A. Introduction

This mode of condensation occurs when a vapour condenses on a surface which is not wetted by the condensate. It has long been recognised that, for non-metal vapours, dropwise condensation gives much higher heat-transfer coefficients titan found with film condensation. For instance, in the absence of non-condensing gases, the heat-transfer coefficient for dropwise condensation of steam is around ten times that for film condensation at power station condenser pressures and more than twenty times that for film condensation at atmospheric pressure. In circumstances where the filmwise coefficient is of similar magnitude to that on the cooling side, a change of mode to dropwise condensation offers a potential improvement in overall coefficient by a factor of around 2. This has been verified by experiment as shown in Figure 1.

Figure 1 Overall heat-transfer coefficient for steam at atmospheric pressure condensing on a water-cooled horizontal aluminium tube (I.D. 9.7 mm, O.D. 12.8 mm). Vapour velocity 0.7 m/s. Non-condensing gas content < 30 ppm. Coolant temperature 20 °C. Promoter — 9 μm copper plate, dioctadecyl disulphide. [From data in Rose (1978)]

Clean metal surfaces are wetted by non-metallic liquids and film condensation is the mode which normally occurs in practice. Non-wetting agents, known as dropwise promoters, are needed to promote dropwise condensation. Unfortunately, sufficiently reliable promoting techniques have not yet been developed and dropwise condensation has not so far been used on an industrial scale to any significant extent. On the laboratory scale, however, dropwise condensation can be reliably obtained for long periods for steam and a few high surface tension organic fluids. Reviews of dropwise condensation heat transfer have been given by Le Fevre and Rose (1969), Tanasawa (1991), and Marto (1994).

B. Dropwise promoters