Refrigerant Cycle Performance

Various thermodynamic properties of a refrigerant combine when operating within a cycle to influence the performance. Tables 12.5 and 12.6 show several important quantities that give added insight into the choice of refrigerant. To provide a common basis of comparison, the standard vapor-compression cycle with an evaporating temperature of -15°C (5°F) and a condensing temperature of 30°C (86°F) has been chosen for the refrigerants in Table 12.5. For the low-temperature refrigerants in Table 12.6, -50°C (-58°F) and -15°C (5°F) are the evaporating and condensing temperatures, respectively.

fig 1 310 - Refrigerant Cycle Performance

The refrigerants listed in Table 12.5 often serve large systems, so their energy characteristics are important. A high latent heat at the evaporating temperature reflects itself in the mass rate of refrigerant flow per unit refrigeration capacity. The latent heats of the halocarbons shown in Table 12.5 are of the same order of magnitude, but the latent heat of ammonia is much higher. Another quantity that affects the size of components is the volume flow rate leaving the evaporator per unit refrigerating capacity. Ammonia, HCFC-22, and R-507 show comparable magnitudes of the volume flow rate per unit refrigerating capacity in comparison to the higher value of HFC-134a. The consequence of this comparison is that the compressor required in an HFC-134a unit must have greater volume-pumping capacity than compressors serving systems with the other refrigerants.

fig 1 311 - Refrigerant Cycle Performance

Systems with different refrigerants perform with somewhat unique COPs, so it is fair to say that some refrigerants are more efficient than others. A comparison of the COPs for the refrigerants shown in Table 12.5 indicates that all these refrigerants are about equally efficient, with the possible exception of R-507, which has a COP of about the same order of magnitude as R-502, for which it is a potential replacement.

The choice of low-temperature refrigerants is now quite limited, as was discussed in Section 12.8. Table 12.6 shows a comparison of three candidate refrigerants, HFC-23, ethane (R-170), and carbon dioxide (R-744). The use of HFC-23 as a single refrigerant is just now proving itself, but the results so far have been promising. Carbon dioxide has certain desirable properties, but cannot be used with evaporating temperatures lower than -56.6°C (-69.9°F), which is its triple point.