Oil Inventory in the Evaporator

If you were to do an oil mass balance analysis on an operating flooded evaporator as described above, by measuring liquid line flow and concentration and oil return line flow and concentration, you might yet experimentally find more oil in the evaporator than you expect. The discussion which follows offers a possible explanation. The point of the discussion is that the design of the evaporator itself and the location of the oil return pickup can have a major impact on the success or failure of an oil recovery system. This is relevant because it can mean that replacing a poorly operating oil return system of one kind with another (e.g. pump with eductor) may not fix the problem, the real problem being that the oil return pickup point is poorly located.

Consider a one pass flooded evaporator. Warm water enters tubes at one end and exits as chilled water at the other end. Refrigerant liquid surrounds the tubes and is introduced by a pipe at the cold water end of the shell. Liquid refrigerant is withdrawn from the shell by the oil return system from the middle of the shell (or worse, from the cold end by the liquid inlet).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 above, the refrigerant entering the evaporator contains 0.2% oil, and refrigerant is drawn by the oil return system at a rate of 100 lbs/hr and the cbd capsules canada concentration at the point of withdrawal is 2%. The arrival and removal rates are identical at 2 lbs per hour. If the evaporator refrigerant charge were 100 lbs, one would be tempted to conclude that the evaporator contained 2 lbs of oil. Yet, if you were to measure the oil concentration at the ends of the shell, you might find that the concentration was 10% at the warm end and 0.2% at the cold end. Why would this be? The answer is that most of the evaporation of liquid refrigerant takes place at the warm end of the shell where the temperature difference between water and refrigerant is the 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 which is evaporating, but slowly. Hence, there will be a slow axial flow of liquid refrigerant from the cold end of the shell to the warm end and it will take oil with it that will not return while the chiller operates. But that oil will not evaporate at the warm end nor will it be picked up by the oil return system which draws from the middle of the shell. Hence, oil will tend to concentrate in a place where the oil return system does not pick it up. And where the oil return system does pick 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. So it is important to know where in the evaporator the oil tends to concentrate and to draw return liquid from that point. That location varies by design of the evaporator and any associated internal liquid distribution system.

Leave a Reply

Your email address will not be published. Required fields are marked *