COMPUTING AMOUNT OF ICE NECESSARY TO COOL A 

 GIVEN AMOUNT OF MILK 



The study of refrigeration is essentially a study of heat. The unit 

 of heat used by refrigeration engineers is called the British Thermal 

 Unit, and it is the amount of heat required to raise one pound of water 

 one degree Fahrenheit at its maximum density, 39.1 F. 



Not all substances require the same amount of heat to raise their 

 temperature per unit of mass. Water is taken as a standard, and the 

 ability of a substance to absorb heat in comparison with water is 

 called the specific heat of the substance. The specific heat of water 

 is called i , and most substances have a specific heat less than that of 

 water. The specific heat of milk is variable, depending to a large 

 extent upon the temperature and the variation in fat content. The 

 lower the fat content the higher the specific heat. The specific heat 

 of normal milk is variously given by different investigators as ranging 

 between .92 and .95. In computations on refrigeration, the melting 

 point of ice is used as a basis, because this is definite and constant. It 

 requires 144 B. T. U. to melt one pound of ice, and knowing this fact, 

 the amount of ice necessary to cool a given amount of milk may be 

 readily computed. The method of computation may best be illus- 

 trated by a specific problem. 



How many pounds of ice will be required to cool 500 pounds of 

 milk from 80 F. to 32 F. ? In this case, the specific heat of the milk 

 is considered as .93. 



Five hundred X .93 = 465, the number of heat units required 

 to raise the 500 pounds of milk one degree as compared with the same 

 amount of water. Since the milk is to be cooled 48 F. (80 32 =48) 

 multiplying 465 by 48 will give 22,320 heat units to be extracted from 

 the milk, (465 X 48 = 22,320). Since it requires 144 B. T. U. to 

 melt one pound of ice, it would require as many pounds of ice to cool 

 the 500 pounds of milk as 144 is contained in 22,320, or 155, (22,320 -f- 



*44 = 155)- 



If the milk is cooled to a temperature above 32 F. the available 

 refrigeration of one pound of ice is increased and the difference 

 between 32 F. (the temperature of the melting ice), and the tempera- 

 ture to which the milk is cooled, must therefore be added to the 

 B. T. U. necessary to melt a pound of ice (144). 



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