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► VOLUTE | Diffuser

The geometry of the outlet or inlet diffuser can be designed and calculated in this design step.

Dimensions x,y-plane

In general, 3 basic shapes are available:

Tangential diffuser

Radial diffuser


The tangential diffuser is easier to manufacture, the radial diffuser has the advantage of minimizing tangential forces. The spline diffuser is similar to the radial but with extended flexibility.

Tangential diffuser

For the tangential diffuser the eccentricity can be specified:

The right side is parallel to the center line (perpendicular to the last spiral cross section).
The diffuser opens to left side only.

The diffuser opens to both sides (default).

The left side is parallel to the center line (perpendicular to the last spiral cross section).
The diffuser opens to right side only.

The eccentricity can be specified manually.

Radial diffuser

In the case of a radial diffuser, the angle ε between the outlet branch and the line connecting impeller-center and outlet branch center can be selected.

Spline diffuser

For the Spline-diffuser the angle φ between connecting line impeller-center  outlet branch center and diffuser start section has to be defined. Points 0 and 4 are start and endpoint of the middle line on the inlet and outlet cross section, point 2 is fixed by the intersection of appropriate perpendiculars of these sections. Position of points 1 and 3 influence the curve shape of the middle line.


By clicking on Default, you can return to the default values for the diffuser geometry.

The extension H of the diffuser can be defined in panel Dimensions x,y-plane.

For all diffuser shapes the extension is defined by the diffuser height H, which is the distance from the diffuser outlet to a parallel line through the center point.

The distance C from the H-line to the center point is displayed for information, both in the diagram and numerical in the Information panel.

The starting position of the diffuser is defined by the angle φ0, whereas 0° is horizontal right. The whole volute can be rotated by this value. By using the button Vertical outflow direction the volute can be rotated for vertical direction of the pressure joint.

Additionally, the diffuser can be shifted in radial direction by the radial offset  ΔrC to reduce the intersection of spiral and diffuser.

This radial offset can increase the available space for the cut-water in order to locate it closer to the desired radius.

Radial offset is available for strictly external volutes with 360° wrap angle only.

Dimensions z-direction

Two options are available for aligning the diffuser in z-direction, bend and offset (rake).


The diffuser bending in z-direction is described by the parameters shown in the sketch. There exist 2 straight segments 1, 3 and a circular segment 2.

The lengths L1, L2 and L3 are specified as percentage.

The curvature is defined by the radius R, the direction by the angle α.

The z-bend is illustrated in the diagram by a green center line.


There are 4 offset options:

The diffuser sections are shifted in negative z-direction in order to avoid higher z-coordinate compared to the last spiral section

The diffuser is developing centric with respect to the z-direction (default).

The diffuser sections are shifted in positive z-direction in order to avoid lower z-coordinate compared to the last spiral section

The offset can be specified manually in both directions. Additionally the progression of the diffuser can be chosen: linear or quadratic with respect to the length of the diffuser.

The following image illustrates this feature: left centric design, right negative offset.

End cross-section

The end cross-section of the diffuser can be either round or rectangular. The diameter D can be directly defined or selected from standard tables. In the case of a rectangular end cross-section the height and width can be chosen.

Section progression

The position of end shape specifies the percentage position along the diffuser, where the type of end cross section is reached (default = 100%). To reach certain cross section areas a scaling of those sections is necessary. Instead of just scaling uniformly in both directions (z and r) a scaling ratio (z/r growth) can be defined.

The choice of the area progression influences the scaling of the morphed cross sections.

Linear blending

The morph between two different cross sections is linear which results in an quadratic area progression. (unscaled)

Linear area

The size of the morphed cross sections is scaled to achieve a linear area progression.

Quadratic area

The size of the morphed cross sections is scaled to achieve a quadratic progression from the diffuser inlet to the end shape position. The progression to diffuser outlet is linear again.

Custom area

The size of the morphed cross sections is scaled with respect to a Beziér curve.

Splitter of Double Volute

The position of splitter end defined the relative length of the splitter inside the diffuser.