22 May 2008

DEFROSTING THE FIN TUBE EVAPORATORS OF AIR/WATER HEAT PUMPS USING AMBIENT AIR


Air/water heat pumps for heating systems are becoming increasingly popular. A
major challenge for these systems is the formation of ice and frost in the fin-tube evaporator.
Defrosting is usually accomplished by the reverse-cycle or hot-gas defrosting technique. In
both defrosting modes, the compressor is in operation. An alternative method is ambient-air
defrosting, where the heat pump’s compressor is simply turned off and defrosting is achieved
by exposure to ambient air, which is conveyed through the fin tube evaporator by the fan. A
model has been developed which permits the calculation of the simultaneous heat and mass
transfer during the melting of ice and frost deposits. The theoretical results have been
validated by numerous defrosting experiments. The theoretical and experimental
investigations revealed that both temperature and humidity of the ambient air strongly
influence the total heat flow and the time needed for defrosting. Already at an ambient air
temperature of 2°C and a dew point temperature of 0°C, the heat flow of ambient-air
defrosting is comparable to that of hot-gas defrosting; energy consumption is decreased by a
factor of 8–10, however. On the other hand, ambient-air defrosting is less effective at air
conditions below the extended fog-isotherm at 0°C. The application of this technique can
significantly reduce defrosting energy consumption and improve the overall efficiency of
air/water heat pumps.