How oil-injected rotary screw air compressors work

As the name suggests, there is oil injected in this type of screw compressor (as opposed to oil-free screw compressors).

But where is it injected, why and how? Let me explain..

As said before, the screw element is the most important part of the air compressor.

But it can't function by itself.

There are a lot of other parts that make up the air compressor.

Although there are some differences between manufacturers, the basic parts and principles are the same.

How it works

Outside air

Air is sucked in by the screw element (air-end) through the air inlet filter. The inlet filter makes sure that all the dust en dirt stays outside. It protects the screw element (which is very expensive and can be damaged.

It's also the first step in making sure that the compressed air is clean: all the dust that is sucked in will eventually end up in the compressed air system.

Load/unload control

Before the air enters the screw element, it passed the inlet valve or unloader valve. This valve opens and closes the air supply to the screw element.

When it's open, the compressor is in 'loaded' condition: it is actually compressing air and pumping it into the compressed air system. When the valve is closed, is shuts off the air supply to the compressor element: the motor and screw element are turning, but the compressor is not sucking any air in and is not pumping any air to the system.

The screw compressor element

When the inlet/unloader valve is open, the air enters the compressor screw element.

The screw element works like a pump and it compressed the air. During this process, oil is injected in the element. The oil is there to cool the air, as the air gets very hot during compression. It's also there for lubrication and sealing off the clearances between the screws.

More detailed information can be found on our screw air compressor element page.

Now we have a mixture of compressed air and compressor oil.

Compressed air/oil mixture

This mixture leaves the screw element through a one-way valve. This valve makes sure that the oil cannot flow back into the compressor element through the exit pipe (this could happen when the compressor stops).

The oil separator

Now we need to separate the oil and the air.

This is done in the separator tank. Most of the oil is separated from the compressed air by centrifugal force (just like what happens in a clothes tumble dryer). The remaining oil (mostly small droplets and oil mist) is separated by the separator element which looks like a big filter.

The air with oil flows through the separator element. The element separates the oil from the compressed air. The separated oil is collected at the bottom of the separator and is removed by the scavenge line. It sucks the collected oil back to the compressor element.

The now clean compressed air is almost ready to leave the compressor. But first is passed the minimum pressure valve and the after cooler.

The minimum pressure valve

The minimum pressure valve is a spring-loaded valve that opens at a certain pressure, about 2.5 bar. The minimum pressure valve makes sure that there is always a minimum pressure inside the compressor (hence the name).

This pressure is needed for the corrected operation of the air compressor (for pumping the oil around).

The after cooler

The compressed air is still very hot at this point, about 80 degrees Celsius. The compressed air is now cooled by the after cooler before it leaves the compressor. The air temperature after the cooler is around 25 – 40 degrees Celsius.

Because of the cooling down of the air, a lot of water vapor has condensed against the inside of the after cooler. This water is carried with the compressed air towards the air outlet of the compressor.

Of course we don't want all this water in our compressed air system.

Moisture trap

For this reason, a water trap (also called a moisture trap, moisture separator or condensate traps).

This can be a mechanical or an electrical version. The condensate trap basically separates the water from the compressed air. The water is drained through a small hose.

The compressed air now finally leaves the compressor.

Oil flow

But what about the oil? Remember the compressed air/oil mixture was separated by the separator?

The separated oil is hot. Is absorbed the heat of the compression and can be as hot as 120 degrees Celsius (anything more and the compressor will shut down).

The oil is cooled by the oil cooler. The amount of cooling is controlled by a thermostatic valve. If the oil is still cold, the oil cooler is completely by-passed. If the oil is very hot, all the oil is led through the oil cooler. The thermostatic valve regulates the oil temperature.

The Oil filter

Finally, the oil flows through the oil filter. The oil filters removes all the dirt and dust that has collected in the oil.

Too much dirt in the oil will damage the screw element. The oil filter has an internal by-pass valve which opens when the pressure difference over the filter becomes too high (when the filter is vey dirty or when the oil is still very cold).

The oil is now again injected in the screw element to do its job again.