for avoiding unfavorable frost conditions is to lower the setpoint temperature of the dock to increase the level of moisture removal at a higher evaporator temperature (when compared to the freezer). Reducing the dock setpoint temperature to something in the range of 35°F (2°C), will permit air defrosting while providing significantly more moisture removal when compared to a 50°F (10°C) space setpoint. In some cases, hot-gas reheat is Figure 5: Low density frost forming on an evaporator due to high coil temperature difference added at the dock evaporator to further and presence of supersaturated air. increase the space sensible heat ratio. Figure 4 shows a dock maintained at a 35°F (2°C) dry-bulb capacity decrease due to airflow blockage. Unfavorable frosttemperature with a relative humidity of 60%. A process line ing conditions leads to the formation of ice crystals directly from the entering coil condition to the ADP shows it to be just in the airstream. There is a tendency for these ice crystals to favorable as it approaches but does not cross the saturation precipitate onto cold surfaces within the space; however, they curve. Adding reheat at the dock door can further increase the will ride along on air currents created by operating evaporator dry-bulb temperature of infiltrating air, driving the process to fans. The frost crystals will readily adhere to the coil surface an increasingly “favorable” frost condition process line. by physical impaction or interception; thereby, blocking airflow. Cleland3 offers other strategies for avoiding unfavorable Figure 5 shows the structure of unfavorable frost adhering to frosting conditions but rightly places a particular emphasis on the surfaces of a variably finned, low-temperature evaporator preventing the infiltration using door protection devices. freezing unpacked product and operating with a moderately Frost Type. Somewhat related to discussions in the previous high TD (difference in temperature between the entering air section, the type of frost has an influence on the rate of coil and the evaporating refrigerant). In this case, the structure of www.info.hotims.com/23932-2 www.info.hotims.com/23932-12 February 2009 ASHRAE Journal 31