22 May 2008

PREDICTION OF ICE AND FROST FORMATION IN THE FIN TUBE EVAPORATORS FOR AIR/WATER HEAT PUMPS


In the next few years, the seasonal performance factor of air/water heat pumps will be greatly
increased. Success in doing this will depend on two factors: The adaptation of the operating
characteristic of the heat pump by the use of continuous power control and the reduction of ice
and frost formation. Frost leads to a considerable reduction of the airflow through the evaporator
and, therefore, of heat and mass transfer. The formation of ice and frost is influenced by a
complex interaction between evaporator, fan characteristic and the characteristic of the heat
pump itself. In order to further improve the efficiency of the overall system, a precise prediction
of the operation behaviour is required. Based on the theory of simultaneous heat and mass
transfer, a mathematical-physical simulation program was developed in the course of the
LOREF project. This program allows forecasts of the non-stationary the operation behaviour of
the air/water heat pump during ice and frost formation in the evaporator to be made. Here, the
greatest challenge was to develop a physical correlation for the properties of the ice and frost
deposits during this non-stationary process. Numerous experiments allowed four fields to be
distinguished with different characteristics – dependent on the temperature on the boundary layer
between air and ice or frost. Comparison with the results of measurements made show that the
simulation program developed represents the processes involved in an air/water heat pump
during its operation with a high degree of accuracy.