The design of the diffuser system immediately downstream of the impeller is considered. The diffuser transforms the kinetic energy at its inlet into a rise in the static pressure. Centrifugal compressors are usually fitted with either a vaned or a vaneless diffuser leading to a collector. The diffuser meridional channel comprises an annular channel extending radially outwards from the impeller outlet, usually of the same width as the impeller. The simplest diffuser system is a radial vaneless annular channel where the radial velocity component is reduced by the increase in the area of the channel with radius (conservation of mass) and the circumferential velocity component is reduced by the increase in radius in the diffuser (conservation of angular momentum). In a vaned diffuser, of which several types are considered, there is a small vaneless region upstream of the diffuser vanes. The vanes themselves form flow channels designed to decelerate the flow more than is possible in a vaneless diffuser by turning the flow in a more radial direction. The different zones of pressure recovery in vaned diffusers are examined and compared with the equivalent planar diffuser.

Aspects of impeller design are explained taking into account the constraints from mechanical and aerodynamic considerations. A one-dimensional steady flow analysis is used to obtain a general understanding of the effects of the impeller design parameters on the geometry. This analysis provides some clear design guidelines for values of specific nondimensional flow parameters for optimum performance. The effects of the impeller blade inlet design on the inlet relative Mach number are considered together with that of the throat on flow capacity. The effect of the outlet velocity triangle on the work input and degree of reaction is explored. The considerations that lead to the choice of backsweep at the impeller outlet are explained. The steps required to adapt an impeller designed for one task to fulfil other requirements by means of trimming or flow cuts are explained. Guidance on the selection of mixed flow impellers is given. Some important differences are explained between the velocity triangles in radial flow compressor impellers and those in the rotors of centrifugal pumps, axial compressors and radial turbines.