ASHRAE Journal - July 2009 - 48

ASHRAE 19 50 59–2009 YE A R S JOURNAL Therefore, the receiver must be so located and the drain piping from C into the receiver must be so designed as to create an open passage, which will permit condensate drainage into the receiver without filling the drain line. There must be no traps so that vapor from the receiver can have open passage into the coldest area C. All condensers provide some liquid subcooling. This provides a permissible pressure drop in the liquid feed line to the system without generating flash gas in the liquid feed at the evaporator control valve, (Fig. 10 D). Obviously, the open drain between C and the top of the receiver permits refluxing of the vapors in the receiver into the area of lowest temperature in the condenser, thus maintaining the sub-cooling of the liquid in the receiver. For example: If the condensing temperature of 96°F and 184.2 psig in the condenser was reduced to 183.8 psig through pipe pressure drop in the condenser; and if there was 2°F of actual liquid subcooling, the liquid temperature of 94°F in the receiver has a vapor pressure of 178 psig. Therefore, 183.8 to 178 = 5.8 psi permissible liquid line pressure drop (without any temperature increase) in the liquid delivery line. F I G . 11 Horizontal drain line. PARALLEL DRAINAGE Parallel condenser drainage, not fully understood, has caused serious problems in numerous installations. These problems stem All evaporators have the ability to accumulate excess liquid refrigerant at low load conditions and means must be provided to prevent this kind of accumulation. from the fact that no two condensers will have the exact same leaving condensate temperature. There will be a migration problem unless proper structure is used, which will permit migration without drainage flow interruption. The horizontal drain line (Fig. 11) shown enlarged from condenser 2 has become filled because of the necessary head required to cause horizontal flow. Assuming that the outlet temperature of condenser 2 is lower than that of condenser 1, there will be migration flow from 1 to 2 and the pipe line being filled with liquid acts as a dam against migration flow from condenser 1 to condenser 2. Condenser 2 will thus stop draining and becomes bottled—a term generally applied to that situation. A plate type of freezer may, by virtue of the operational program, cause serious compressor damage. An excess liquid refrigerant return in the operational program occurs when the freezing day is discontinued, the freezer is left filled with product, and refrigeration is continued during the night without any limitation of the amount of liquid refrigerant being accumulated and stored. When this freezer is opened and warm product added, the excess liquid thrown out may well exceed the capabilities of the separator and thereby pass directly to the compressor. A corrective measure is to limit the liquid supply during the night so that the evaporator area will not be filled with liquid at the time of starting the next freezing day. DELAYS Product freezers designed for a –30°F freezing level are often brought down to temperature prior to product entrance. Very often a product arrival estimated for 7:00 a.m. may extend to 8:00 a.m. Meanwhile, under many present-day control arrangements, the freezer will remain on line and continue to lower in temperature. There is very little heat load, the liquid content of the coils becomes excessive, and immediately when the warm product load arrives there is a serious liquid refrigerant return to the compressor. A proper control would have stopped refrigerant flow the instant –30°F was obtained, and a two degree differential thermostat would renew refrigerant flow at –28°F. 48 ASHRAE Journal COMPRESSOR DAMAGE Fig. 8 illustrates the low temperature section of a pump recirculation system. The pumping tank, at evaporator presa s h r a e. o rg July 2009

ASHRAE Journal - July 2009

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