How to Use the Tank Agitator Power Calculator
Agitator power depends far more on impeller diameter and speed than on how big the tank is. In the governing equation P = Np × ρ × N³ × D⁵, speed enters cubed and diameter to the fifth power, so widening the impeller by only 20 percent more than doubles the power draw.
The power number Np is a shape factor. In turbulent flow it is essentially constant: about 5.0 for a Rushton six-blade disc turbine, 1.3 for a 45° pitched four-blade turbine and 0.35 for a marine propeller. When viscosity pushes the impeller Reynolds number Re = ρND² ÷ μ below 10, flow turns laminar and Np rises inversely with Re, so this tool takes whichever of the laminar and turbulent values is larger, which keeps the estimate on the conservative side.
Power per 1,000 gallons is the sanity check engineers actually use. Mild blending typically runs 0.25 to 0.5 hp per 1,000 gallons, general blending 0.5 to 1.5, and viscous or solids suspension duty considerably more. A tank without baffles lets the whole batch swirl with the impeller, which drops real power draw well below the calculation while mixing poorly, so baffles belong in the same discussion. Have the final selection reviewed by the manufacturer including shaft strength and starting torque.
Frequently Asked Questions
For turbine impellers, roughly one third of the tank diameter is the usual starting point. A larger impeller moves more volume for the same power, which favors blending, but it must turn slower, so match the choice to whether the process needs shear or circulation.
In laminar flow the power number varies inversely with Reynolds number. Multiply viscosity by ten and Reynolds number drops by ten while Np climbs by the same factor, so shaft power rises steeply even at unchanged speed.
After applying drive efficiency, add margin for starting torque and for variations in density and viscosity during the batch. Selecting the next standard motor size above the calculated figure is common practice.