ECONOMICAL USE OE FUEL 1ST CREAMERIES. 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° F. into steam at atmospheric pressure the fac- 
tor of evaporation is F= — ^rT^r^, in 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 TO pounds, the factor 
of evaporation, F= — ' 70 . =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 -ot\ =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 
steam directly from the boiler, it will require QQ ' 7Q 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+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 
