198 DAIRY TECHNOLOGY 



when wanted. In the direct-expansion system, as soon 

 as the compressor stops refrigeration ceases. However, 

 for dry hardening rooms the direct-expansion system is 

 absolutely necessary to secure and maintain a sufficiently 

 low temperature. 



Strength of Brine. The proper degree of concentra- 

 tion of the salt solution depends upon the temperature 

 desired. Low temperatures demand a stronger brine to pre- 

 vent freezing; but an unnecessarily strong brine is undesir- 

 able, because the stronger the brine is, the less is its specific 

 heat; that is, it has less ability to absorb heat, and too 

 concentrated brine is likely to clog the pipes. 



Size of Compressor. The size of a refrigeration ma- 

 chine is expressed as a certain number of tons' capacity. 

 For instance a machine of four tons' capacity means that 

 that machine would produce in 24 hours as much cold as 

 is given off by four tons of ice melting to water at 32 F. 

 Its actual ice-making capacity is about half this much; 

 a four-ton machine will make about 2 tons of ice per day. 



In selecting the size of machine needed it must be 

 remembered that the capacity is rated on a 24-hour run. 

 If conditions are such that it will be desirable to run the 

 compressor only four hours per day then the machine must 

 have six times the daily capacity needed. The larger 

 machines produce a ton of refrigeration at less cost than 

 small ones, but in a general way, for small and medium- 

 sized machines, the power required is about two and one- 

 quarter horse power per ton of refrigeration. 



Operation of an Ammonia Plant. Charging and operat- 

 ing an ammonia plant are very ably discussed by H. H. 

 Kelley in The Engineer, from which the following is taken. 



" When about to start an ice or refrigerating plant, the 

 first thing necessary is to see that the system is charged 



