The heat-rejection ratio (HRR) is defined as the ratio of the rate of heat rejected at the condenser to that absorbed at the evaporator.

The designer and operator of the refrigeration system will usually characterize plant size by the refrigeration capacity. This capacity can be translated to a condenser capacity through the condenser-to-evaporator heat rejection ratio (HRR). The HRR is a function of the evaporating and condensing temperatures, but is also influenced by the compressor type and any supplementary cooling arrangements. The standard procedure for computing the HRR from catalog data of the compressor is to propose that the heat rejected at the condenser is composed of two contributions—the refrigerating capacity and the thermal equivalent of the power supplied to the compressor. The standard equation for computing the HRR is, therefore,

where all the energy flow rates are expressed in the same units.

Figure 7.4 shows HRRs as functions of the evaporating and condensing temperatures. Changes of either of these temperatures affect both the refrigerating capacity and the power requirement of the compressor. The ideal HRR can be derived from knowledge of the Carnot cycle (Section 2.17), in which the ratio of area under the condensing line to that under the refrigeration line represents the HRR,

where the temperatures T are in absolute, thus °C+273.1 (°F+459.7). Equation 7.4 assumes a 100% efficiency of the cycle and the compressor, and an improved expression that can be used when compressor catalog data are not readily available is

Equation 7.3 is correct except for heat losses to the ambient or supplementary transfers of heat to other devices. The curves in Fig. 7.4 apply to open-type compressors, and the HRR will be higher for hermetic compressors servicing small halocarbon systems, because some of the motor heat enters the refrigerant stream. Also the HRR will be lowered if a reciprocating compressor uses watercooled heads where the heat is rejected to a separate cooler or in a screw compressor where oil is cooled by a separate water or antifreeze circuit.