www . refrigerationandaircon . co . za RACA Journal I September 2024 37
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GRANT LAIDLAW
Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy ( ACRA ) in Edenvale . He holds a Bachelor of Business Administration and an associate degree in educational administration . He has a National Technical Diploma and completed an apprenticeship with Transnet . He has dual-trades status : refrigeration and electrical . He has been involved with SAIRAC for over two decades and served on the Johannesburg committee as chairman and was also president between 2015 and 2018 . Currently he is the SAIRAC national treasurer .
WELCOME TO THE SOLUTIONS PAGE
By Grant Laidlaw
Many people ask for assistance in the understanding of theoretical and practical aspects of the industry . We are going back to basics as I have questions coming in that indicate that the basic understanding necessary to work in the industry is not in place .
Ryan asks : Good day Grant , we have issues when servicing refrigeration systems relating to compressor oil . It is often difficult to get information on specific compressor oil requirements . What is so special about refrigeration / air conditioning oils ? Your input would be appreciated . Thanks .
Hi Ryan , it is always best to let the compressor manufacturers specify what oil to use in any application . If you are unsure of what oil to use , call the compressor manufacturer or use the specification data if available .
What is so special about refrigeration compressor oils you ask . Well , let us look into this in more detail . In refrigeration systems the purpose of the oil is to provide adequate lubrication between metal surfaces . Oil often serves as a seal to prevent refrigeration from leaking between high- and low-pressure regions of the compressor and performs a cooling function that is significant in some compressors . In addition , oil can form an integral part of capacity control on larger compressors . Some oil will leave
the compressor during the cooling cycle and circulate with the refrigerant through the system . The system should be piped to return this oil back to the compressor on a continual basis .
Other oils used to lubricate motor vehicle engines , hydraulic systems , etc . do not have the correct properties and as such are not suitable for refrigeration systems . For example , the hydraulic oils used in a forklift will never be exposed to the low temperatures found in a low temp evaporator .
In the past most of the lubricants for refrigeration systems have been mineral oil , which is relatively inexpensive , has desirable solution characteristics with the traditional refrigerants , is reasonably stable and has been widely available . With modern refrigerants we have modern oils available , although mineral oils remain in use .
However , mineral oils are not suitable for use with the HFC / HFO refrigerants . The main reason for this incompatibility is the insufficient lubricity ( lubricating ability ) and miscibility ( mixing ability ) of mineral oils . Much work has been done to find acceptable lubricants to use with HFC / HFO refrigerants . Mineral oils could remain in use with some of the hydrocarbon refrigerants . Care must be taken when filling oil into a compressor that only the correct oil is used . In refrigeration systems oil circulates with the refrigerant . As the temperature and pressures vary greatly in the system , refrigeration oil must have very special characteristics .
Some of these are :
At high temperature and pressures :
• The oil must maintain its lubrication properties at condensing conditions
• The oil must not carbonise into valve gear , pistons , etc , due to the gas delivery temperature in the compressor
At low temperature and pressures :
• The oil must remain fluid so that it can still lubricate
• The oil must be dry and not contain wax that can form deposits at system operating temperatures
At all temperatures and pressures :
• The oil must not react chemically with the refrigerant and any other materials used in the system ( i . e . it must be compatible with the refrigerant and the system components such as gaskets and seals ). Mineral oils were used for HCF ’ s refrigerants but are not compatible with the new refrigerants due to their low solubility factors . Polyester oils are mostly used for the HFC refrigerants - however , the oil is hydroscopic and will readily absorb water .
www . refrigerationandaircon . co . za RACA Journal I September 2024 37