Choosing the right type of compressor is a difficult job as the machine selection would depend on not only capacity and discharge pressure, but also on pressure ratio, service, gas composition, duty (continuous or intermittent), criticality of service, different operating cases etc The figure 1 shows the basic pressure-capacity constant speed characteristics of positive displacement, dynamic type and centrifugal compressors.
Thecentrifugal compressor is essentially a variable capacity constant pressure
machine. The axial compressor and the positive displacement compressors are
essentially constant capacity, variable pressure machines.
These basic characteristics however represents only part of the problem of choosing the type of compressor best suited to a specific application. Equally important is the capacity range that can be built into a single machine. As a general rule, positive displacement machines are for smaller capacities. Centrifugal compressors are for medium capacities and axial compressors are for large capacities per machines. As in all other general statements, however,it must be realized that there is considerable overlap of capacity range between these different types of compressors.
Although flow limits for compressors are dictated somewhat by the specific conditions involved and design philosophy of different manufacturers, general range of compressor selection goes as below.
1. Positive displacement machines below 3000 m3/hr
2. Axial compressors above 30,000 m3/hr
3. Flow region between 3000-12,000 m3/hr can be considered over-lapping between positive displacement and centrifugal compressors.
4. Flow region between 30,000-1,20,000 m3/hr can be considered as overlapped
between centrifugal and axial compressors.
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