How to distinguish between screw air compressors and piston air compressors?

Screw-type air compressors and piston-type air compressors can be said to be the two models with a high market share in the current air compressor market. Piston-type air compressors were mainly used in the past. In recent years, with the characteristics of screw-type air compressors, they have gradually replaced piston-type air compressors and become the mainstream in the air compressor market. So, what are the differences between piston-type air compressors and screw-type air compressors?

1. Structure and working principle.
1. Piston-type oil-free lubricated air compressor.
The piston-type oil-free lubricated air compressor consists of the compressor main unit, cooling system, regulation system, lubrication system, safety valve, motor, and control equipment. The compressor and motor are bolted to the base, which is fixed with anchor bolts. During operation, the motor directly drives the crankshaft through a coupling, which drives the connecting rod, crosshead, and piston rod, causing the piston to reciprocate in the compressor cylinder, completing the processes of intake, compression, and discharge. This machine is a double-acting compressor, meaning that the upward and downward movement of the piston involves air intake, compression, and discharge.

2. Screw-type air compressor.
The screw-type air compressor consists of a screw head, motor, oil-gas separation tank, cooling system, air conditioning system, lubrication system, safety valve, and control system. The entire machine is installed in a box, forming a self-contained unit that can be placed directly on a flat concrete surface without the need for anchor bolts. The screw head is a type of dual-shaft volumetric rotary compression head. During operation, the motor directly drives the main rotor through a coupling (or belt). Due to the intermeshing of the rotors, the main rotor directly drives the auxiliary rotor to rotate together. The cooling liquid is directly injected into the rotor meshing area through a nozzle at the lower part of the compressor housing, mixing with the air to carry away the heat generated during compression, achieving a cooling effect. At the same time, a liquid film is formed to prevent direct metal-to-metal contact between the rotors and the gap between the closed rotor and the housing. The injected cooling liquid can also reduce the noise generated by high-speed compression.

2. Characteristics.
1. Piston-type oil-free lubricated air compressor.
The piston rings in the cylinder of the oil-free lubricated air compressor and the packing in the packing device both use polytetrafluoroethylene with self-lubricating properties as sealing elements. Therefore, the cylinder and packing device do not require lubrication oil to be injected for lubrication. Under normal circumstances, the compressed gas is basically pure, free of oil contamination, and does not require an additional oil removal device. The disadvantages of this machine are that the motor power is large, the exhaust pressure is unstable, the exhaust temperature is high, the noise is loud, and the maintenance workload and costs are high.
2. Screw-type air compressor.
The fit between the male and female rotors of the screw-type air compressor reduces gas backflow leakage and improves efficiency; there is only intermeshing of the rotors without the reciprocating motion of a cylinder, which reduces sources of vibration and noise. The unique lubrication method has the following advantages: continuously injecting cooling liquid into the compression chamber and bearings simplifies the complex mechanical structure; the injected cooling liquid can form a liquid film between the rotors, allowing the auxiliary rotor to be directly driven by the main rotor without the need for high-precision synchronous gears; the injected cooling liquid can increase gas tightness, reduce noise generated by high-frequency compression, and absorb a large amount of compression heat. Therefore, screw-type air compressors have advantages such as low vibration, no need for anchor bolt fixation, low motor power, low noise, high efficiency, stable exhaust pressure, and no easily worn parts.