An experimental study on even frosting performance of an air source heat pump unit with a multi-circuit outdoor coil

When the surface temperature of the outdoor coil in an air source heat pump (ASHP) unit is lower than both the air dew point and freezing point of water, frost can be formed and accumulated over outdoor coil's surface. On the other hand, an outdoor coil normally has multiple parallel circuits on its refrigerant side for minimized refrigerant pressure loss and enhanced heat transfer efficiency. Currently, most attentions were paid to the experimental and numerical studies on defrosting with different methods, without however paying attention to the distribution of frost accumulation on the multi-circuit outdoor coil. The phenomenon of uneven frosting exists, which also maybe the most important reason of uneven defrosting for an ASHP unit with a multi-circuit outdoor coil. Therefore, in this study, a comparative and experimental study on system frosting performance when frost accumulated on a three-circuit outdoor coil's surface in an ASHP unit at different frosting evenness values (FEVs) has been carried out. Experimental results indicated that, when the FEV was increased from 75.7% to 90.5% for an ASHP unit with a three-circuit outdoor coil, the COP could be increased from 4.10 to 4.26 at a 3600 s frosting process, and increased from 3.18 to 4.00 at its last 600 s.