Difficulty: Easy
Correct Answer: Mixing highly viscous pastes
Explanation:
Introduction / Context:Choosing the right impeller or agitator is critical in chemical, polymer, and food processing. The helical screw (also called helical ribbon) agitator is a specialty device for moving extremely viscous materials that conventional turbines cannot handle effectively.
Given Data / Assumptions:
Concept / Approach:At high viscosity, inertial effects diminish and laminar flow dominates. Standard propellers lose efficiency because they rely on turbulence. Helical screw agitators create axial and radial flow in laminar regimes by physically conveying material along the vessel wall and back through the core, ensuring turnover without excessive shear heating.
Step-by-Step Solution:
Identify the rheology: pastes with very high viscosity require positive-displacement-like motion.Match impeller: a helical screw agitator provides wall-sweeping action, reducing stagnant zones.Rule out alternatives: turbines/propellers are suited to low-to-moderate viscosity and gas–liquid dispersion.Verification / Alternative check:Industry practice confirms ribbon/helical screw mixers in adhesive compounding, battery pastes, plastisols, sealants, and dough mixing, which are all high-viscosity duties.
Why Other Options Are Wrong:
Blending immiscible low-viscosity liquids usually uses Rushton turbines, pitched-blade turbines, or high-shear mixers, not helical screws.Very high temperature (> 250 °C) does not alone determine impeller type; material rheology is the key factor.High-turbulence gas–liquid dispersion needs radial turbines, not ribbon/screw agitators.Common Pitfalls:Assuming temperature dictates impeller choice; ignoring laminar vs turbulent regime; or selecting high-speed propellers for pastes (inefficient and heat-intensive).
Final Answer:Mixing highly viscous pastes
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