Compressors are lubricated for three main purposes:
- To reduce frictional wear on bearings and other moving parts of the compressor
- To cool the refrigerant gas during compression
- To seal against refrigerant gas leakage
Different compressors use different lubrication techniques. In screw compressors, the oil is often pumped into the moving parts, while pistons and scrolls often employ splash lubrication using oil from a vessel in the bottom of the compressor.
If the viscosity of the oil-refrigerant mixture is too low, it leads to incomplete or inefficient separation of the metal surfaces, which increases friction and wear. Various anti-wear additives can counteract this to some extent, but this solution cannot be used to its full extent in refrigeration systems due to the risk of reaction between the additives and the refrigerant.
With high-viscosity oil-refrigerant mixtures, there may be problems, such as obstructed flow, that may lead to poor pumping efficiency. In order to work properly, oil-refrigerant mixtures should have a dynamic viscosity that is sufficiently high to give satisfactory sealing and lubrication in the compressor. In addition, the mixture must be thermally and chemically stable, so as not to react with components and materials in the refrigeration system.
Lubricating oil may have negative impacts on other parts of the refrigeration system. An oil separator is therefore often mounted directly after the compressor outlet to reduce the flow of lubricant into the condenser and evaporator. Heat transfer will be impaired if oil droplets become trapped in these components. The refrigerant is protected from most of the oil, because the oil separator continuously returns lubricant to the crankcase of the compressor.
Partly miscible oils and refrigerants may separate in the condenser. If so, a refrigerant-rich phase is carried over to the expansion valve, while the oil accumulates in an oil-rich phase in the refrigerant reservoir. This may restrict the return of oil to the compressor, leading to insufficient lubrication. In the evaporator, the lubricant is subject to low temperatures, which may lead to problems with wax formation and phase separation. If the solubility of the refrigerant in oil at low temperatures is low, there may be problems with returning the oil to the compressor.