Optical Demand Defrost
This report presents laboratory test data investigating the operation of an optical demand defrost system in a medium temperature open vertical refrigerated display case. In open vertical display cases, infiltration of warm and moist air into the cabinet is responsible for a significant portion of the case cooling load. The air curtain typically is severely impacted by the buildup of frost on the coil, thus increasing the amount of infiltration to the refrigerated case. Demand defrost technologies use various methods to monitor frost buildup on evaporator coils and initiate defrost when it is needed. Conventional defrost methods typically operate on a strict time schedule that does not change when the case defrost requirements change. Demand defrost should reduce the variance in air curtain stability seen in typical display case operation. Typically, when display cases go into defrost the product temperatures increase significantly. In theory, the shorter defrost cycles of a demand defrost system would result in a greatly reduced product temperature swing. This would likely lead to longer shelf-life and enhanced product quality for items merchandised in display cases. Optical demand defrost technology relies on signals received from a visible red light (650 nm) emitted from an LED (Figure ES-1 Item ). The light is transmitted through some of the strands of a multi-core fiber optic cable to an output point, or lens, which is pointed at the edge of a metal tab. The output point with integral tab is clipped to an area of the evaporator that will accumulate frost, e.g. a fin. As frost accumulates on the evaporator fins, it also accumulates on the tab in front of the fiber optic output. In theory, the more frost accumulates, the more light is reflected back onto the fiber optic output point. The remaining strands of the multi core fiber optic cable are used to transmit reflected light back to a photo detecting element. When the intensity of the reflected light reaches a preset threshold, a transistorized output is triggered to close the contacts on an external relay indicating the threshold of frost buildup has been reached.