REMOVING OIL FROM A COMPRESSOR

  Occasionally problems in line sizing or system operation may caused oil to trap in the evaporator or suction line, and large amounts of oil may be added to the system in an effort to maintain a satisfactory oil level in the compressor. When the basic oil logging problem is corrected, the excess oil will return to the compressor crankcase, and unless removed from the system, can cause oil slugging, excessive oil pumping, and possible compressor damage. Also in cases where the system has been contaminated, for example by a broken water tube in a water cooled condenser, or in cleaning  a system after a bad motor burn, it may be necessary to completely remove the oil from the compressor crankcase.

  To some extent choice of a method for removing oil depends on the degree of system contamination. For removing excess oil or on system with only slight contamination, almost any method is acceptable. However if the system is badly contaminated, it may be advisable to remove the compressor bottom plate and thoroughly the interior of the crankcase.

1. Removing by Oil Drain Plug
  Some compressor are equipped with oil drain plugs. If so, this provides an easy method for removing oil.


  Close the suction service valve, and operate the compressor until the crankcase pressure is reduced to approximately 1 to 2 psig. Stop the compressor and isolate the crankcase by closing the discharge service valve. Carefully loosen the oil drain plug, allowing any pressure to bleed off before the threads are completely disengaged. Drain oil to the desired level by seepage around the threads without removing the plug.

  When draining is complete, tighten the drain plug, open the compressor valves, and restore the compressor to operation. The oil seal at the drain hole and the residual refrigerant pressure in the crankcase will effectively block the entrance of any measurable quantities of air or moisture into the system.

2. Removing by Oil Fill Hole
  If a drain plug is not convenient or is not furnished on the compressor, oil may be removed by means of the oil fill hole.

  Close the compressor suction service valve, reduce the crankcase pressure to 1 to 2 psig, and isolate by closing the discharge service valve.

  Carefully loosen the oil fill plug, allowing any pressure to bleed off before the threads are completely disengaged. Remove the oil fill plug, and insert a 1/4¨O.D. copper tube, so that the end is at or near the bottom of the crankcase. If possible use a tube of sufficient length so that the external end can be bent down below the crankcase, thus forming a syphon arrangement. Wrap a waste rag tightly around the oil fill opening, and crack the suction service valve, pressurizing the crankcase to approximately 5 psig, and then reclose the valve.

  Oil will be forced out the drain line, and will continue to drain by syphon effect until the crankcase is emptied. If the syphon arrangement is not possible, repressurize the crankcase as necessary to remove the desired amount of oil.

  The residual refrigerant pressure in the crankcase will prevent the entrance of any serious amounts of moisture or air into the system. Purge the crankcase by cranking the suction service valve off plug, tighten, open the compressor valves, and restore the compressor to operation.

  In large system where a large amount of excess oil must be removed, or where oil must be removed at intervals over a prolonged period, considerable time can be saved by brazing a dip tube in valve so that oil can be removed as desired as long as the crankcase pressure is above 0 psig. To speed up separation, the oil should be removed to a 1/4 sight glass level. After oil removal is complete, the oil level may then be raised to the normal operating level.


3. Removal by Means of Baseplate
  On accessible compressor, it may be necessary to remove the base plate if complete crankcase cleaning is necessary.

  Pump the system down to isolate the compressor, remove the base plate, clean as necessary, and reinstall with new gasket. Since both air and moisture can enter the crankcase during this operation, the crankcase should be evacuated with a vacuum pump before restoring to operation. In an emergency, the crankcase may be purged by cracking the suction service valve and venting through the oil fill hole and the discharge service port. Replace the plug in the oil fill hole and jog the compressor a few times by starting and stopping, discharging through the discharge service port. Cap the discharge service port, the discharge valve, and the compressor can be restored to operation.

4. Removing Oil From Welded Compressors
  If the oil must be removed from a welded compressor, for example to recharge with a measured amount of oil, the compressor must be removed from the system, and the oil drained out the suction line stub tilting the compressor

  After compressor is reinstalled the system must then be evacuated by means of an access valve or the process tube before recharging with refrigerant and restoring to operation.