In all positive-displacement rotary compressors with fixed port
location, the degree of compression in the rotor thread is determined
by the location of suction and discharge ports. The built-in volume
(compression) ratio of screw compressors is defined as the ratio of
volume of the thread at the start of compression to the volume of the
same thread when it begins to open to the discharge port. The suction
port must be located to trap the maximum suction charge;
hence, the compression ratio is determined by the location of the
discharge port.