Studies in condensation of vapours

Abstract

for the same identical conditions, dropwise condensation gives higher heat transfer coefficients compared to filmwise type of condensation. This shows that if dropwise condensation is assured, the condensation surface can be significantly reduced, thus saving in the cost of condensers. Because of this great practical advantage, considerable attention has been paid to the study of dropwise condensation. It is therefore important to study the factors which are responsible for the formation of dropwise condensation. Clearly, the concept of surface energy of the system is closely related to the type of condensation. If the wetting of the surface by condensate is somehow prevented, dropwise condensation is invariably obtained. This is sometimes achieved by the addition of promoters to the system or coating promoters like mercaptans on the condenser surface. Emmons (13) reports that a unimolecular layer of promoter is sufficient for the purpose of bringing about dropwise condensation. The presence of external agents thus brings about a considerable variation in heat transfer coefficients. Though a large number of substances can be used for making the surface temporarily non-wettable, only a few adhere to the surface firmly and give dropwise condensation for longer periods. The life of promoters depends on the nature of vapour to be condensed. Though a number of attempts have been made to express the dropwise condensation on the basis of assumed mechanisms, they have not been very successful. The main reason for this is lack of basic knowledge about this phenomenon. This is extremely essential before any attempt can be made to derive a reasonably realistic model for the above phenomenon. Even the questions like the sequence of events leading to the formation and subsequent movement of drops have not been completely resolved. Further, it is still not known whether the surface energy alone decides the nature of condensation. It has been reported that surface tension of condensate is important; sometimes it is assumed that wettability of surface is important. Therefore, it was decided to study the effect of surface tension and contact angles on the type of condensation.