Adaptation of an Existing Impeller Design to Large Bore Requirements: Aerodynamic Considerations

Multistage process centrifugal compressor applications with single shaft rotors supported by only two bearings are quite common. It is sometimes desirable to operate impellers at higher rotational speeds, resulting in relatively compact and cost-effective machines. Such high-speed rotors can, however, pose rotordynamic challenges, and therefore require larger shaft or impeller bore diameters to increase rotor stiffness and rotordynamic stability. This work explores aerodynamically favorable ways to adapt an existing standard bore impeller design to large bore requirements. First, the stage aerodynamic performance and flow range implications of increasing bore diameter are discussed using meanline modeling and vector diagram arguments. Some strategies for adapting a standard bore design to large bore variant are then presented. Attempts are made to identify and clarify technical limitations to the degree of adaptability of an existing impeller to large bore requirements. Finally, a CFD-backed case study on a large-bore adaptation of a particular stage is presented to clarify practical considerations.