Rate of Combustion. It is a well-known fact that each pound of fuel is capable of giving out a certain definite amount of heat. Therefore, the more rapid the combustion, the greater the amount of heat produced in a given time. In stationary boilers, where the grate is practically unlimited, the rate of combustion per square foot of grate area per hour varies from 15 to 25 pounds. In locomotives, however, where the grate area is limited, the fuel consumption is much greater, rising at times as high as 200 pounds per square foot of grate area per hour. This rapid combustion results in a great loss of heat and a reduction in the amount of water evaporated per pound of coal. It has been shown that when coal is burned at the rate of 50 pounds per square foot of grate area per hour, 8¼ pounds of water maybe evaporated for each pound of coal. While if the rate of combustion is increased to 180 pounds per square foot of grate area per hour, the evaporation will fall off to about five pounds, a loss of water evaporated per pound of coal of nearly 40 per cent. This loss may be due to a failure of the heating surface to absorb properly the increased volume of heat passing over them, or to the imperfect combustion of the fuel on the grate, or it may be due to a combination of these causes.
The results of experiments show that the lower the rate of combustion the higher will be the efficiency of the furnace, the conclusion being that very high rates of combustion are not desirable and consequently that the grate of a locomotive should be made as large as possible so that exceptionally high rates of combustion will not be necessary.
With high rates of combustion, the loss by sparks is very serious and may equal in value all of the losses occurring at the grate. Fig. 57 is a diagram representing the losses that occur, due to an increase in the rate of combustion. The line a b illustrates graphically the amount of water evaporated per pound of coal for the various rates of combustion. Thus, with a rate of 50 pounds per square foot of grate area per hour, 8¼ pounds of water are evaporated. When the rate of combustion is raised to 175 pounds, only about 5 1/3 pounds of water are evaporated. It is thus seen that the efficiency of the locomotive from the standpoint of water evaporated per pound of coal decreases as the rate of combustion per square foot of grate area increases. If it could be assumed that the heat developed in the furnace would be absorbed with the same degree of completeness for all rates of combustion, the evaporation would rise to the line a c. If, in addition to this, it could be assumed that there were no spark losses, the evaporation would rise to the line a d. Finally, if in addition to these, it could be assumed that there were no losses by the excess admission of air or by incomplete combustion, then the evaporation would remain constant for all rates of combustion and would be represented by the line a e. That is, with the boiler under normal conditions, the area a b c represents the loss occasioned by deficient heating surface; the area a c d represents that caused by spark losses; and the area a d e represents that due to excessive amounts of air and by imperfect combustion.
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