EVAPORATIVE POWER. 



To find the theoretical evaporating power of a fuel, then, 

 divide the number of thermal units it generates on combus- 

 tion by 965.7. For instance, the heat of combustion of a 

 sample of Illinois coal was determined by Prof. Carpenter to 

 be 13200. Its evaporative power would be 



13200 



= 13.67 pounds. 



This means that under the proper conditions one pound 

 of the coal in question would evaporate 13.67 pounds already 

 heated to 212 F. 



But this amount of duty 'is rarely realized. The boiler 

 may not be well built, the setting may be faulty, and there 

 are numerous other chemical or mechanical conditions which 

 modify the yield. With these no rule can be established ; 

 each individual case must be allowed for specially. With 

 ashes and moisture, chemical constituents of the coal, the 

 case is different. A percentage allowance for these will usually 

 suffice. 



For instance, in the above coal there was 5.12 per cent of 

 water and 15.2 per cent of ash. Then 



100 (15.2 -f- 5.12) X 13.67 = 12.23 pounds. 



If deemed necessary, a further correction can be made for 

 the water of the coal, which would reduce the evaporation by 

 its own amount. This correction would become 



12.23 0.05 = 12. 18 pounds 



as the quantity which should be evaporated with the coal as 

 analyzed. 



The quantity of ash produces an effect on the evaporative 

 power aside from its proportional reduction in combustible. 

 This is due to the fact that where a large percentage of ash 

 occurs, the particles of carbon of the fuel are not burnt com- 



