Performance and exergy destruction analyses of optimal subcooling for vapor-compression refrigeration systems

Performance enhancement and exergy destruction analyses were conducted numerically for vapor-compression refrigeration systems using R22, R134a, R410A, and R717. The first law of thermodynamics combined with finite-temperature-difference heat transfer theory and the second law of thermodynamics were applied to calculate the COP, heat-exchanger area, irreversibility, and friction loss that occurs in heat exchangers used in this system at various condensation and evaporation temperatures. The effects of cooling water in a subcooler, refrigerant pressure drop among heat exchangers, and superheating in an evaporator were also considered. Two dimensionless parameters, which represented initial cost saving and the total exergy destruction of the system, were introduced and evaluated to obtain the optimal degree of subcooling. Compared with evaporation temperature, condensation temperature plays a considerably more crucial role in determining the optimal degree of subcooling for achieving maximal initial cost saving. In addition, the optimal degrees of subcooling obtained according to the second law of thermodynamics were consistently higher than those obtained according to the first law of thermodynamics.