Optimizing Evaporator Runtime and Defrost Frequency
Optimizing Evaporator Runtime and Defrost Frequency (2015)-When cooling air to temperatures below freezing, the surfaces (fins and tubes) of evaporators unavoidably accumulate frost which must periodically be removed by defrosting. The defrosting process is inherently bad because it: a) reduces system refrigeration capacity by interrupting the air cooling process, and b) increases system power consumption by adding heat to the space which must then be removed by the refrigeration system. The room temperature and relative humidity in combination with the evaporating temperature defines the Sensible Heat Ratio (SHR) for the air cooling process. The SHR in turn indicates exactly how much moisture will be removed from the airstream and end up as frost on the evaporator (the “frost load”). To date, manufacturers of evaporators have not really offered much guidance to refrigeration system designers regarding how to properly select evaporators for a given frost load. Most of the time, manufacturers’ application guidance consists of “Well, when the frost load is heavy increase the number of defrosts, Oh and by the way, watch out.” The paper quantifies the rate of moisture removal as a function of SHR and shows the effect of frost accumulation on evaporator airflow and cooling capacity. The predicted reduction in evaporator cooling capacity over time is used to determine an appropriate value for runtime hours (hours/day) used for equipment selection as well as selecting an appropriate defrost frequency (no. of defrosts/ day).
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