2 
BULLETIN 85, U. S. DEPARTMENT OF AGRICULTURE. 
amount of milk than the holder process. Furthermore, the milk 
must be cooled through a correspondingly wider range. These 
figures, however, deal only with the heat absorbed by the milk, and 
do not take into consideration that radiated to the air and absorbed 
by the metal and other materials used in the construction of the 
apparatus. 
TESTS OF MILK-PASTEURIZING APPARATUS. 
The following tests were made on the pasteurizing equipment of 
five city milk plants. They were considered as representing average 
city plants. The pasteurizing equipment in each case consisted of 
heater, holding tank, regenerator, and cooler. In plants 1 and 2 the 
heater and regenerator were combined in one unit, and in plants 3, 
4, and 5 they were separate. In plant 3 the regenerator was in the 
form of an ordinary tubular cooler, and the hot milk from the holding 
tank was pumped through the coils while the cold raw milk flowed 
over the tubes. In plants 4 and 5 the regenerators consisted of 
double-pipe arrangements, the hot milk flowing through the inner 
pipe and the cold milk through the outer and therefore surrounding 
the inner pipe. 
The boilers were in good condition and were provided with exhaust 
steam feed-water heaters which heated the boiler feed water from an 
initial temperature of 60° F. to a final temperature of 180° F. The 
boiler pressure in all cases was approximately 80 pounds. The 
efficiency of the boiler and setting is assumed to be 50 per cent in all 
cases, which is believed to be a fair average. However, if there was 
a variation of 10 per cent in the estimated efficiency of the boiler and 
setting, it would affect the cost of pasteurization by approximately 
one-half of 1 per cent. It is further assumed that the coal cost $4 per 
ton (2,240 pounds) delivered in the bunker and that it had a heat 
value of 12,500 B. t. u. per pound. 
The condensed steam was caught as it came from the heater and 
weighed and its temperature taken, the average temperature being 
180° F. The pressure of the steam entering the heater was reduced 
from the boiler pressure of 80 pounds gauge to from 3 to 5 pounds. 
Therefore, the heat absorbed in the heater per pound of steam sup- 
plied was 1,155- (180-32) = 1,007 B. t. u. For the sake of simplicity 
the heat absorbed in the heater per pound of steam supplied is taken 
as 1,000 B. t. u. 
The temperatures of the milk were taken at each stage of the process 
and are recorded in Table 1, "Temperature balance." It will be 
noted from an inspection of the temperature balance that the cycle of 
operation consisted in starting with the initial temperature of the raw 
milk and raising its temperature to the pasteurizing point, about 145° 
F., then cooling the milk down to the temperature of the raw milk. 
