Demand Defrost Controls for Walk-in Freezers

The refrigeration systems of walk-in coolers and freezers operate continuously to maintain proper food product temperatures within the conditioned space. One primary heat gain constituent in walk-ins is the infiltration of warm and moist air from the adjacent space. A key factor associated with infiltration in walk-ins is the opening and closing of the walk-in doors. Ultimately, the added moisture due to infiltration leads inevitably to frost formation on evaporator coils. Frost formation occurs when the moist air comes in contact with a surface temperature that is below the dew point temperature of the air. Hence, frost accumulates on the evaporator coil. The insulating properties of frost slow down the heat transfer and impede airflow through the evaporator coil, thereby decreasing the refrigeration capacity. Therefore, the evaporator coils need to be defrosted in order to maintain performance and product temperatures within the refrigerated zones. The most common defrost methods are off-cycle and electric defrost. Generally, off-cycle is used for medium-temperature (cooler) and electric defrost is used for low-temperature (freezer) application. In both methods, the refrigeration compressor shuts off, stopping the flow of refrigerant to the evaporator, while the evaporator fan motors stay running. Fan motors blow air over the evaporator coil surface, melting the frost. In electric defrost, electrical heating elements are energized to warm the circulating air over the coil. The conventional defrost method for walk-ins is time initiated and temperature terminated. The consequence of defrost, however, is an increase in the energy usage of the refrigeration system. Hence, defrost technologies that can detect frost on the evaporator and accordingly initiate and terminate defrosts can greatly improve the efficiency of defrost cycles. The purpose of this project is to investigate the viability of a demand defrost technology compared to a conventional defrost method for a walk-in freezer application. The study focuses on the technology’s capability to initiate defrost based on ice or frost build-up on the evaporator coil. For defrost termination, the technology relied on conventional temperature termination method during the evaluation. The system evaluated in the laboratory was comprised of a condensing unit, an evaporator coil assembly, and all the necessary controls associated with a programmable controller. According to the manufacturer, refrigerant temperature at the inlet and outlet of the evaporator coil, as well as refrigerant pressure at the outlet of the evaporator, are measured to calculate the degradation in refrigeration capacity for initiating defrost. When the system performance or capacity drops below a predetermined efficiency, defrost is triggered to initiate. Tests intended to isolate the performance of the demand defrost technology with respect to conventional defrost at outdoor dry-bulb temperatures (DBTs) of 75°F, 90°F, and 110°F. For all 6 test runs, the adjacent space DBT and relative humidity (rh) were maintained fixed at 80°F and 60%, respectively, representing kitchen environment. Additionally, the interior walk-in freezer box temperature was set at 0°F. All tests were performed under controlled laboratory setting at Southern California Edison’s (SCE’s) Technology Test Centers (TTC) controlled environment chambers. Overall, results indicated that the demand defrost technology under evaluation had no or little impact on the power demand and energy usage of the refrigeration unit. For all three test conditions, the demand defrost technology underwent less defrost cycles per day compared to conventional four defrost per day schedule. The impact of reduced defrost frequency was evident on the product temperatures, especially for test runs at higher outdoor ambient DBTs. Ultimately, data obtained here suggests no tangible power demand and energy benefits associated with the evaluated demand defrost technology. A properly commissioned walk-in freezer with time initiated, temperature terminated defrost controls will have similar energy impacts as demand defrost controls. A recommended next step would be to broaden the evaluation to control systems that optimize the refrigeration system beyond demand defrost.