In most refrigeration systems oil escaping from the compressor is not a major problem and oil separators are usually not necessary. This is because most systems use halocarbons as refrigerants.
Except at low temperatures, oil is quite soluble in halocarbons. As such, oil returns to the compressor with the refrigerant before significant adverse effects occur.
Systems in which oil separators are needed include:
1. Systems using ammonia as a refrigerant because ammonia and oil do not mix.
2. Systems using halocarbons at low temperatures, again because of the poor miscibility with oil.
3. Systems with flooded evaporators which can trap large quantities of oil.
4. Systems which cannot prevent the crankcase oil from absorbing the refrigerant during compressor shutdown.
5. Systems where the discharge velocity of the refrigerant is occasionally reduced, thus allowing the oil to separate instead of being carried along with the refrigerant.
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