35 FARM MOTORS 



carbon, there must be 2 2/3 pounds of oxygen ; hence 

 one pound of carbon requires 2 2/3 pounds of oxygen. 

 As we must hav.e 4 1/2 pounds of air to get one pound of 

 oxygen to burn one pound of carbon to CO 2 , it requires 



pounds of air. 



2^ X 4^ 12 



As there are impurities in all fuels, so that a pound 

 of fuel is not necessarily a pound of pure carbon, there 

 are variations which have to be considered. 



480. Volume of air for combustion. As before stated, 

 an insufficient amount of air burns the carbon only to 

 CO, while a sufficient amount burns it to CO 2 . Instead 

 of having the exact 12 pounds of air for each pound of 

 carbon, as previously computed, it requires an excess for 

 complete combustion. This excess varies from one-half 

 the quantity required for combustion to an equal quan- 

 tity. Roughly, for each pound of carbon there should be 

 from 18 to 24 pounds of air. 



By experiment it has been found that it requires 10 

 pounds of air for each pound of certain coals, and since 

 13 cubic feet of air at the temperature it generally enters 

 the fire box weighs I pound, for each pound of coal it 



requires 



10 X 13 = 130 



cubic feet of air without excess. If the excess is 50 per 

 cent, it requires about 200 cubic feet. 



Loss from improper amount of air. If one pound of car- 

 bon be burned to CO, there will be 4,400 B.T.U. liberated. 

 If it be burned to CO ? , there will be 14,650 B.T.U. set 

 free. Hence there will be a loss of 



14,650 4,400 = 10,250 B. T. U. 



100 X 10,250 -f- 14,650 70 per cent. 



This would be a case too rare to be considered and is 

 used only for simplicity. If due caution is practiced in 



