must therefore be on the air that rises through the grates and passes upwards through the bed of fuel. These conditions make it essential, for complete and economical combustion, that a sufficient supply of air be admitted through the grate itself, and that the supply be well distributed over the whole grate area, so that it may become mixed with the gases almost as soon as they are formed. It is also important that the grate be placed far enough below the tubes to permit of a thorough mixture of the gas and air and of complete combustion of the gas before it enters the space between the tubes.

HEAT LOSSES AND THEIR PREVENTION

MISCELLANEOUS HEAT LOSSES

21. A portion of the heat generated in the furnace is usefully expended in evaporating water, but a large percentage of the heat is often wasted. Some of these heat losses are unavoidable, while others are due to poor management or poor design of the boiler.

The heat losses due to incomplete combustion of the carbon and hydrocarbons, the formation of smoke, excessive air supply, etc. have already been pointed out, and the methods of preventing them have been explained. In addition to these losses, there is the loss of heat inseparable from the use of natural draft. Since the draft depends entirely on the difference in density between the gases within the smokestack and the air surrounding the smokestack, it follows that the heat required to produce the difference in density (that is, to produce the draft), while not available for the generation of steam, is essential to the operation of a boiler; while the heat thus expended may be called a heat loss, it is nevertheless an unavoidable loss.

Some heat is lost by radiation from the boiler itself and some from the connections. This loss, while it cannot be prevented entirely, can be minimized by covering the exposed parts with some good non-conducting material.

MOISTURE AND DISTILLATION HEAT LOSSES

22. There are processes accompanying the combustion of coal in the boiler furnace that in themselves absorb great quantities of heat and, in consequence, have an important bearing on the question of complete and economical combustion. All moisture that enters the furnace with the coal must be evaporated at the expense of the heat developed by the combustion of the coal; the vapor thus formed passes out of the smokestack at a temperature seldom less than 400° F. Assuming that the moisture enters the furnace at a temperature of 70° F. and leaves at 400° F., 1,200 British thermal units, nearly, will be lost for each pound of moisture in the coal. Moisture in coal can only be driven off by heating it above ordinary temperatures, but it will be readily absorbed at ordinary temperatures; hence, it is important that coal when stored is not exposed to rain-it should always be stored under cover. In some cases, it may be advantageous to wet bituminous coal; when wet, especially if the coal is fine, it cokes better, and hence there is less waste from coal falling into the ash-pit. Wetting is recommended only for bituminous slack and anthracite culm, and should be as moderate as will secure the results desired. It would be much better, however, when the draft is moderate, to use grates having smaller openings. When the draft is exceptionally strong and very fine coal is burned, wetting becomes almost a necessity. If not done, the strong draft will actually carry a large percentage of the fine coal up the smokestack.

The distillation of the volatile matter is a process that, with all bituminous coals, and to a lesser degree with anthracite, absorbs a great deal of heat, as is shown by the drop in the steam pressure when a large quantity of fresh coal is thrown on the fire; this drop is largely due to the lowering of the furnace temperature through the heat absorbed by the distillation of the volatile matter. While the absorption of heat is necessary to drive off the volatile combustible, the losses attendant upon a lowering of the furnace temperature can be minimized by frequent light charges of coal.

HIGH-TEMPERATURE HEAT LOSSES IN SMOKESTACKS

23. In many cases, there is a large amount of heat passing out of the smokestack in excess of that required to produce the necessary intensity of draft. In practice, it has been found that, when the temperature of the escaping gases has been lowered to about 500° F., the draft will be ample. If their temperature is in excess of this, it usually indicates a serious heat loss. The high temperature may be due to several causes, either singly or combined, among which may be mentioned insufficient heating surface, inefficient heating surface, and poor water circulation.

It is rather hard to decide where to place the blame in case the temperature of the escaping gases is excessive. In general, the trouble is that the efficiency of the heating surfaces has become impaired by reason of the collection of soot and condensable tarry vapors on the fire side, and the deposit of scale on the water side. The obvious remedy is to clean the surfaces, and to clean them thereafter at such intervals as will keep them in a state of high efficiency.

If the heating surfaces are clean and the temperature of the escaping gases is excessive, the trouble, with fire-tube boilers, may be due to poor circulation. It is difficult to state just exactly what should be done to improve the natural circulation, since boilers vary so much in design. In general, it is cheapest to use some suitable apparatus designed to give a forced circulation. In flue boilers and water-tube boilers, the circulation is usually free and strong; an excessive temperature of the escaping gases with these types of boilers is usually due to dirty or insufficient heating surfaces.

Insufficiency of heating surface is generally found in cases where, due to the exigencies of service, boilers are forced. beyond the steam-making capacity for which they were installed. This calls for an increased combustion rate per square foot of grate surface, in consequence whereof an increased volume of gas passes through the tubes and over the heating surface at a higher velocity. Since the transfer of heat from the heated gases to the water depends to a

large extent on the time during which they are in contact with the heating surfaces, a proportionately smaller amount of heat per pound of gases is transferred to the water. A partial remedy for loss due to this cause is to lengthen the time the gases are kept in contact with the heating surfaces, as may be done by using spiral retarders in the tubes of fire-tube boilers; or, if feasible, by fitting suitable baffle plates between the tubes of water-tube boilers, in order to lengthen the path of the gases. Another partial remedy is a feedwater heater placed in the path of the waste gases before they pass out of the smokestack.

RULES FOR BOILER EFFICIENCY

24. The efficiency of a boiler is the ratio of the difference between the heat in the steam delivered by the boiler and the heat in the feedwater to the heat that would be developed by the perfect combustion of the fuel, and is expressed by dividing the former quantity by the latter. Thus, if a test shows a total supply of heat of 270,187,000 British thermal units, and a useful application of 186,429,030 British thermal units to the evaporation of water into dry 186,429,030 steam, the efficiency of the boiler is .69. 270,187,000

Boiler efficiency thus determined consists of two factors not readily separable-the efficiency of the furnace as a heat producer and the efficiency of the boiler as a heat absorber. It is possible to have a furnace so well constructed and managed that the combustion is nearly perfect, and still have a low efficiency of the boiler as a whole, owing to inefficient heating surfaces, large radiation losses, etc. In order to secure a high efficiency of the boiler, as a whole, it is necessary to pay strict attention to each and every detail and to keep it in the most perfect condition possible.

25. Having seen that complete, i. e., economical, combustion depends on a sufficient air supply intimately mixed with the combustibles, and a high furnace- and combustion

chamber temperature to insure ignition, the following rules will be self-evident:

1. Fire light and often.

2. Keep the fire as thin as circumstances will permit.

3.

Keep the fire clean.

4.

Keep the space between the grate bars clear.

5. Keep the ash-pit clear.

6. When using bituminous coal, use the coking firing system, if possible.

7. Regulate the draft so that it will be strongest when a fresh charge of coal is put into the furnace.

8. Do not let the fire burn out in spots.

9. Do not let the fire burn too low before charging.

10. If possible, fire at regular intervals and in regular charges.