Power numbers for impellers

For improved cultivation it’s good to be aware of

Power Number is a dimensionless parameter used for estimating the power consumed by the agitating impeller.



For a standard cylindrical vessel geometry, the power consumed by the 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 ρn3Da5

Where
P = Impeller power, Watts
Np = Power Number
ρ = Density of liquid, kg/m3
n3 – Agitator speed measured in revolutions / second
Da5 – Impeller diameters in meters

Power number data for different type of impellers/turbines, 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.

Experimental results can be downloaded here