Rotary refrigeration compressor is a volumetric compressor that rotates its working volume. Gas compression and pressure changes are achieved through volume changes, which in turn are achieved through the rotational motion of one or several rotors of the compressor in the cylinder. Unlike reciprocating compressors, their volume periodically expands and contracts while their spatial position constantly changes. As long as the intake and exhaust ports are reasonably configured on the cylinder, basic working processes such as intake, compression, and exhaust can be achieved. Screw compressors are divided into twin-screw and single screw compressors. For simplicity, twin-screw compressors are also commonly referred to as screw compressors. A single screw compressor, also known as a worm compressor, consists of a screw and two star wheels. It is similar to twin-screw compressors in many aspects and has a more ideal force balance, so it has developed rapidly at home and abroad. However, it is not widely used in refrigeration at present.

Section 2 Working Principle and Characteristics of Screw Refrigeration Compressor

1、 Working principle
The screw type (i.e. twin-screw) refrigeration compressor has a pair of helical teeth rotors that mesh with each other and rotate in opposite directions. The rotor with protruding teeth is called a male rotor, and the rotor with concave teeth is called a female rotor. The teeth of the rotor are equivalent to the piston, and the working volume formed by the teeth of the rotor, the inner wall surface of the body, and the end caps at both ends is equivalent to the cylinder. There are suction and exhaust ports arranged diagonally at both ends of the body. As the rotor rotates inside the body, the working volume continuously changes due to the invasion or disengagement of teeth, thereby periodically changing the volume between each pair of teeth slots of the rotor to achieve the purpose of suction, compression, and exhaust.
(Quoted from Oriental Simulation)










2、 Job characteristics
As a type of rotary refrigeration compressor, screw refrigeration compressor has both piston type and power type (speed type) characteristics.
1) Compared with reciprocating piston refrigeration compressors, screw refrigeration compressors have a series of advantages such as high speed, light weight, small size, small footprint, and low exhaust pulsation.
2) Screw refrigeration compressors do not have reciprocating mass inertia force, have good power balance performance, operate smoothly, have small base vibration, and can be made smaller.
3) Screw refrigeration compressors have a simple structure, few components, and no vulnerable parts such as air valves and piston rings. Their main friction parts, such as rotors and bearings, have high strength and wear resistance, and good lubrication conditions. Therefore, they require less machining, have low material consumption, long operating cycles, reliable use, simple maintenance, and are conducive to achieving automated operation.
4) Compared with speed compressors, screw compressors have the characteristic of forced gas delivery, that is, the displacement is almost not affected by the exhaust pressure, and there is no surge phenomenon at small displacement. In a wide range of operating conditions, high efficiency can still be maintained.
5) Adopting slide valve adjustment, it can achieve stepless energy regulation.
6) Screw compressors are not sensitive to liquid intake and can use oil injection cooling. Therefore, at the same pressure ratio, the discharge temperature is much lower than that of piston compressors, resulting in a higher single-stage pressure ratio.
7) There is no clearance volume, so the volumetric efficiency is high.

The screw refrigeration compressor still has the following defects:
1) The refrigerant gas periodically passes through the suction and exhaust ports at high speed, causing significant noise in the compressor due to leakage through gaps and other reasons. Therefore, noise reduction measures need to be taken.
2) The machining accuracy of the spatial surface of the spiral rotor is required to be high, and equipment and cutting tools are needed for machining.
3) Due to limitations such as gap sealing and rotor stiffness, screw compressors currently cannot achieve high final pressures like reciprocating compressors.

3、 Screw refrigeration compressor with economizer







Although screw refrigeration compressors have the advantage of high single-stage pressure ratio, as the pressure ratio increases, the leakage loss increases rapidly, resulting in a significant decrease in efficiency during operation at low temperatures. In order to expand its usage range, improve the performance of low-temperature conditions, and increase efficiency, the characteristic of unidirectional suction, compression, and exhaust of screw refrigeration compressors can be utilized. A supplementary air port can be opened at an appropriate position on the casing or end cover, so that the volume of the rotor element is connected to the supplementary air port at a certain angle range during the compression process, and the flash gas in the intermediate container added to the system can enter the element volume through the supplementary air port. In this way, the single-stage screw compressor operates in a two-stage refrigeration cycle, achieving energy-saving effects. This additional intermediate container is called an economizer.