The scavenge line ( Oil Return Line ) is found on all oil injected rotary screw compressors and plays a crucial part in the removal of oil from the compressed air. Most people know that the oil separator filter/element is the part that separates the oil from the compressed air. But that is only part of the story. The oil separation process consists of three steps.

1) Centrifugal separation.

Oil separation on rotary oil flooded screw air compressors:

The air/oil mixture from the screw element outlet enters the separator vessel at an angle. The air/oil mixture flows in circles around the circular inner surface of the vessel (think of centrifugal). The air/oil mixture enters the vessel at a pretty high velocity, but will lose momentum quickly. To facilitate this 'centrifugal action' and to protect the oil separator from direct contact with the incoming air/oil mixture, there is an oil screen, shroud or splash screen installed.

When the air/oil mixture loses velocity, the oil falls down into the oil sump, while the compressed air finds it ways to the exit. This process removes already 95% of the oil from the air.

2) Oil separator filter/element. 

The compressed air that finds it way up reaches the oil separator element. The 5% oil that is left is in the form of tiny little droplets (but lots of them). The oil separator is made of a special fabric that makes the oil droplets stick together to form bigger drops. These drops become so big that they separate from the air flow and drip down to the bottom of the separator filter. Over a period of time a pool of oil will collect on the bottom of the separator.

3) The scavenge line 

The scavenge line sucks up this oil from the bottom of the separator filter. The oil is sucked back to the air compressor element (low pressure side of air end). The scavenge line actually consists of two parts: the scavenge line and the scavenge pipe. The scavenge line is the flexible hose coming from the vacuum side of the compressor element to the top of the separator vessel. Inside the separator vessel it continues as the scavenge pipe.

The scavenge pipe hovers about; maximum of 4mm above the bottom of the oil separator. Sometimes there is a filter and/or sight glass installed in the scavenge line. Often there is also a check valve (CV) installed where the scavenge line connects to the separator vessel.

When things go wrong?

There can be a lot of sources for the common 'oil in compressed air' ( Oil Carry-over ) problem, but since this post is about the scavenge line, I will focus on this part.

Common problems: - The scavenge line or scavenge pipe is blocked or plugged with dirt.

• The scavenge pipe is bent so it does not reach the bottom
• The scavenge pipe is too long, it touches the bottom.
• The scavenge line filter is dirty

All of these causes have the same result: the scavenge line stops sucking up oil from the bottom of the oil separator.

Result: the oil level in the oil separator rises. 

At some point you will see lots of oil carry over to the compressed air system.

Do you have oil-in-air problems?

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