<< Click to Display Table of Contents >>

Navigation:  Impeller > Main dimensions > Centrifugal, Mixed-flow Pump/ Fan >


Previous pageReturn to chapter overviewNext page

On page Parameters you have to put in or to modify parameters resulting from approximation functions in dependence on specific speed nq or flow rate Q.
See Approximation functions.

For details of how to handle the parameter edit fields please see Edit fields with empirical functions.


Parameter and efficiency values can be handled manually or can be switched to automatic update by the checkbox on top of the page. Then the default values are used always, even after design point modifications (see Global setup).

If the automatic mode is not selected the current default values can be specified by one of the following options:

globally by the button on top of the page

regionally by the default button within the Parameters or Efficiency region

individually by the default button within the input field when selected


The panel Parameters allows defining alternative parameters in each case for the calculation of the following impeller main dimensions:

for pumps

for fans

suction diameter dS

LE diameter dLE


LE width bLE

impeller diameter d2

impeller width b2

For dS-calculation (pumps)

Intake coefficient ε

Ratio between meridional inflow velocity and specific energy

0.05…0.4 (rising with nq)

(km1 at Stepanoff)

Rel. inlet flow angle β1

high smaller dimensions, lower friction losses

< 20° prevent the risk of cavitation

> 15° with regard to efficiency

12°...17° recommended for good suction capability

Minimal relative velocity  w

small friction and shock losses

only if no cavitation risk !

fdS=1.15...1.05 standard impeller, nq=15...40

fdS=1.25...1.15 suction impeller

suction specific speed  nSS

(European definition for illustration)

Standard suction impeller

u1<50 m/s


Suction impeller, axial inflow

u1<35 m/s


Suction impeller, cont. shaft

u1<50 m/s


High pressure pump

u1>50 m/s


Standard inducer

u1>35 m/s


Rocket inducer




λc suction pressure coefficient for absolute velocity c (inflow acceleration and losses): 1.1 for axial inflow; 1.2…1.35 for radial inflow casing

λw suction pressure coefficient for relative velocity w (pressure drop at leading edge): 0.10…0.30 for standard impeller; 0.03…0.06 for inducer


for dLE calculation (fan)

Diameter ratio dLE/d2


for bLE calculation (fan)

Merid. deceleration



For d2-calculation

Work coefficient Ψ

(= pressure and head coefficient)

dimensionless expression for the specific energy:

0.7  ...1.3 centrifugal impeller

0.25...0.7 mixed-flow impeller

0.1  ...0.4 axial impeller

high small d2, flat characteristic curve
low high d2, steep characteristic curve

Specific diameter δ

according to Cordier diagram (see Dimensions)

Rel. outlet flow angle β2

6°...13°: recommended for stable performance curve (with nq rising)

βB2 = 90°
for impeller type "Barske (low nq)" only

empirical factor kd2 = 1.15 ... 1.29


For b2-calculation

Outlet width ratio


0.04...0.30 (rising with nq)

for pumps:

Mer. deceleration


0.60...0.95 (rising with nq)

for pumps:

Outlet coefficient ε2

Ratio between meridional outlet velocity and specific energy

0.08…0.26 (rising with nq)

(km2 at Stepanoff)

for fans:
Shroud angle εShr


In panel Efficiency you have to specify several efficiencies. You have to distinguish between design relevant efficiencies and efficiencies used for information only:

Design relevant

hydraulic efficiency ηh

volumetric efficiency ηv

tip clearance efficiency ηT

additional hydraulic efficiency ηh+ (displayed for information only, see Global setup)

Information only

side friction efficiency ηS

mechanical efficiency ηm

motor efficiency ηmot

The additional hydraulic efficiency ηh+ is used additionally for impeller dimensioning in order to compensate the flow losses.

The losses resulting in energy dissipation from the fluid form the internal efficiency.

Internal and mechanical efficiency form the overall efficiency (coupling efficiency) of the stage ηSt.

When considering motor losses additionally the overall efficiency of the stage incl. motor ηSt* is defined.

PQ: pump output, see above

PD: mechanical power demand (coupling/ driving power)

Pel: electrical power demand of motor


The following summary illustrates the single efficiencies and their classification:



Relevant for impeller design





additional hydraulic

yes: for energy transmission







yes: for flow rate


side friction

no: for overall information only




stage incl. motor




The obtainable overall efficiency correlates to specific speed and to the size and the type of the impeller as well as to special design features like bypass installations and auxiliary aggregates. Efficiencies calculated by approximation functions are representing the theoretical reachable values and they should be corrected by the user if more information about the impeller or the whole pump are available.

The hydraulic efficiency (or blade efficiency) describe the energy losses within the pump caused by friction and vorticity. Friction losses mainly originate from shear stresses in boundary layers. Vorticity losses are caused by turbulence and on the other hand by changes of flow cross section and flow direction which may lead to secondary flow, flow separation, wake behind blades etc.. The hydraulic efficiency is the ratio between specific energy Y and the energy transmitted by the impeller blades:

The volumetric efficiency is a quantity for the deviation of effective flow rate Q from total flow rate inside the impeller which also includes the circulating flow within the pump casing:

(rising with impeller size)

The tip clearance efficiency is only relevant for unshrouded impellers. It contains losses due to the flow through the gap between blade tips and housing from the pressure to the suction side of the blades. The flow losses mainly depend on the tip clearance distance xT and decrease with rising number of blades and rising blade outlet angle β2.

The side friction efficiency contains losses caused by rotation of fluid between hub/ shroud and housing:

The mechanical efficiency mainly includes the friction losses in bearings and seals:

(rising with impeller size)

Hydraulic and volumetric efficiency as well as the tip clearance efficiency are most important for the impeller dimensioning because of their influence to and/or . Mechanical and side friction efficiency are affecting only the required driving power of the machine.


If the check box "Use η for main dimensions" is set, then main dimension calculation is done on the basis of Yeff= 0.5(Y/η+Y). Otherwise Y - specific work without losses - is used.


In the right area of the register Parameter you can find again some calculated values for information:

Required driving power

Power loss

Internal efficiency

Stage efficiency

Stage efficiency incl. motor