Question

Select the evaporator for the heat sensitive materials:

• A. Forced Circulation Evaporator
• B. Rising Film Evaporator
• C. Falling Film Evaporator
• D. Vertical Evaporator

Hint:

For heat-sensitive products, remember: Short Residence Time = Falling Film. These evaporators are designed to prevent product degradation by ensuring the liquid "zips" through the heat exchanger as fast as gravity allows.

Answer 1

The Correct Option is C

Concept: Evaporation of heat-sensitive materials (like fruit juices, milk, or pharmaceuticals) requires minimizing the "thermal history" of the product. This is achieved by reducing the temperature of evaporation (using vacuum) and, crucially, minimizing the residence time (the duration the liquid stays in contact with the heated surface).

Step 1: Analyze the mechanism of a Falling Film Evaporator.
In a falling film evaporator, the liquid feed enters at the top of the heating tubes and flows downward as a thin film along the inner walls due to gravity. Because the liquid moves rapidly and does not fill the entire tube, the residence time is extremely short (often just a few seconds).

Step 2: Compare with other types.

• Rising Film: Requires a high temperature difference to generate enough vapor to "lift" the liquid, which can lead to overheating.
• Forced Circulation: Involves high-speed pumping which is better for viscous or scaling liquids, but typically results in longer residence times than falling film types. The falling film evaporator provides the best combination of high heat transfer coefficients and the lowest possible residence time for sensitive materials.