Use of Eductors in Oil Return Systems

From time to time it is reported that on screw chillers using eductors for oil return, when operating at low load conditions, it appears that the eductor does not operate efficiently enough to return a sufficient amount of oil to the oil separator or sump to maintain its oil level, which then causes the chiller to shut down on low oil, a consequence of oil retained in the refrigerant charge in the evaporator.

For such a chiller which uses an eductor for oil return, the cause of the failure may not be low load, but rather low lift. In a comfort cooling environment, chiller load is responsive to outdoor temperature. That is, when it is hot outside, heat flows rapidly into the building and chiller load is high. Simultaneously, the chiller must reject its heat to a high ambient temperature. Hence the chiller operating at a high load condition is also operating at a high lift condition. Lift is defined as the difference between the suction and discharge saturation temperatures (or pressures).

When the outdoor temperature is cool, little heat needs to be removed from the conditioned space and so chiller load is low. The low load is accompanied by a low lift condition since the ambient temperature is down from its high value. The low lift is the cause of the loss of effectiveness of the eductor. The eductor is driven by the pressure difference between the condenser and the evaporator. When this pressure difference falls, the flow inducing capacity of the eductor is reduced. The flow inducing capacity of the eductor is approximately proportional to the square of the pressure difference. Hence, a pressure difference reduction to 50% of design will lead to an induced flow reduction to 25% of design.

Not all chillers serve the comfort cooling market. There are chillers applied to chemical processes, for example, that may have varying load but constant lift; i.e. constant suction and discharge temperatures. These chillers would not likely have oil loss problems related to load if served by an adequately sized eductor based oil return system.

Possible remedies for poor eductor 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 the Compressor Suction

Ideally, any liquid entering the compressor suction will be rich enough in oil and lean enough in refrigerant that lubrication will be satisfactory. Yet, if any liquid ingested into a compressor has too low a concentration of oil, lubrication may be compromised and wear leading to compressor failure can ensue. All compressors are vulnerable to lack-of-lubrication failure, either from lack of oil or from too much refrigerant in the oil..

A second type of failure is the result of injecting too much liquid refrigerant/oil into a compressor that can damage or destroy the compressor by “liquid slugging”. Screw and scroll compressors are rather more tolerant of liquid in the suction stream than are reciprocating compressors. This is due to the differing nature of the compression processes.

In a reciprocating compressor designed for a three to one compression ratio, the gas may reach the discharge pressure when the piston is only at half stroke. At this point the discharge valve opens and gas is discharged as the piston continues to rise even though gas pressure in the cylinder no longer rises. The final clearance volume may be only one tenth of the total swept volume. This clearance volume is not discharged, but is re-expanded on the suction stroke. One might say at this point that the true compression ratio is ten to one considering a closed discharge valve (swept volume divided by swept volume plus clearance volume). If a volume of liquid of 110% of clearance volume is in the cylinder when compression begins, the piston will be compressing only liquid at the end of its stroke and the liquid may not be able to exit the discharge valve fast enough to avoid developing a very high pressure in the cylinder. This high pressure can cause failure of the connecting rod or failure of the head bolts. For a reciprocating compressor to be efficient, a small clearance volume is required. Yet, it is the small clearance volume that makes reciprocating compressors susceptible to liquid slugging damage. Allowable levels of liquid in the suction are determined by the ratio of clearance volume to swept volume.

In contrast, screw and scroll compressors designed for a three to one compression ratio capture a volume of suction gas (and some oil and maybe some liquid refrigerant) and reduce its volume to one third 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 rise is slower than in the reciprocating compressor. For example, assume that the suction stream for a screw compressor consists of 1 part liquid and 8 parts gas by volume. The compressor will reduce these 9 parts to 3 parts. At the completion of compression, one part will still be liquid and two parts will be gas. The pressure in the compressor when the discharge port opens will be four times suction pressure (8 parts gas going in divided by 2 parts gas going out). The one part of liquid remains one part because the liquid is essentially incompressible. Thus, the effect of liquid in the suction stream is to increase true compression ratio. But a four to one true compression ratio in a compressor designed for three to one is probably safe to operate. Allowable levels of liquid in the suction stream are determined by the design pressure ratio and the maximum pressure that can be tolerated in the compression chamber.