How They Work

Suction Phase
The pair of lobes unmesh on the suction port side. Gas flows into the increasing volume formed between the two lobes.

Transfer Phase
As the rotors mesh, the gas is trapped between the rotors and the casing.

Compression Phase
Due to the continued rotation of the rotors the trapped volume is decreased, resulting in gas compression.

Discharge Phase
Compression continues until the trapped volume becomes exposed and exits into the discharge port.

Screw compressors are positive displacement rotary devices. Compression is achieved by successive volumes of suction gas being enclosed in a confined space between the two rotors and the casing. Due to the rotation of the shafts the pressure of the trapped gas increases until it reaches and exits the compressor discharge. See the diagrams and the descriptions (at left) for the details of each phase of the compression cycle.

Capacity Control Mechanism
Capacity control is accomplished by a slide valve which moves parallel to the rotor axis and changes the area of the opening in the bottom of the rotor casing. This, in effect, lengthens or shortens the region of compression of the rotor and further acts to return gas to the suction side, while bypassing compressed gas.