valve without having to replace the entire assembly.When replacement of an expansion valve is necessary,you must replace the unit with a valve of the samecapacity and type.ADDITIONAL SYSTEM MAINTENANCEIn addition to the maintenance of the componentspreviously described, other parts of the system will needperiodic maintenance to keep the plant operatingproperly.Vibration may cause leakage in the piping system.This leakage may allow air and moisture to be drawn inor a loss of refrigerant charge. If this happens, the plantoperation will become erratic and inefficient, and thecause of trouble must be corrected.CHARGING THE SYSTEMInformation concerning the charging ofrefrigeration systems may be found in NSTM, Chapter516, “Refrigeration System.” The amount of refrigerantcharge must be sufficient to maintain a liquid sealbetween the condensing and evaporating sides of thesystem. Under normal operating conditions, when thecompressor stops, the receiver of a properly chargedsystem is about 85 percent full of refrigerant. The propercharge for a specific system or unit can be found in themanufacturer’s technical manual or on the ship’sblueprints.A refrigeration system should not be charged if ithas leaks or if you have a reason to believe the systemhas a leak. The leaks must be found and corrected.Immediately following-or during-the process ofcharging, you should carefully check the system forleaks.A refrigeration system must have an adequatecharge of refrigerant at all times; otherwise, itsefficiency and capacity will be impaired.PURGING THE SYSTEMTo determine if the system contains noncondensablegases, operate the system for 30 minutes. Stop thecompressor for 10 to 15 minutes, leaving all the valvesin their normal positions. Observe the pressure andtemperature as indicated on the high-pressure gauge.Read the thermometer in the liquid line, or read thetemperature of the cooling water discharge from thecondenser. Compare the temperature reading with thetemperature conversion figures shown on the dischargepressure gauge. If the temperature of the liquid leavingthe receiver is more than 5°F lower than the temperaturecorresponding to the discharge pressure, the systemshould be purged. Pump the system down and secure thecompressor; then open the purge valve on the condenser.Purge very slowly, at intervals, until the air is expelledfrom the system and the temperature difference dropsbelow 5°F.CLEANING LIQUID LINE STRAINERSWhere a liquid line strainer is installed, it should becleaned at the same intervals as the suction strainer. If aliquid line strainer becomes clogged to the extent that itneeds cleaning, a loss of refrigeration will take place.The tubing on the outlet side of the strainer will be muchcolder than the tubing on the inlet side.To clean the liquid line strainer, secure the receiveroutlet valve and wait a few minutes to allow any liquidin the strainer to flow to the cooling coils. Then closethe strainer outlet valve and very carefully loosen thecap that is bolted to the strainer body. (Use goggles toprotect your eyes!) When all the pressure is bled out ofthe strainer, remove the cap and lift out the strainerscreen. Clean the strainer screen with an approvedsolvent and a small brush. Reinstall the spring andscreen in the strainer body; then replace the strainer caploosely. Purge the air out of the strainer by blowingrefrigerant through it; then tighten the cap. After theassembly is complete, test the unit for leaks.CLEANING OIL FILTERS AND STRAINERSCompressors arranged for forced-feed lubricationhave lubricating oil strainers in the suction line of thelube-oil pump. An oil filter may be installed in the pumpdischarge line. A gradual decrease in lubricating oilpressure indicates that these units need cleaning. Thiscleaning is done in much the same manner as describedfor cleaning suction strainers.When cleaning is necessary, drain the lubricating oilin the crankcase from the compressor. Add a new chargeof oil, equal to the amount drained, before restarting theunit. When the compressor is put back into operation,adjust the lube-oil pressure to the proper setting byadjusting the oil pressure regulator.MAINTAINING COOLING COILSYou should inspect the cooling coils regularly andclean them as required. Defrost the cooling coils as oftenas necessary to maintain the effectiveness of the coolingsurface. Excessive buildup of frost on the cooling coils5-9
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