ECONOMICAL USE OF FUEL IX CBEAMEEIES. 29 



ing point, and that steam is generated at atmospheric pressure. If 

 the water is fed to the boiler at a temperature other than 212° F., or 

 if steam is generated at a pressure other than atmospheric, it is 

 obvious that a correction factor must be employed in order to reduce 

 the results to an equivalent evaporation from and at 212° F. This 

 factor is known as the " factor of evaporation." The heat actually 

 required to evaporate a pound of water into steam is the total heat 

 at the boiler pressure less the sensible heat at feed-water temperature. 

 Since 970.4 B. t. u. are required to evaporate a pound of water from 

 a temperature of 212° JB\ into steam at atmospheric pressure the fac- 

 tor of evaporation is F= 9704 ' ? ^ n which Th is the total heat in 

 steam at boiler pressure and Sh the sensible heat in the feed water. 



If the boiler feed water is at a temperature of 60° F. and steam 

 is generated in the boiler at a gauge pressure of 70 pounds, the factor 



of evaporation, F= — ■ — ' -. ^ ' =1.191. Therefore, if the 720 



pounds of steam required per hour by the 12-horsepower engine were 

 generated from a feed-water temperature of 60° F. into steam, at 70 

 pounds' gauge pressure, the equivalent evaporation from and at 212° 

 F. would be 720X1-191=857.5 pounds and the boiler capacity would 



be -sji =24.8 boiler horsepower. 



With a pasteurizer efficiency of 80 per cent it requires 382,500 

 B. t. u. to heat 4,000 pounds of milk from 60° F. to a final tempera- 

 ture of 145° F. If the heating is done in 30 minutes, by using live 



382 500 

 steam directly from the boiler, it will require „, * ^ X2=22.8 boiler 



horsepower. This added to the boiler horsepower required for fur- 

 nishing steam to the engine makes the total capacity of the boiler, 

 24.8-4-22. 8=47.6 horsepower. In practice about 25 horsepower is used 

 and it is forced during pasteurization. Before beginning to pasteur- 

 ize a full head of. water is fed to the boiler and the fires crowded. 

 As pasteurization proceeds the water level in the boiler falls until at 

 the end of the operation the water level has fallen perhaps to the 

 bottom of the gauge. This method of operation puts, a severe strain 

 on the boiler and is attended with more or less danger. 



On the other hand, if the heat in the exhaust steam is used for pas- 

 teurizing, the necessity of forcing the boiler during pasteurization is 

 avoided, for there are 720X800=576,000 B. t. u. per hour, or 288,000 

 B. t. u. per half hour available in the exhaust steam. By storing 

 this heat in a specially designed tank it becomes unnecessary to draw 



