Some Modeling Improvements for Unitary Air Conditioners and Heat Pumps at Off-Design Conditions

Models for unitary air conditioners and heat pumps typically have difficulties in providing accurate predictions for the following situations: 1) charge modeling over a large range of charge levels, 2) two-phase refrigerant entering fixed-orifice expansion devices, 3) two-phase refrigerant entering compressors, and 4) non-uniform refrigerant mass flow allocation among evaporator circuits at low indoor air flow rates. This paper provides a detailed evaluation of the impact of non-uniform refrigerant mass flow allocation among evaporator circuits on performance predictions. In addition, it gives brief descriptions and overall assessments of improved modeling methodologies for the other three issues. The study utilizes data obtained from extensive testing at a wide range of operating conditions for two R-410A units and one R-407C unit.