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Impeller

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:

Impeller types



Impeller: Radial impeller with purely radial vanes, stagnation point S (shown with front shroud removed) Fig. 1 Impeller: Radial impeller with purely radial vanes, stagnation point S (shown with front shroud removed) Fig. 2 Impeller: Radial impeller with vanes extending into the suction eye (shown with front shroud removed) Fig. 2 Impeller: Radial impeller with vanes extending into the suction eye (shown with front shroud removed) Impeller: Mixed flow impeller (diagonal impeller) (shown with front shroud removed) Fig. 3 Impeller: Mixed flow impeller (diagonal impeller) (shown with front shroud removed) Impeller: Mixed flow impeller (mixed flow propeller) Fig. 4 Impeller: Mixed flow impeller (mixed flow propeller)

Impeller: Axial impeller (axial propeller) Fig. 5 Impeller: Axial impeller (axial propeller) Impeller: Variants of a mixed-flow impeller showing the difference between a closed and an open impeller Fig. 6 Impeller: Variants of a mixed-flow impeller showing the difference between a closed and an open impeller, a single-entry and a double-entry impeller a) Closed, single-entry impeller b) Open, single-entry impeller c) Closed, double-entry 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

Impeller: Closed single-vane impeller (shown with shroud removed) Fig. 7 Impeller: Closed single-vane impeller (shown with shroud removed) Impeller: Open single-vane impeller Fig. 8 Impeller: Open single-vane impeller Impeller: Closed single-channel impeller (shown with front shroud removed) Fig. 9 Impeller: Closed single-channel impeller (shown with front shroud removed)

Impeller: Closed two-channel impeller (shown with shroud removed) Fig. 10 Impeller: Closed two-channel impeller (shown with shroud removed) Impeller: Open two-channel impeller with S-shaped vanes Fig. 11 Impeller: Open two-channel impeller with S-shaped vanes

Impeller: Closed three-channel impeller (shown with shroud removed) Fig. 12 Impeller: Closed three-channel impeller (shown with shroud removed) Impeller: Open three-channel impeller with cylindrical vanes Fig. 13 Impeller: Open three-channel impeller with cylindrical vanes

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

The blades of axial and mixed flow propellers (see Propeller pump) can be either fixed, adjustable (when the pump is dismantled) or of variable pitch type (see Impeller blade pitch adjustment).

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

Impeller: Free-flow impeller Fig. 14 Impeller: Free-flow impeller

Impeller: Peripheral impeller Fig. 15 Impeller: Peripheral 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


Impeller: Open, diagonal single-vane impeller (D impeller) Fig. 16 Impeller: Open, diagonal single-vane impeller (D impeller)


Fig. 17 Impeller: Single-entry, in-line (tandem) impeller arrangement  a) Single-stage b) Two-stage c) Six-stage Fig. 17 Impeller: Single-entry, in-line (tandem) impeller arrangement a) Single-stage b) Two-stage c) Six-stage

Impeller: Double-entry, back-to-back impeller arrangement  a) Double-entry, single-stage b) Four-entry, single-stage c) Double-e Fig. 18 Impeller: Double-entry, back-to-back impeller arrangement a) Double-entry, single-stage b) Four-entry, single-stage c) Double-entry, three-stage

Impeller: Single-entry, back-to-back impeller arrangement  a) Two-stage (back-to-back) b) Four-stage (crossover) c) Six-stage Fig. 19 Impeller: Single-entry, back-to-back impeller arrangement a) Two-stage (back-to-back) b) Four-stage (crossover) c) Six-stage (back-to-back)