Calculated Power Numbers for impellers

For improved cultivation it’s good to be aware of Np

Power Number is a dimensionless parameter used for estimating the power consumed by the rotor/agitating rotor / delivered to the media.

Rotor code created for a SUB like this: Impeller (IMP), principal type (Kidney), diameter (040) in mm, number of blades (3), blade angle in degree (45°), direction (Left or Right).

For a standard cylindrical vessel geometry, the power consumed by the rotor/impeller(s) at a specified rotational speed, for a liquid with known density, can be determined from the Power Number correlation.

Power = Power Number x Density x Agitator speed3 x Impeller Diameter5

P = Np ρ n3 Da5


P = Impeller power, Watt
Np = Power Number
ρ = Density of liquid, kg/m3
n3 = Rotor/agitator speed measured in revolutions / second
Da5 = Rotor/impeller diameters in meters

Calculated Power Number data for different type of rotors / impellers, under a give set of conditions are documented in various technical literature. Typical values of Power Number (Np) are:

  • Axial flow impellers, such as marine, helical : 0,25 to 0,75
  • Mixed flow impellers, such as kidney, pitch: 0,75 to 2,0

Power Number decreases with an increase in Reynolds number.

See also Glossary

Experimental results can be downloaded here