• The condensation sets in, whenever a saturation vapour comes in contact with a surface whose temperature is lower than the saturation temperature corresponding to vapour pressure.
• As the vapour condenses, latent heat is liberated and there is flow of heat to the surface.
• The liquid condensate may get somewhat sub-cooled by contact with the cooled surface and that may eventually cause more vapour to condense on the exposed surface or upon the previously formed condensate.
• Depending upon the condition of cold surface, condensation may occur in two possible ways: Film condensation and Drop wise condensation.

Film condensation 

• If the condensate tends to wet the surface and thereby forms a liquid film, then the condensation process is known as ‘Film condensation’.
• Here, the heat from the vapour to the cooling medium is transferred through the film of the condensate formed on the surface.
• The liquid flows down the cooling surface under the action of gravity and layer continuously grows in thickness because of newly condensing vapours.
• The continuous film offers thermal resistance and checks further transfer of heat between the vapour and the surface.
• Further, the heat transfer from the vapour to the cooling surface takes place through the film formed on the surface.
• The heat is transferred from the vapour to the condensate formed on the surface by convection and it is further transferred from the condensate film to the cooling surface by the conduction.
• This combined mode of heat transfer by conduction and convection reduces the rates of heat transfer considerably (Compared to Drop wise condensation).
• This is the reason that heat transfer rates of film condensation are lower than drop wise condensation.

Drop wise condensation:

• In Drop wise condensation, the vapour condenses into small droplets of various sizes which fall down the surface in random fashion.
• The drops form in cracks and pits on the surface, grow in size, break away from the surface, knock off other droplets and eventually run off the surface under the influence of gravity, without forming a film.
• In this type of condensation, a large portion of the area of solid is directly exposed to vapour without an insulating film of condensate liquid, consequently higher transfer rate are achieved.
• Drop wise condensation has been observed to occur either on highly polished surfaces, or on surfaces contaminated with impurities like fatty acids and organic compounds.
• This type of condensation gives coefficient of heat transfer generally 5 to 10 times larger than with film condensation. Although Drop wise condensation would be preferred to film condensation, yet it is extremely difficult to achieve or maintain.
• Drop wise condensation can be obtained under controlled conditions with the help of certain additives to the condensate and various surface coatings but its commercial viability has not yet been approved. For this reason, the condensing equipment in use is designed on the basis of film condensation.