Screw Compressor

Screw Compressor Points : screw compressor, definition Rotary screw compressors also belong to the category to positive displacement compressors. The machine essentially consists of two helically grooved rotors which rotate in housing. The male motor consists of lobes & is normally the driving rotor. The female rotor has gullies & is normally the driven rotor. A four lobe male rotor will drive a six —gully female rotor at two thirds of its speed. At 3600 rpm, the number of compressed gas discharges of a four-lobe rotor will be 4 x 14400 per mm.

As in the case of other positive displacement machines, there are three basic continues phases of the working cycle, suction, compression & discharge. When the male rotor turns clockwise, an inter-lobe space b/w a pair & housing nearest to the suction end opens & is filled with the gas. There are four such pairs to be filled during one revolution in a four lobe rotor & the suction periods overlap one another.

When remeshing starts, the volume decreases & the pressure rises. The charge is removed helically & compressed until the trapped volume reaches the discharge end. The compression ratio is thus fixed.

Further rotation simply empties the rotors of the high pressure gas until the traces of the gas are squeezed Out, irrespective of the pressure in the condenser.

On completion of the discharge phase, there is no residual gas remaining in the rotors. As a result, there is no expansion of clearance gases. The compressor has no suction & discharge valves.

There are leakage paths in a screw compressor mainly across the line of mesh 1/w the rotors & across the clearance b/w the rotors & the housing. To eliminate leakage, oil is injected in a number of small jets directed towards the mesh oil injection also serves the purpose of cooling & lubricating along with that of sealing the leakage paths.