A discussion of the flow path through the centrifugal compressor will provide a better understanding of the compression process.
There is often confusion about the term “stage” when applied to centrifugal compressors. The process designer thinks of a stage as a compression step made up of an uncooled section, usually consisting of several impeller/diffuser units. The mechanical engineer or machine designer defines a stage as one impeller/diffuser set, and a section as a single compressor casing containing several stages. In this section of the manual:
• Stage is defined as one impeller/diffuser set
• Process stage is defined as an uncooled section (or casing) containing several impellers/diffusers
Based on this, a centrifugal compressor is made up of one or more stages; each stage consisting of a rotating component or impeller, and the stationary components which guide the flow into and out-of the impeller. Figure 200-1 shows the flow path through a section of a typical multistage unit.