Condensation occurs when a saturated vapour comes in contact with a surface whose temperature is below the saturation temperature. There are fundamentally two type of condensation: film-type and dropwise condensation.
At first, a film of condensate is formed on the surface and increases with increase in surface area. In the later condensation occurs when the wall is not uniformly wetted by the condensate, with the result that the condensate appears in very small droplets at the various points on the surface which grow, coalesce with each other, and finally form rivulets.
Adhesion force is overcome by gravitational force, the rivulets flow quickly to the bottom of the surface, capturing and adsorbing all droplets in its part and leaving dry surface in its wake.
Condensation can be applied to the control of gaseous vapour close to their dew points. The method is most suited for hydrocarbons and organic compounds that have relatively high boiling points compared to ambient conditions and present in the vapour phase in appreciable concentrations.
Pollutants having relatively low vapour pressures at ambient temperatures may be controlled satisfactorily in water or air cooled condensers. For somewhat more volatile solvents, two stage condensation may be required, using cooling water in the first stage and refrigeration in the second. Refrigeration to extremely low temperature levels is seldom attractive for pollution control alone unless needed for other process reasons.
Minimizing the presence of inert or non-condensable gases in the effluent will increase the practicality and economic attractiveness of condensation, because it will become less necessary to cool to very low dew point.
The two primary types of flue gas condenser are: