Modeling of Compressors and Expansion Devices With Two-Phase Refrigerant Inlet Conditions

Simulation models for unitary air conditioners and heat pumps usually can not provide accurate predictions in the case of two-phase refrigerant entering the compressor or fixed-area expansion device. This paper focuses on improving modeling methods in these situations. The study utilized extensive laboratory tests and detailed modeling of two unitary air conditioners, one using R-410A and the other using R-407C. Heat exchanger models were used to give upstream qualities of the compressor and fixed orifice. It was found that it is best to use a two-phase suction density correction for compressor maps to predict mass flow rate. However, using a two-phase suction density correction with a compressor map to predict the power consumption leads to significant under-predictions. It is best not to correct the compressor map power consumption for a two-phase entering condition. For the case of twophase refrigerant entering a fixed-area expansion device, mass flow rate predictions from short-tube orifice models are extremely sensitive to the upstream quality. A two-point charge tuning method is recommended, as it can lead to an accurate prediction of upstream state for the expansion device at all refrigerant charge levels.