www . refrigerationandaircon . co . za RACA Journal I October 2024 39
Support refrigerant is sharply decreased . There will be a rush of gas out of the oil over a short period of time . The drop in crankcase pressure causes oil foaming .
When this occurs , an excessive amount of oil may be carried with the refrigerant through and beyond the compressor . In fact , this slugging can damage compressor components .
In addition , we find that when the compressor is idle , the oil cools down . Refrigerant concentration begins to increase . If the ambient temperature around the compressor is lower than elsewhere in the system , refrigerant rapidly transfers into the crankcase . In this situation , a crankcase heater is useful .
CRANKCASE HEATERS A small electric heater immersed in the crankcase oil is often used to maintain adequate oil temperatures . The amount of refrigerant dissolved in crankcase oil depends on the temperature of the oil and the temperature of pure refrigerant elsewhere in the system .
The ideal situation would be to maintain the temperature of the oil , when a system is shut down , so that the concentration of refrigerant is no greater than it is when the system is operating .
Crankcase heaters are usually part of the original compressor equipment . The manufacturer has determined the most effective size and location .
To prevent refrigerant buildup in the crankcase just be sure that the oil temperature is kept above that of the liquid refrigerant elsewhere in the system or make use of a pump down cycle . gradually while the compressor is running . This avoids the sudden rush of refrigerant out of the oil . The amount of oil circulating with the refrigerant is greatly reduced .
OIL SEPARATORS Oil separators can effectively remove oil from refrigerant vapours . The oil removed usually collects at the bottom of the separator . From there , it is returned periodically to the crankcase . The relationships of solubility , pressure and temperature between oil and refrigerant in the crankcase also apply here . Too much refrigerant can dissolve in oil and present a real problem when oil gets to the crankcase . The separator should be warm enough to keep oil at a higher temperature than the refrigerant in the condenser . For this reason , the oil separator should be close to the compressor . It might be good to insulate or heat the lower part of the separator .
HYGROSCOPIC NATURE OF OIL All refrigerant oils absorb moisture when they are exposed to the atmosphere . However , synthetic oils are hygroscopic by nature . They absorb many more times the amount of moisture than mineral oils do , and as a result you must exercise special care when changing a system or transferring such lubricants from one
PUMP DOWN CYCLE Pumping down seals off the compressor and evaporator from the rest of the system with a liquid-line solenoid valve keeps refrigerant from dissolving in oil . The evaporator and crankcase then can be pumped down before the compressor stops running . Very little refrigerant remains in the part of the system containing oil . This is a very effective way to prevent refrigerant buildup in the crankcase during system off cycles .
ENTRAINMENT Entrainment in refrigeration terms occurs when the refrigerant in the compressor picks up and carries oil out of the compressor and into the system . To reduce entrainment of oil by the refrigerant when the compressor starts you may find that compressor , check valve and oil separators are used .
COMPRESSOR CHECK VALVES Some compressors have a check valve in the compressor crankcase . It is designed to close when the crankcase pressure is higher than the pressure in the suction line leading to the compressor cylinder . However , there is a small bleed line around or through the valve . It allows crankcase pressure to reduce
www . refrigerationandaircon . co . za RACA Journal I October 2024 39