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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 dualtrades status : refrigeration and electrical . He has been involved with SAIRAC for 17 years and has been a Johannesburg committee member for the past eight ( chairman in 2011 and 2012 ). Currently he is the treasurer ( Johannesburg council ) as well as president ( national council ).
OIL IN THE EVAPORATOR
By Grant Laidlaw
Looking at the effect of this situation on the system , components , and capacity .
Gordon asks : Hi Grant , we have recently lost several compressors and it would seem that the compressor oil is all ending up in the evaporator . This then causes the compressor to fail . We can possibly fit oil cut out switches , but this does not solve the root problem . Can you please shed some light on this situation , thanks !
Hi Gordon . This situation sometimes occurs when an oil refrigerant mixture enters the evaporator but during the evaporation process the refrigerant separates from the oil leaving the oil behind in the evaporator . This reduces system capacity ( evaporator capacity ) and may drain the compressor sufficiently to cause lubrication issues as you have experienced , and possible oil pressure switch cut out .
There are several important factors to consider around keeping oil from accumulating in the evaporator . Typically , these problems can be avoided through proper design , including :
• Proper system capacity balance with the compressor
• Proper evaporator design
• Avoidance of improper traps
• Installation of properly designed 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 to the evaporator . HFC systems are particularly prone to this problem .
As you are aware , 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 is desirable and is of some benefit in lubricating valves . Large amounts of oil , however , are a problem .
The following are the areas of concern : Oil circulating with the refrigerant and is not in the compressor crankcase , where it should be , can be a 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 startup to compensate is certainly not a solution , because the amount of oil circulating with the refrigerant is not consistent throughout the system . Excess oil may accumulate in the evaporator and then rapidly return to the compressor possibly resulting in an overfilled crankcase and possibly damage the compressor .
" A small amount of oil in circulation is not harmful , in fact it is desirable and is of some benefit in lubricating valves ."
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 factors produce a change in temperature difference ( TD ) between load and evaporating refrigerant and consequently 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 . If suction pressure controls the compressor , it stops running at a higher temperature when vapour pressure is reduced .
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 problem , but restrictions may still occur in certain circumstances .
www . hvacronline . co . za RACA Journal I October 2021 37