Conference Year



hybrid control


The proposed hybrid control employs a primary expansion valve that provides most of the pressure drop, while small secondary balancing valves in the distributor lines to the evaporator circuits adjust the refrigerant flow to provide equal circuit exit superheats. This paper presents experimental results for application of hybrid control of expansion valves to a 5-ton R410A domestic heat pump. Baseline performance data was taken with an electronic expansion valve (EXV) to determine the best possible performance without using individual circuit flow control. After that, secondary balancing valves were inserted into the distributor lines to complete the hybrid control scheme. The same tests as done with the EXV were repeated to determine the achievable performance improvement. Ice-up tests at high outdoor humidity were conducted to determine the influence of the control scheme on frost build up and system performance. For repeatable results, additional tests with a partially blocked evaporator coil were conducted. It was found that during operation with the EXV, even with a clean coil, substantial maldistribution occurred leading to individual circuits with significantly different refrigerant exit conditions. With the EXV control scheme maintaining an overall exit superheat, some circuits had two-phase refrigerant at the exit while other circuits had a high degree of superheat. As a result, the evaporation temperature was lower than would occur for more even distribution leading to a performance penalty. In addition, the maldistribution resulted in uneven ice build-up during the ice-up test. The tests with the hybrid control scheme indicated that the maldistribution can be avoided and better overall performance achieved. Similar tests as for the domestic heat pump have been conducted for a 3-ton large room cooling system; refer to companion paper Bach et al.(2012).