The compressor contains lubricating oil (refrigeration oil), which is essential for lubricating the rotors and bearings to prevent wear.
When the refrigerant circulates through the system, part of the oil is inevitably carried out of the compressor, especially during evacuation or when the refrigerant is charged and discharged repeatedly.
If the system is kept under vacuum for too long or if refrigerant is frequently removed and recharged, the oil will be carried away with the refrigerant into the piping, evaporator, or condenser. As a result, the compressor may be left with insufficient lubrication, which can cause severe damage or even compressor failure.