Question

Which type of flow is required in holding tube for uniform heat treatment?

• A. Laminar
• B. Streamline
• C. Turbulent
• D. Hyper laminar

Hint:

Turbulent flow produces a flat velocity profile that minimizes the difference in velocity between the center and the wall of the tube, ensuring uniform holding times.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
In continuous thermal processing systems like High-Temperature Short-Time (HTST) pasteurizers, raw milk is held in a holding tube for a precise duration (e.g., 15 seconds) to ensure pathogen destruction.
For uniform heat treatment, all particles of milk must remain in the holding tube for nearly the same length of time.

Step 2: Detailed Explanation:
The fluid velocity profile inside a pipe depends on the regime of flow, characterized by the Reynolds number.
Under laminar or streamline flow conditions ($Re < 2100$), the velocity profile is parabolic.
The fluid at the center of the pipe travels at twice the average velocity, while the fluid near the pipe walls is nearly stationary.
This velocity variation would allow some milk particles to pass through the holding tube too quickly, resulting in under-pasteurized milk.
Under turbulent flow conditions ($Re > 4000$), intense mixing and eddies create a much flatter, plug-like velocity profile.
In this regime, the ratio of the minimum holding time to the average holding time is highly uniform (usually above 0.8 to 0.85).
This flat velocity profile ensures that almost all milk particles receive an equal, uniform, and safe heat treatment.
Therefore, turbulent flow is required in pasteurizer holding tubes.

Step 3: Final Answer
The correct option is (C).