Question

Dropwise condensation,

• A. Requires smooth, clean uncontaminated surfaces
• B. Is stable and easy to maintain
• C. Have higher heat transfer coefficients
• D. Have lower heat transfer coefficients

Hint:

Dropwise condensation achieves higher heat transfer coefficients (5–10 times greater than filmwise) due to the absence of a continuous liquid film.

Answer 1

The Correct Option is C

Step 1: Understand dropwise condensation.
Condensation occurs when a vapor cools and changes into a liquid on a surface. There are two types of condensation:
Filmwise condensation: The condensate forms a continuous liquid film on the surface, which acts as a thermal resistance, reducing heat transfer.
Dropwise condensation: The condensate forms discrete droplets on the surface, which roll off, exposing the surface to the vapor again. This minimizes thermal resistance from the liquid layer.
Step 2: Analyze the characteristics of dropwise condensation.
Heat transfer coefficient: In dropwise condensation, the absence of a continuous liquid film reduces the thermal resistance between the vapor and the surface. Droplets form, grow, and roll off, allowing direct contact between the vapor and the surface. This results in a significantly higher heat transfer coefficient compared to filmwise condensation (often 5–10 times higher).
Surface requirements: Dropwise condensation requires non-wetting surfaces (e.g., smooth, clean, or coated with promoters like oleic acid) to prevent the formation of a continuous film. However, this is not the focus of the correct answer.
Stability: Dropwise condensation is difficult to maintain in practice because surfaces tend to become contaminated or wet over time, reverting to filmwise condensation.
Step 3: Evaluate the options.
(1) Requires smooth, clean uncontaminated surfaces: While true, this is a requirement for achieving dropwise condensation, not a characteristic of the process itself. Incorrect.
(2) Is stable and easy to maintain: Incorrect, as dropwise condensation is unstable and difficult to sustain in industrial settings due to surface contamination. Incorrect.
(3) Have higher heat transfer coefficients: Correct, as dropwise condensation significantly enhances heat transfer due to reduced thermal resistance. Correct.
(4) Have lower heat transfer coefficients: Incorrect, as dropwise condensation has higher heat transfer coefficients compared to filmwise condensation. Incorrect.
Step 4: Select the correct answer.
Dropwise condensation has higher heat transfer coefficients, matching option (3).