Refrigeration Lubricants - Lubricants for use in Refrigerant-134a (R-134a) Applications
Several different types of lubricants have been evaluated for use with highly fluorinated refrigerants such as R-134a (1, 1, 1, 2 tetrafluoroethane). At the present time, two different types of products have emerged as the primary long-term fluids for these applications. These products came as a result of several years of development, and have been found to supply the needed lubricity and stability for the system.
The RPAG SERIES fluids are made from a family of soluble polyglycol based lubricants. These products are inversely miscible with R-134a. At elevated temperatures the fluids will separate from the refrigerant, producing a significantly higher viscosity fluid, which helps to improve the efficiency of the compressor by its increased sealing capacity. Less dissolved refrigerant also leads to increased lubricity. The RPAG fluids have been designed for use primarily in automotive applications. The fact that these fluids exhibit the property of inverse miscibility also makes them an ideal candidate for use in applications such as rotary screw compressors. The RPAG fluids are generally not recommended for use in retrofit due to incompatibility with residual mineral oil, as well as residual chlorine from the R-12. The RPAG fluids are covered under US Patents 5,027,606 and 4,916,914
The Solest SERIES products are formulated from modified ester base stocks to provide controlled miscibility, as well as solubility with all HFC based refrigerants, including R-134a. They are mineral oil compatible, and can be used to effectively retrofit an existing CFC based system. The controlled miscibility and solubility properties of the lubricants have lead to improved volumetric efficiency in compressor tests, as well as providing efficient oil return from the system. In addition, tests conducted on the fluid have shown no loss of lubricant film under diluted conditions with rolling ball elements.
Hydrocarbon refrigerants, such as propane, are considered highly soluble in a lower viscosity and reduced oil film thickness. The washing away or lubricant absorption of lubricants into the gas phase in the compression cylinder may result in loss of lubrication. These problems have led to the widespread use of CP-1516 SERIES polyglycols. The CP-1516 SERIES are specially selected polyglycols which are "resistant to dilution" by hydrocarbons as they have lower solubility. The polar nature of these fluids help to preferentially wet lubricated surfaces. The high viscosity index of the CP-1516 SERIES fluids help to provide a higher viscosity for sealing compressor rotors while remaining a lower viscosity needed for good oil return from the low temperature side of the refrigeration system. Reports by rotary screw compressor developers have indicated up to an eighteen- percent improvement in volumetric efficiency over mineral oil in propane compression. High-pressure reciprocating compressors have benefited by up to a twenty-fold increase in the life of pressure packings while cylinder oil feed rates have been reduced. Other more miscible synthetics have been used in very low temperature systems (such as -180°F (-118°C) ethylene) for their low volatility. Separators can effectively control the amount of lubricant in the low temperature side of the system. Careful selection of oil viscosity provides enough lubricant film to lubricate compressor components in the presence of the gas while the small amount of miscible lubricant is diluted in the evaporator and can be easily returned. There is no problem of wax or viscous liquid formation found with mineral oils.
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