Flat tube heat exchangers: Experimental analysis of frosting and water retention

In air source heat pumps, frost may form on the surface of the evaporator at low ambient air temperatures. If significant amounts of frost accumulate, the heat pump efficiency decreases so the heat exchanger needs to be defrosted. The aim of this work is an experimental analysis of different flat tube heat exchanger designs for use as direct evaporators in heat pumps. The analysis will quantify heat pump efficiency potentials in the context of cyclic frosting and defrosting conditions. For the purpose of direct evaporators in heat-release controlled heat pumps, the heat exchanger designs are studied in a heat carrier circuit under constant air heat absorption control during the frosting processes. Previous published investigations assume a constant inlet temperature, whereby the heat absorption degrades as frosting occurs. The effect of heat exchanger design on water retention from defrosting and the impact this has on the length of the refrosting phase, is analyzed experimentally. The evaluation criteria for a heat exchanger are the rate of obstruction of the air path due to frost formation and the retention of as small a mass of melt water as possible after defrosting. The heat exchanger properties fin pitch, fin overlap, and heat exchanger depth, as well as the omission of louvers and different surface coatings, are analyzed experimentally using the evaluation criteria. It is found that heat exchangers with louvered fins and an uncoated surface are advantageous for an efficient cyclic frosting and defrosting process. In the executed experiments a larger fin pitch of 1.4 mm can extend the frosting process up to 36% compared to a fin pitch of 1.15 mm. An increase in heat exchanger depth from 28 mm to 34 mm lengthens the frosting process by 26%.