With few exceptions, all compressors that are oil lubricated will discharge oil into the gas stream. The discharge rate can be as little as parts of oil per million parts of refrigerant for direct drive hermetic centrifugal compressors and as much as several percentages for screw compressors. Oil discharge rates are usually expressed in terms of lbm of oil discharged per lbm of compressed refrigerant or mass percent of oil in discharge gas.
Oil in compressor discharge gas occurs in two forms: fine oil droplets (mist) in the gas stream; and liquid oil driven by gas velocity, creeping 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 to the condenser. The liquid leaving the condenser consists mainly of refrigerant with a certain amount of dissolved oil (assuming the oil is miscible in the refrigerant). The oil content in the liquid refrigerant at this point is the same as the compressor/separator oil discharge rate.
Liquid refrigerant containing oil flows through the expansion valve to the evaporator. In the evaporator, the refrigerant boils delivering its cooling effect. The oil, however, does not evaporate since its boiling temperature is very high in relation to the temperatures existing in the evaporator. In the absence of an oil return system, the oil will continue to accumulate and concentrate in the evaporator, which will have two negative consequences: the heat transfer in the evaporator will gradually degrade and the compressor will eventually run out of oil and shut down. Therefore, an effective oil return system is essential.
Mass flow balance of refrigerant and oil in a flooded evaporator
Consider the evaporator of a water cooler in operation. The oil arrives at a certain rate, specifically: the compressor oil discharge rate minus the oil separator removal rate, if equipped. For illustrative purposes, assume the rate of mass arrival at the evaporator is 2 lb of oil along with 1000 lb of refrigerant in one hour. The compressor/separator has an oil discharge rate of 0.2%, ie mass of oil per mass of compressed refrigerant expressed as a percentage. This would be a good discharge rate for a screw compressor/separator.
Oil also leaves the evaporator through the oil return system. The amount of oil that exits through the oil return system is a function of the liquid removal rate and the concentration of oil in that liquid. Assume the oil return system draws 50 pounds 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 pound per hour. Since this rate of departure is less than the rate of arrival, the oil will accumulate further in the evaporator and the oil concentration will increase. Under the conditions stated above, the oil concentration in the evaporator will rise and stabilize at 4%.
Four percent is unacceptably high. There are two things we can do to reduce this concentration. The first is that we can increase the rate of oil return liquid extraction. If we double the oil return flow rate to 100 lbs/h and the oil concentration is 2%, the oil arrival and withdrawal rates will equal 2 lbs/h and the concentration will be stable at 2%. However, we can lower the concentration of oil in the liquid entering the evaporator (perhaps by installing a more efficient oil separator). These two possibilities also suggest the cause of unacceptably high oil concentrations in the evaporators and cooler shutdowns due to oil loss. The first is a failure of the compressor (leaky o-rings, missing plugs, etc.) and/or oil separator causing unusually high and unacceptable oil discharge rates. The second is an oil return system failure, such as clogged lines, inadequate pump capacity, or improper line pressure differential for an eductor. Taking the above into account, it should be obvious that the most effective improvement to any oil return system is to reduce the oil arrival rate; that is, reduce the compressor oil discharge rate and/or improve the efficiency of the oil separator.
Oil inventory in the evaporator
If you were to perform an oil mass balance analysis on an operating flooded evaporator as described above, measuring liquid line flow and concentration and oil return line flow and concentration, you could still experimentally find more oil in the evaporator than expected. The discussion that follows offers a possible explanation. The point of contention is that the design of the evaporator itself and the location of the return oil collection can have a major impact on the success or failure of an oil recovery system. This is relevant because it may mean that replacing a malfunctioning oil return system of one type with another (eg pump with eductor) may not fix the problem, the real problem is that the oil return collection point is poorly located.
Consider a single pass flooded evaporator. Hot water enters the tubes at one end and exits as cold water at the other end. The liquid coolant surrounds the tubes and is introduced through a pipe at the cold water end of the shell. Liquid refrigerant is drawn from the shell by the oil return system from the middle of the shell (or worse, from the cold end by the liquid inlet). As stated above, the refrigerant entering the evaporator contains 0.2% oil, and the refrigerant is extracted by the oil return system at a rate of 100 lb/h and the concentration at the point of extraction is 2%. Arrival and departure rates are identical at £2 per hour. If the evaporator refrigerant charge were 100 pounds, one would be tempted to conclude that the evaporator contained 2 pounds of oil. However, if you were to measure the oil concentration at the ends of the cover, you might find that the concentration was 10% at the hot end and 0.2% at the cold end. Why would this be? The answer is that most of the liquid refrigerant evaporation takes place at the warm end of the shell, where the temperature difference between the water and the refrigerant is greatest. Gravity will see to it that this liquid is replaced with liquid from a higher elevation: liquid at the cold end of the shell that is evaporating, but slowly. Therefore there will be a slow axial flow of liquid refrigerant from the cold end of the casing to the hot end and it will carry oil with it which will not return while the cooler is running. But that oil will not evaporate at the hot end or be picked up by the oil return system that is drawn from the center of the casing. Therefore, the oil will tend to collect in a place where it is not picked up by the oil return system. And where the oil return system picks up liquid, that liquid will not contain much oil. This will result in a “stored inventory” of oil in the evaporator which can be substantial. Therefore, it is important to know where in the evaporator the oil tends to concentrate and draw return liquid from that point. That location varies depending on the design of the evaporator and any associated internal liquid distribution system.