Experimental investigations on destroying surface tension of melted frost for defrosting performance improvement of a multi-circuit outdoor coil

When frost forms and accumulates over the outdoor coil's surface in an air source heat pump (ASHP) unit, system operating performance will be adversely affected. Currently, reverse cycle defrosting is the most widely used standard defrosting method. A previous related study suggested that, for an ASHP unit with a multi-circuit outdoor coil, water collecting trays installed between circuits could eliminate the negative effects of downwards flowing melted frost due to gravity. However, for a vertical multi-circuit outdoor coil with separations, there is also a lot of melted frost remained on the downside surface of each circuit due to surface tension. The remained water would consume energy, and thus deteriorate system defrosting performance. To quantitatively study the negative effects of surface tension, in this paper, an experimental study changing the vertical multi-circuit outdoor coil into horizontally installed in an ASHP unit was conducted. Compared with the surface tension kept, defrosting duration could be shorted from 186 s to 167 s, or 20 s less, and defrosting efficiency improved from 49.4% to 61.4%, or 12.0% more, when the remained water cleaned as the surface tension destroyed.