or for the sake of even figures and a quantity which can easily be remembered, we will say 150 CUBIC FEET OF AIR ARE NEEDED FOR THE COMBUSTION OF EACH POUND OF COAL. This is the theoretical quantity of air which is needed for combustion. Now, unfortunately, the process of combustion in the fire-boxes of locomotives is one in which any very exact combination of the substances which unite is not possible with the appliances which are now employed. If, therefore, we admitted the exact amount of air given above, while some portions of the fire where combustion was not very active might have more air than is needed, other portions would have too little; and if the air is not very thoroughly mixed, the flame and burning coal may be surrounded with the products of combustion, which would exclude the air and thus reduce its effect upon the fire. For this reason, besides the air required to furnish the oxygen necessary for the complete combustion of the fuel, it is also necessary to furnish an additional quantity of air for the dilution of the gaseous products of combustion, which would otherwise prevent the free access of air to the fuel. The more minute the division and the greater the velocity with which the air rushes among the fuel, the smaller is the additional quantity of air required for dilution. In locomotive boilers, although this quantity has not been exactly ascertained, there is reason to believe that it may on an average be estimated at about one-half of the air required for combustion.[96] We would therefore have as the quantity of air needed for combustion

This estimate is roughly made, but it is the nearest approximation at present attainable. It is probable that the supply of air required for dilution varies considerably in different arrangements of the fire-box and for different kinds of fuel, and it is possible that by admitting the air for combustion in small enough jets, and deflecting the currents of smoke and gases so as to cause them to mingle with the air, the quantity required for dilution might be reduced below that indicated by the above calculation. Undoubtedly all the air which is admitted into the fire-box which does not combine with the chemical elements of the fuel lessens the amount of steam generated in the boiler, both with reference to time, that is to say per minute, and to fuel, that is per pound of coal consumed. But with the present locomotive boiler it is simply a choice of two evils. If no more air is admitted than theory indicates to be needed for combustion, then, owing to the imperfect means which are usually employed to cause the air and fuel to combine, a portion of the latter will escape unconsumed; and if more air is admitted, the temperature of the products of combustion is lowered and their volume increased, the evils of which have already been pointed out. It therefore becomes a matter in which we are obliged to consult experience and determine by experiment what amount of air it is necessary to admit to the fuel to produce the most economical results.

Question 395. What proportion of the air should be admitted through the grate, and how much above the fire?

Answer. This, too, is a question which can probably be answered best by consulting experience. The relative quantity of air required above and below the fire depends very much on the nature of the fuel. Coal which “runs together” or cakes very much or has a great deal of clinker in it, doubtless, will need more air above the fire than other coal which is said to be “dryer,” for the reason that it will be found impossible to admit so much air through the caking coal in the grate as through the other kind. An idea of the relative quantity which should be admitted above and below the fire may be found if we know how much air is needed to burn the solid carbon or coke which is left after the gas is expelled from it, and how much for the gas itself. The gas which is expelled from a pound of coal consists of about 0.05 lb. of hydrogen and 0.15 lb. of carbon. Now, it has been shown that hydrogen requires 36 times its weight of air to burn it perfectly, so that 0.05 lb. would need 0.05 × 36 = 1.8 lbs.; and carbon requires 12 times its weight of air, so that for 0.15 lb. of carbon 0.15 × 12 = 1.8 lbs. is needed, so that for both 3.6 lbs. of air is required for perfect combustion. As has been shown, 12 lbs. is needed to consume the whole of the fuel, so that 30 per cent. of the whole supply is required for the combustion of the gas alone. If this is diluted in the same proportion as that required for the combustion of the carbon, and it probably should be even more so, we would have 30 per cent. of 225 = 67.5 cubic feet of air required for the combustion of the gas. It is certain, however, that the solid coke on the grates is not perfectly consumed, or, in other words, converted into carbonic dioxide, especially when the layer of it on the grates is very thick. When this is the case the air coming in contact with the lower layer of coke forms carbonic dioxide, but as it rises through the burning coke another equivalent of carbon unites with the carbonic dioxide, and thus forms carbonic oxide. If, now, enough air is admitted above the fire, this carbonic oxide will combine with it, and, as has been explained before, a second combustion will take place if there is time and opportunity for combination before the gases enter the flues. It is therefore probable that more than 30 per cent. of the whole supply of air should be admitted above the fire. It is at any rate best to provide the means for admitting more, and also appliances for regulating the supply, so that it can be governed as experience may indicate to be best.

Question 396. Is it not possible by enlarging the grate to admit enough air to the fire to produce perfect combustion?

Answer. Yes; when no air is admitted above the fire, large grates are found to produce the best combustion. But while it is true that the same amount of heat will be produced by the union of each equivalent of oxygen and fuel, yet if we can force more air and fuel to unite in the same place, a higher temperature is produced in that place, just as a fire in a blacksmith’s forge is hotter because of the forced blast than that in an ordinary stove, or a smelting furnace than a parlor grate. If, then, we can concentrate the draft in the fire of a locomotive, we secure a greater intensity of combustion; and when the air is urged against the solid carbon with considerable force, it comes in contact with every point of its surface, and therefore less dilution of the air is needed, and consequently the products of combustion have a higher temperature; and, as has been explained, a larger proportion of the heat is then transferred to the water than if the temperature is lower and the volume greater.

Intensity of combustion also has the effect of maintaining an igniting temperature; whereas, if the same amount of fuel is burned slowly, its heat may not be high enough to ignite the gases as they are produced.

It is desirable, however, to have all the space that is possible in the fire-box, so as to give room for the mixing of the gases; but with a large fire-box and large grate a decided improvement and economy will often result by diminishing the effective area of the grate by covering a part of it with dead plates, but at the same time making provision for the admission of air above the fire.

Question 397. What is meant by the “Total Heat of Combustion?”

Answer. It is the number of units of heat given out by the combustion of a given quantity (usually a pound) of fuel.

Question 398. How is this determined?

Answer. The heat given out by the combustion of one pound of the chemical elements of which coal is composed has been determined by experiment, and from such data, knowing the substances of which fuel is composed, we can determine the amount of heat which would be developed if they were each perfectly consumed. Thus the total heat of combustion of one pound of hydrogen is 62,032 units, and of the same quantity of carbon 14,500 units.[97] Therefore, if a pound of coal contains 5 per cent. of hydrogen, the heat given out by the combustion of that element will be 62,032 × 0.05 = 3,101.60 units, and if it has 80 per cent. of carbon, the combustion of the latter would develop 14,500 × 0.80 = 11,600 units, so that the total heat of the combustion of these two elements would be 3,101.6 + 11,600 = 14,701.6 units. It was shown in answer to Question 40 that it required 1,213.4 units of heat to convert water at zero to steam of 100 pounds pressure. As steam is usually generated from water at a temperature of about 60 degrees, the total heat required to convert it into steam of 100 pounds pressure would be 1,213.4 - 60 = 1,153.4 units. A pound of average bituminous coal, therefore, contains heat enough to convert 12¹⁄₄ lbs. of water into steam of 100 lbs. absolute pressure. Ordinarily only about half that amount of water is evaporated in locomotive boilers per pound of fuel.

Question 399. What are the chief causes of this waste of heat?

Answer. It is due, first, to the waste of unburnt fuel in the solid state. This occurs when fuel which is very fine falls through the grates, or is carried through the tubes and out of the stack in the form of cinders.[98]

Second, to the waste of unburnt fuel in the gaseous or smoky state. The method of preventing this waste by a sufficient supply and proper distribution of air has been explained in the answer to preceding questions.

Third, to the waste or loss of heat in the hot gases which escape up the chimney or smoke stack. The temperature of the fire in a locomotive fire-box in a state of active combustion is probably from 3,000 to 4,000 degrees. This heat is in part radiated and conducted to the heating surface of the fire-box, and it is found that more water is evaporated by this portion of the heating surface in proportion to its area than by any other in the boiler. The gases when they enter the tubes transmit a portion of their heat to the surfaces with which they are first in contact. The amount of heat thus transmitted, as has been stated, is in proportion to the difference in temperature of the gases inside the tubes and that of the water outside. After passing over the part of the tube with which the gases are first in contact, they then arrive at another portion of the tube surface with a diminished temperature, and the rate of conduction is therefore diminished; so that each successive equal portion of the heating surface transmits a less and less quantity of heat, until the hot air at last leaves the heating surface and escapes up the chimney with a certain remaining excess of temperature above that of the water in the boiler, the heat corresponding to which excess is wasted.[99] It is, therefore, desirable to extract as much heat as possible from the gases before they escape from the tubes. Now it will be impossible to heat the water outside of the tubes hotter than the gases inside. When the temperature of the water is equal to that of the gases, no more heat will be transmitted from one to the other. If the temperature of the water is 350 degrees, that of the gases in the tubes will never be any lower, but will escape into the smoke-box with not less than that amount of heat. If, however, the cold water is introduced at the front end of the tubes, so that the surface with which the gases are last in contact has a temperature considerably lower than 350, then an additional amount of heat will be transmitted before they escape. It is, therefore, important that the cold feed-water should be admitted near the front end of the boiler, so that the products of combustion will be in contact with the coldest part of the heating surface last, and thus give out as much of their heat as possible before they escape. As a matter of fact, the gases escape at a much higher temperature. Experiments made by the writer showed that the temperature in the smoke-box of a locomotive when first starting was 270 degrees, and when working at its maximum capacity on a steep grade and with a heavy train it was as high as 675 degrees. The average temperature while running was, in three trials on different parts of the road, as follows:

In making these experiments a record was made of the indications of a pyrometer and of the steam gauge once every minute while the engine was running. The distance run was 19 miles for the first experiment, 13 for the second and 6 for the third, with 30 loaded freight cars in the train. The last experiment was made while the engine was working on a heavy grade and very nearly up to its maximum capacity.

It will thus be seen that a great deal of heat is wasted by escaping up the chimney.

Fourth, by external radiation from the boiler. This occurs chiefly from the fact that it is not sufficiently well protected or covered with non-conducting material. The practice, or rather the neglect, of not covering the outside of the fire-box with lagging doubtless causes a very considerable loss of heat by radiation and convection from the hot boiler plates.

Question 400. What is the ordinary form of fire-box employed for burning bituminous coal?

Answer. It is that represented in plate II and figs. 41 and 44, and is simply a rectangular box, and for that reason it is called a plain fire-box. Sometimes provision is made for admitting air into such fire-boxes through hollow or rather tubular stay-bolts, which are put into the sides and front. In most cases, too, the fire-box door has perforations for admitting air.

Question 401. What other appliances are used for burning bituminous coal?

Answer. The most common appliance which is added to the plain fire-box is what is called a fire-brick arch. This is shown in fig. 214. B C is the arch which, as its name implies, is formed of fire-brick and extends backward and upward from a point in the tube-sheet below the tubes. In order to be self-supporting it is built in the form of an arch, the two sides of the fire-box acting as abutments for its support. The engraving represents with sufficient clearness the direction of the flames and smoke. These must take a more circuitous “run,” as it is called, after leaving the fire, in order to reach the tubes. Time is thus given for the gases to combine and combustion to take place. The fire-brick becomes heated, and thus to some extent prevents the gases from being cooled down below an igniting temperature by contact with the cold surface of the fire-box before combustion is complete. The fire-brick, however, soon burns out, and must be replaced, but owing to its cheapness and the ease with which it can be removed, this does not make a serious objection to its use. Air is nearly always admitted above the fire when the brick arch is used, either by tubular stay-bolts, a, a, a, or perforations in the door, or both.

In order to avoid the inconvenience and expense of replacing the fire-brick arch, what is known as the “Jauriet water-table” has been extensively used on some roads. This is the invention of Mr. C. F. Jauriet and is represented in fig. 215, and consists of a flat “table,” B C, formed of two boiler plates placed about 4¹⁄₂ in. apart, with the space between filled with water. The two plates are stayed with ordinary stay-bolts in the same way as the sides of the fire-box. The form of the water-table is similar to that of the fire-brick, excepting that it is not arched, this form not being necessary, as the plates are riveted to the sides of the fire-box. Air is admitted above the fire both by hollow stay-bolts and holes in the door, as shown at A. Tubes, f, are put into the front and lower portion of the water-table to allow the ashes and cinders, which would otherwise be deposited above, to fall down on the grates.

When air is admitted at the furnace door of an ordinary fire-box, it is very apt to rush directly into the tubes without mingling with the gases. It was found by some of the firemen on English railroads that by placing a shovel over the top of the furnace door, the current of air which entered could thus be deflected downward, and in this way smoke could be almost entirely prevented. This led to the adoption of a hood or deflector, A, fig. 216, which is made of sheet iron and is placed over the fire-box door and is arranged with a lever, B, so that it can be raised in order to be out of the way when coal is thrown on the fire. It is suspended from a hook, C, from which it can easily be detached and taken out for repairs. This is frequently necessary, as the intense heat of the fire-box burns away the sheet iron of which it is made very rapidly. It can be made of old boiler plate, so that the expense of renewal is very slight. When this plan is used, a double sliding door, shown in fig. 217, is commonly used with it. These doors are opened by the levers f d; and e g, which are all connected together. With these sliding doors the opening for the admission of air can easily be regulated, and the opening through which the lever, B, is attached to the deflector, A, can be arranged more conveniently than with a swinging door. This plan has been employed by the Rogers Locomotive Works.

Another plan of fire-box, which was designed and patented by Mr. William Buchanan, Master Mechanic of the Hudson River Railroad, and used extensively on that line, is shown in fig. 218. This consists of a water-table, but it extends completely across the fire-box from the tube sheet to the back-plate, thus dividing the fire-box into two compartments, M and N. In order to afford communication from the lower one to the upper one a round hole, D, about 24 in. in diameter, is put in the water-table in the position shown. It will thus be seen that all the currents of gas, smoke and air must unite in passing through this opening, and are thus brought into close contact with each other. After they enter the upper chamber and before they enter the tubes, there is room and time for combustion. The position of the lower side of the table, it will be seen, is similar to that of the deflector shown in fig. 216, so that it acts in somewhat the same way, by directing the currents of air, which enter through the furnace door, downward on the fire.

Question 402. How do these different plans operate?

Answer. They will all burn coal more perfectly, and therefore more economically, if they are carefully and skillfully managed, than is possible in ordinary plain fire-boxes; but it is probable that more economy in the consumption of coal would result from the improvement of the practice and knowledge of firemen than can be expected from the use of any of the appliances described, if they are used without care, or knowledge of the principles of combustion.

Question 403. In what respect does anthracite coal differ from bituminous?

Answer. It differs chiefly in the fact that it contains a much larger proportion of carbon and less of hydrogen, and in the fact that it consequently gives off very little or no coal gas. Its combustion is therefore more simple than that of bituminous coal, as there is very little else than solid carbon to burn.

Question 404. In what kind of a fire-box is anthracite usually burned?

Answer. It is usually burned in a very long grate, and as the heat is very intense, the grate-bars are usually made of iron tubes, through which a current of water circulates, so as to prevent them from melting.

Question 405. Is it important to admit air above an anthracite fire to facilitate combustion?

Answer. As there are no gases to be burned, it is not so important as it is with bituminous coal, but if the layer of anthracite on the grates is very thick, it will be impossible to get enough air through the coal to convert all the carbon into carbonic dioxide, and the carbon and oxygen will therefore unite so as to form carbonic oxide. If air is admitted above the fire, as has already been explained, another equivalent of oxygen will unite with the carbonic oxide, and a second combustion will then take place above the fire, and the carbonic oxide will thus be converted into carbonic dioxide. If, under these circumstances, no air was admitted above the fire, the second combustion would not occur, and all the heat produced thereby would be lost.

Question 406. How can we determine the relative value of different kinds of fuel for use in locomotives?

Answer. This can only be determined satisfactorily by actual experiment. The chemical composition, excepting so far as it indicates the presence of deleterious substances, such as sulphur, ashes, clinkers, etc., affords but little assistance in determining the value of fuel. Nearly the same quantities of elements in different fuels may arrange themselves, before and during combustion, so as to produce very different series of compounds. It is true that the composition of coal gives us some indication of its heat-producing capacity, but the extent to which that capacity can be converted into actual steam in locomotive boilers, depends to a very great extent upon the conditions under which the fuel is burned. It should also be remembered that the rapidity with which steam can be generated is a very important matter in locomotive practice. Whether a heavy freight train can be taken up a given grade, or a fast express make time, often depends upon the amount of steam which can be generated by the fuel in each second of time that the boiler is worked to its maximum capacity. Therefore any appliance for improving combustion, which reduces the quantity of steam which can be generated by the boiler in a given time, is quite sure to fall into disuse or be abandoned. It is of course often necessary to adapt the appliances for burning fuel to the fuel itself; and when a poor quality of the latter must be used, more boiler capacity must be given than is needed to do the same work with better fuel.

The table in the appendix will no doubt be valuable as indicating the properties and relative value of several different kinds of fuel used in this country. The table is copied from a report made to the Navy Department of the United States by Professor Walter B. Johnson in 1844, and the conclusions are deduced from a series of very elaborate experiments made for the Navy Department. This report furnishes the most full and reliable data regarding the value of American fuel thus far (1874) published; but it contains little or no information concerning the fuels which are now used on railroads in our Western States. The first eight specimens of coal given in the table are anthracite; all the rest are bituminous coals.