GIFSQ Tools · Module 17 — Food Processing, Thermal Engineering & Utilities · Tool 339 of 500
Work out the power a mixer impeller draws from the power number, fluid density, impeller speed and diameter — plus the impeller tip speed.
Try sample values — fills the form with the worked example below.
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Mixing power (W)
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Mixing power (kW)
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Tip speed (m/s)
Equation: P = Np × ρ × N³ × D⁵, with N in revolutions per second and D in metres; tip speed = π × D × N.
Method: Impeller power correlation using the dimensionless power number, standard mixing practice (Paul, Atiemo-Obeng and Kresta, Handbook of Industrial Mixing; Geankoplis).
Assumptions: fully turbulent flow in a baffled vessel; the power number is constant for the impeller type and geometry; Newtonian fluid.
Limitations: the power number depends on impeller type, baffles and Reynolds number — use the published Np for your impeller (about 5 for a Rushton turbine, 0.3–1 for a pitched-blade turbine); laminar mixing needs an Np–Re curve.
Rounding: power to 1 decimal place in watts and 3 in kW, tip speed to 2; intermediates unrounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Method: Impeller power correlation using the dimensionless power number, standard mixing practice (Paul, Atiemo-Obeng and Kresta, Handbook of Industrial Mixing; Geankoplis).
Assumptions: fully turbulent flow in a baffled vessel; the power number is constant for the impeller type and geometry; Newtonian fluid.
Limitations: the power number depends on impeller type, baffles and Reynolds number — use the published Np for your impeller (about 5 for a Rushton turbine, 0.3–1 for a pitched-blade turbine); laminar mixing needs an Np–Re curve.
Rounding: power to 1 decimal place in watts and 3 in kW, tip speed to 2; intermediates unrounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: Rushton turbine, Np = 5, density 1000 kg/m³, 120 rpm, 0.5 m impeller: power = 1250.0 W (1.250 kW), tip speed = 3.14 m/s.
Enter values above to see results.
Power rises with the cube of speed and the fifth power of diameter: doubling the speed needs eight times the power. Tool 339 of 500 — GIFSQ
