Q.31 Which one of the following is the most suitable type of impeller for mixing high viscosity
(viscosity > 10⁵ cP) fluids?

(A) Propeller
(B) Helical ribbon
(C) Paddle
(D) Flat blade turbine

Helical ribbon impellers excel for mixing ultra-high viscosity fluids above 10^5 cP due to their close wall clearance and helical flow pattern. This ensures thorough blending in laminar regimes common to such thick materials like polymers or pastes. Other options fall short in these conditions, as detailed below.​

Correct Answer

Helical ribbon (Option B) stands out as the most suitable impeller for high viscosity fluids exceeding 10^5 cP (100,000 centipoise). These impellers feature twisted ribbon blades that sweep nearly the entire tank diameter, promoting axial flow and preventing dead zones near walls.​

Propeller Impellers (Option A)

Propellers generate strong axial flow ideal for low-viscosity liquids under turbulent conditions. They struggle with high-viscosity fluids because their small diameter and high-speed rotation fail to move thick materials effectively, leading to poor bulk blending.​

Paddle Impellers (Option C)

Paddles, often anchor-style, handle moderate viscosities by scraping walls and providing radial flow. For viscosities over 10^5 cP, they lack the continuous helical motion needed for uniform mixing, often resulting in stagnation in tank centers.​

Flat Blade Turbines (Option D)

Flat blade turbines produce high shear via radial flow, suiting transitional viscosities and suspensions. In extreme high-viscosity scenarios, they demand excessive power without achieving full circulation, making them inefficient for laminar mixing.​

Key Selection Factors

High-viscosity mixing prioritizes low shear, high torque, and wall-to-center flow over speed. Helical ribbons operate best at low RPMs (10-50), aligning with Reynolds numbers below 10 in pseudoplastic fluids common in biotech fermentation.​