The effect of favourable and unfavourable frost on air cooling coil performance : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology, at Massey University
The most common type of air cooling coil used in the refrigeration industry is the finned tube heat exchanger. The performance of such coils can be greatly hindered by frost formation, which will occur when the coil surface temperature is both below the dewpoint of the air passing over it, and below 0°C. Frost reduces performance, both through the increased thermal resistance of the frost layer, and by reduction of the air flow through the coil. Whilst frosting on coils is influential on performance, there is comparatively little information available on the performance of finned tube heat exchangers under frosting conditions. Smith (1989) has proposed an "unfavourable" frost formation theory. The theory states that unfavourable frost formation occurs when the line representing the temperature and humidity of the air passing through the coil, crosses the saturation line of the psychrometric chart. This criteria is more likely to occur under conditions of high relative humidity, low sensible heat ratio (SHR), and/or high refrigerant-to-air temperature difference (TD). Under unfavourable conditions it is suggested that the frost will be of particularly low density, which would cause coil performance to decline to a much greater extent for the same total frost accumulation, than under "favourable" frosting conditions. The objectives of this study were to measure the change in performance of a cooling coil under frosting conditions, and to assess the validity of the unfavourable frost formation theory. A calorimeter style coil test facility was used, that allowed coil performance to be measured as frost accumulated in a manner consistent with coil operation in industrial practice (i.e. declining air flowrate and a wide range of SHR's). The data collected supported the concept of unfavourable frost formation with a more rapid decline in performance for operation with low SHR, than that at high SHR, for the same total frost accumulation. Some recovery of coil performance was observed when operation at low SHR (with rapid performance deterioration) was followed by a period of high SHR operation. Equations were developed that allowed the theoretical conditions for the formation from favourable to unfavourable frosting to be quantified. The measured change in the rate of coil performance deterioration with frost buildup was dependent on air and coil conditions, in a manner consistent with these equations. The transition between favourable and unfavourable frost formation appeared to be related to the lowest temperature on the coil surface rather than the mean surface temperature. Satisfactory predictions of frost formation types were obtained by using the refrigerant evaporation temperature as an approximation to the lowest coil surface temperature.