Power Number

Power Number

  • Power number is dimensionless parameter used for estimating the power consumed by the mixing impeller.

  • For a standard vessel geometry, the power consumed by an impeller at a specified rotational speed, for a liquid with known density, can be determined from the power number correlation.

            P  = Np ρn3Da

            Where

            P = Impeller power, Watts

           Np = Power number

           ρ = Density of liquid, kg/m3

           n – Agitator speed measured in revolutions / second

           Da – Impeller diameters in meters

  • Power number data for different type of impellers, under a give set of conditions are documented in technical literature. Typical values of power number are as under:

    • Axial flow impellers such as propellers, hydrofoils : 0.3 < Np < 0.75
    • Mixed flow impellers such as paddles, pitched blade turbines : 0.75 < Np < 3.0
    • Radial flow impellers such as Rushton turbines : 3.0 < Np < 6.
  • Power number decreases with an increase in Reynolds number.


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