Agitator / Mixer Selection

Quick pick (TL;DR)

Pick the impeller by duty: hydrofoil/pitched-blade for flow-controlled blending & solids suspension; Rushton turbine for high-shear gas dispersion; anchor/helical for viscous fluids.
Compute power from P = Np·ρ·N³·D⁵ (turbulent), where Np is the impeller power number.
Add wall baffles to convert swirl into top-to-bottom mixing, choose a seal for the service, and add a motor margin.

Step 1 — Impeller by duty

Duty	Impeller	Why
Blending / suspension	Hydrofoil / PBT	Flow-controlled
Gas dispersion	Rushton	High shear
Viscous	Anchor / helical	Close clearance

Step 2 — Worked power

Rushton, water, N 1, D 1 m

Power-number equation P = Np · ρ · N³ · D⁵ Given Np = 5.0 (Rushton, baffled) ρ = 1000 kg/m³ , N = 1.0 rev/s , D = 1.0 m Step 1 — Impeller power P = 5.0 × 1000 × 1.0³ × 1.0⁵ P = 5000 W = 5.0 kW Step 2 — Motor + gearbox loss + margin → 7.5 kW motor Note — sensitivities P ∝ N³ and D⁵ → small change, big power

P ∝ N³·D⁵ — small change, big power. Np depends on impeller & baffling. Use the lowest workable speed.

Step 3 — Baffles & mechanical

Four wall baffles for real mixing.
Seal by service (mech / packing / mag-drive).
Check shaft critical speed; AGMA gearbox.

Agitator power calculator

Turbulent impeller power

P = Np·ρ·N³·D⁵ (turbulent, baffled).

ρ (kg/m³)

N (rev/s)

D (m)

Turbulent regime. N in rev/s (rpm ÷ 60). Viscous fluids use another correlation.

Common mistakes

No baffles → swirl, no mixing.
High shear on sensitive product.
Ignoring N³/D⁵ sensitivity.

Requisition & datasheet

Duty, fluid, viscosity, volume.
Impeller, power, baffles, seal.

Frequently asked questions

What should I know about Agitator / Mixer Selection?

Pick the impeller by duty: hydrofoil/pitched-blade for flow-controlled blending & solids suspension; Rushton turbine for high-shear gas dispersion; anchor/helical for viscous fluids. Compute power from P = Np·ρ·N³·D⁵ (turbulent), where Np is the impeller power number. Add wall baffles to convert swirl into top-to-bottom mixing, choose a seal for the service, and add a motor margin.

Which standards or codes apply?

Process mixing correlations (power number / pumping number), AGMA (gearbox), API 682 (seals where applicable), vendor design.

Standards — Mixing correlations; AGMA; API 682.