Condensation is the change of the physical state of matter from the gas phase into the liquid phase, transferring heat to the external environment (the inverse process of vaporization). It is a phase transition not characterized by a specific value of temperature for each substance, nor of pressure; can be performed:
- at constant temperature by compression, if the temperature of the aeriform is lower than the critical temperature;
- by cooling with an external source;
- for adiabatic expansion.
Condensation can generally occur in two ways:
- volumetric condensation: normally occurs in natural phenomena, such as in clouds;
- condensation on surfaces: used mainly for technical and engineering aspects, for example in heat exchanges. Condensation on surfaces can occur in two ways:
- film condensation: the vapor condenses on the surface creating a liquid film, which percolates downwards by gravity; the presence of the liquid film hinders the formation of new condensate.
- condensation drops: the vapor condenses in the form of small droplets, which can be detached from the surface leaving it free for the formation of new droplets.
Film condensation generally occurs on clean and smooth surfaces. The presence of impurities on the surface and in the vapor and the roughness of the exchange area leads to the fractionation in drops of the film.
The condensation drops can develop much higher heat transfer coefficients than film since the effective heat exchange surface, in the case of drops, becomes much higher and the thermal resistance of the condensate is lacking. For technical calculations, since drop condensation cannot be accurately predicted, reference is made to film condensation: this is to the advantage of safety.
In the field of industrial chemistry and experimental chemistry, condensation is a unit operation, that is, a stage in a set of chemical-physical processes that have the purpose of transforming a substance. For example, the condensation of a fluid stream is carried out in the upper part of a distillation column or inside a refrigeration cycle. Condensation is a crucial component of distillation, an important laboratory, and industrial chemistry application.