Introduction

Centrifugal and axial-flow compressors are classified as dynamic compressors, or simply as turbomachines. In contrast to positive displacement compressors, the dynamic compressor achieves its pressure rise by a dynamic transfer of energy to a continuously flowing fluid stream. There is a substantial increase in radius across the rotating blade rows of the centrifugal compressor, which is its primary distinguishing feature from the axial-flow compressor. For this reason, the centrifugal compressor can achieve substantially higher stage pressure ratios than the axial-flow compressor. But the axial-flow compressor can achieve a much larger flow rate per unit frontal area. As illustrated in Fig. 1-1, these two types of compressors also have quite different performance characteristics. The centrifugal compressor approximates a constant head-variable flow machine, whereas the axial-flow compressor is closer to a constant flow-variable head machine. Neither description is strictly correct, but they serve to differentiate between the performance characteristics of these two types of dynamic compressors.