With few exceptions, all compressors that are lubricated with oil will discharge oil into the gas stream. The rate of discharge can be as small as parts of oil per million parts of refrigerant for direct drive hermetic centrifugal compressors and as much as several percent for screw compressors. Oil discharge rates are usually expressed in terms of lbm of oil discharged per lbm of refrigerant compressed or in mass percent of oil in the discharge gas.
Oil in compressor discharge gas is in two forms: fine magnolia oil droplets (mist) in the gas stream; and liquid oil driven by the gas velocity, crawling along the tube walls. Oil flows from the compressor with the discharge gas through the oil separator (if equipped and always less than 100% efficient), and into the condenser. The liquid leaving the condenser consists mostly of refrigerant with some amount of dissolved oil (assuming that the oil is miscible in the refrigerant). The oil content in the liquid refrigerant at this point is the same as the oil discharge rate of the compressor/separator.
The liquid oil-containing refrigerant flows through the expansion valve and into the evaporator. In the evaporator, the refrigerant boils off delivering its refrigerating effect. The oil, however, does not evaporate as its boiling temperature is very high relative to the temperatures existing in the evaporator. In the absence of an oil return system, oil will continue to collect and concentrate in the evaporator which will lead to two negative consequences: heat transfer in the evaporator will be progressively degraded and the compressor will eventually run out of oil shutting it down. Hence, an effective oil return system is essential.
Refrigerant and Oil Mass Flow Balance in a Flooded Evaporator
Consider the evaporator of an operating water chiller. Oil is arriving at a certain rate, specifically: the oil discharge rate of the compressor less the removal rate of the oil separator, if equipped. For illustration purposes, assume the mass arrival rate in the evaporator to be 2 lb of oil along with 1000 lb of refrigerant liquid in one hour. The compressor/separator has an oil discharge rate of 0.2%, i.e. mass of oil per mass of refrigerant compressed expressed as a percent. This would be a good discharge rate for a screw compressor/separator.
Oil is also leaving the evaporator via the oil return system. The amount of oil leaving via the oil return system is a function of the liquid removal rate and the concentration of oil in that liquid. Let us assume that the oil return system draws 50 lbs of refrigerant/oil mixture from the evaporator per hour. If the concentration of oil in the evaporator liquid is say 2%, then the oil returned is 1 lb per hour. Since this leaving rate is less than the arrival rate, oil will further accumulate in the evaporator and the oil concentration will rise. Under the conditions stated above the oil concentration in the evaporator will rise to and stabilize at 4%.