Positive displacement compressors ‒ as are predominantly used in commercial and industrial refrigeration, air conditioning and heat pump systems ‒ are commonly oil-lubricated. Despite appropriate constructional measures and/or installation of an oil separator, a small amount of oil is pumped into the circuit together with the compressed gas flow. To stabilise the oil balance, suitable measures for continuous oil return must be taken. Oils that are soluble and miscible with the refrigerant are advantageous. The refrigerant dissolved in the oil significantly reduces the viscosity, improving oil fluidity and minimising the negative influence on heat transfer in heat exchangers.
In the past, so-called naphthenic mineral oils and synthetic alkylbenzenes were preferred. For systems with CFC and HCFC refrigerants (for example R22) and hydrocarbons, they are very favorable with regard to solubility and miscibility. On the other hand, owing to their low polarity, they are insufficiently miscible with the highly polar HFC and HFO refrigerants and are therefore not properly and sufficiently drawn into the refrigeration cycle.
Immiscible oils can accumulate in the heat exchangers and hinder the heat transfer so much that operation of the system is no longer possible.
Therefore, new lubricants with appropriate solubility/miscibility have been developed for systems with HFC and HFO refrigerants. These are oils based on polyol ester (POE) and polyalkylene glycol (PAG).
They have similar or better lubricating properties than previously customary oils, but are more or less hygroscopic, depending on the refrigerant solubility. This requires special care in manufacturing (including drying), transport, storage and charging, so that chemical reactions in the plant – such as hydrolysis in POE – are avoided.
PAG-based oils are particularly critical concerning water absorption. In addition, they have a relatively low dielectric strength and are therefore less suitable for semi-hermetic and hermetic compressors. They are primarily used in mobile air conditioning systems with open drive compressors, where special requirements for lubrication and best solubility/miscibility are required because of a high oil circulation rate. To avoid copper plating, non-ferrous metals are used in these systems.
The remaining refrigeration industry so far prefers POE oils. The extensive experience gained with them is positive if the water content in the oil does not significantly exceed 100 ppm. However, only oils specified by the compressor manufacturer may be used. Because of the increased reactivity of HFOs with oil, this is especially true for systems with these refrigerants.
Compressors for factory-made air conditioners and chillers are also increasingly being charged with poly-vinyl ether (PVE) oils. Although they are more hygroscopic than POE, they are very resistant to hydrolysis, thermally and chemically stable, have good lubricating properties and high dielectric strength. In contrast to POE, they are less prone to the formation of metal soaps and thus offer more security against blockage of capillaries.
Special requirements for the lubricants exist with CO2 systems. Specially formulated POEs are also suitable for use in widely ramified pipe networks due to their particularly good solubility/miscibility. However, these properties have a negative effect on viscosity and lubricity (tribology) and therefore require compressors with an extremely robust and wear-resistant drive gear. At very high loads, e.g. heat pumps, PAG oils specially developed for CO2 applications ensure even more favorable lubrication conditions.
Due to the thermodynamic properties of ammonia (NH3) and the resulting plant engineering, non-soluble/miscible oils are advantageous. These include for example mineral oils and polyalphaolefins (PAO). However, they require a special technique for oil separation and oil recirculation. For further explanation as well as additional information on applications when using partially soluble PAG oils see NH3 (Ammonia) as alternative refrigerant and supplementary information (Supplementary BITZER information concerning lubricants).
A complete overview of lubricants is shown in Overview lubricants for compressors.