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problem , but restrictions may still occur in unusual circumstances .
• In practice , it is difficult to find out how much oil is circulating with the refrigerant . On larger equipment , you can observe the oil level in the crankcase . However , if oil has been added the crankcase level may not indicate the correct oil charge .
• A loss of capacity might indicate excess oil in the evaporator .
• Finally , circulating oil may break down at hot spots in the system and cause varnishing and valve sticking . It may react with the refrigerant at high temperatures and form acids .
FACTORS WITH REGARDS TO OIL IN THE EVAPORATOR When there is oil in the evaporator , several factors determine whether or not it remains or is circulated by the refrigerant returning the oil to the compressor :
• The geometry of the piping : line sizing , bend , elbows , restrictions , vertical risers .
• The velocity of the refrigerant gas .
• The viscosity or fluidity of the oil . Oil itself is quite fluid and has a relatively low viscosity at ordinary temperatures . At lower temperatures , the oil becomes more viscous . It becomes more difficult to pour or flow . At very low temperatures , it becomes solid . Whether refrigerant vapour can push oil out of the evaporator depends on how fluid or viscous the oil is . Remember that refrigerant itself has very low viscosity . Solutions of oil and refrigerant have a viscosity between that of the oil and that of the refrigerant . As more refrigerant dissolves in the oil , the solution becomes less viscous .
You might assume that oil is most difficult to move in the coldest part of the evaporator , but viscosity measurements show that this is not so . Refrigerant gas is more soluble in oil at low temperatures than at high temperatures at the same pressure , or in other words , near saturation conditions . In many systems , the point of highest viscosity may be outside the evaporator . In this case , it would help to raise the suction-line temperature quickly .
As stated previously , the compressor may not be properly lubricated if too much oil circulates with the refrigerant .
EXCESSIVE REFRIGERANT IN THE COMPRESSOR CRANKCASE Lubricating oil tends to absorb appreciable amounts of refrigerants at temperatures found in the compressor crankcase . But the amounts vary for different refrigerants . When a cold compressor is started , pressure drops rapidly and the temperature of the oil rises . As a result , the solubility of the
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RACA Journal I October 2024 www . refrigerationandaircon . co . za