An impeller is a rotating component equipped with vanes or blades used in turbomachinery (e. g. centrifugal pumps). Flow deflection at the impeller vanes allows mechanical power (energy at the vanes) to be converted into pump power output.In accordance with EUROPUMP TERMINOLOGY and DIN 24250, a distinction is made between counter-clockwise and clockwise impellers, as viewed in inlet flow direction.
Depending on the fluid flow pattern in multistage pumps and the impellers' arrangement on the pump shaft, impeller design and arrangements are categorised as: single-stage, multistage, single-entry, double-entry, multiple-entry, in-line (tandem) or back-to-back arrangement. Typical impeller arrangements are illustrated in Figs. 17 to 19 Impeller.
To accommodate the vanes, all impellers are equipped with a back shroud, and in the case of closed impellers also a front shroud (see Disc friction); depending on the perspective, these can also be viewed as an inner shroud and, in the case of closed impellers, an outer shroud. If an impeller has no front (outer) shroud, it is classed as an open impeller.
In order to achieve optimal pump efficiencies and minimum NPSHr values, the impeller must be provided with a certain number of vanes. Employing a low number of vanes increases the free, unimpeded flow cross-section through the impeller. This enables impellers to handle more or less contaminated fluids (waste water pumps, pulp pumps) and solids (solids transport).
In practice, the number of vanes of radial flow and mixed flow impellers handling liquids containing sludge or solids is reduced to one, two or three vanes. These impellers are called channel impellers or single-vane impellers and can be either open or closed impellers.