A screw compressor is a rotary compressor that compresses air through the action of a screw. The major benefit of this type of compressor is that it incessantly supplies the compressed air with minimal fluctuations in the supply pressure. These compressors are utilized for low pressure applications up to 8 bar.

Types of Screw Compressors

1) Twin-screw compressors

The principle of a double screw positive displacement rotary compressor was developed in the 1930s when a rotary compressor with a high and stable flow rate under various pressure conditions was required.

The main components of the biaxial scroll element are female and male rotors that revolve in reverse directions, but with a reduced volume between them and the housing. Every part of the screw has a built-in constant pressure ratio, depending on the shape, the screw pitch and the length of the outlet connection. The built-in pressure ratio must adapt the desired working pressure for extreme efficiency,

Screw compressors are generally valveless and do not create unbalanced mechanical forces. As a result, it works with high shaft speeds and can merge large flow rates with small external dimension. The bearing has to overcome the axial force, which depends on the pressure difference between inlet and outlet.

2) Oil-free screw compressors

The rotor profile of the first twin screw compressor was asymmetrical and no refrigerant was used in the compression chamber. These are called dry or oil-free screw compressors. The latest high-speed oil-free screw compressors feature an asymmetrical screw profile that reduces internal leakage and greatly improves energy efficiency.

External gears are most commonly used to synchronize the position of the rotor as it rotates backwards. No lubrication is required in the compression chamber because the rotors are not in contact with each other and do not touch the compressor housing. The compressed air is therefore completely oil-free. The rotor and housing are manufactured with extremely high precision in order to minimize leakages from the pressure side to the inlet. The built-in pressure ratio is limited by the critical temperature difference between inlet and outlet. That is why oil-free screw compressors are often built with multi-stage and inter-stage cooling.

3) Liquid-injected screw compressors

In a screw compressor with liquid injection, the liquid is injected into the compression chamber, usually the compressor bearing. Its function is to cool and lubricate the moving parts of the compressor component, cool the compressed air inside, and reduce leakage at the return inlet.

Due to its excellent lubricating and sealing properties, oil is the most common type of injection today, but other liquids such as water and polymers are also used. Liquid injection screw compressor components can be manufactured with high pressure ratios. Typically, a compression step can withstand pressures of up to 14 bar or 17 bar, but is less energy efficient.

Advantages Of Screw Compressor

  • Screw compressors combine many of the advantages of centrifugal compressors and compressors with some of their unique advantages. Since this is a positive displacement machine, it uses high pressure refrigerants such as R-22 and ammonia. Since it is a high-speed rotary machine, a large amount of processing is possible. Therefore, it is ideal for a wide variety of low temperature applications such as refrigerating food.
  • There are no overvoltage problems like with reciprocating compressors. Due to the use of high-pressure refrigerant, the pipe size is small and there is overpressure. As with centrifugal compressors, there are no problems with high compression efficiency, stepless capacity control, idle start and balance. The compressor is also suitable for high-performance systems.
  • Low maintenance costs
  • It is usually not loud.
  • Can work under full load for a long time
  • Helps to recover heat from the heating system.
  • Highly efficient oil immersion model

By Hemant Kumar

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