Effects of fan-starting methods on the reverse-cycle defrost performance of an air-to-water heat pump

The effects of fan starting methods on the defrost performance in an air-source heat pump were investigated experimentally. Experiments were conducted on a 50 kW unitary air-to-water heat pump. The dynamic characteristics of both the coil fan pre-start and normal-start tests during the defrost cycle were discussed. The peak of discharge pressure for the fan normal-start test was up to 2595.6 kPa during defrost termination and recovery time, which was close to the discharge protection value (2650 kPa). The discharge pressure for the fan pre-start test was 742.3 kPa lower at the end of the drain time, so the peak of the discharge pressure was 687 kPa lower during the defrost termination and recovery time than that for the fan normal-start test. We found the phenomenon seemed to be related to both the small inner volume of the plate heat exchanger and larger refrigerant flow rate during the defrost termination and recovery time. Pertinent performance data (pressure, temperature, superheat, sub-cooling, etc) was plotted and discussed to determine the effects of the coil fan pre-start.