Carryover occurs when liquid refrigerant droplets travel into the compressor from the cold, low-pressure vapor produced in the evaporator (Figure 1). Generally there are two types of carryover. The first type is by design and controlled with the intent of enhancing the lubrication process in the compressor. The second type is not by design and can be detrimental to the performance and reliability of a chiller. If excessive liquid droplets enter the compressor system, they vaporize on the metal internals, stripping away lubricant.
Ultimately this oil stripped from the compressor becomes entrained in the hot, high-pressure refrigerant vapor and enters the condenser. Once this happens, oil from the sump is used to replace the stripped oil, causing the oil level to drop in the compressor oil reserve.