A Catechism of the Steam Engine

by C. E. John Bourne

A.--Anthracite consists almost entirely of carbon, having 91 per cent. of carbon, with about 7 per cent. of volatile matter and 2 per cent. of ashes. Newcastle coal contains about 83 per cent. of carbon, 14 per cent. of volatile matter, and 3 per cent. of ashes.

143. Q.--Will you recapitulate the steps by which you determine the quantity of air required for the combustion of coal?

A.--Looking to the quantity of oxygen required to unite chemically with the various constituents of the coal, we find for example that in 100 lbs. of anthracite coal, consisting of 91.44 lbs. of carbon, and 3.46 lbs. of hydrogen, we shall for the 91.44 lbs. of carbon require 243.84 lbs. of oxygen--since to saturate a pound of carbon by the formation of carbonic acid, requires 2-2/3 lbs. of oxygen. To saturate a pound of hydrogen in the formation of water, requires 8 lbs. of oxygen; hence 3.46 Fibs. of hydrogen will take 27.68 lbs. of oxygen for its saturation. If then we add 243.84 lbs. to 27.68 lbs. we have 271.52 lbs. of oxygen required for the combustion of 100 lbs. of coal. A given weight of air contains nearly 23.32 per cent of oxygen; hence to obtain 271.52 lbs. of oxygen, we must have about four times that quantity of atmospheric air, or more accurately, 1164 lbs. of air for the combustion of 100 lbs. of coal. A cubic foot of air at ordinary temperature weighs about .075 lbs.; so that 100 lbs. of coal require 15,524 cubic feet of air, or 1 lb. of coal requires about 155 cubic feet of air, supposing every atom of the oxygen to enter into combination. If, then, from one-third to one-half of the air passes unconsumed through the fire, an allowance of 240 cubic feet of air for each pound of coal will be a small enough allowance to answer the requirements of practice, and in some cases as much as 300 cubic feet will be required,--the difference depending mainly on the peculiar configuration of the furnace.

144. Q.--Can you state the evaporative efficacy of a pound of coal?

A.--The evaporative efficacy of a pound of carbon has been found experimentally to be equivalent to that necessary to raise 14,000 lbs. of water through 1 degree, or 14 lbs. of water through 1000 degrees, supposing the whole heat generated to be absorbed by the water. Now, if the water be raised into steam from a temperature of 60°, then 1118.9° of heat will have to be imparted to it to convert it into steam of 15 lbs. pressure per square inch. 14,000 / 1118.9 = 12.512 Lbs. will be the number of pounds of water, therefore, which a pound of carbon can raise into steam of 15 lbs. pressure from a temperature of 60°. This, however, is a considerably larger result than can be expected in practice.

145. Q.--Then what is the result that may be expected in practice?

A.--The evaporative powers of different coals appear to be nearly proportional to the quantity of carbon in them; and bituminous coal is, therefore, less efficacious than coal consisting chiefly of pure carbon. A pound of the best Welsh or anthracite coal is capable of raising from 9-1/2 to 10 lbs. of water from 212° into steam, whereas a pound of the best Newcastle is not capable of raising more than about 8-1/2 lbs. of water from 212° into steam; and inferior coals will not raise more than 6-1/2 lbs. of water into steam. In America it has been found that 1 lb. of the best coal is equal to 2-1/2 lbs. of pine wood, or, in some cases to 3 lbs.; and a pound of pine wood will not usually evaporate more than about 2 1/2 lbs. of water, though, by careful management, it may be made to evaporate 4 1/2 lbs. Turf will generate rather more steam than wood. Coke is equal or somewhat superior to the best coal in evaporative effect.

146. Q.--How much water will a pound of coal raise into steam in ordinary boilers?

A.--From 6 to 8 lbs. of water in the generality of land boilers of medium quality, the difference depending on the kind of boiler, the kind of coal, and other circumstances. Mr. Watt reckoned his boilers as capable of evaporating 10.08 cubic feet of water with a bushel or 84 lbs. of Newcastle coal, which is equivalent to 7 1/2 lbs. of water evaporated by 1 lb. of coal, and this may be taken as the performance of common land boilers at the present time. In some of the Cornish boilers, however, a pound of coal raises 11.8 lbs. of boiling water into steam, or a cwt. of coal evaporates about 21 cubic feet of water from 212°.

147. Q.--What method of firing ordinary furnaces is the best?