Solid Suspension Njs Calculator (Zwietering Method)

Calculate the minimum just-suspended impeller speed (Njs) to prevent particle settling and filleting using the classic Zwietering correlation.

Tank & Impeller Geometry

Tank Diameter T (ft):

Impeller Diameter D (ft):

Off-Bottom Clearance C/T:

Impeller Type:

Slurry Solids & Fluid Properties

Mean Particle Diam dp (µm):

Solids Loading (% wt):

Solid Particle SG (g/cm³):

Liquid Carrier SG (g/cm³):

Kinematic Viscosity (cSt):

Zwietering Njs Sizing Results

Just-Suspended Speed (Njs)

-- RPM

Tip Speed: -- ft/min

Zwietering Factor S

Geometric Constant

Power Draw @ Njs

-- HP

-- kW shaft power

D/T Impeller Ratio: --

Suspension Quality: 1-Sec Complete Off-Bottom

Hydrofoil Power Savings: --% vs Rushton

Axial-flow hydrofoils are by far the most energy-efficient impellers for solid suspension, requiring 60-70% less motor power than radial turbines.

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Frequently Asked Questions

What is the Zwietering 1-second off-bottom suspension criterion (Njs)?

The just-suspended speed (Njs) is officially defined as the lowest impeller RPM at which no particle remains stationary on the vessel bottom for more than 1 to 2 seconds. Operating at or above Njs ensures 100% of particle surface area is fully wetted and exposed to the liquid phase for mass transfer and chemical reaction.

Why are axial hydrofoils preferred for solid suspension over radial turbines?

Radial turbines direct liquid sideways against the vessel wall, leaving a quiescent stagnant dead zone in the center of the dish bottom where solids readily settle into heavy fillets. Down-pumping axial hydrofoils blast flow directly downward against the tank floor, sweeping solids upward into suspension with 60% to 75% less power input.

How does off-bottom clearance (C) affect Njs?

Lowering the impeller closer to the tank bottom (C/T = 0.15 to 0.25) reduces the distance the fluid jet must travel before scouring settled solids off the floor. This lowers the Zwietering geometric constant S, significantly reducing required speed and motor HP.