Improvement of heat transfer during steam condensation in horizontal tubes

Continuous hydrophobic coatings, which cause the condensation of water vapor in droplets, make it possible to significantly increase the heat transfer: however, from the economic point of view, their high price absorbs most of the gain obtained. In order to remedy this inconvenience, the interest of certain discontinuous gold coatings is examined on horizontal tubes; these coatings mainly affect the form of bands parallel to generating lines, of helical bands and of pellets. The experimental study, conducted in the vicinity of the atmospheric pressure, made it possible to determine the heat transfer coefficient on the vapor side on these various coatings and, by way of reference, on continuous coatings of gold, N parylene and uncoated tubes. It turned out that the use of discontinuous coatings values the amount of gold used in a very important way. As a result, the tubes studied appear interesting, notably for seawater desalination horizontal condensers through successive expansion. Different structures of discontinuous coatings have been patented. Chapter II is mainly devoted to a bibliographic study of condensation; an original theoretical model is proposed, and results discussed in the literature concerning the formation and growth of an isolated droplet; a review of articles dealing with heat transfer related to condensation in drops is then presented. Chapter III attempts a theoretical approach to the various discontinuous coatings tested during the study; the proposed model, which generalizes Nusselt's theory of film condensation, will allow to interpret all the obtained experimental results. The description of the apparatus and the different techniques used is the subject of Chapters IV and V. After discussion of the preliminary measurements concerning the contact angle of water on permanent coatings (Chapter VI) and the condensation on tubes of various kinds (Chapter VII), the different discontinuous coatings studied are presented in Chapter VIII: the experimental results obtained are successfully compared with the theoretical model. Chapter IX finally draws the conclusions of the study by comparing the interest of these various coatings by means of a function which takes into account the performances and the probable cost of realization of each one of them