SECT. III.] 



CONDENSATION OF STEAM. 



115 



SUPPLY OF Am AND AREA OF FIRE GRATING. 



197. The most effective method of supplying fuel regularly with air, that has 

 yet been tried, is that of burning it on a grate placed over a pit to receive the 

 ashes. 



And in examining this subject, we have first to inquire what quantity of air must 

 pass through the fire for the combustion of each species of fuel. It has been shown 

 that the different species of fuel require different quantities of oxygen. For the 

 different kinds of coal it varies from 1-87 to 31bs. for each pound of coal, and 

 twelve cubic feet of oxygen weigh one pound ; also to obtain one pound of 

 oxygen five pounds of air must pass through the fire ; consequently, sixty cubic 

 feet of air will be necessary to afford one pound of oxygen. But it is not possible 

 to render the whole of the air effective ; part of it will escape unchanged by com- 

 bustion, and the allowance I have usually made is that only two-thirds is effective ; 

 therefore we require ninety cubic feet of air for each pound of oxygen, and the 

 product when carbon alone is consumed is carbonic acid, and the specific -gravity of 

 the air after thus changed by combustion will be 1'05. But a fuel sometimes 

 contains hydrogen, and in that case the oxygen and hydrogen form steam of 

 double the volume of the oxygen ; and the bulk of the mixture of air and vapour 

 will be 102 feet for each pound of oxygen combining with hydrogen, and its 

 specific gravity will be 0'9. The last column of the following is computed from 

 the numbers given in the last column of the preceding table, (art. 190.) 



It appears therefore that we may state the quantity of air and smoke in round 

 numbers, for coal and coke, at 2000 cubic feet, for each cubic foot of water 

 converted into steam, and for wood at 4000 cubic feet. 



198. The grate must be sufficient to admit the air required for combustion in 

 the state of expansion due to the temperature of the burning fuel ; and it is moved 

 through the fire by the joint effect of the draught of the chimney and the ash-pit ; 

 hence, as deep an ash-pit below as possible should be procured, the ash-pit 

 narrowing to a uniform breadth, the same width as the grate before it arrives at 



