why is the film coefficient for dropwise condensation higher than that for film wise condensation?

Answer 1

Filmwise Condensation
• When liquid formed by condensation wets the surface. It is a more common type of condensation to occur.
• In filmwise condensation liquid condensate forms a continuous film over the surface, this film flows down the surface under the action of gravity, shear force due to vapor flow, or other forces.
• The layer of liquid condensate acts as a barrier to heat flow due to its very low thermal conductivity and hence low heat transfer rate.

Dropwise Condensation
• Dropwise condensation takes place when the liquid condensate does not wet the solid surface.
• The condensate does not spread, but forms separate drops.
• These drops in turn coalesce to form large drops and sweeping clean a portion of the surface, where again new droplets are generated.
• The average heat transfer coefficient for dropwise condensation is much higher than filmwise condensation.

You know that,
Overall heat transfer coefficient of the heat exchanger:

U0 = 1/(Ao*((1/(h0 * Ao)) + Rw + (1/(hi * Ai)))


– Uo = Overall heat transfer coefficient
– hi= Tube side heat transfer coefficient
– ho = Condensation heat transfer coefficient
–Rw = Wall resistance

If the area of condensation decreases, coefficient will increase, therefore drop wise condensation have more condensation than film wise condensation.

Answer 2

Jamand may be right, you need to check all the things that Jamand has mentioned. In addition, though, there is a certain amount of water released into your home by the normal daily acts of breathing, sweating, cooking, washing, showering etcetera. This moisture normally escapes through normal ventilation, up chimneys, around windows and thorough extractor fans. If you reduce draughts and ventilation, by fitting double glazing, this moisture may not be able to escape and will condense on any cold surface. If you have removed all the "cold windows" it will condense somewhere else. When you have checked for penetrating damp, rising damp, leaks and leaky roof and corrected any faults, you may need to improve the ventilation in your home. Use the ventilator setting in your double-glazed windows. This allows the window to let a little air to pass through but still remain locked. You may need to fit an extractor fan in your kitchen, bathroom or both. These only need to run when the room is in use. As a last resort, you can use an electric de-humidifier to remove excess moisture from the air in your home. Only consider this when everything else has failed as they are expensive to buy and need to run for several hours per day to have any effect. You also need to empty the reservoir occasionally as it fills up.

Answer 3

Filmwise

Answer 4

cool drops carry heat away from the vapour phase more efficiently than the film (dispersal and high surface area)