Question

Heat transfer coefficients for a vapor condensing on a surface can be increased by promoting (a) film condensation (b) dropwise condensation (c) rolling action (d) none of them

Short answer

Answer: Dropwise condensation.

Step-by-step solution

Understanding film condensation

Film condensation occurs when the vapor condenses on the surface and forms a continuous liquid film. The heat transfer in this case is mainly through the flowing liquid film, which has relatively lower thermal conductivity compared to the vapor. As the film thickness increases, the overall heat transfer coefficient decreases, leading to lower efficiency.

Understanding dropwise condensation

Dropwise condensation occurs when the vapor condenses as individual droplets on the surface, which then combine and roll off the surface when they reach a certain size. The heat transfer in this case is much more efficient as the droplets expose more surface area between the vapor and the coolant. This allows for quicker and more effective heat transfer, resulting in higher heat transfer coefficients compared to film condensation.

Understanding rolling action

Rolling action refers to the movement of liquid droplets on the surface, which also aids in heat transfer. However, this movement is more associated with dropwise condensation, as it involves the movement and combination of droplets on the surface. Thus, rolling action itself is not an independent process that enhances heat transfer coefficients in vapor condensation on a surface.

Identify the correct option

Now that we have understood film and dropwise condensation, and their effects on heat transfer coefficients, we can choose the correct option. It is clear that promoting dropwise condensation increases the heat transfer coefficient. Hence, the correct option is (b) dropwise condensation.