COST OF POWER 133 



possess practical fuel value. The heating power of fuels con- 

 taining carbon and hydrogen is approximately expressed by the 

 formula: 



A manufacturer purchasing a quantity of fuel will do well 

 in having a sample thereof subjected to chemical analysis, and 

 the heat value is thus readily determined. Take as an illustra- 

 tion the analysis of Hocking Valley Coal, Table III, and the 

 heat value is as follows: 



h = 145 (67 + 4.28 X 4.8) = 12,694 B. T. U. per pound. 



Heat Required for Producing Steam. Water at 212 F. 

 takes up 966 B. T. U. when converted into steam of the same 

 temperature. Therefore if one pound of coal produces 12,694 

 heat units then a pound of such coal will produce 12,694/966 or 

 13.14 pounds of steam of 212 F. from water of the same tem- 

 perature. The amount of heat thus required to produce a 

 pound of steam is known as the latent heat of vaporization. 



In practice more heat is required for producing a pound of 

 steam, as the water that is converted into steam is usually at 

 a temperature much below 212 F. Possibly 60 F. will come 

 nearer to the temperature of the boiler feed water as used in 

 most creameries. Furthermore the steam in the boiler is heated 

 to a temperature exceeding that of 212 F. Consider that a 

 creamery is using boiler feed water at 60 F. and carrying a 

 steam-gauge pressure of 80 pounds, it will then require 1,153 h ea -t 

 units for each pound of steam produced, or one pound of the 



above-mentioned coal will produce '-^ or 1 1 pounds steam. 



1,153 



Losses of Heat When Producing Steam. If there were no 

 losses to consider it would be comparatively easy to determine 

 the amount of fuel of a known composition required for produc- 

 ing a definite amount of steam. It will be impossible even for 

 the most careful engineer to reach the theoretical efficiency. It 

 should be possible, however, for many engineers to materially 



1 Siebel's Compend. of Mechanical Refrigeration and Engineering, 19 n, p. 96. 



