Similarly as in the previous chapter, in this chapter we will discuss heat transfer with phase change, but in this case we will discuss condensation of gas phase (vapor-to-liquid phase change).
In general, condensation is the change of the physical state of matter from gas phase into liquid phase, and is the reverse of vaporisation. Flow processes associated with condensation on a solid surface are almost a mirror image of those involved in boiling. Condensation occurs when the temperature of a vapor is reduced below its saturation temperature or when the pressure of a vapor is increased above its saturation parameters (see phase diagram of water).
In dropwise condensation, the condensed vapor forms droplets on the surface instead of a continuous film. Dropwise condensation can occur when the surface is nonwetting or these droplets are taken away from the surface by external flow or by gravity. The vapour is in direct contact with the surface over most of the area and heat transfer rates are much higher (more than 3 – 10 times higher) as there is very little resistance for heat flow between the vapour and the surface. The droplets develop at nucleation sites (points of surface imperfections such as pit, scratch and cavities), and grow in size as more vapour condenses on its exposed surface. For steam condensers, it is common practice to use surface coatings (silicons, teflons or waxes) that inhibit wetting, and hence stimulate dropwise condensation. But coatings gradually lose their effectiveness due to removal, oxidation etc. In practice no surface is found to continue as nonwetting over any length of time. Since in most engineering applications it is often difficult to maintain this condition, using the value of heat transfer coefficients assuming dropwise condensation for design purposes is not advisable.