Which Condensation is More Efficient? Dropwise vs. Filmwise

Condensation is the process by which a substance changes from its gaseous state to its liquid state when it loses heat. This occurs when the temperature of a gas decreases, causing its particles to slow down and come closer together, forming liquid droplets. 

Condensation is commonly observed when water vapour in the air cools and turns into liquid water droplets, such as when dew forms on the grass in the morning or when steam from a boiling kettle turns into water droplets on a cold surface.

Condensing Film Coefficient

The condensing film coefficient depends upon the nature of the surface and whether the surface, on which Condensation occurs is mounted vertically or horizontally. For filmwise Condensation on a vertical surface, the mean heat transfer coefficient is given by 

hm = 0.943[p2gλK3/μL∆Tf]1/4

Where 

hm = mean heat transfer coefficient over the tube length.

L = tube length

g = gravitational acceleration.

μ = liquid viscosity.

k = thermal conductivity of the liquid.

ΔTf = temperature difference between vapour and metal.

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Dropwise Condensation

When a saturated vapour comes into contact with a cold surface, it condenses and if condensate does not wet the surface, the droplet is formed on the surface. These droplets grow and ultimately fall from or fall down the surface under the influence of gravity leaving behind the bare metal surface on which further consideration takes place. The condensation occurring by the mechanism is known as dropwise Condensation.

Dropwise Condensation needs a physical preparation of the surface which is very difficult and needs to be promoted by introducing an impurity into the vapour stream. This type of condensation is very unstable and also the design methods are not available.

Because of these cited points, dropwise Condensation is not common in the industry. In general, a smooth, clean surface seems to promote Filmwise Condensation and oily or greasy surfaces and the presence of dirt on the surface, when impurities are present and the surface is contaminated seems to promote dropwise Condensation.

Film-wise Condensation Explain

When a saturated vapour comes into contact with the cold surface, it condenses and if condensate wets the surface, it forms a continuous film of condensate through which heat must be transferred. The additional vapour is then required to condense into the liquid film rather than directly on the surface. The condensate ultimately flows down the surface under the influence of gravity. The condensation occurring by this mechanism is called Filmwise Condensation.

In filmwise condensation, the film covering the surface acts as a resistance to heat transfer, while in dropwise condensation, a large portion of the surface is directly exposed to the vapour. Because of this the heat transfer coefficient (hence the heat transfer rates) in dropwise Condensation is four to eight times larger than those for filmwise condensation.

Condensation used

Condensation is widely used in the chemical industry. For example, in the separation of the constituents of a liquid mixture by distillation, a condenser converts vapours from the top of the column into liquid distillate and reflux. In power plants, surface condensers are used for condensing steam from the exhaust of turbines into liquid. The home air conditioning unit makes use of an air-cooled condenser for the liquefication of refrigerant 

Effects of non - condensable gases

If a non - non-condensable gas is also present, for example, the air in the condensing vapours in a condenser because of say leakage it hinders the process of heat transfer. The non-condensable gas collects in the vicinity of the condensate surface, and the condensing vapours must have to diffuse through the gas film. 

The presence of diffusion resistance in the process of Condensation decreases the rate of condensation far below that for a pure material. The presence of air of about 1% by volume can almost all the condensers to eliminate air in the system.

Difference between drop-wise and film-wise condensation

Dropwise condensation

1. In dropwise Condensation the condensate does not wet the surface and collects in the form of droplets. These droplets grow for a while and then fall from the surface, leaving a bare metal surface for further consideration.

2. Heat transfer coefficients are very high as the heat does not have to flow through the film by conduction.

3. Oily or greasy surfaces tend to tend towards dropwise condensation.

4. Dropwise Condensation is very difficult to achieve.

5. Dropwise condensation is unstable and difficult to maintain, uncommon and so not desired industrially.

6. Methods for predicting the film coefficient are not available.

Filmwise Condensation

1. In film-wise condensed the liquid wets the surface and forms a continuous film of condensate through which heat transfer takes place. This condensate flows down under the action of gravity.

2. Heat transfer coefficients are relatively very low since the heat does have to flow through a film by conduction.

3. Smooth, clean surface seen to tend towards film-wise condensation.

4. Filmwise Condensation is easily obtainable.

5. Flimwise condensation is stable, easy, reliable, common and so desired industrially.

6. Methods for predicting the film coefficient are available.

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane