Introduction

There is a significant difference between centrifugal compressor and axial-flow compressor. The centrifugal compressor could achieve higher pressure ratios because of the big increase in radius across the rotating blade rows. But the axial flow compressor could achieve much larger flow rate per unit frontal area. The centrifugal compressor is considered constand head-variable flow and axial is considered constant flow-variable.

Stage

The stage consists of rotating impeller to energize the fluid and diffuser to recover kinetic energy before it enters volute. The diffuser converts kinetic energy to pressure by slowing its velocity. (Refer Bernoulli's principle.) The diffuser could be vaned, vaneless or a combination of both. The next part is the volute which collects the flow smoothly. Volute will be replaced by return channel for multistage compressor. The return channel remove the swirl or tangential velocity from impeller.

There are two impeller styles which are unshrouded or open impeller and covered or shrouded impeller. The covered impellers are commonly used in multistage industrial compressors which requires extra effort to hold tight clearances between impeller blades and a stationary shroud.

One aspect of impeller is classified from the inducer part. The full-inducer impeller is when the impeller blade extends all the way into axial portion of the passage. Other type of impeller just extends partway into the axial portion of the passage and called semi-inducer impeller. the third type would have impeller blades only in radial portion of the passage, no inducer and called radial impeller.