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Use of Eductors in Oil Return Systems
From time to time it is reported that when using a screw chiller for oil return, operating under low load conditions, it appears that the eductor is not operating efficiently enough to return sufficient oil to the oil separator or sump. The oil level, which causes the chiller to shut down at low oil, results in oil retention in the refrigerant charge in the evaporator.
For a chiller that uses an eductor for oil return, the failure may be due to low lift rather than low load. In a comfort cooling environment, the chiller load responds to the outside temperature. That is, when it is hot outside, heat flows faster into the building and the chiller load is greater. At the same time, the chiller must reject its heat at high ambient temperatures. Hence a chiller operating at high load condition is also operating at high lift condition. Lift is defined as the difference between suction and discharge saturation temperature (or pressure).
When the outside temperature is cold, less heat has to be removed from the conditioned space and therefore the chiller load is less. A low load is accompanied by a low lift condition as the ambient temperature is lower than its peak value. Less lift is the reason for teacher effectiveness to decrease. The eductor is driven by the pressure difference between the condenser and the evaporator. When this pressure difference occurs, the teacher’s ability to induce flow is reduced. The ability of an eductor to induce flow is roughly proportional to the square of the pressure difference. Therefore, reducing the pressure difference to 50% of design will reduce the induced flow to 25% of design.
Not all chillers serve the comfort cooling market. Chillers are applied to chemical processes, for example, which may have varying loads but constant lift; ie constant suction and discharge temperatures. If these chillers are served by an adequately sized ejector based oil return system, load related oil loss will not be a problem.
Possible remedies for poor teacher performance in low lift applications include reducing the oil discharge rate of the compressor/separator and modifying the control system to increase the minimum lift of the system.
Liquid in compressor suction
Ideally, any fluid entering the compressor suction would be rich in oil and sufficiently lean in the refrigerant that lubrication would be satisfactory. However, if the concentration of oil in any fluid entering the compressor is too low, lubrication can be compromised and wear can occur which can lead to compressor failure. All compressors are vulnerable to lack of lubrication due to lack of oil or excess refrigerant in the oil.
Another type of failure is the result of injecting too much liquid refrigerant/oil into the compressor which can damage or destroy the compressor through “liquid slugging”. Screw and scroll compressors are more tolerant of liquid repulsion in the suction stream than compressors. This is due to the different nature of the compression process.
In a reciprocating compressor designed for a three to one compression ratio, the gas can reach discharge pressure when the piston is at half stroke. At this point the discharge valve opens and the gas is discharged as the piston continues to rise even though the gas pressure in the cylinder is not increasing. The final clearance volume may be only one tenth of the total swept volume. This clearance volume is not discharged, but re-expanded on the suction stroke. At this point one can say that the true compression ratio is ten to one considering the closed discharge valve (swept volume divided by swept volume and clearance volume). If the cylinder contains 110% of the clearance volume at the start of compression, the piston will only compress the liquid at the end of its stroke and the liquid will not exit the discharge valve as fast. Too high pressure in cylinder. This high pressure can damage the connecting rod or cause the head bolt to fail. For a reciprocating compressor to be efficient, a small clearance volume is required. However, it is this small clearance volume that makes reciprocating compressors susceptible to liquid slugging damage. The permissible level of liquid in the suction is determined by the ratio of the clearance volume to the swept volume.
In contrast, screw and scroll compressors designed for a three-to-one compression ratio capture a quantity of suction gas (and some oil and perhaps some liquid refrigerant) and reduce its volume to one-third of its original value. But the compression process is completed before the discharge port opens. Any liquid in the suction stream will cause the compression ratio to rise above the design value of three, but the increase is less than that of a reciprocating compressor. For example, assume that the suction stream for a screw compressor consists of 1 part liquid and 8 parts gas by volume. A compressor will reduce these 9 parts to 3 parts. When compression is complete, one part will still be liquid and two parts will be gas. When the discharge port opens, the pressure in the compressor will be four times the suction pressure (8 parts gas divided by 2 parts exhaust). A part of a liquid remains a part because the liquid is essentially incompressible. Thus, the effect of fluid in the suction flow is to increase the true compression ratio. But a compressor designed for a three to one true compression ratio of four to one is probably safe to operate. The permissible level of liquid in the suction stream is determined by the design pressure ratio and the maximum pressure that can be tolerated in the compression chamber.
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