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In a compressor, atmospheric air is usually brought to a higher pressure potential by supplying mechanical energy. The enclosed air molecules are compressed, "compressed" by reducing the space.
In screw compressors, the ambient air is condensed to compressed air by rotors and belong to the group of rotary compressors, the most important category of positive displacement compressors.
Screw compressors are equipped with two spindle-shaped intermeshing rotors. The main rotor converts approx. 85 - 90% of the energy absorbed at the coupling of the drive unit into pressure and heat energy. Together with the main rotor, the secondary rotor ensures that a compression space is created between the suction and discharge sides.
During compression, the air inlet opening is closed by progressive rotation of the rotors and the volume is reduced as the pressure increases. At the same time, oil is injected into the system for lubrication, sealing and heat removal.
The air enters the compressor housing through the inlet opening.
When the rotors have turned past the inlet opening, they form a sealed compression chamber between the threads and the housing. This is reduced by the rotary movement of the rotors, the enclosed air is compressed.
Compressed air flows from final compression pressure: max. 15 bar (g).
The main difference between two-stage and single-stage compression is that the compression process at the two-staged compression is splitted on two airends. After compression in the first airend, the air is intercooled. Then the air in the second compressor airend is compressed to the final pressure.
The compression process is close to isothermal compression, i.e. the outlet temperature is almost as high as the inlet temperature, thus avoiding energy losses.
With two-stage compression, the pressure differences in the individual stages are smaller. This reduces leakage losses.
Overall, two-stage compressors consume between 11 and 15% less energy than single-stage compressors of comparable size.
During the compression of air, it is strongly heated - depending on the compression principle, temperatures of up to 200°C are generated. The heated compressed air must be cooled before it can be leased into the compressed air lines.
With two-stage compression, the air is heated less, i.e. the compression temperature is lower. Due to the lower compression temperature, the heated compressed air must be cooled to a lesser extent before it is fed into the pipes.
This means less energy has to be used for cooling.