The oil injected rotary screw compressor is a positive displacement type compressor.
A given quantity of air or gas is trapped in a compression chamber and the space that it occupies is mechanically reduced, causing a corresponding rise in pressure prior to discharge.
A rotary screw compressor has a pair of intermeshing rotors housed in a suitable casing to produce compression.
With the rotation, the leading strip of the male lobe reaches the contour of the female groove and traps the air in the pocket previously formed.
The air is moved down the female rotor groove and is compressed as the volume is reduced.
When the male rotor lobe reaches the end of the groove, the trapped air is discharged from the airend.
This type of twin-screw compressors can be oil free or oil injected. In the case of the oil lubricated compressor oil is injected.
The oil performs four crucial functions:
The purpose of a compressor is to convert shaft work into a useful output, that is, air flow. As compressing air generates heat, all of the heat is retained within the compression chamber; this is adiabatic compression. If heat is added or taken away during the compression process this is called isothermal compression.
Oil injected screw compressors have a near isothermal compression process as the heat generated by the compression process is almost dissipated by the oil.
All air compressors are less than 100% efficient.
The best way to measure the efficiency of the compression is to look at its isentropic efficiency.
The Isentropic efficiency of compression is the ratio of the ideal isentropic work to the actual work:
nc = isentropic compressor work (ws)
Actual compressor work (wa)
Therefore the specific energy that equals to the work required to compress a given amount of air (cubic meter) in a specific amount of time (minute) to a specified pressure (barg) is used. It is measured in kW.
Specific Energy = energy / unit volume kW
The specific energy and the compressor package’s efficiency is dependable on all its component’s efficiency as well as the complete system’s pressure drop. The pressure drop can be measured on the intake valve and suction box, air filter, piping and oil separator.