Question

The falling-film evaporator allows a greater number of effects than the rising-film evaporator because
Statement I: Rising-film evaporators require large temperature differential across the heating surface
Statement II: The falling-film evaporator can handle more viscous liquids than the rising-film type
In light of the above statements, choose the correct answer from the options given below

• A. Statement I is true
• B. Statement II is true
• C. Both Statement I and Statement II are true
• D. Both Statement I and Statement II are false

Hint:

Falling-film evaporators are preferred in the dairy industry because they can operate with small temperature differences, allowing for multi-effect systems (up to 7 or more effects) that improve energy efficiency.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Evaporation is used to concentrate milk prior to spray drying.
Multi-effect evaporation systems reuse the vapor generated in one stage as the heating medium in the subsequent stage, which improves steam economy.

Step 2: Detailed Explanation:
Let us evaluate the physics and operation of both evaporator types:
1. Statement I: Rising-film evaporators rely on the rapid generation of vapor bubbles to carry a liquid film upward against gravity along the tube wall.
This mechanism requires a relatively high temperature difference (typically 15--20$^{\circ}$C) across the heating surface to maintain vigorous boiling and upward momentum.
Falling-film evaporators utilize gravity to move the liquid film downward.
Consequently, they can operate with small temperature differences (as low as 3--5$^{\circ}$C).
This smaller temperature difference allows for a larger number of effects to be configured within a given overall temperature range, validating Statement I.
2. Statement II: Falling-film evaporators are generally better suited for moderately viscous liquids than rising-film evaporators.
Because gravity assists the flow, a falling-film evaporator can process liquids of higher viscosity before the film flow stalls.
In a rising-film evaporator, high viscosity increases drag, preventing the vapor from lifting the film and causing dry patches or tube blockage.
This validates Statement II.

Step 3: Final Answer:
Both Statement I and Statement II are true.