Experimental study on frost unevenly distributed and melted frost downwards flowing during defrosting for ASHPs

Abstract

For an air source heat pump (ASHP) unit, uneven frost distribution would result in mal-defrosting, or adversely degrade system defrosting efficiency. Previous investigations demonstrated that, for an ASHP unit with a multi-circuit outdoor coil, when the frosting evenness value (FEV) on its surface of outdoor coil is higher, the defrosting efficiency would also be higher. On the other hand, as reported, the downwards flowing melted frost during defrosting would also play negative effects on defrosting performance. Although the comprehensive influence of the two factors, uneven frost distribution and the downwards flowing melted frost, had been experimentally investigated, the influence of uneven frost distribution or the FEV on negative effect of downwards flowing melted frost was still unknown. As a fundamental and interesting problem, it is meaningful to be explored. It was therefore quantitatively analyzed here basing on six experimental cases. Water collecting trays were used to take away the melted frost during defrosting in three cases, and the other three ones without trays installed stood for the traditional status. Meanwhile, their FEVs of two groups were separately adjusted to be at three different levels. Results clearly showed the positive effects of increasing FEV and taking away the melted frost on system defrosting efficiency. After the melted frost was taken away, the FEV effects was increased. The increase of defrosting efficiency seems unchanged, but the reduction of defrosting duration was obviously increased from 17 s to 22 s. However, the differences of defrosting duration and efficiency changed were too small when melted frost was locally drained or not. More experimental cases with lower FEVs should be carried out.