Fermenter impellers are critical components in bioreactors, responsible for creating the fluid motion and mixing needed for efficient fermentation. Their main functions include:

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1. Homogeneous Mixing & Mass Transfer

   • Evenly distributes substrate, nutrients, and cells throughout the vessel.

   • Enhances oxygen transfer (kLa) by dispersing gas bubbles into the culture medium.

2. Heat Transfer & Temperature Control

   • Transfers metabolic heat uniformly to prevent hot spots.

   • Improves contact with heating or cooling jackets and coils.

3. Gas Dispersion

   • Breaks up incoming air or oxygen into fine bubbles, maximizing gas-liquid interfacial area.

4. Shear (Mechanical Stress) Management

   • Impeller type and speed must be matched to cell robustness.

   • Low-shear designs (e.g. paddle, helical ribbon) protect sensitive mammalian cells, while robust cells tolerate high-shear turbines.

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Key Selection Criteria

• Flow Pattern

  • Radial-flow impellers (e.g. Rushton turbine): high turbulence, excellent gas dispersion

  • Axial-flow impellers (e.g. marine propeller): strong bulk circulation, low power consumption

• Viscosity & Density

  • Low-viscosity broths favor fast-running turbines or propellers

  • High-viscosity media require helical ribbons or anchor impellers

• Shear Sensitivity

  • Robust bacterial or yeast cultures can handle high shear

  • Fragile mammalian cells need gentle paddle or low-shear designs

• Scale & Power Density

  • Smaller lab fermenters vs. large industrial tanks have different power-to-volume requirements

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Common Impeller Types

• Rushton (Flat-Blade) Turbine

  • Excellent for gas dispersion and mixing intensity

• Marine Propeller

  • Axial flow, energy-efficient bulk circulation

• Paddle Impeller

  • Gentle mixing, low shear stress

• Helical Ribbon

  • Ideal for very viscous fluids and slurries

• Anchor Impeller

  • Scrapes vessel walls to prevent settling in high-viscosity media

 

Selection Criteria

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• Viscosity

  • Low (water-like): Rushton, marine propeller

  • Medium (syrup-like): Concave-blade, pitched-blade

  • High (gel/paste): Helical ribbon, ancho

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• Shear Sensitivity

  • Low shear required: Paddle, helical ribbon, anchor

  • High mixing intensity required: Rushton, disc turbines

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• Gas Dispersion & Oxygen Transfer

  • High O₂ transfer: Rushton turbine

  • Moderate transfer: Concave-blade, pitched-blade

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• Energy Efficiency

  • Low power consumption: Marine propeller, gas-lift

  • Maximum mixing: Turbines