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Lubricants are therefore less suitable for semi-herme- tic 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 speci- fied by the compressor manufacturer may be used. Because of the increased reactivi- ty of HFOs with oil, this is especially true for systems with these refrigerants. Compressors for factory-made air conditio- ners and chillers are also increasingly being charged with polyvinyl ether (PVE) oils. Although they are more hygroscopic than POE, they are very resistant to hydro- lysis, thermally and chemically stable, have good lubricating properties and high dielec- tric strength. In contrast to POE, they are less prone to the formation of metal soaps and thus offer more security against block- age of capillaries. Special requirements for the lubricants exist with CO2 systems. Specially formulat- ed POEs are also suitable for use in widely ramified pipe networks due to their particu- larly good solubility/miscibility. However, these properties have a negative effect on viscosity and lubricity (tribology) and there- fore 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 par- tially soluble PAG oils see chapter NH3 (Ammonia) as alternative refrigerant (page 28) and supplementary information (see below). Further information see Fig. 37 “Overview lubricants”, page 45 and explanations for the particular refrigerants. Supplementary BITZER information con- cerning lubricants (see also https://www.bitzer.de) o Technical Information KT-500 „BITZER refrigeration compressor oils for reciprocating compressors” o Technical Information ST-500 „BITZER refrigeration compressor oils for screw compressors“ o Technical Information EST-500 „BITZER refrigeration compressor oils for scroll compressors – statio- nary applications" o Technical Information AT-640 „Use of ammonia (R717) in BITZER compressors” – chapter: "Oils and their influence on the system design" o Technical Information AT-660 „Use of propane (R290) and propene (R1270) in semi-hermetic BITZER compressors” – chapter: "Oils“ o Operating Instructions KB-120 and KB-130 „Semi-hermetic reciprocating com- pressors for CO2 applications“ Lubricants for compressors Positive displacement compressors ‒ as are predominantly used in commercial and industrial refrigeration, air conditioning and heat pump systems ‒ are commonly oil- lubricated. Despite appropriate construc- tional measures and/or installation of an oil separator, a small amount of oil is pumped into the circuit together with the compress- ed 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 advantage- ous. The refrigerant dissolved in the oil sig- nificantly reduces the viscosity, improving oil fluidity and minimising the negative influence on heat transfer in heat exchan- gers. In the past, so-called naphthenic mineral oils and synthetic alkylbenzenes were pre- ferred. For systems with CFC and HCFC refrigerants (for example R22) and hydro- carbons, 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 refriger- ants. These are oils based on polyol ester (POE) and polyalkylene glycol (PAG). They have similar or better lubricating pro- perties 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 con- cerning water absorption. In addition, they have a relatively low dielectric strength and 41PDF Image | REFRIGERANT REPORT 21
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