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.

Screw compressors market situation

Screw compressors are manufactured from small up to medium output volume ranges and thus overlap market areas which, until recently, were still reserved for turbo compressors. Only a few manufactures can supplier screw compressors above 300kW. The smaller ranges of screw compressors are supplied in air cooled form whereas the larger ones are constructed either in air cooled or in water cooled versions.

Most common Types of compressors, displacement compressors and dynamic compressors

There are two generic working principles for the compression of air (or gas), so compressors can be subdivided into two groups: The first group operates in the principle of dynamic compression; and the second group of compressors operate in positive displacement principle:

In dynamic compression group, compressors are oil free compressing systems. The effective quantity delivered by a dynamic compressor depends on the operating pressure.

For examples: compressors using the dynamic compression principle are turbocompressors of axial or radial design. In a dynamic compressor, the pressure increasing takes place while the gas flows. Single or multi bladed wheels are used to impart a high velocity of flow to the air drawn in. The flow rate of the gas is subsequently transformed into static pressure when it is forced to decelerate under expansion in a diffuser. Thus converting the kinetic energy of the flow velocity into pressure. All dynamic compressors are designed for large volume flow rates.

The second group consists of compressors which operate in accordance with the positive displacement principle. The air is drawn into one or more compression chambers, which are then closed from the inlet. Gradually the volume of each chamber decreases and the air is compressed internally. So static pressure is built up through this positive displacement process, later the compressed air can be discharged to compressed air distribution system.

Positive displacement compressors include piston compressors and rotary compressors (vane compressors and screw compressors).

These displacement compressor systems are supplied in three versions: oil free (“non lubricated”), oil lubricated and oil flooded versions.

Displacement compressors by far are the most widely used types, and the common types are listed as follows:

• Piston compressors: oil free and oil lubricated versions. 
• Vane compressors: oil lubricated and oil flooded versions. 
• Screw compressors: oil free and oil flooded versions. 

Oil free compressors are very commonly used in clean industries, such as food, pharmaceutical, electronics and semiconductor industries, etc.

But in other industries it is also a better choice to use oil free compressors, in where it is possible, to keep oil pollution away from the compressed air.

It is also not correct to say that the oil free compressors make the compressed air oil free, so the compressed air purification could be completely removed. In most cases, the atmosphere air is not clean enough, it contains solid particles, air borne molecules, oil vapor and oil aerosol. But the treatment of compressed air is much simpler than for compressed air from the other type of compressors.