The performance deterioration of an air cooling coil with an air-on temperature of 0°C (32°F) was measured as it frosted for a range of heat loads and sensible heat ratios (SHR). At high SHR (low air relative humidity, RH) and low heat load (low air to refrigerant temperature difference, TD), decline in airflow rate and heat transfer performance was more gradual than at lower SHR (high RH) and high heat load (high TD) for the same total frost accumulation. The rate of decline was consistent with the theory suggesting unfavorable frost formation if the air becomes supersaturated as it is cooled. Equations to predict the transition between favorable and unfavorable frost formation are developed and shown to satisfactorily predict frost type if the coil surface temperature is approximated by the refrigerant evaporation temperature. Some recovery of coil performance was observed when operation at low SHR (with rapid deterioration) was followed by a period of operation with high SHR.

Units: Dual