Frosting and Defrosting of Air-Coils
Doctoral thesis, 1996
Experience indicates that air-source heat pumps have more operating problems than ground-source heat pumps. One major factor pertains to the effects of frosting and the subsequent need for defrosting. This thesis reviews previous research and practical experience and presents novel experimental facilities to investigate the process of frosting. This includes a test chamber to achieve the necessary control of ambient conditions as well as equipment to monitor frost growth. The thesis also includes a new approach on how to obtain practical values of frost density and thermal conductivity from experimental investi-gations of ordinary air-coils. Finally, the problems of defrost control are discussed and a number of alternative methods are compared.
Experience from real installations as well as from laboratory tests indicates that defrosting, in general, is carried out much too frequently. This is often the case even with demand control systems. Furthermore, there are problems with direct malfunctioning of defrost controllers, unsuccessful defrosts and frozen drip-trays. Laboratory tests show that, on average, the mean COP, including defrosting, is 10 % lower than the COP with a non-frosted coil.
The test chamber features displacement ventilation to achieve high condi-tioning air flows with low air velocity (240 ACH with a velocity < 0.5 m/s). Specially designed dampers and a special control strategy ascertains that de-frosting of the conditioning coils of the chamber will exert no undue influence on the room climate (±0.3 K during a defrost). A support frame with a strain-gauge load-cell facilitates continuous monitoring of frost mass.
Frost density and thermal conductivity are two of the most important factors in calculating the effect of frost on coil performance. A new method is proposed on how to derive effective values of these properties by means of measured frost mass and calculated pressure drop and heat transfer. This method provides results in reasonable agreement with previous research within the experimental uncertainties (which may be up to 50 % in previous work). Furthermore, the new methodology has the advantage of yielding on-line measuring possibilities and being applicable in the determination of mean values for real-world coils.
Finally, the thesis compares a number of alternative defrost indicators with a proposed optimized control. This control can be realized by combining information from sensors of sophisticated safety control systems of heat pumps or refrigeration equipment to actually optimize COP or capacity, as the need may be, i.e. a form of Integrated Refrigeration Management.