Chapter 6—REFRIGERATION AND AIR CONDITIONINGspecific system or unit can be found in themanufacturer’s technical manual or on the ship’sblueprints.A refrigeration system should not be chargedif there are leaks or if there is reason to believethat there is a leak in the system. The leaks mustbe found and corrected. A system should bechecked for leaks immediately following—orduring—the process of charging.A refrigeration system must have an adequatecharge of refrigerant at all times; otherwise itsefficiency and capacity will be impaired.PURGING THE SYSTEMTo determine if there are noncondensablegases in the system, close the liquid line stopvalve. By-pass all evaporator pressure regulatorvalves and allow the system to pump down oneor more times. Stop the compressor. By-pass thewater regulating valve and circulate cooling waterthrough the condenser. When discharge pressurestops dropping,convert the pressure totemperature and from this subtract thetemperature of the injection or overboard. (Theyboth should be equalized.) A variation of over 5 °Fwill indicate that air and noncondensable gasesare present in the system. Crack open the purgevalve for 2-3 seconds at 2 to 3 minute intervalsuntil the temperature is within 5 °F.CLEANING LIQUIDLINE STRAINERSWhere a liquid line strainer is installed, itshould be cleaned at the same intervals as the suc-tion strainer. If a liquid line strainer becomesclogged to the extent that it needs cleaning, a lossof refrigeration effect will take place. Thetubing on the outlet side of the strainer will bemuch colder than the tubing on the inlet side.To clean the liquid line strainer, secure thereceiver outlet valve and wait a few minutes toallow any liquid in the strainer to flow to the cool-ing coils. Close the strainer outlet valve and verycarefully loosen the cap which is bolted to thestrainer body. (Use all appropriate safety gear.)When all of the pressure is bled out of the strainer,remove the cap and lift out the strainer screen.Clean the strainer screen with a small brush,using an approved solvent. Reassemble the springand screen in the strainer body. Replace thestrainer cap loosely. Purge the air out of thestrainer, by blowing refrigerant through it, thentighten the cap. After assembly is complete, testthe unit for leaks.CLEANING OIL FILTERSAND STRAINERSCompressors arranged for forced feed lubrica-tion are provided with lubricating oil strainers inthe suction line of the lube oil pump and an oilfilter installed in the pump discharge line. Agradual decrease in lubricating oil pressure in-dicates that the units need cleaning. Cleaning isaccomplished in much the same manner asdescribed for cleaning suction strainers.When cleaning is necessary, the lubricating oilin the crankcase should be drained from the com-pressor and a new charge of oil, equal to theamount drained, should be added before re-starting the unit. When the compressor is put backinto operation, the lube oil pressure must beadjusted to the proper setting by adjustment ofthe oil pressure regulator.MAINTAINING COOLING COILSCooling coils should be inspected regularly andcleaned as required. The cooling coils should bedefrosted as often as necessary to maintain theeffectiveness of the cooling surface. Excessivebuild up of frost on the cooling coils will resultin reduced capacity of the plant, low compressorsuction pressure, and a tendency for the com-pressor to short-cycle. The maximum time inter-val between defrosting depends on such factorsas refrigerant evaporating temperature, conditionof door gaskets, moisture content of suppliesplaced in boxes, how frequently the doors areopened and atmospheric humidity.Cooling coils should be defrosted before thefrost thickness reaches three-sixteenths of an inch.When defrosting, do not scrape or break off thefrost, as this may cause damage to the coils.EVACUATING AND DEHYDRATINGTHE SYSTEMThe major cause of system failures is moisture(H2O) which is brought in through air leaks.6-13
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