Solid Suspension Critical Speed (Zwietering Njs) Calculator

Determine minimum impeller rotational speed Njs required to ensure no solid particle remains stationary on the vessel bottom for >1–2 seconds.

Slurry & Particle Properties

Solid Density ρ_s (kg/m³): Sand / Quartz ~ 2650 | Catalyst ~ 1800

Liquid Density ρ_L (kg/m³):

Mean Particle Size d_p (μm):

Solids Mass Concentration X (%):

Impeller Diameter D (m):

Zwietering S-Factor (Geometry): PBT (C/T=1/4): ~6.5 | Hydrofoil: ~5.0

Critical Just-Suspended Parameters

Just-Suspended Speed Njs:

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Impeller Tip Velocity (v_tip):

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Density Difference Δρ:

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Particle Settling Terminal Vel:

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Hydrodynamic Slurry State:

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Frequently Asked Questions

What is the "1-to-2 second" Zwietering criterion for solid suspension?

Just-suspended speed Njs is defined as the impeller speed where no solid particle remains stationary on the tank bottom for longer than 1 to 2 seconds, exposing 100% of particle surface area for mass transfer or dissolution.

What is the dimensionless geometric constant S in the Zwietering equation?

S depends strictly on tank and impeller geometry: impeller type, D/T ratio, off-bottom clearance (C/T), and bottom dish shape. Down-pumping hydrofoils typically have the lowest S-factors, requiring the least power for suspension.

How does particle size affect required agitator speed?

Particle size has a mild power-law exponent of 0.20 (Njs ∝ dp^0.2). An 8-fold increase in particle diameter requires only ~50% higher rotational speed for off-bottom suspension.