decrease in temperature. When the temperature reaches35°F (0.2 in.Hg, absolute), dry air should be admittedthrough a chemical dehydrator into the system at a pointfarthest from the pump. Continue operating the pump sothe dry air will mix with and dilute any remainingmoisture. Secure the opening that feeds the dry air intothe system. Continue evacuating the system until theindicator again shows a temperature of 35°F. Thedehydration process is complete. Close the valves anddisconnect the vacuum pump.Sometimes obtaining a temperature as low as 35°Fin the vacuum indicator will be impossible. Theprobable reasons for such a failure and the correctiveprocedures to take are as follows:l Excess moisture in the system. The dehydrationprocedure should be conducted for longerperiods.l Absorbed refrigerant in the lubricating oilcontained in the compressor crankcase. Removethe lubricating oil from the crankcase beforeproceeding with the dehydration process.l Leakage of air into the system. The leak must befound and stopped. You must then repeat theprocedure required for detecting leaks in thesystem.l Inefficient vacuum or defective vacuumindicator. The defective unit(s) should berepaired or replaced.Immediately after each period of use or after thesystem has been opened for repairs, replace the dryingagent in the dehydrator. If a replacement cartridge is notavailable, reactivate the drying agent and use it until areplacement is available.You can reactivate the drying agent by removingand heating it for 12 hours at a temperature of 300°F tobake out the moisture. Place the drying agent in an ovenor circulate a stream of hot air through the cartridge.Both methods are satisfactory for reactivatingcommonly used dehydrating agents such as activatedalumina or silica gel. The specific instructions furnishedby the manufacturer should be followed to reactivatespecial drying agents.After reactivation, replace the drying agent in thedehydrator shell and seal it as quickly as possible. Thisprevents absorption of atmospheric moisture. When thedrying agent becomes fouled or saturated withlubricating oil, replace it with a fresh charge, ordehydrator cartridge, taken from a sealed container.Remember that the dehydrators permanentlyinstalled in refrigeration systems of naval ships aredesigned to remove only the minute quantities ofmoisture unavoidably introduced into the system. Youmust be careful to prevent moisture or moisture-ladenair from entering the system.CLEANING THE SYSTEMSystems may accumulate dirt and scale as a resultof improper techniques used during repair or installationof the system. If such dirt is excessive and a tank-typecleaner is available, connect the cleaner to thecompessor suction strainer. When such a cleaner is notavailable, a hard, wool felt filter about five-sixteenthsinch thick should be inserted into the suction strainerscreen. Run the plant with an operator in attendance forat least 36 hours or until the system is clean. The lengthof time required for a clean system depends upon thesize and condition of the plant.AIR-CONDITIONING SYSTEMMost of the information presented so far applies tothe refrigeration side of a system, whether it is used fora refrigeration plant or for air conditioning. Thecompressor controls for both types of systems are nearlyidentical; however, the devices used to control spacetemperatures differ, The two-position dual control,called 2PD, is used for the automatic control of mostshipboard air-conditioning systems.TWO-POSITION DUAL CONTROL (2PD)This control is used on three types of systems:Type 1.Systems employing a simple thermo-statically controlled single-pole switchto control flow of refrigerant to thecooling coilType 2.Systems using reheaters, employing ather- mostatic element actuating twointerlocked switchesType 3.Systems using reheaters in the samemanner as those in type 2, with controlof humidity added where specifiedThe type 1 system, because of its simplicity, requireslittle explanation. The thermostat consists of atemperature-sensing element actuating a single-pole,single-throw switch. It opens and closes a magneticvalve to start and stop the flow of refrigerant-chilledwater or commercial refrigerant. This type of control is5-11
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