Refrigeration

REFRIGERATING and ICE-MAKING. “Refrigeration”

(from Lat. frigus, frost) is the cooling of a body by the transfer of a portion of its heat to another and therefore a cooler body. For ordinary temperatures it is performed directly with water as the cooling agent, especially when well water, which usually has a temperature of from 52° to 55° F., can be obtained. There are, however, an increasingly large number of cases in which temperatures below that of any available natural cooling agent are required, and in these it is necessary to resort to machines which are capable of producing the required cooling effect by taking in heat at low temperatures and rejecting it at temperatures somewhat above that of the natural cooling agent, which for obvious reasons is generally water. The function of a refrigerating machine, therefore, is to take in heat at a low temperature and reject it at a higher one.

This involves the expenditure of a quantity of work W, the amount in any particular case being found by the equation W = Qi — Qi,where W is the work, expressed by its equivalent in British thermal units; Qf the quantity of heat, also in B.Ther.U., given out at the higher temperature Ti; and Qi the heat taken in at the lower temperature Ti. It is evident that the discharged heat Qi is equal to the abstracted heat Q~, plus the work expended, seeing that the work W, which causes the rise in temperature from T~ to T2, is the thermal equivalent of the energy actually expended in raising the temperature to the level at which it is rejected. The relation then between the work expended and the actual cooling work performed denotes the efficiency of the process, and this is expressed by Qi/(Qi — Qi); but as in a perfect refrigerating machine it is understood that the whole of the heat Qi is taken in at the absolute temperature T~, and the whole of the heat Q,, is rejected at the absolute temperature Ti, the heat quantities are proportional to the temperatures, and the expression T,/(T2—T1) gives the ideal coefficient of performance for any stated temperature range, whatever working substance is used.