13 - Casing Component Design

Summary

The design of the stationary components upstream of the impeller and downstream of the diffuser is considered. The inlet nozzle accelerates the fluid from the compressor flange to the impeller inlet, keeping losses low and avoiding distortion in the velocity profile. The impeller inlet may be axial or radial and it may be fitted with inlet guide vanes to change the swirl velocity as a way of controlling performance. Downstream of the diffuser, the flow is guided to the outlet flange, a downstream component or an intermediate cooler. The scroll, or volute, collects the flow, leaving the diffuser to take it to the outlet flange. In middle stages of multistage inline compressors, a crossover bend and a vaned return channel lead the flow to the next stage. In gas turbine applications, axial exit guide vanes remove the swirl at outlet and diffuse the flow to the low velocity needed in the combustor. Special applications may also include side-stream inlets or a secondary inlet and outlet nozzles, allowing the flow to gain access to a cooler and be returned to the compressor. The rotor–stator cavities determine the pressure field around the impeller and play a role in the axial thrust.