www . refrigerationandaircon . co . za RACA Journal I October 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 .
This Solutions article is a continuation of the question posed in the September issue .
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 .
In refrigeration systems the purpose of the oil is to provide adequate lubrication between metal surfaces . Oil often serves as a seal to prevent refrigerant 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 .
Picking up from the last issue , let us look into a situation in refrigeration called oil logging . Oil logging occurs when an oil refrigerant mixture enters the evaporator but the refrigerant separates from the oil leaving the oil behind in the evaporator . This situation reduces system capacity and may drain the compressor sufficiently to cause lubrication issues and possible oil pressure switch cut out .
Several important factors help to keep oil from accumulating in the evaporator . These problems can be avoided through proper design including , proper capacity balance with the compressor , proper evaporator design , avoidance of improper situations where oil is unintentionally trapped , installation of properly designed oil traps , correct refrigerant line sizing
High-side oil separators may help in some installations , especially low-temperature applications . Such separators are not 100 % efficient , but they can keep the compressor from passing excessive amounts of oil into the system .
In a refrigeration system , oil is carried through the compressor by the refrigerant . It circulates with the refrigerant throughout the system . A small amount of oil in circulation is not harmful : in fact , it may be of some benefit in lubricating valves . However , large amounts of oil are a problem . The following are the areas of concern :
• Oil circulating with the refrigerant is not in the compressor crankcase where it should be . This can be a major problem . A shortage of oil may develop in the crankcase . As a result , bearings and other parts may not be properly lubricated . Adding more oil after start-up to compensate is not a solution , because the amount of oil circulating with the refrigerant is not consistent throughout the system .
• Excessive amounts of oil in an evaporator decrease the capacity of a system . Heat transfer through the coil wall decreases . The coil area is less if part of the space is occupied by oil . Both of these factors produce a larger temperature difference ( TD ) between load and evaporating refrigerant . Increased TD in the evaporator indicates a decrease in capacity . Capacity is further reduced to the extent that the vapour pressure of a refrigerant / oil solution is less than that of a refrigerant alone .
• Oil in the refrigerant may cause restrictions in expansion valves or capillaries , and thus reduce refrigerant flow . Improvements in oil quality have greatly reduced this
www . refrigerationandaircon . co . za RACA Journal I October 2024 37