Optimum Refrigerants for Non-Ideal Cycles: An Analysis Employing Corresponding States

The principle of corresponding states is used to evaluate the effects of the thermodynamic characteristics of the working fluid on the performance of refrigeration cycles. The desired characteristics, expressed in terms of the critical temperature and ideal gas heat capacity cc;) using propane as the reference fluid, are examined for various departures from the theoretical (ideal) vapor compression cycle. These departures from the ideal cycle approximate a refrigerator. Cycle performance and the resulting conclusions regarding the optimum refrigerant are strong functions of the assumptions made in modeling the cycle. The baseline cycle for the comparisons includes compressor efficiency and heat transfer limitations in the condenser and evaporator. For the baseline cycle, there are trade-offs between a high coefficient of performance (COP) and high volumetric refrigerating capacity; low values of C~, corresponding to simple fluids such as the single-carbon CFCs, yield the highest COP. When simple representations for subcooling and/or heat exchange between the liquid and suction lines are added to the baseline cycle, the dependence of COP on the refrigerant critical temperature is greatly reduced, and the optimum C~ is much higher. These results indicate that modifications to the basic vapor compression cycle should be considered for more complex refrigerants such as the two-carbon HFCs and HCFCs.