Oil Carry-over

An oil separator of the typical screw compressor has two sections. The first section encountered by the oil-laden refrigerant allows the large drops of oil to fall to the sump. This separating section removes a larger quantity of oil than the coalescing section that follows. The refrigerant vapor passes to the coalescing section and flows through the mesh of fine metal. Here the tiny oil droplets impinge on the mesh and consolidate with other small droplets until the size is sufficient for them to fall to the bottom of the coalescing sump.

Sometimes, at low condensing pressure, screw compressors have oil carry-over. This is the barrier to improving the efficiency of the refrigeration plants.

What is the reason for oil carry-over?

A low condensing pressure will reduce the density of the discharge gas. At relatively constant mass flow, reduced discharge gas density will increase volume flow and thus the velocity through the oil separator. This increased velocity is the reason for oil carry-over. To reduce the discharge gas velocity, two actions can be done.

1. Increase size of oil separator.

2. Reduce refrigerant mass flow.

Replacement of oil separator is expensive and very often it is not practical.

Refrigerant mass flow can be reduced by unloading a compressor and/or by reducing suction pressure. These two actions will reduce the compressor efficiency. But at the same time, a lower condensing pressure will increase the compressor efficiency. These factors are opposite. To choose the right action, an evaluation of the whole system performance should be done. For one plant it will better to reduce suction pressure, while for another it will be better to unload the compressor. To prevent oil carry-over, many refrigeration plants operate at a high condensing pressure. This is a simple, but inefficient solution. I think that we have better options and the right one can be chosen.