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.
Depending on the flow line pattern in the impeller (especially in the outer impeller diameter area), impellers are subdivided into the following types:
- Radial impeller See Figs. 1, 2 Impeller
- Mixed flow impeller See Figs. 3, 4, 6 Impeller
- Axial flow impeller See Figs. 5 Impeller
- Peripheral impeller See Figs. 15 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.
See Figs. 7 to 13 Impeller
Closed single-vane impellers are used to pump fluids containing very coarse solids. They are characterised by a non-clogging free passage. The drawback of these impellers is the so-called hydraulic unbalance due to the asymmetrical pressure field. See Fig. 7 Impeller
Open channel or single-vane impellers are used to handle gaseous liquids. A single-vane impeller is referred to as an open, diagonal single-vane impeller (D impeller) if the flow lines in the impeller run diagonally outward. It is particularly suitable for untreated, solids-laden and gaseous waste water, as well as for fluids with a higher viscosity. See Fig. 8 Impeller
In the case of adjustable or variable pitch blades, the contour or profile of the pump casing and of the hub in the adjustment region is usually spherical. This ensures that the internal and external clearance gap width at the hub remains constant for all blade pitch adjustment angles. See Fig. 4 Impeller
The free-flow impeller and the peripheral impeller represent special impeller types. See Figs. 14 to 15 Impeller
When selecting a pump for a given flow rate (Q) and a given head (H)the impeller type is decisive. Free selection of an axial, mixed flow, radial or peripheral impeller type is restricted by the fact that the values for the anticipated rotational speed (n) and the anticipated impeller diameter (D) must not be too extreme. The ability to achieve optimum pump efficiencies or stage efficiencies at a specific speed (ns) is therefore dependent on specific impeller designs:
- Radial impeller ns ≈ 12 to 80 rpm
- Mixed flow impellers ns ≈ 80 to 160 rpm
- Axial flow impellers ns ≈ 160 to 400 rpm and higher