Quantcast Evacuating and Dehydrating the System

will  result  in  reduced  capacity  of  the  plant,  low compressor  suction  pressure,  and  a  tendency  for  the compressor to short-cycle. The maximum time interval between   defrostings   depends   on   such   factors   as condition of door gaskets, moisture content of supplies placed  in  boxes,  frequency  of  opening  doors, atmospheric  humidity,  and  refrigerant  evaporating temperatures. You should always defrost the cooling coils before the frost thickness reaches three-sixteenths of an inch. When defrosting the coils, be sure that you do NOT try to scrape or break the frost off. Improper defrosting will cause serious damage to the coils. EVACUATING AND DEHYDRATING THE SYSTEM In  areas  where  moisture  accumulation  must  be corrected,  the  system  should  first  be  cleared  of refrigerant and air. The time required will depend upon the  size  of  the  system  and  the  amount  of  moisture present.  It  is  a  good  engineering  practice  to  circulate heated  air  through  a  large  dehydrator  system  for  several hours, or as long as the dehydrator drying agent remains effective,   before   proceeding   with   the   evacuation process. If possible, the dehydrated air should be heated to about 240°F. Large   dehydrators,   suitable   for   preliminary dehydration   of   refrigeration   systems,   are   usually available at naval shipyards and on board tenders and repair  ships.  After  the  preliminary  dehydration,  the remaining   moisture   is   evacuated   by   means   of   a two-stage,   high-efficiency   vacuum   pump   having   a vacuum indicator. (These vacuum pumps are available on board tenders and repair ships.) The vacuum indicator shown in figure 5-4 consists of  an  insulated  test  tube  containing  a  wet-bulb thermometer with its wick immersed in distilled water. The  indicator  is  connected  in  the  vacuum  pump  suction line.  The  suction  line  from  the  vacuum  pump  is connected  to  the  refrigeration  system.  The  refrigerant circuit  should  be  closed  to  the  atmosphere  and  the charging connection opened to the vacuum pump. A two-stage vacuum pump is started for operation in PARALLEL so that maximum displacement may be obtained during the initial pump-down stages. When the indicator  shows  a  temperature  of  about  55°F  (0.43 in.Hg,  absolute),  the  pumps  are  placed  in  SERIES operation (where the discharge from the first step enters the suction of the second step pump). The dehydration process will produce a temperature drop of the vacuum Figure 5-4.—Dehydrator vacuum indicator. indicator  as  shown  in  figure  5-5.  Readings  will  initially reflect ambient temperatures, then show rapidly falling temperatures until the water in the system starts to boil. When  most  of  the  evaporated  moisture  has  been evacuated  from  the  system,  the  indicator  will  show  a Figure 5-5.—Vacuum indicator readings plotted during dehydration. 5-10


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