2 BULLETIN 85, U, S. DEPARTMENT OE 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. 



