screw compressors have replaced reciprocating compressors in large installations for two reasons.
firstly, fewer and more compact machines are used: secondly, a reduced number of working parts results in greater reliability with reduced maintenance requirements.
There are two types of screw compressor; one employs two rotors side by side and the other, which is a more modern development, is a single rotor with two star wheels , one on either side.
As the str wheels compress the gas in opposite directions, the thrust on this type of rotor is balanced.
Such a compressor in shown in figure 9.3.
The principle of operation for both types is similar to a screw-type positive displacement pump (see chapter 6)