An experimental study on the uneven refrigerant distribution over a vertically installed multi-circuit outdoor coil in an air source heat pump unit during reverse cycle defrosting

For an air source heat pump (ASHP) unit with a vertically installed multi-circuit outdoor coil, many parameters would affect system defrosting performance, especially the uneven distribution of refrigerant for each circuit and downwards flowing of melted frost due to gravity from up circuit(s) to down circuit(s). Currently, the negative effects of melted frost have been demonstrated and quantitatively studied, without however giving a fundamental study on the effects of uneven refrigerant distribution due to gravity and tube internal resistance for an ASHP unit with a vertically installed multi-circuit outdoor coil. Therefore, an experimental study on system defrosting performance when refrigerant was evenly or unevenly distributed into each circuit has been carried out and a comparative and quantitative analysis conducted using the experimental data. In this paper, the negative effects of refrigerant uneven distribution on system defrosting performance were demonstrated and an increase of 6.9% in defrosting efficiency when refrigerant evenly distributed, compared with the case of refrigerant unevenly distributed, was reported.