This figure shows a structural diagram of a horizontal split centrifugal compressor. Multiple impellers are attached to a single rotating shaft, for a multi-stage structure.
The gas enters through the intake nozzle, and is compressed and pressurized by centrifugal force as it passes through the impellers, which are rotating at high speed, before being ejected through the ejection nozzle. With this type of compressor, the outer casing can be split in the horizontal direction, facilitating the maintenance of internal parts.
The red parts are the critical shaft and impellers. The photos below show magnified views of the impeller, clearly illustrating the three-dimensional design of the internal blades.