When H2O combines with R- 12 it forms sulfricacid which will attack the entire system. Goodengineering practice dictates that evacuation,dehydration and fixing all air leaks will in the longterm prevent an acid attack upon the system.evacuation process. If possible, the dehydratedair should be heated to about 240°F.Large dehydrators, suitable for preliminarydehydration of refrigeration systems, are usuallyavailable at naval shipyards and aboard tendersand repair ships.Where moisture accumulation must be cor-rected, the system should first be cleared ofrefrigerant and air. The time required for theseprocesses will depend upon the size of the systemand the amount of moisture present. It is goodengineering practice to circulate heated airthrough a large dehydrator for several hours,or as long as the dehydrator drying agentremains effective, before proceeding with theAfter the preliminary dehydration, the remain-ing moisture is evacuated by means of a two-stagehigh-efficiency vacuum pump. (These vacuumpumps are available aboard tenders and repairships.)A vacuum indicator (figure 6-9) is attached tothe two-stage high efficiency pump. It consists ofan insulated test tube containing a wet bulb ther-mometer with its wick immersed in distilled water.This indicator is connected to the vacuum pumpsuction line, which in turn, is connected to therefrigeration system. The refrigerant circuit shouldbe closed to the atmosphere and the charging con-nection should be opened to the vacuum pump.Two-stage pumps are started for operation inPARALLEL so that maximum displacement maybe obtained during the initial pump-down stages.When the indicator shows a temperature of about55 °F (0.43 inch Hg, absolute), the pumps areplaced in SERIES operation (wherein thedischarge from the first step enters the suction ofthe second step pump). The dehydration processwill be reflected in the temperature drop of thevacuum indicator (as shown in figure 6-10).Readings will initially reflect ambienttemperatures. Then they will show rapidlyFigure 6-10.—Vacuum indicator readings plotted duringFigure 6-9.—Dehydrator vacuum indicator.dehydration.ENGINEMAN 1 & C6-14
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