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DROPWISE CONDENSATION
In dropwise condensation the condensate liquid collects in the form of countless droplets of varying diameters on the condensing surface instead of forming a continuous film and does not wet the solid cooling surface. The droplets develop at a point of surface imperfection (pit, scratch), called nucleation sites and grow in size as more vapour condenses on its exposed surface. When the size of droplets is large there comes a time the droplet breakaway from the surface and knock off other droplets and carries it downstream. The moving droplet devours the droplets of smaller size. Dropwise condensation is one of the most effective mechanism of heat transfer and extreamly large heat transfer coefficients can be achieved with this mechanism. There is no liquid film in this case to resist heat transfer as a result heat transfer coefficients can be achieved more than 10 times larger than those associated with film condensation. Heat transfer coefficients is large so designers to achieve a specified heat transfer rate with a smaller surface area and thus a smaller and less expensive condenser.

Dropwise condensation is achieved by adding a promoting chemical into the vapor, highly polished surface and surface contaminated with impurities like fatty acids and organic compounds, known as dropwise promoters. Dropwise condensation is provoked artificially with the help of silicons, teflons, assortment of waxes and fatty acids. These promoters are used to promote dropwise condensation but promoters are highly unstable and lose their effectiveness with time due to oxidation, fouling and removal of the promoter from the surface. Dropwise condensation sustain for a long time by the combined effects of surface coating and periodic injection of the promoter into the vapor neither it does not continue long time and converts to filmwise condensation. So most condensers are designed on the assumption that film condensation will take place on the surface.

The total amount of heat transfer through a single droplet is a function of it’s radius and the size distribution over the condensation surface. Some important factor factors which are involved in the mechanism of heat transfer through a single droplets. These factors are:

1.	Conduction through a single droplet 2.	Conduction in the substrate material 3.	Interphase matter transfer at the vapour liquid interface 4.	Curvature of vapour liquid interface