How Oil-Inject Rotary Screw Compressors Work

All oil-injected screw compressors have the same working principle.

Let me show you exactly how oil-injected rotary screw air compressors work.

It doesn’t matter what make or model of screw compressor you have. Atlas Copco, Ingersoll Rand, Sullair, Kaeser (just ot name a few) … they are all the same. Same working principle, same parts, similar design.

The oil-injected screw compressor

Ambient air is taken in by the screw compressors and is compressed in the screw element. In this screw element, oil is injected during the compression.

Before the compressed air leaves the compressor, the oil is separated from the air/oil mixture (we want compressed air – the oil must stay inside the compressor).

The air leaves the compressor as compressed air and the oil is filtered, cooled and returned back to the screw element.

The screw element

The screw element is the heart of the air compressor. It’s also called “air-end”. The screw element consists of two ‘helical screw rotors’ in a housing. There’s a male and a female rotor that fit each other exactly.

As they turn (in opposite directions), air is trapped between the rotors and pushed towards the pressure side of the screw.

Compressor Oil

During compression, oil is injected in between the screws. The oil has various functions, the most important ones are:

  1. Absorption of the heat of compression
  2. Sealing between the rotors
  3. Lubrication

Compressing air generates huge amounts heat. This heat needs to be removed to prevent overheating. The compressor oil absorbs the heat during compression. The oil is filte

Oil-Free Screw Compressors

There are also oil-free screw compressors. Obviously, there is no oil injected in oil-free rotary screw compressors.

For this reason, oil-free screw elements are manufactured with very tight tolerances, since there is no oil to help seal the gaps between the two rotors

And, since there’s no oil to cool the compressed air – oil-free screw compressors work with two stages, with an intercooler in between, to help keep temperatures down.

Oil-injected Rotary Screw Compressor Overview

Here’s an overview of an oil-injected rotary screw compressors, with all major parts.

A screw compressor can be divided into 3 main sub-systems:

  • Air system
  • Oil system
  • Control system

In the diagram below, the air system is colored blue. The oil system is yellow and the control system is red.

Detailed explanation of the screw compressor

Air is taken in through the inlet filters (far left in the diagram). It pases through the inlet valve and is compressed in the screw element. Oil is injected during compression. A mixture of air and oil leaves the screw element (the yellow/blue-ish line).

The oil is separated from the compressed air in the separator vessel. Most of the oil is separated by centrifugal force. The rest is removed by the oil separator filter.

The air leaves the compressor through the minimum pressure valve (on top of the oil separator vessel) and through the aftercooler. Water condensate is removed in a condensate trap with automatic drain.

The separated oil is filtered and cooled in the oil-cooler. A thermostatic valve by-passes the cooler when the oil is cold. The cooled and clean oil is again injected in the screw element.

Controlling compressor output

There are multiple ways to control compressor output. The most common way is ‘load/unload’ control, which I will describe here.

In the case of ‘load/unload’ control, the output capacity of a rotary screw compressor is controlled by the inlet valve.

The inlet valve opens and closed the ambient air supply to the screw element.

When it is closed – the compressor runs ‘unloaded’ – it is not pumping any air.

When the inlet valve is open – the compressor runs ‘loaded’.

The inlet valve is opened and closed by the control system. It opens the inlet valve when the lower pressure setpoint is reached. It closed the inlet valve when the upper pressure setpoint is reached.

"Load" vs "Unload"

In most machines, the inlet valve is operated by compressed air. An electronically controlled solenoid valve opens and closes the air to the inlet valve.

The biggest differences between air compressors of different manufactures (and different models of the same brand), are with the control system.

There are some extra functions that the control system performs, like blowing-off the internal pressure when the compressor runs unloaded.

Besides that, we have normally open inlet valves, and normally closed inlet valves.

So the exact layout and working principle of the control system can be different – but the main principles are always the same.

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