22 MASS. EXPERIMENT STATION BULLETIN 360 



which is maintained at a relatively high pressure. The combination of high 

 pressure and low temperature in the condenser serves to return the vapor to a 

 liquid form. The condenser is cooled b}' air in the small models and with cool 

 water in the larger sizes. 



Refrigerants 



There are a number of substances which can be used in the refrigeration cycle. 

 These are known as refrigerants. Those commonly used in cooling apple storages 

 are ammonia, freon, methyl chloride, and sulfur dioxide. Their properties differ 

 considerably but all can be used with reasonable success. Ammonia operates at 

 considerably higher pressure than the other refrigerants and must be kept in 

 iron, steel, or aluminum pipes as it attacks copper and copper alloys. For other 

 refrigerants, evaporating and condensing coils made from copper or copper alloy 

 can be used. These are made up at lower cost than steel units. Ammonia re- 

 quires a somewhat smaller compressor than the others and the power require- 

 ments are slightly less; but for all practical purposes it can be assumed that the 

 same power will be required with all refrigerants. The pressure-temperature 

 characteristics of the common refrigerants are given in Table 5. 



Large apple storages are cooled with ammonia, while the small ones use low- 

 pressure gases. At present the dividing line seems to come at about five tons 

 capacity, but the low-pressure refrigerants are gradually encroaching on territory 

 formerly held by ammonia. Apparently both lines of equipment are giving 

 satisfaction in their respective fields. Blower and piping costs are lower with 

 low-pressure systems, inasmuch as copper piping can be used; but the com- 

 pressor is usually more expensive, so that the total cost of the two types does 

 not differ a great deal. 



Rating 



The capacity of refrigeration units is measured in terms of tons. A one-ton 

 machine is one which will produce the same cooling effect in operating 24 hours 

 as is obtained when one ton of ice is allowed to melt. As each pound of ice takes 

 up 144 B. t. u. (British thermal unit) in melting, the melting of one ton of ice 

 will absorb 288,000 B. t. u. One B. t. u. is the heat required to raise the tem- 

 perature of one pound of water one degree Fahrenheit. A one-ton machine, 

 therefore, removes heat at the rate of 288,000 B. t. u. per 24 hours, or 12,000 

 B. t. u. per hour. 



