Dropwise promoters, condensation

It should be noted that the only valid application of dropwise condensation is for steam condensers, since nonwetting substances are not available for most other working fluids. For example, no dropwise condensation promoters have been found for refrigerants (i.e., no dropwise condensation promoters seem to be Freon-phobic ) [46]. The enhancement of dropwise condensation, beyond inducing the process by selection of an effective, durable promoter, is fruitless, since the heat transfer coefficients are already so high. 

The present results indicate that the spray evaporation technique provides a rapid and sensitive method for the investigation of scale formation on evaporator surfaces. Very high heat transfer rates are obtainable, especially when dropwise condensation promoters are used on the steam side, so that the effect of a thin film of scale becomes rapidly apparent. 

Film-type condensation is more common and more dependable. Dropwise condensation normally needs to be promoted by introducing an impurity into the vapor stream. Substantially higher (6 to 18 times) coefficients are obtained for dropwise condensation of steam, but design methods are not available. Therefore, the development of equations for condensation will be for the film type only. 

Patent 3,384,154, May 21, 1968). Both of these tubes also had steam-side coatings to promote dropwise condensation—paryleue for tube 47 and gold plating for tube 39. 

These properties, coupled with the metal s ability to promote bubble-type vapour formation on the surface when heating liquids, and dropwise condensation when condensing vapours, make the metal an ideal constructional material for heat-transfer equipment for use with strong acids. 

Additionally, the surfactant properties of filmers reduce the potential for stagnant, heat-transfer-resisting films, which typically develop in a filmwise condensation process, by promoting the formation of condensate drops (dropwise condensation process) that reach critical mass and fall away to leave a bare metal surface (see Figure 11.2). This function, together with the well-known scouring effect on unwanted deposits keeps internal surfaces clean and thus improves heat-transfer efficiencies (often by 5-10%). 

Some commonly used filming amines are better at promoting dropwise condensation, whereas others, notably octadecylamine (stearylamine), are better at reducing friction and improving the drop runoff rate. 

Fig. 4.17 shows the results for heat transfer in dropwise condensation that have been produced over the last 20 years. Most condensation areas were made of copper. Different liquids served as promoters they were spread over the cooling surface. Depending on the promoter and the material for the condensation surface very different results were found. As we can see,... 

The quantity of contaminant or promoter required to cause dropwise condensation is minute, and apparently only a monomolecular film is necessary. 

Effective drop promoters are strongly adsorbed by the surface, and substances that merely prevent wetting are ineffective. Some promoters are especially effective on certain metals, e.g., mercaptans on copper alloys other promoters, such as oleic acid, are quite generally effective. Some metals, such as steel and aluminum, are difficult to treat to give dropwise condensation. 

Steam-film coefficients. The steam-film coefficient is characteristically high, even when condensation is filmwise. Promoters are sometimes added to the steam to give dropwise condensation and a still higher coefficient. Since the presence of noncondensable gas seriously reduces the steam-film coefficient, provision must... 

As noted in Ref. 367, surface treatment for the promotion of dropwise condensation in vapor space environments has been extensively investigated. If dropwise condensation is achieved, the enhancement is 10 to 100 times the filmwise condensation coefficient. Numerous promoters and coatings have been found effective however, a number of practical problems relate to the method of application, permanence, and compatibility with the rest of the system. Tana-sawa s review [47] includes a good discussion of the difficulties that must be overcome if industry is to adopt this condensation process. 

T. G. Sundararaman and T. Venkatram, Dropwise Condensation Using Newly Developed Promoters on Copper Substrates, Indian Chem. Engr., 23(4), pp. 35-38,1981. 

Some recent work has also been done on increasing the rate of transfer on the condensing-steam side by promoting dropwise condensation. This type of condensation results from surface-tension effects and is promoted in three ways ... 

The grooves on the outside surface of the copper tube, which had a height of 0.026 to 0.040 inch and a pitch of 0.04 to 0.06 inch, evidently promoted dropwise condensation, since in some tests the steam-film coefficient was estimated to be as high as 10,000 in the usual units. The power requirement for the wiper was estimated to be 1/4 hp. for an 8000-gallon-per-day still, so that the cost of the power is negligible. The total energy requirement will depend mainly on the power to compress the steam and this would be no different than for any other vapor-compression process. This statement was made because a recent news report claims that this evaporator requires only one fourth as much energy as other conversion systems. 

Promotion of Dropwise Condensation. Preliminary heat transfer experiments, using distilled water as feed, gave values of over-all heat transfer coefficient around 1000 B.t.u./sq. ft hr. ° F. and analysis indicated that the jacket condensate film provided the main resistance to heat transfer, due to filmwise condensation of the jacket steam. Following reports by Garrett (4) of successful tests using drop-wise condensation promoters, it was decided to use oleic acid for this purpose in the present experiments. The resulting improvement in jacketside coefficients enabled over-all heat transfer coefficients in excess of 5000 B.t.u./sq. ft hr ° F. to be achieved. 

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