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• The oil must be chemically stable
• The oil must be very dry - contain the minimum amount of water
• The oil must be miscible with the refrigerant
• It must be specially made for refrigeration applications
• It must have the right viscosity
• It must have good wear characteristics
• It must have chemical , thermal and hydrolytic stability at both low and high temperatures
The selection of the refrigeration oil depends on the operating conditions of the system , the type of refrigerant ( HCFCs require mineral oil and HFCs require polyester oil ) as well as the type of compressor .
Lubricating oils for refrigeration compressors are specifically designed for that application and selection of these lubricants requires special attention and knowledge . Do not mix minerals and polyester oils , as they are not compatible .
Consequences of using the wrong oil :
• Due to bad mixing properties , oil returned to the compressor is reduced resulting in insufficient lubrication of the compressor
• Reduced heat transfer in the evaporator and the condenser resulting in reduced system efficiency
• Excessive foaming of the oil , which can result in liquid slugging
• Formation of sludge and coke causing blockages and corrosion wear protection , as well as chemical and thermal stability . Compatible with R23 , R134a , R404a , R407c , R410a , R410b , R417a , R422a , R422d , R427a , R507 , R507a .
• Alkyl Benzene compressor oils ( AB ): Extreme performance synthetic lubricant for refrigeration compressors with and without ammonia refrigerant . Wax-free for excellent lowtemperature fluidity and evaporator efficiency . Outstanding thermal / oxidative and chemical stability for long oil life , extended drain intervals , less routine maintenance . Compatibility with seals previously used with mineral lubricant for limited risk of oil leakage .
• Polyalkylene glycol ( PAG ) compressor oils : Synthetic PAG lubricant suitable for specific refrigeration applications . PAG – based shear-stable lubricant with outstanding resistance to thermal degradation and the formation of sludge and deposits . Suitable for HC ( hydrocarbon ) or CO2 refrigerant fluid applications . Compatible with R290 , R600 , R600a , R134a ( automotive compressor applications ) and R1234yf .
• Polyalphaolefin ( PAO ): These refrigeration oils have a good chemical and thermal stability . PAO also has excellent viscosity temperature characteristics . The miscibility with refrigerant is low , which is why the major use of PAO in refrigeration is in applications where miscibility is not a concern . PAO can cause seals to shrink .
• Mineral oils : Naphthenic oils have a low pour point , which make them more suitable at lower temperatures . Naphthenic oil is derived from crude oil and as processed using a separation process removing all contaminants .
Let us look at which oil types are typically used for refrigeration systems :
• Polyester compressor oil ( POE ): for refrigeration compressors and systems . Specifically designed for use with ozone-friendly HFC refrigerant fluids ; formulated from synthetic Polyol esters to provide outstanding lubricity and
Remember , that you should use only the lubricant recommended by the compressor manufacturer . Oil producers and compressor manufacturers go to great lengths and expense to develop and test new lubricants . There are tests for viscosity , floc point , pour point , flash point , etc . You should understand the terms and why they are important .
Supplied by Grant Laidlaw
Rough Guide for refrigeration oil usage
www . refrigerationandaircon . co . za RACA Journal I September 2024 39