will result in reduced capacity of the plant, lowcompressor suction pressure, and a tendency for thecompressor to short-cycle. The maximum time intervalbetween defrostings depends on such factors ascondition of door gaskets, moisture content of suppliesplaced in boxes, frequency of opening doors,atmospheric humidity, and refrigerant evaporatingtemperatures.You should always defrost the cooling coils beforethe frost thickness reaches three-sixteenths of an inch.When defrosting the coils, be sure that you do NOT tryto scrape or break the frost off. Improper defrosting willcause serious damage to the coils.EVACUATING AND DEHYDRATING THESYSTEMIn areas where moisture accumulation must becorrected, the system should first be cleared ofrefrigerant and air. The time required will depend uponthe size of the system and the amount of moisturepresent. It is a good engineering practice to circulateheated air through a large dehydrator system for severalhours, or as long as the dehydrator drying agent remainseffective, before proceeding with the evacuationprocess. If possible, the dehydrated air should be heatedto about 240°F.Large dehydrators, suitable for preliminarydehydration of refrigeration systems, are usuallyavailable at naval shipyards and on board tenders andrepair ships. After the preliminary dehydration, theremaining moisture is evacuated by means of atwo-stage, high-efficiency vacuum pump having avacuum indicator. (These vacuum pumps are availableon board tenders and repair ships.)The vacuum indicator shown in figure 5-4 consistsof an insulated test tube containing a wet-bulbthermometer with its wick immersed in distilled water.The indicator is connected in the vacuum pump suctionline. The suction line from the vacuum pump isconnected to the refrigeration system. The refrigerantcircuit should be closed to the atmosphere and thecharging connection opened to the vacuum pump.A two-stage vacuum pump is started for operationin PARALLEL so that maximum displacement may beobtained during the initial pump-down stages. When theindicator shows a temperature of about 55°F (0.43in.Hg, absolute), the pumps are placed in SERIESoperation (where the discharge from the first step entersthe suction of the second step pump). The dehydrationprocess will produce a temperature drop of the vacuumFigure 5-4.—Dehydrator vacuum indicator.indicator as shown in figure 5-5. Readings will initiallyreflect ambient temperatures, then show rapidly fallingtemperatures until the water in the system starts to boil.When most of the evaporated moisture has beenevacuated from the system, the indicator will show aFigure 5-5.—Vacuum indicator readings plotted duringdehydration.5-10
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