ASHRAE 19 50 59–2009 YE A R S JOURNAL of frost. The result was a sufficient accumulation of liquid in the separator to shut down the compressors because of high liquid level. However, other areas of the plant needed refrigeration, and that prompted the attendant to short out the safety switch, thus getting a compressor on line. Serious compressor damage resulted from this maneuver. Fig. 9 shows a typical defrost system using hot gas and depending on accumulating a partial amount of liquid in the separator. Separators are not always of sufficient size to accomodate the volume of liquid to be returned from the the coil. Many installations rely on a timed pumpout so as to decrease the liquid volume to a safe level. The fault associated with this pumpout method is the variable of heat transfer because of frost buildup. A frost buildup of 50% of fin space closure might provide for a sufficient liquid pumpout in one hour. But if the fin space is permitted to become fully closed with frost it could require as much remove the volume of liquid normally as four hours to pumped out in one hour with 50% fin frost closure. A recent investigation of a compressor problem, after five years of trouble-free operation, showed that a change in the number of defrosts was the cause. For five years the programming system caused three defrosts every 24 hours. New management changed the defrost programming to two defrosts every 24 hours. The result was an insufficient liquid removal in the one-hour pumpout. The separator was overfilled at each defrost and a severe liquid return to the compressor resulted with each defrost. The volume of liquid returning to any compressor may not in itself be sufficient to damage the compressor, but liquid in any small quantity may cause oil foaming and thus lower the lubricating quality of that oil. F I G . 9 Typical defrost system using hot gas. trolled for all possible load changes, programmed for defrost before excess frost accumulation, and with a main line suction line separator for collection and prompt disposition of any accidental returned liquid refrigerant. SUMMARY Positive action for full compressor protection begins by designing into the system correctly sized evaporators, conwww.info.hotims.com/25206-9 July 2009 ASHRAE Journal 51