velocity, the result being that there is not sufficient time for the heat to be imparted to the water; consequently the gases pass into the stack at a higher temperature than would be the case if the movement of the gases were slower. One can get a good personal illustration of this by passing his hand through the flame of a gas-burner.

A thoughtless remedy so readily tried with locomotives that do not steam freely is the use of smaller nozzles. That produces bad results in two ways. It causes increased back pressure in the cylinders through the restrictions put upon the escape of the steam, thus reducing the power that the engine can exert and causing more steam to be used to perform a given measure of work. It also increases the velocity of the fire-gases with the result that less of the heat is imparted to the water in the boiler.

Our engine is drawing in 600 cubic feet of air per minute through each square foot of grate, that is, 600 x 17 equals 11,200 cubic feet for the whole grate area. The act of combustion is turning 40 pounds of coal per minute into gas, adding about 300 cubic feet more to the volume. This cloud of gas has to pass out through 202 two inch flues that give a total opening Of 485 square inches, equal to 3.36 square feet. The body of gas reduced to this diameter makes a column over 3,400 feet long, so it must pass through at a velocity of at least 3,400 feet per minute.

THREATENED LOSS OF HEAT.
From these figures it will be understood that in firing, loss of heat is threatened from two opposite directions. If there is not enough air admitted, a gas of inferior heating power will be generated, and a waste of heat will take place equal to the difference between 261 pounds of water evaporated by the heat from one pound of coal burned as carbonic oxide, and 85 pounds of water evaporated when the same weight of coal is burned to carbonic acid gas. If the admission of air is greater than what is necessary, heat will be wasted in proportion to the quantity needed to raise the temperature of the superfluous air up to the heat of the furnace. Those who have noted the difference in the fuel needed to heat a small and a large room, thirty or forty degrees, may readily understand the quantity of coal that must be wasted raising about 1,000 degrees the temperature of the blizzard of extra air that is often passing through the fire-box of a locomotive. Then, as has been mentioned, an extra supply of air causes an increased speed of draft, and this prevents the sheets and flues from abstracting as much heat as they would if the speed of the gases were slower.

IGNITING TEMPERATURE OF THE FIRE.
The igniting temperature of the fire has been repeatedly mentioned. Everybody meets daily with illustrations of the fact that fuel will not burn till it has been raised to a certain heat. If you put a piece of wood or coal on the fire it remains unchanged for a time till the temperature at which it combines with oxygen is reached, when it begins to burn. The point of heat at which it begins to burn is called the igniting temperature. Different kinds of fuel have different igniting points. Coal gas does not burn below a red heat of iron, and carbon has a still higher igniting point. If you take a piece of iron, heated dim red, and try to light an illuminating gas jet with it you will not succeed. Increase the heat till the iron approaches orange color, and it will then light the gas. From this it will be learned that the igniting temperature of hydrocarbon gas is about the cherry heat of iron. As the igniting temperature of carbon is still higher, it will be understood that coal must be kept at a higher temperature still to make it burn.

When wood, coal, or gas will not begin to burn outside till they have been raised to the heat mentioned, it may be readily understood that they will not burn in a locomotive fire-box if they are not up to the igniting temperature. As the active portion of the fire is constantly distilling gases from the fuel that rise upwards, and require a high temperature for their combustion, it will readily be seen, that a great waste of heat must happen, when the temperature of any part of the firebox gets so low that the gases pass away unconsumed.

So the fireman ought to make it his business, to see that the fuel in any part of the fire-box is not permitted to fall below the temperature of combustion. It may be said or believed, that the heat in the fire-box is so high that it is always up to the igniting temperature. This would be a mistake. The rush of cold air is so great, that a thin part of the fire readily permits air that is not up to the igniting temperature to pass through, and it chills all the gas it touches. When a heavy charge of coal is thrown into the fire-box, the cold material reduces for a time part of the fire-box below the igniting temperature, and the gases distilled by the hot fire beneath are ruined by the cold place they have to go through above, and they pass into the flues in the shape of worthless smoke and coal gas. The fire-box sheets abstract the heat so quickly, that waste will occur from the fuel close to the sheets, or the gases passing up beside them, getting below the igniting temperature, unless the fireman watches to see that a bright fire is kept up in the vicinity of the sheets.

BURNING ANTHRACITE COAL.
Thus far we have considered principally the conditions met with in burning carbon alone, such as may be encountered in burning coke, or in the firing of anthracite coal burning engines. Anthracite burns more slowly than bituminous coal, and consequently a larger grate area has to be provided, in order that sufficient coal may be burned to keep up the steam required. As cylinders of a given size draw from the boiler the same volume of steam per minute, no matter what kind of coal is used, and as soft coal which burns freely produces about the same quantity of steam per pound consumed as anthracite which burns slowly, means must be devised to make the bard coal burning engine consume the same quantity per minute as the other, and no better way has been found than that of making a large fire-box.

Anthracite coal has to be fired to suit the size of the lumps used. If the coal is in coarse lumps weighing in the neighborhood of eight pounds each, a thick fire must be carried, for the lumps lie so open that the air would pass so freely through that it would chill the firebox. A thin fire of this kind of coal cannot be carried in a locomotive furnace, for the same reason that you cannot keep a fire burning in a small stove with three or four big lumps of hard coal. In firing lump coal of large size, even when a thick fire is carried, constant care has to be exercised to prevent loss of heat from excessive quantities of air passing through holes. There is a constant tendency for air passages to form close to the sheets, and good firemen provide against this by keeping the fire heavier close to the sheets than at other parts. When too much air is admitted through the fire, the tendency is to reduce parts of the fire-box below the igniting temperature with the, results already mentioned.

Firing with large lumps is wasteful both with anthracite and bituminous coal.

When the smaller broken qualities of anthracite coal are used, a very large grate area is necessary, because the fire must be burned thin, and a thin fire will not stand the action of a sharp exhaust unless the blast is divided over a wide area. The man who makes a highly successful fireman of hard coal, whether it be in lumps or of the small quality, is constantly on the lookout for spots where an over supply of air is beginning to work through, and he promptly checks this by applying fresh coal at the proper point.

BURNING BITUMINOUS COAL.
The burning of bituminous coal is a much more complex operation than that of burning anthracite. The volatile gases in this kind of coal contain great heat-generating power, but they are difficult to burn so that none of the heating elements will be lost. Average bituminous coal contains 65 per cent of carbon and 25 per cent of hydrocarbons. About ¼ by weight of the latter is hydrogen gas, which makes the hottest fire that can be burned; but it ignites only at a very high temperature, as has been alluded to, and if the fire-box or any part of it gets cooler than this, all or a part of the gas passes away unconsumed. In that case there is direct loss by the gas not being used to create heat, and also loss due to the work done by the burning carbon in gasifying the hydrocarbons. To turn a solid into a gas uses up heat in the same way that evaporating water into steam does.

To burn, hydrogen gas unites in the proportion of two parts by weight (two atoms) to sixteen parts by weight of oxygen (one atom), and the product is water. It may appear strange that water is formed by the burning of a fire; but such is the case, and a tremendous heat is evolved by the operation. The water passes away in the form of colorless steam; but when it touches a cool place the vapor instantly condenses into water. When a fire is newly lighted in the boiler of a locomotive the drops of water that may be seen oozing out of the smoke-box joints is the water formed from the hydrogen of the fuel.

HEAT VALUE OF THE VOLATILE GASES.
The combustion of each pound of hydrogen gas, if it combines with eight pounds of oxygen taken from the air, produces about 62,000 heat-units, or enough to raise about 365 pounds of water from the tank temperature to the boiling-point. It will be noted that one pound of hydrogen calls for eight pounds of oxygen (2 to 16) for perfect combustion, while each pound of carbon requires only 2q pounds of oxygen (12 to 32). As the hydrocarbon gases are released at the top of the fire, it is difficult getting this very large volume of air needed for combustion to the proper place, unless means are taken for admitting air above the fire.

Where there is much volatile gas in the coal, it is an economical arrangement to admit air above the fuel; but the means of its admission ought to be under the control of the fireman, or there is likely to be loss of heat by the ingress of cold air when it is not needed.

It is important in the economical combustion of coal, to keep the fire as bright on the top as possible. Experimenters on combustion have found that "the efficiency of fuel to heat by radiation, depends directly upon the luminosity of the products of combustion." That means, that a smoky or cloudy fire wastes a great part of the heat, because the heat rays cannot strike the heating surfaces. The "luminosity" or brightness of the flames of a fire, is said to be due to the free carbon liberated by the hydrocarbons of the flame being heated up to the temperature of the flame itself. The solid particles becoming incandescent, act like tiny incandescent gas-lights, each particle of free carbon throwing off heat and light in all directions until consumed and converted into carbonic acid gas. This free carbon is the last component of the flame to burn, and it only burns at a very high temperature; so if the fire-box is not maintained very hot there will be little bright flame, the volatile gases will pass off as smoke, and those burned will lose part of their value through not being able to send through the mist of smoke their steam-making rays.

HEAT LOSSES THAT RESULT FROM BAD FIRING.
Our engine is laboring along with a heavy, thick fire on the grates. The air that passes up into the fire has the atoms of oxygen seized on by the glowing carbon first encountered, and the heat generated keeps distilling the hydrocarbon gas from the green coal above. There being no means of admitting air above the fire, and there being very little oxygen left in the air, after it worked up through the body of the burning fuel, the volatile gases fail to receive their supply of oxygen, and with their great steam-making possibilities, they pass away in the form of worthless smoke and unconsumed coal gas. The fire being so thick and compact that the air cannot diffuse freely through the mass, a considerable part of the solid carbon does not receive its full share of oxygen, so it passes away in the inferior heating condition of carbonic oxide.

An inferior fireman, who maintains a thick fire, will often use up an enormous quantity of coal without making an engine steam freely. This is caused by the air failing to reach the 25 per cent of the fuel that exists as hydrocarbons, and which is in consequence utterly wasted; and because part of the solid carbon is burned to carbonic oxide, which produces 4,500 heatunits, as compared with 14,5oo heat-units that would result from the carbon being consumed as carbonic acid gas. A fire run in this wasteful manner is always smoky, and the fire-box looks dull and cloudy, with a tendency for the sheets to hold a covering of soot. Other losses due to a smoky fire have already been explained.

Some firemen have acquired the habit of firing at times when the fire-door ought to be kept closed. As soon as the engineer opens the throttle to pull out of a station, these men begin filling up the fire-box. Cold air is pumped through the flues without any need for it, and the charge of fresh coal put in at the wrong time helps add to the chilling effect. When approaching a heavy pull these men generally let the fire get thin, and then they are ready to begin shoveling industriously when the engine is toiling hard up the grade.

EFFECT OF SMALL NOZZLES.
Thick, heavy firing, with all the losses described, is not always caused by ignorance or want of skill on the part of the fireman. It is very frequently the case that an engine will not steam freely unless a heavy fire is carried. This state of things is nearly always due to the use of very small nozzles, which make the blast so sharp that a thin fire could not be used, as the fierce rush of air would be constantly tearing holes in places through which the cold air would pass directly into the flues. When an engine does not steam freely, the tendency always is to call for smaller nozzles; yet it often happens that the nozzles are already too small for free steaming. The diverse character of the coal supplied on most roads is responsible for great waste of fuel. With the average coal an engine will steam while using a large nozzle. But occasionally some cars of coal will be sent in that contains a large percentage of slate and other incombustible material. When an engine gets a tenderful of this stuff, there will be trouble in making steam freely enough to take the train along on time. The men know that a sharp blast would help them in such a case, and it is natural that they should be ready always to provide against this emergency.

BOILER-DESIGNING.
The mistakes and prejudices of enginemen often lead to the use of extravagantly small nozzles; but what in most cases makes the use of small nozzles necessary is badly proportioned locomotives. Where the cylinders are too large for the boiler, or where the fire-box is badly proportioned, the defect must be overcome by employing small nozzles.

For burning bituminous coal economically, means should be provided for regulating the supply of air above and below the fire, the same to be under control of the fireman. The dampers should also be so constructed that the supply of air through the grates could be regulated to suit the needs of the fire. A light fire could often be carried if the fireman could restrict the air to the exact volume wanted. If greater attention were directed to this part of locomotive construction, firemen would feel more encouraged to find out what supply of air best suited a fire for the economical combustion of coal.

A good brick arch when properly cared for is a very valuable aid to economical combustion. The great mass of hot brick helps to maintain the temperature of the fire-box even, and is often the means of raising gases to the igniting temperature before they pass into the flues. Projected as it is into the middle of the fire-box, it lengthens the journey of part of the fire-gases and acts as a mixer of the elements that must combine to effect combustion.

ORDINARY FIRING.
Suppose the engine we are dealing with has the best devised and well-adjusted draft appliances, and it is practicable to carry a medium light fire, even when doing fairly heavy work. When the fireman throws a shovelful of coal upon the fire, the effect is similar to that of pouring a dipperful of cold water into a boiling kettle: it reduces the temperature of the fire-box. A small quantity of cold water does not check the boiling of a kettle much, and one or two shovelfuls of coal is little felt on a big bright fire. When the coal newly thrown into the fire-box strikes the incandescent fire, part of the heat of the latter is absorbed, raising the temperature of the fresh fuel to the igniting point. This takes just an instant, and the hydrocarbons of the coal are released. If the necessary oxygen is present, the gases combine and give forth the intense heat due to their perfect combustion. When the fire is kept thin, and firing is done at frequent intervals, the hydrocarbons are fairly well utilized, and the engine steams well with a small supply of coal, and there is little smoke produced. If, instead of throwing one or two shovelfuls of coal into the fire at one time, the fireman throws in ten or a dozen shovelfuls, the great mass of cold coal reduces the surface of the fire below the igniting temperature of gas, and all the gases of the fresh coal that are expelled by the hot fire beneath pass through the flues without doing any good; and the heat of the bright burning coal that has been expended turning the green coal into gas is wasted.

The most common way of firing soft-coal engines is, to carry as heavy a fire as the engine will steam with. At each firing six or seven shovelfuls of coal are thrown into the fire-box. When the door is closed a stream of black smoke keeps pouring out of the stack, which tells that the hydrocarbons of the fuel are passing away without being burned—a source of heat-loss due to the want of oxygen at the point where the gases are distilled, or through spots of the fire being below the igniting temperature. The fireman watches the smoke-stack, and as soon as he finds the current of smoke begins to get clear, he concludes that it is time to begin firing again, so he pitches in another heavy charge of green coal, and the blackness of the smoke is kept up.

In most cases this method of firing is pursued because it requires the least amount of personal exertion and attention. More coal is burned than would be used with good, skillful firing, but very little attention has to be bestowed upon the work done.

GOOD FIRING.
Firemen who are anxious to do their work well, and who know something about what good firing means, keep up steam after a different method. They keep sufficient fire on the grates to suit the way the engine is working, and enough to prevent loss from air passing up so freely as to reduce the temperature of the firebox. They keep up the fire by throwing in a shovelful or two of coal at short intervals; and the result is that the greater portion of the hydrocarbon gases is burned, and very little smoke is seen issuing from the stack. When the engine is stopped at a station or any other place, the fireman has planned ahead to have a fire in ready for the start. When the train is pulling out he is not found tumbling in the coal as fast as the scoop can transfer it from the tender. He is quietly looking out for signals and switches, and when the engineer hooks up the links and the pull of the exhaust begins to get light, he begins to replenish the fire. This style of firing is the way to make steam with the least possible consumption of coal, and a man who follows the practice does his work in a first-class workmanlike manner. Although he replenishes and keeps up his fire at stations and stopping-places, this fireman does not make his engine a nuisance to the people by pouring out a cloud of black smoke. He prevents this by never putting in a heavy charge of coal at one time. This enables him to maintain a flame on top of the fire, which consumes the gases that would make smoke. When it is necessary to put in considerable coal while standing at a station, he closes the dampers, opens the fire-door slightly, and starts the blower lightly. By this exercise of care and intelligence he makes his locomotive a light consumer of coal and a perfect consumer of smoke.


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