An experimental study of the thermal performance of R-134a and refrigerant blends in an air conditioning system
Howard N. Shapiro
This document presents the findings of an experimental study of the effects of changing refrigerants on the performance of a refrigeration system. The different refrigerants studied are considered to be less harmful to the environment than R-12 which is currently widely used. Environmentally harmful effects include global warming and ozone depletion. The findings are based on the results of over one hundred independent tests using four refrigerants, namely R-12, R-134a, and two R-22 blends;The test equipment consisted of a 3-ton vapor compression refrigeration system, an air flow loop used to control evaporator conditions, and a water flow loop used to control condenser conditions. The instrumentation provided information needed to determine the system COP, cooling capacity, mass flow rates, and thermodynamic states of the refrigerant at the inlet and exit of each component in the system. The parameters varied in the study were the evaporator and condenser sink temperatures and the amount of refrigerant used in the system. The evaporator exit superheat was maintained constant at 13.5 F for all tests;The performance with each refrigerant at several refrigerant charges is presented as a function of operating conditions. Curve fits of several system variables (COP, capacity, mass flow rate, and system temperatures and pressures) allow for comparisons to be made among refrigerants. The performance with each alternative refrigerant was compared to the performance with R-12 at the same operating conditions. The other system variables were also compared with R-12. These comparisons are used to explain the observed differences in performance, and they provide information needed for design and for making system modifications;The experimental results show equal or slightly better COPs with the alternative refrigerants compared to R-12. The capacities are also similar with the exception of MP-52 which had a lower capacity than R-12. The differences in performance were somewhat dependent on the operating conditions;Several models for the compressor are presented and compared with experimental data. The models are based on the ideas of polytropic compression and isentropic efficiency. Heat exchanger models are also examined and compared with experimental data. The validity of some of the models is questioned as they do not represent the test data well.