Volume (Compression) Ratio
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.
Only suction pressure and compression volume ratio determine
the internal pressure achieved before opening to discharge. However,
condensing and evaporating temperatures determine discharge
pressure and compression ratio in piping that leads to the compressor.
Any mismatch between internal and system discharge pressures
results in under- or overcompression loss and lower efficiency.
If the operating conditions of the system seldom change, it is
possible to specify a fixed-volume-ratio compressor that will give
good efficiency. Compressor manufacturers normally make compressors
with three or four possible discharge port sizes that correspond
to system conditions encountered frequently. Generally, the
designer is responsible for specifying a compressor that most
closely matches expected pressure conditions.
The required compression ratio for a particular application can
be determined as follows: