Application of two hybrid control methods of expansion valves and vapor injected compression to heat pumps

This paper presents the benefits in capacity and performance of a hybrid control of expansion valves for two 5-ton (17.6 kW) domestic heat pumps (HPs) based on experimental results. The hybrid control, as proposed by Kim et al. (2008a), employs a primary expansion valve that provides most of the pressure drop, while small secondary/balancing valves in the distributor lines to the circuits of the evaporator control the refrigerant flow to provide equal circuit exit superheats. Initial baseline data for Heat Pump 1 was taken with an electronic expansion valve. After that, secondary/balancing valves were inserted into the distributor lines and the same tests were repeated. Frost build-up tests at high outdoor humidity were conducted with Heat Pump 1 to determine the influence of the control scheme on the frost uniformity and system performance. For repeatable results, additional tests with partially blocked evaporator coils were done for both heat pumps. Heat Pump 2 was equipped with a vapor injected compressor and had two interchangeable evaporator coils of the same type. One of the evaporator coils was equipped with a more cost effective reduced hybrid control scheme and employed prototype 2-step balancing valves.