Page 274 - Compression Machinery for Oil and Gas

Screw Compressors Chapter 6 259

TABLE 6.1 Pocket Passing Frequency of Different Screw Compressor Types

Small Dry Large Dry Oil-Flooded
Screw Screw Screw
Compressor Type Compressor Compressor Compressor
Male-rotor speed 18,000 3000 3600
(rpm)
Number of lobes on 4 4 5
male rotor
Number of working 72,000 12,000 18,000
cycles per minute
Pocket passing 1200 200 300
frequency (Hz)

The thermodynamic behavior of screw compressors is similar to recip-
rocating compressors. The power consumption is nearly independent of the
gas molecular weight. The inlet gas volume flow does not change much
when the pressure ratio or the molecular weight of the gas changes. There-
fore, screw compressors do not have a surge line. Due to the insensitivity
against mole weight changes a screw compressor can operate with a variety
of gases.
A screw compressor can operate at very high-pressure ratios as long as the
allowable discharge temperature or mechanical limits like bearing loads or shaft
stress limits are not exceeded. With oil-flooded screw compressors pressure
ratios of as high as 25 in one stage can be achieved. With dry screw compressors
a pressure ratio up to 10 is possible with liquid injection.
The compact rotor design with a small number of lobes gives a robust rotor.
The first lateral critical speed is always higher than the maximum allowable
speed (“stiff rotor design”). Due to the purely rotating motion the vibrations
of screw compressors are similar to centrifugal compressors and much lower
than for reciprocating compressors. While vibration in the machine itself is pos-
sible (due to imbalance, bearing failure, torsional resonance, etc.), vibration in a
screw compressor package (piping, vessels, baseframe, etc.) is much more
likely to come from the effects of gas pulsations than from the motion of the
compressor itself.
The clearances between the rotors and the casing are only fractions of a mil-
limeter which means that any buildup of process gas residues is scraped away
and the balance status of the rotors is not affected.
In contrast to reciprocating compressors screw compressors do not have
valves for controlling the gas inlet and outlet of the working chamber. The
gas inlet and outlet are controlled by ports in the casing. Therefore, failure
of valves is not an issue with screw compressors.