Because refrigerants and refrigeration oils are miscible in one another, there will always be some oil that leaves the compressor with the refrigerant being circulated throughout the refrigeration system. Also, any time flooding or migration occurs, the crankcase oil is sure to be diluted with refrigerant. This will cause oil foaming at start-up. The crankcase will then build excessive pressures, often forcing oil and refrigerant around the rings of the compressor's cylinders to be pumped into the discharge line.
Oil separators remove oil from the compressor's discharge gas, temporarily store the oil, and then return it to the compressor's crankcase. Oil separators are located close to the compressor in the discharge line. (See Figure 1.) Even though most oil separators are designed to be mounted vertically, there are some horizontal models available on the market.