What are Screw Compressors?

Did you know that the principle on which the rotating displacement compressor in twin screw form is based on was developed around the 1930s, when a rotating screw compressor with high flow rate and stable flow under different pressure conditions was required.

The main parts of a twin screw are the male and female rotors, which rotate in opposing directions during which the volume and housing between them decreases.

There is a fixed, build-in pressure ratio in each screw element. This pressure ratio depends on the length, pitch of the screw and form of the discharge port. To attain maximum efficiency, the build-in pressure ratio must be adapted to the working pressure.

Screw compressors don’t have valves or any other mechanical forces that can cause an imbalance. And since it does not have factors that can cause imbalance, screw compressors are equipped to work at very high shaft speeds that can combine a large flow rate with small exterior dimensions.

History of the oil-free screw compressors

The very first twin screw compressors that were developed had a symmetric rotor profile and did not use any cooling liquid inside the compression chamber. The name given to this form of twin screw compressor was oil-free or ‘dry’ screw compressors.

Today’s screw compressors have assymetric screw profiles unlike its forefathers that were of the symmetric orientation. This results in greater energy efficiency due to reduced internal leakage. Since the rotors don’t meet each other at any stage, no lubrication is needed inside the compression chamber. .

The compressed air is therefore, totally oil-free. The rotors and housing are made with complete precision so that there is minimal leakage from the pressure side to the inlet. The build-in pressure is also limited by the limiting temperature difference between the inlet and the discharge.

Due to this factor, oil-free screw compressors are manufactures with several stages and interstage cooling to reach high pressures.

Liquid-injected screw compressors:

In this type of screw compressor, a liquid is injected into the compression chamber and the compression bearings.

The function of this liquid is to cool and lubricate the moving parts, to cool the air that is being compressed and to minimise the re
turn leakage to the inlet. .

The liquid injected is mostly oil but sometimes other liquids like water or even polymers are used. This type of screw compressor can be manufactured for very high pressure rations with one compression stage being sufficient for pressure up to 14 and even 17. However, this comes with reduced energy efficiency.

Fixed speed compressors and variable speed drive compressors

As the name suggests, these compressors run at a fixed speed and are super efficient when operating at 100% capacity. However, its inefficiency is a downside. Before the motor comes to a stop, it continues to run even when no air is being produced thereby expending both energy and money.

On the other hand, variable speed drive (VSD) runs the motor at a speed that is needed according to the amount of air required. VSD is much more popular today given the energy savings and the reduction in expenses too!