Analysis of the working principle of screw air compressor and the working principle of each system

A single-screw air compressor, also known as a worm gear air compressor, consists of a 6-screw shaft and two 11-tooth star wheels. The worm gear meshes with both star wheels simultaneously, balancing the forces on the worm and doubling the displacement. The term "screw compressor" usually refers to a twin-screw compressor.

A screw compressor (also known as a twin-screw compressor) has a pair of rotors with meshing, oppositely rotating helical teeth. The rotor with raised teeth is called the male rotor, and the rotor with recessed teeth is called the female rotor. As the rotors rotate within the compressor body, the working volume continuously changes due to the insertion and disengagement of the teeth, thus periodically altering the volume between each pair of tooth slots to achieve the purposes of intake, compression, and exhaust.

The main unit is the core component of a screw compressor, and the main unit structure and working mechanism are similar for screw compressors of any brand.

As the rotor rotates, one tooth of the male rotor continuously disengages from one tooth slot of the female rotor, gradually expanding the inter-tooth volume and connecting it with the intake port. Gas enters the inter-tooth volume through the intake port until the inter-tooth volume reaches its maximum value, at which point it disconnects from the intake port. The inter-tooth volume is then sealed by the combined action of the tooth and the inner shell, ending the intake process. It is worth noting that at this time, the inter-tooth volumes of the male and female rotors are not connected to each other.

During the compression process, the rotor continues to rotate. Before the volumes between the male and female rotor teeth are connected, the gas in the volume between the male rotor teeth is compressed first by the intrusion of the female rotor teeth. After a certain angle, the volumes between the male and female rotor teeth are connected, forming a "V"-shaped pair of inter-tooth volumes (basic volumes). As the two rotor teeth squeeze into each other, the basic volumes are gradually pushed and the volume gradually shrinks, thus realizing the gas compression process. The compression process continues until the basic volumes are connected to the exhaust port.

As the rotor rotates, the volume of the basic unit continuously shrinks, and the compressed gas is sent to the exhaust pipe. This process continues until the volume is at its minimum.

As the rotor rotates continuously, the above-mentioned intake, compression, and exhaust processes are carried out in a cycle, and each basic volume works in turn, forming the working cycle of the screw refrigeration compressor.

From the analysis of the above process, it can be seen that when the two rotors rotate to the side where they meet each other, that is, the side where the convex and concave teeth meet and embed themselves, the gas is compressed and forms a higher pressure, which is called the high-pressure zone. Conversely, when the screws rotate to the side where they move away from each other, that is, the side where the convex and concave teeth disengage, the inter-tooth volume expands and forms a lower pressure, which is called the low-pressure zone. These two regions are separated by the contact line between the casing and the rotors. It can be roughly considered that the axial plane of the two rotors is the interface between the high and low pressure zones. In addition, the helical channel formed by the meshing line between the male and female rotors causes the gas in the basic volume to move in a helical motion from the intake end to the exhaust end while being compressed.

As a type of rotary compressor, the screw compressor has the structural characteristics of a centrifugal compressor, but its working principle falls under the category of positive displacement compressors.

1. High precision is required for machining the spatial curved surface of the helical rotor;
2. Due to limitations in rotor stiffness and the inability to achieve good interstage sealing, screw compressors cannot currently achieve high final pressures;
3. The periodic high-speed passage of the medium through the suction and discharge ports, and leakage through gaps, results in high compressor noise, necessitating noise reduction measures;
4. Due to these characteristics, screw compressors are generally used in the following operating conditions: 1. Low flow rate; 2. Requires long-term stable operation; 3. Low discharge pressure.

consisting of the body, intake end seat, and exhaust end seat, is a major component of the compressor. The body is the central component connecting all parts, providing the correct assembly positions and ensuring the male and female rotors mesh reliably within the cylinder. Its end face is ∞-shaped, conforming to the outer cylindrical surfaces of the two meshing rotors, allowing for precise rotor insertion into the housing. Radial intake ports, sized to match the rotor's rotation angle, are provided on the inner wall of the housing, ensuring smooth intake during rotor rotation.

The intake and exhaust end seats are sealing connectors located at the front and rear ends of the machine body. In addition to sealing the end faces of the machine body, they also provide the assembly positions for the male and female rotors and the bearings supporting the rotors.

The main component for variable displacement compression, consisting of male and female rotors. The rotor teeth are machined using high-precision specialized machine tools and cutting tools, making it one of the key components of the compressor. The rotor profile is often a single-sided asymmetrical cycloid—an arc profile. The male and female rotors have the following two structural designs:

1. The male rotor is connected to the motor as the driving rotor, transmitting torque, and at the same time, it drives the female rotor (driven rotor) to rotate through meshing.
2. The male and female rotors transmit torque by meshing with the driving gear driven by the motor through their respective driven gears.

Bearings are components that support the male and female rotors and ensure their high-speed rotation. Roller bearings are typically used at the motor end for support. Secondly, as the rotor rotates and compresses gas, it generates axial thrust. To overcome this axial force, a slant-piston bearing is used at the other end of the rotor, which not only overcomes the axial force of the rotor's rotation but also withstands the radial force.

Most screw compressors use oil injection lubrication. Lubricating oil mixes with the compressor fluid and enters the compressor, lubricating and sealing the machine as the medium flows. Its advantages include:

1. lower exhaust temperature;
2. reduced working fluid leakage and improved sealing;
3. enhanced lubrication of components, extending their lifespan;
4. absorption and damping of sound waves, reducing noise; and
5. flushing away mechanical impurities, reducing wear.

However, due to the large amount of oil injected, a lubrication system must be added, and an oil-gas separator must be installed at the compressor outlet, increasing the unit's size and complexity. Furthermore, this method cannot be used for media that cannot tolerate contamination.

Therefore, some screw compressors use oil-free lubrication.

Compressors using oil-free lubrication have high requirements for the meshing clearance of the male and female rotors, the rigidity and quality of the rotors, and the overall machining quality of the unit.