Thermodynamic Optimization of Evaporators with Zeotropic Refrigerant Mixtures

A study was conducted to determine the influence of heat exchanger design on the performance of residential air-conditioning systems using zeotropic mixtures as HCFC-22 alternatives. A computer simulation of the evaporator was developed to model various evaporator designs, and was validated with experimental data collected under controlled air and refrigerant conditions with both HCFC-22 and a zeotropic mixture of HFC-32/HFC- 134a/HFC-125 (23%/52%/25%). The model predictions of heat transfer, pressure drop, and temperature profIles were in very good agreement with the experimental data. An effectiveness-type model for combined heat and mass transfer was developed and implemented in the simulation, resulting in increased computational speed and stability. The model was compared to two others in the literature, and their predictions of the total rate of heat transfer were found to be in reasonable agreement. A discretized solution of the differential heat and mass transfer equations resulted in the best latent load predictions. An irreversibility-based objective function, chosen to quantify evaporator thermodynamic performance, showed a clear dependency on design and operating conditions. A trade-off between irreversibilities due to heat transfer and air pressure drop was found with an increasing number of exchanger rows, and the presence of a minimum suggested a possible optimum design. The effects of refrigerant circuitry and glide matching on exchanger performance were also investigated. Finally, a link was established between the second-law optimization of the evaporator and its actual performance in a full air-conditioning system. The evaporator model was implemented in a system simulation, and its thermodynamic performance was found to significantly affect that of other components under certain operating conditions. A complete analysis of the interaction between all system irreversibilities is recommended, based on the results of this study.Air Conditioning and Refrigeration Center Project 2