Frost growth, mitigation, defrost
When the operating conditions are extremely cold and humid and the surface temperature of the heat exchanger well below the freezing point (lower than the dew point temperature of the air) moisture from the air stream will freeze on the surface after condensation and the frost will start growing. The frost growth degrades the performance of the system considerably. It hinders the airflow and increases the pressure drop through the coil which means more fan power is requires for to maintain the desired flow rate. With reduced flow rate due to the increase of pressure drop, system’s capacity drops rapidly. In the case of heat pump the capacity of the evaporator decreases due to the airflow drop, which reduces the overall heating capacity and coefficient of performance of the heat pump. Additionally, the frost layer increases the thermal resistance to the heat transfer between the air and refrigerant. The reduction in airflow and increased thermal resistance reduces the heat energy extracted by the evaporator and decreases the heat pump capacity and efficiency. Similar process is observed for the cooling coils of commercial refrigeration system where the frost growth can dramatically reduce the system capacity. Once the performance reaches its minimum acceptable stage, a defrost process is introduced to remove the frost layer and to achieve the performance at the start of the cycle. The frost defrost process is repeated continuously. Overall the frost growth is highly undesired phenomena which can cause considerable reduction in performance of the system. This study overviews different procedures to counteract the frost growth. Various frost mitigation procedures have been reviewed and compared to access their feasibility. The methods such as air treatment before entering the heat exchanger are used to effectively eliminate or at least minimize the frost growth rate. Such procedures are discussed under two major categories, air treatment processes to mitigate the frost and appropriate system modification to minimize or eliminate the frost growth.