Phase change transfer - Condensation on smooth surfaces

The phase change from vapor to liquid is known as condensation. This phenomenon is frequently encountered in industrial processes, and plays an important role in steam power plants, refrigeration machines and heat pumps. In industrial condensers, the steam to be condensed is separated from the cold fluid by an intermediate surface. When steam comes into contact with a surface whose temperature is below the saturation temperature of the steam, a vapor-liquid phase change takes place, giving rise to significant heat transfer.

The condensation of steam on a cooled wall gives rise to two types of phenomena, characterized by the visual appearance of the condensate formed. In the first case, the condensate covers the surface in the form of a continuous film, known as film condensation. In the second case, liquid droplets form on the surface and are referred to as droplet condensation. Industrial equipment is designed for film condensation. Heat exchange with droplet condensation is higher than with film condensation, but droplet condensation is difficult to maintain over the long term. This is because the condensate film is unable to wet the solid due to the presence of organic molecules on the surface. The deposition of a thin layer of plastic (Teflon) is sometimes used to develop droplet condensation, but this introduces additional thermal resistance and the improvement in exchange is not as spectacular as might be hoped. At present, there is no reliable theoretical analysis method for dealing with direct condensation, and only the results of tests on specific coatings are used to determine heat exchange. These results will not be presented in this dossier, which is devoted to film condensation.

Basic theory is presented, as well as condensation on smooth surfaces, with application to condensation either outside or inside circular cylindrical tubes.