Compressors are commonly classified as either positive displacement or dynamic compressors. The positive displacement compressor achieves its pressure rise by trapping fluid in a confined space and transporting it to the region of higher pressure. The dynamic compressor develops its increase in pressure by a dynamic transfer of energy to a continuously flowing fluid stream. There are two basic types of dynamic compressors: axial-flow compressors and centrifugal (radialflow) compressors. The flow streamlines through rotating rows in an axial-flow compressor have a radius that is almost constant, whereas they undergo a substantial increase in radius in a centrifugal compressor. For this reason, the centrifugal compressor can achieve a much greater pressure ratio per stage than the axial-flow compressor. But the axial-flow compressor can achieve a significantly greater mass flow rate per unit frontal area. Figure 1-1 compares normalized discharge pressure, P, versus flow rate, Q, for these two compressor types to illustrate the differences in their performance characteristics. The axial-flow compressor approximates a variable pressure ratio—constant flow machine, whereas the centrifugal compressor is closer to a constant pressure ratio—variable flow machine. The performance data displayed in Fig. 1-1 are for a singlestage centrifugal compressor and a five-stage axial-flow compressor, both of which have about the same design pressure ratio. This demonstrates the superior pressure ratio-per-stage capability of the centrifugal compressor. Traditionally, the centrifugal compressor has been the more rugged and lower-cost type, while the axial-flow compressor has offered better efficiency. Those differences have become much less significant in recent years due to advances in technology, particularly with regard to efficiency. Presently, the compressor type selected is more likely to be based on the performance characteristics, size and cost that is best suited to the application.
