Detection of Evaporator Frost

Frost growth is a major concern in many air conditioning and refrigeration systems. Frost buildup causes an increase in the airside pressure drop and thermal resistance of the evaporator, yielding a substantial decrease in operational efficiency. To date, methods for reliable detection of frost have not been available. Approaches in which ???point??? sensing of frost is used for detection have difficulties when the growth pattern of the frost changes due to manufacturing variability, installation variations, and differences in operational conditions. Defrost cycles rely almost exclusively on timers set by the manufacturer based on previous tests. In this project, two different methods were investigated as possible frost detection techniques. Small accelerometers were used to record and compare acceleration power spectrum signals from the evaporator structure as frost adhered to the evaporator surface. The white noise vibration from the expansion valve, used as an excitation source for the accelerometers, did not propagate through the evaporator tubes because the liquid of the two-phase flow refrigerant damped the vibrations. It appears that the method is not useful as a means of frost detection for home refrigerators although it may have utility for other types of refrigeration systems. In the case of home refrigerators, other sources of vibrational excitation of the evaporator should be explored. The electro-mechanical impedance method is being currently studied as a structural health monitoring technique. In this project, the method was tested as a mean of monitoring frost formation on the evaporator. The impedance signal obtained using small piezoceramic wafers showed significant changes as frost was formed on a test structure (an isolated evaporator fin). These results suggest that the electro-mechanical impedance method could be used as a frost detection technique and further investigation should be performed.Air Conditioning and Refrigeration Project 14