Question

Helical and ribbon type of agitators are used for mixing the material having viscosity in the range of

• A. 500-1000 Pa.s
• B. <100 Pa.s
• C. 50 – 100 Pa.s
• D. <3 pa.s

Hint:

For low-viscosity fluids ($<$ 3 Pa.s), use high-speed turbine or propeller agitators; for highly viscous, non-Newtonian dairy products (50--100 Pa.s), use slow-speed, close-clearance helical ribbon agitators.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Agitation and mixing are key unit operations in dairy processing.
The choice of impeller or agitator design is determined by the rheological properties, primarily the viscosity, of the product being processed.

Step 2: Detailed Explanation:
Different agitator designs are suited for different viscosity ranges:
1. Propellers and Turbines: These high-speed impellers are designed for low-viscosity fluids (typically below 3 Pa.s, and often below 0.1 Pa.s) like raw milk or whey.
They generate high shear and axial or radial flow currents.
2. Paddle Agitators: These are used for medium-viscosity fluids (up to 10--20 Pa.s).
3. Helical and Ribbon Agitators: These close-clearance agitators are designed for high-viscosity materials, typically in the range of 50 to 100 Pa.s.
The ribbon or helix rotates close to the vessel wall, creating a positive displacement-type flow that moves highly viscous material along the wall to prevent stagnant zones.
This action provides uniform heat transfer and mixing without requiring high shear rates.
They are commonly used for processing viscous dairy products such as processed cheese, sweetened condensed milk, and concentrated ice cream mixes.
Fluids with viscosities much higher than 100 Pa.s (such as 500--1000 Pa.s) require specialized heavy-duty kneaders or extruders rather than standard ribbon agitators.

Step 3: Final Answer:
Therefore, helical and ribbon type agitators are typically used for materials with viscosities in the range of 50 to 100 Pa.s.