When it comes to the screw type air compressor, generally, there are two kinds, single-screw type and twin-screw(or double-screw) type. We will make a comparison between these two kinds of air compressors.

Differences Single-screw air compressor Twin-screw air compressor

Structure
A screw rotor and two symmetrically starwheels.
The screw groove and the inner wall of the casing constitute a closed elementary volume. So that the gas can reach the required pressure.
It has the characteristics of simple structure, small volume, and so on.
The twin-screw compressor is a double-shaft positive displacement compressor.
The main (male) pair (female) two rotors cooperate to form a meshing pair.
The tooth profile of the main and auxiliary rotors forms a closed elementary volume with the inner wall of the casing.

Working principle
The motor transmits power to the screw shaft through coupling or a belt. The screw drives the star wheel to rotate.
The gas enters the screw groove from the suction chamber. The exhaust port and the exhaust chamber compress the gas, and it is discharged.
The role of the star wheel is equivalent to the piston of a reciprocating piston compressor. When it moves relative to each other, the enclosed volume is gradually reduced. Then the gas is compressed.
The motor drives the main rotor through a coupling, speed-increasing gear, or belt. The main rotor directly drives the auxiliary rotor to rotate together.
Under the action of relative negative pressure, the air is sucked in. The gas is transported and compressed under the anastomosis of the tooth crest and tooth groove.
When the rotor meshing surface turns to communicate with the exhaust port of the casing, the compressed gas begins to be discharged.

Technical history

 The invention of the single screw type is more than ten years later than the twin-screw type. The invention of the twin-screw compressor was earlier.

Force balance

 The radial and axial force of the screw is automatically balanced. The gas force of the planetary gear requires sufficient strength and rigidity. The screw rotor bears large radial and axial force. The screw rotor is required to have sufficient strength and rigidity. The bearing needs to bear a large load at last.

Reliability
The star wheel is a vulnerable part. Because of material problems, it usually has a life span of only a few thousand hours.
If it is regarded as a consumable, there will be no other reliability problems.
 Due to the heavy bearing load, the overhaul time of most screw compressors is 20,000 to 50,000 hours. The bearing needs to be replaced and the value is high.

Noise and vibration

 Good force balance, low noise, and vibration, generally 60~68dB(A). Poor force balance, high-frequency noise when the rotor meshes, generally 64~78dB(A).

Manufacturing costs

 Ordinary bearings can be used for screw and star wheel bearings. The manufacturing cost is lower. Due to the relatively large load of the two-screw rotors, bearings with higher precision are required. The manufacturing cost is higher.

Maintenance and repair

 It is not difficult to maintain and repair. The maintenance of the twin-screw is very simple. As the large-scale use of the twin-screw, there are more and more technicians for the overhaul of the main engine. Currently, there is no need to return to the factory or the support of the manufacturer.

Scope of application
It can work under high pressure and is suitable for occasions with high exhaust pressure.
The single-screw rotor and the star wheel are in contact sealing. It is easier to achieve oil-free at low speeds.
It only can be used in medium and low-pressure applications, for power, refrigeration, and gas transportation. It is generally less than 4.5MPa.
Since the rotors cannot directly mesh without oil lubrication, they require a high speed (usually above 10,000 rpm). The efficiency is low.

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