How to use the calculator
- Choose the mode: Power from Po to get the power draw from a known power number, or Po from measured power to get the power number from a measured shaft power.
- Enter the impeller diameter D in m or mm and the rotational speed N in rpm or 1/s. Speeds in rpm are divided by 60 to give N in 1/s.
- Enter the liquid density ρ in kg/m³ and the dynamic viscosity μ in mPa·s or Pa·s.
- Enter the power number Po, or in the second mode the measured shaft power P in W or kW.
- Optionally enter the liquid volume V in m³ or L to get the power per unit volume P/V.
- Press Calculate. You get Re = ρND2/μ, the power P = Po·ρ·N3·D5 in W and kW (or Po = P/(ρN3D5)), the tip speed U = πND and P/V, each with the formula and your numbers substituted.
About the power number
- The power number depends on the impeller geometry and on the flow regime: the Reynolds number, the impeller diameter, the blade width and number of blades, the impeller position and the baffles.
- In the turbulent regime Po is constant. For a Rushton disc turbine Po is about 5.00 (Grenville); a DTU PhD thesis uses Np = 5 per Rushton turbine.
- In the laminar regime P = Kp·μ·N2·D3, so Po falls as 1/Re.
- The curves of Po against Re go back to Rushton et al., Chem. Eng. Prog., 1950. The constant Po applies only where your impeller curve is flat at the Re shown.
- One manufacturer, CerCell, gives an un-gassed Po of 4.5 to 6.5 for its RT6-90 Rushton turbine.
- The Po values quoted here are for un-gassed liquid.
Sources: R.K. Grenville, Characterizing Impeller Performance in Stirred Tanks, IChemE South Wales Branch, 2017; T. Jirout, Agitation and Mixing of Fluids, CTU Prague course notes.
Disclaimer: the results are an estimate for preliminary sizing, not a substitute for the impeller supplier data.