The influence of both the evaporating and condensing temperatures on the refrigerating capacity and the power requirement of reciprocating compressors has now been studied. Figure 4.10 is a summary of the influence of these temperatures on the refrigerating capacity and Figure 4.13 the influence on the power requirement. The designer and operator of a plant might be interested as well in the efficiency, as expressed by the coefficient of performance, COP, or the horsepower per ton. The importance of these variables lies in the fact that the plant has a certain refrigeration duty to perform, and the desire is to accomplish that duty with the minimum power requirement. The data in Figures 4.10 and 4.13 are the bases for computing the COP:
Figure 4.15 also shows values of horsepower per ton (hp/ton), which varies inversely as the COP through the equation:
The COP always increases with an increase in evaporating temperature and decreases with an increase in condensing temperature. Designers and operators frequently want to know the possible reduction in power for a given refrigerating capacity through an increase in the evaporating temperature or a decrease in the condensing temperature of one degree. Table 4.1 shows some approximate values, based on the data of Figure 4.15, that could serve as guidlines for potential savings. As expected, the improvement per degree in temperature expressed as a percentage will be greatest when the temperature lift from the evaporating to condensing temperatures is small and least when one degree change is a small fraction of the total temperature lift.