Screw compressors

In the 1930s, a rotating compressor with high flow rate and stable flow under varying pressure conditions was required. Then the principle for a rotating displacement compressor in twin screw form was developed. it took nearly 75 years until the wide ranging opportunities for this type of compressor were recognized.

The twin screw element’s main parts are the male and female rotors, which rotate in opposite directions while the volume between screws and the housing decreases. Each screw element has a fixed, build-in pressure ratio that is dependent on its length, the pitch of the screw and the form of the discharge port. To attain maximum efficiency, the build-in pressure ratio must be adapted to the required working pressure.

The screw compressor is generally not equipped with valves and has no mechanical forces that cause unbalance. This means it can work at a high shaft speed and can combine a large flow rate with small exterior dimensions. An axial acting force, dependent on the pressure difference between the inlet and outlet, must be overcome by the bearings.

Oil-free screw compressors

The first twin screw compressors had a symmetric rotor profile and did not use any cooling liquid inside the compression chamber. These were called oil-free or dry screw compressors. Modern, high-speed, oil-free screw compressors have asymmetric screw profiles, resulting in significantly improved energy efficiency, due to reduced internal leakage.

External gears are most often used to synchronize the position of the counter-rotating rotors. As the rotors neither come into contact with each other nor with the compressor housing, no lubrication is required inside the compression chamber. Consequently, the compressed air is completely oil-free.

The rotors and housing are manufactured with ultimate precision to minimize leakage from the pressure side to the inlet. The build-in pressure ratio is limited by the limiting temperature difference between the inlet and the discharge. This is why oil-free screw compressors are frequently built with several stages and inter-stage cooling to reach higher pressures.

Liquid-injected screw compressors

In liquid-injected screw compressors, a liquid is injected into the compression chamber and often into the compressor bearings. Its function is to cool and lubricate the compressor element’s moving parts, to cool the air being compressed internally, and to reduce the return leakage to the inlet.

Today oil is the most commonly injected liquid due to its good lubricating and sealing properties, however, other liquids are also used, for example, water or polymers. Liquid-injected screw compressor elements can be manufactured for high pressure ratios, with one compression stage usually being sufficient for pressure up to 14 and even 17 bar, albeit at the expense of reduced energy efficiency.