Often, systems without good oil separators, or none at all, will have oil build-up points throughout the piping. When manufacturer’s specifications are followed, refrigerant and oil mix with one another in a refrigeration or air conditioning system. The fine oil mist that travels with the refrigerant will usually build up and collect at low points in the system’s piping. Most of the time, the refrigerant vapor will push this slug of oil to the next low spot until the oil finally returns to the compressor. Large oil slugs returning to some compressors can damage valve plates and internal running gear if hydraulic pressure builds too great in the cylinders.
The function of an oil reservoir, commonly known as an oil trap, is to prevent large oil slugs from forming. This is accomplished by trapping a small amount of oil in the reservoir or trap and gradually returning it to the compressor.