Use of Eductors in Oil Return Methods

Occasionally it’s reported that on screw chillers utilizing eductors for oil return, when working at low load circumstances, it seems that the eductor doesn’t function effectively sufficient to return a adequate quantity of oil to the oil separator or sump to keep up its oil stage, which then causes the chiller to close down on low oil, a consequence of oil retained within the refrigerant cost within the evaporator.

For such a chiller which makes use of an eductor for oil return, the reason for the failure is probably not low load, however fairly low carry. In a consolation cooling surroundings, chiller load is attentive to out of doors temperature. That’s, when it’s sizzling outdoors, warmth flows quickly into the constructing and chiller load is excessive. Concurrently, the chiller should reject its warmth to a excessive ambient temperature. Therefore the chiller working at a excessive load situation can be working at a excessive carry situation. Elevate is outlined because the distinction between the suction and discharge saturation temperatures (or pressures).

When the out of doors temperature is cool, little warmth must be faraway from the conditioned area and so chiller load is low. The low load is accompanied by a low carry situation because the ambient temperature is down from its excessive worth. The low carry is the reason for the lack of effectiveness of the eductor. The eductor is pushed by the strain distinction between the condenser and the evaporator. When this strain distinction falls, the circulate inducing capability of the eductor is lowered. The circulate inducing capability of the eductor is roughly proportional to the sq. of the strain distinction. Therefore, a strain distinction discount to 50% of design will result in an induced circulate discount to 25% of design.

Not all chillers serve the consolation cooling market. There are chillers utilized to chemical processes, for instance, which will have various load however fixed carry; i.e. fixed suction and discharge temperatures. These chillers would unlikely have oil loss issues associated to load if served by an adequately sized eductor based mostly oil return system.

Doable cures for poor eductor efficiency in low carry functions embrace lowering the oil discharge price of the compressor/separator and modifying the management system to extend the minimal carry of the system.

Liquid within the Compressor Suction

Ideally, any liquid getting into the compressor suction might be wealthy sufficient in oil and lean sufficient in refrigerant that lubrication might be passable. But, if any liquid ingested right into a compressor has too low a focus of oil, lubrication could also be compromised and put on resulting in compressor failure can ensue. All compressors are susceptible to lack-of-lubrication failure, both from lack of oil or from an excessive amount of refrigerant within the oil..

A second sort of failure is the results of injecting an excessive amount of liquid refrigerant/oil right into a compressor that may harm or destroy the compressor by “liquid slugging”. Screw and scroll compressors are fairly extra tolerant of liquid within the suction stream than are reciprocating compressors. That is because of the differing nature of the compression processes.

In a reciprocating compressor designed for a 3 to 1 compression ratio, the fuel could attain the discharge strain when the piston is barely at half stroke. At this level the discharge valve opens and fuel is discharged because the piston continues to rise regardless that fuel strain within the cylinder now not rises. The ultimate clearance quantity could also be just one tenth of the entire swept quantity. This clearance quantity isn’t discharged, however is re-expanded on the suction stroke. One would possibly say at this level that the true compression ratio is ten to 1 contemplating a closed discharge valve (swept quantity divided by swept quantity plus clearance quantity). If a quantity of liquid of 110% of clearance quantity is within the cylinder when compression begins, the piston might be compressing solely liquid on the finish of its stroke and the liquid could not have the ability to exit the discharge valve quick sufficient to keep away from growing a really excessive strain within the cylinder. This excessive strain could cause failure of the connecting rod or failure of the pinnacle bolts. For a reciprocating compressor to be environment friendly, a small clearance quantity is required. But, it’s the small clearance quantity that makes reciprocating compressors prone to liquid slugging harm. Allowable ranges of liquid within the suction are decided by the ratio of clearance quantity to swept quantity.

In distinction, screw and scroll compressors designed for a 3 to 1 compression ratio seize a quantity of suction fuel (and a few oil and perhaps some liquid refrigerant) and cut back its quantity to 1 third its authentic worth. However the compression course of is accomplished earlier than the discharge port opens. Any liquid within the suction stream will trigger the compression ratio to rise above the design worth of three, however the rise is slower than within the reciprocating compressor. For instance, assume that the suction stream for a screw compressor consists of 1 half liquid and eight components fuel by quantity. The compressor will cut back these 9 components to three components. On the completion of compression, one half will nonetheless be liquid and two components might be fuel. The strain within the compressor when the discharge port opens might be 4 instances suction strain (8 components fuel moving into divided by 2 components fuel going out). The one a part of liquid stays one half as a result of the liquid is actually incompressible. Thus, the impact of liquid within the suction stream is to extend true compression ratio. However a 4 to 1 true compression ratio in a compressor designed for 3 to 1 might be protected to function. Allowable ranges of liquid within the suction stream are decided by the design strain ratio and the utmost strain that may be tolerated within the compression chamber.

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