OIL SEPARATORS

  Proper refrigerant piping design and operation of the system within its design limits so that adequate refrigerant velocities can be maintained are the only cure for oil logging problems, but an oil separator may be a definite aid in maintaining lubrication where oil return problems are particularly acute.

  For example, consider a compressor having an oil charge of 150 ounces, with the normal oil circulation rate being 2 ounces per minute. This means that on a normal system with proper oil return at stabilized conditions, two ounces of oil leave the compressor through the discharge line every minute, and two ounces return through the suction line. If a minimum of 30 ounces of oil in the crankcase is necessary to properly lubricate the compressor, and for some reason oil logged in the system and failed to return the compressor, the compressor would run out of oil in 60 minutes. Under the same condition with an oil separator having an efficiency of 80%, the compressor could operate 300 minutes or 5 hour before running out of oil.

  As a practical matter, there seldom are condition in a system when no oil will be returned to the compressor, and even with low gas velocities, some fraction of oil leaving the compressor will be returned. If there are regular intervals of full load conditions or defrost periods when oil can help to bridge long operating periods at light load condition. Oil separators are mandatory on systems with flooded evaporators controlled by a float valve, on all two stage and cascade ultra-low temperature system, and on any system where oil return is critical.

  Oil separators should be considered as a system aid but not a cure-ail or a substitute for good system design. They are never 100% efficient, and in fact may have efficiencies as low as 50% depending on system operating conditions. On systems where piping design encourages oil logging in the evaporator, an oil separator can compensate for system oil return deficiencies only on temporary basis, and may only serve to delay lubrication difficulties.

  If a system is equipped with a suction accumulator, it is recommended that the oil return from the separator be connected to the suction line just ahead of the accumulator. This will provide maximum protection against returning liquid refrigerant to the crankcase. If the system is not equipped with a suction accumulator, the oil return line on suction cooled compressors may be connected to the suction line if more convenient than the crankcase,but on air cooled compressor, oil return must be made directly to the crankcase to avoid damage to the compressor valves.

  If the separator is exposed to outside ambient temperatures, it must be insulated to prevent refrigerant condensation during off periods, resulting in return of liquid to the compressor crankcase. Small low wattage strap-on heaters are available for oil separators, and if any problem from liquid condensation inn the separator is anticipated, a continuously energized heater is highly recommended.