Engineering calculator reviewed for preliminary design use · Last updated: March 2026
Calculate agitator power P = Np × ρ × n³ × D⁵ from power number Np, fluid density, rotational speed, and impeller diameter. Apply to mixer and agitator selection for reactors and blending tanks.
Agitator Power Calculation
Correctly estimating agitator power is essential for motor selection and process vessel design. Power is calculated using the dimensionless Power Number (Np), which depends on impeller geometry and the mixing Reynolds number.
Formula
P = Np × ρ × n³ × d⁵
P: Required shaft power [W]
Np: Power Number (dimensionless)
ρ: Liquid density [kg/m³]
n: Rotational speed [rev/s]
d: Impeller diameter [m]
Typical Power Numbers (turbulent)
Disc turbine (Rushton): Np ≈ 5
Paddle: Np ≈ 2
Marine propeller: Np ≈ 0.35
Anchor: Np ≈ 0.5
Mixing Reynolds Number
Re_mix = ρ × n × d² / μ
Re > 10,000: turbulent regime; Re < 10: laminar regime.
Np depends on impeller type and Reynolds number. For turbulent flow: Rushton turbine ~5, pitched blade ~1.3, marine propeller ~0.35. Np is approximately constant in the turbulent regime (Re > 10,000).
Does this account for baffles?
The Power number values assume standard baffled tank geometry (4 baffles, width = T/10). Unbaffled tanks produce vortexing and require different correlations. Baffle configuration affects both Np and mixing performance.
These results are preliminary estimates for screening-level design use. They do not replace detailed engineering, code compliance verification, or vendor-certified calculations.