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  same capacity  and  type. ADDITIONAL  SYSTEM  MAINTENANCE In addition to the maintenance of the components previously described, other parts of the system will need periodic  maintenance  to  keep  the  plant  operating properly. Vibration may cause leakage in the piping system. This leakage may allow air and moisture to be drawn in or a loss of refrigerant charge. If this happens, the plant operation  will  become  erratic  and  inefficient,  and  the cause of trouble must be corrected. CHARGING THE SYSTEM Information  concerning  the  charging  of refrigeration systems may be found in NSTM, Chapter 516, “Refrigeration System.” The amount of refrigerant charge  must  be  sufficient  to  maintain  a  liquid  seal between  the  condensing  and  evaporating  sides  of  the system. Under normal operating conditions, when the compressor  stops,  the  receiver  of  a  properly  charged system is about 85 percent full of refrigerant. The proper charge for a specific system or unit can be found in the manufacturer’s  technical  manual  or  on  the  ship’s blueprints. A refrigeration system should not be charged if it has leaks or if you have a reason to believe the system has  a  leak.  The  leaks  must  be  found  and  corrected. Immediately   following-or   during-the   process   of charging,  you  should  carefully  check  the  system  for leaks. A   refrigeration   system   must   have   an   adequate charge   of   refrigerant   at   all   times;   otherwise,   its efficiency and capacity will be impaired. PURGING THE SYSTEM To determine if the system contains noncondensable gases,  operate  the  system  for  30  minutes.  Stop  the compressor for 10 to 15 minutes, leaving all the valves in  their  normal  positions.  Observe  the  pressure  and temperature  as  indicated  on  the  high-pressure  gauge. Read  the  thermometer  in  the  liquid  line,  or  read  the temperature  of  the  cooling  water  discharge  from  the condenser. Compare the temperature reading with the temperature conversion figures shown on the discharge pressure gauge. If the temperature of the liquid leaving the receiver is more than 5°F lower than the temperature corresponding  to  the  discharge  pressure,  the  system should be purged. Pump the system down and secure the compressor; then open the purge valve on the condenser. Purge very slowly, at intervals, until the air is expelled from the system and the temperature difference drops below 5°F. CLEANING LIQUID LINE STRAINERS Where a liquid line strainer is installed, it should be cleaned at the same intervals as the suction strainer. If a liquid line strainer becomes clogged to the extent that it needs cleaning, a loss of refrigeration will take place. The tubing on the outlet side of the strainer will be much colder than the tubing on the inlet side. To clean the liquid line strainer, secure the receiver outlet valve and wait a few minutes to allow any liquid in the strainer to flow to the cooling coils. Then close the strainer outlet valve and very carefully loosen the cap that is bolted to the strainer body. (Use goggles to protect your eyes!) When all the pressure is bled out of the  strainer,  remove  the  cap  and  lift  out  the  strainer screen.  Clean  the  strainer  screen  with  an  approved solvent  and  a  small  brush.  Reinstall  the  spring  and screen in the strainer body; then replace the strainer cap loosely.  Purge  the  air  out  of  the  strainer  by  blowing refrigerant  through  it;  then  tighten  the  cap.  After  the assembly is complete, test the unit for leaks. CLEANING OIL FILTERS AND STRAINERS Compressors  arranged  for  forced-feed  lubrication have lubricating oil strainers in the suction line of the lube-oil pump. An oil filter may be installed in the pump discharge  line.  A  gradual  decrease  in  lubricating  oil pressure indicates that these units need cleaning. This cleaning is done in much the same manner as described for cleaning suction strainers. When cleaning is necessary, drain the lubricating oil in the crankcase from the compressor. Add a new charge of oil, equal to the amount drained, before restarting the unit. When the compressor is put back into operation, adjust  the  lube-oil  pressure  to  the  proper  setting  by adjusting  the  oil  pressure  regulator. MAINTAINING COOLING COILS You should inspect the cooling coils regularly and clean them as required. Defrost the cooling coils as often as necessary to maintain the effectiveness of the cooling surface. Excessive buildup of frost on the cooling coils 5-9