84 SWEETENED CONDENSED MII^K CONDENSING 



of evaporation only as long as enough water passes through the con- 

 denser to maintain a high vacuum. As soon as the steam pressure 

 in the jacket and coils reaches the point where the water in the con- 

 denser fails to promptly reduce the vapors, the vacuum drops, the 

 temperature in the pan rises and evaporation is checked. 



The condensing of milk requires immense quantities of water ; 

 experience has shown that it takes from two to three gallons of 

 water to condense one pound of fresh milk. The water supply is 

 one of the weakest links in most condenseries, so that economy of 

 water is one of the important factors to be considered. The steam 

 pressure in the jacket and coils should, therefore, be so regulated as 

 to make it possible to maintain the maximum vacuum consistent 

 with reasonably economic use of water. The experience of the 

 best pan operators is that about fifteen pounds of steam pressure in 

 the jacket and coils and a vacuum of twenty-five inches is practic- 

 ally the maximum that can be maintained under average conditions 

 without taxing the usual water supply beyond its capacity. With a 

 vacuum of twenty-five inches the temperature in the pan is about 

 135 degrees F., the temperature varying somewhat with the altitude 

 of the factory. In some condenseries the temperature of the pan 

 is kept at 150 degrees F. This practice may economize the water 

 a trifle better, but the rapidity of evaporation is considerably lower. 



Condensing at temperatures lower than 130 degrees F., without 

 reducing the steam pressure in the jacket and coils, increases the 

 rapidity of evaporation, but taxes the water supply beyond the reach 

 of most condenseries. So much water has to be used in the con- 

 denser that it is not used economically, as is shown by the relatively 

 low temperature of the water discharging from the condenser. The 

 temperature of the condenser discharge bears a direct relation to 

 the temperature of the vapors in the pan. Observations made in 

 various factories and under different conditions by Hunziker and 

 others showed that the condenser discharge was anywhere from 5 to 

 25 degrees F. lower in temperature than the vapors in the pan, the 

 difference averaging about 15 degrees F. 



The smaller the difference in temperature between the con- 

 denser discharge and the vapors in the pan, the more economic is 



