How the scavenge line keeps your compressed air oil free

Posted by Cas at 9:02 AM on Aug 10, 2013

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This is the first post in the ‘part of the day’ series. In each post I will discuss another air compressor part. I will explain what they do, how they work and how to troubleshoot when there are problems.

The part of today is:

the scavenge line.

The scavenge line is found on all bigger 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 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. I have added a picture with some beautiful hand-drawn extras to explain the process. :)

1) Centrifugal separation.

Oil separation on rotary screw compressors

Oil separation on rotary screw 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 an tornado). The air/oil mixture enters the vessel at a pretty high velocity, but will lose momentum quickly.

To facilitate this ‘tornado action’ and to protect the oil separator from direct contact with the incoming air/oil mixture, there is an oil screen or splash screen installed.  See the green arrows in the picture where the oil screen is located.

When the air/oil mixtures 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. (the blue dots is oil