Agitator Power and Impeller Reynolds Number Calculator
How much power does your agitator really draw? Enter impeller type, speed, diameter and liquid properties to get shaft power, power per cubic metre and the Reynolds number that tells you whether the formula applies.
Your numbers
Fully turbulent, baffled tank values. Confirm with the vendor.
Water is 1 cP. Heavy oils and polymer solutions can be thousands.
Your results
Power number is only constant in the turbulent regime (Re above roughly 10,000). In viscous or transitional flow Np rises, so use vendor data. Add allowance for seal, bearing and gearbox losses when sizing the motor.
Power drawn vs impeller speed
Sweeps Impeller speed from 20 to 300, holding your other inputs fixed.
Capture process parameters in batch records
Optional: we review how speed and mixing data reach your batch records.
Optional. The tool and every export work without this. No spam.
How to use it
Select the impeller type, which sets a typical power number, then enter speed in rpm, impeller diameter in metres, density and viscosity. Add the batch volume to see power per unit volume, a common scale-up criterion. The sweep shows that power rises with the cube of speed, so a small speed increase costs far more energy than expected.
The relationships
In the turbulent regime power is P = Np x rho x N^3 x D^5, with N in revolutions per second. The impeller Reynolds number is Re = rho x N x D^2 / mu, with viscosity in Pa.s. Because power scales with the fifth power of diameter, a modest change in impeller size has a very large effect on load.
- Double the speed, power rises eightfold
- Increase diameter by 10 percent, power rises about 61 percent
- Typical blending duty is a few tenths of kW/m3
- Gas dispersion and solids suspension need more
Common mistakes
Using the turbulent power number at low Reynolds numbers underestimates power for viscous liquids. Another mistake is ignoring baffles: an unbaffled tank forms a vortex and draws less power. Finally, a variable liquid level during filling changes the effective power drawn, so check the worst case, usually the full vessel with the densest charge.
- Confirm tip speed limits for shear sensitive products
- Check motor size with margin of 10 to 20 percent
- Gassed power is lower than ungassed power
- Record agitator speed in your batch record
Frequently Asked Questions
What is a power number?
It is a dimensionless constant that relates power draw to density, speed and diameter for a given impeller and tank geometry. It is roughly constant in turbulent flow and measured by impeller manufacturers.
What Reynolds number means turbulent?
In a baffled stirred tank flow is considered fully turbulent above about 10,000. Between 10 and 10,000 is transitional and below 10 is laminar, where power number depends strongly on Reynolds number.
Is power per volume a good scale-up rule?
It is widely used for blending and mass transfer, but not always. Tip speed, mixing time and Reynolds number may matter more for shear sensitive, suspension or reaction-limited processes. Choose the criterion by the controlling mechanism.
Does the motor need to match calculated power?
No, it needs margin. Add allowances for gearbox and seal losses and for variation in fluid properties. Also check start-up torque, especially when solids have settled.
If agitator speeds and mixing times are part of your batch records in an ERP or MES, Netray can help review how they are captured. Ask us.
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