Screw Compressor – Diagram, Parts ,Working Principle Of Screw Compressor

A screw compressor is a type of rotary compressor which compresses air due to screw action. The main advantage of using this compressor is that it can supply compresses air continuously with minimum fluctuation in delivery pressure. It is usually applied for low pressure applications up to 8 bars.


A screw compressor comprises of two screw like rotating elements, a casing, an air filter, rubber seals, suction valve and delivery valve mainly.
A picture can help you visualise the parts and their assembly more better.

Screw Compressor parts
Screw Compressor parts

Working principle:

In a screw compressor one of the shaft is driving shaft and the other is driven shaft. The driving shaft is connected to the driven shaft via timing gears which help to match speeds of both the shafts. The driving shaft is powered by an electric motor generally.The two shafts are enclosed in an air tight casing.
Firstly the suction valve is opened to allow air suction. Then the motor is turned on which drives the shaft. The driven shaft, thus, also starts rotating counter to the driving shaft because of timing gears. The air enters the gap between the two screws cut over the shafts. As the two screws turn in opposite direction the air gets trapped in the groove between the two screws. The gap between the two screws decreases gradually from suction end to delivery end, which leads to compression of air. Also due screw action the air moves from the suction end to the delivery end. When the compressed air reaches the delivery end, it passes through the delivery valve to the storage tank.
Also during the compression process, cold water is continuously run around the casing which exchanges heat with the hot compressed air. The reason behind this is that the efficiency of any compression process will be highest if it is carried out isothermally in comparison to adiabatically or polytropically. Thus to maintain the temperature of air during compression, the heat developed is continuously exchanged with flowing water. A graph can help you understand this logic more clearly.

pv screw compressor
pv screw compressor

​As you can see the shared area will be minimum for isothermal compression thus requiring minimum work for compression process which will lend you higher efficiency.
This is the working principle of a screw principle

Types Of Screw Compressor: 

Twin screw compressors

The principle for a rotating displacement compressor in twin screw form was developed during the 1930s, when a rotating compressor with high fl ow rate and stable fl ow under varying pressure conditions was required.
The twin screw element’s main parts are the male and female rotors, which rotate in opposite directions while the volume between them 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.

Advantages Of Screw Compressor : 

  • The screw compressor combines many advantageous features of both centrifugal and compressors, along with some of its own. As it is a positive displacement machine, high pressure refrigerants, such as R-22 and ammonia are used in it. As it is a high speed rotary machine, a large volume can be handled by it. It is,therefore, found extremely suit able for large capacity low temperature applications such as in food refrigeration.
  • Like reciprocating compressors, it has no surging problems. It has small pipe dimensions and positive pressures due to the use of high pressure refrigerants.Like centrifugal compressors, it has high compression efficiency, continuous capacity control, unloaded starting and no balancing problems. Also, the compressor is suitable for large capacity installations.
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