Dropwise condensation, heat transfer

FIGURE 14.2 The influence of surface material and operating pressure on the dropwise condensation heat transfer coefficient of steam. (Adapted from Ref 1 and printed with permission from Academic Press, Inc., Orlando, FL.)... 

All of the preceding has been based on the assumption of film condensation. There is another condensation mode, namely, dropwise condensation (see Figure 7-4). In this case, droplets coalesce and flow in rivulets over the surface. The result is that parts of the surface are not covered and hence can contact the vapor directly (i.e., without the resistance of the liquid film). Because of this we obtain extraordinarily high heat transfer rates with dropwise condensation which can have heat transfer coefficients four to ten times those of film condensation heat transfer coefficients. 

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%). 

Figure 12. Relation of heat transfer coefficient to temperature with dropwise condensation on rotor...

The high heat transfer rates associated with dropwise condensation make it very attractive for engineering applications. High heat transfer rates lead to high condensation rates allowing the use of smaller condensers. The controlling factor for... 

FIGURE 113 Heat transfer rates with film and dropwise condensation of steam on a vertical surface. 

Citakoglu. E. and Rose, J.W., Dropwise Condensation Some Factors Influencing the Validity of Heat Transfer Measurements." Int. J. Heat Mass Transfer, Vol. 11, p. 523,... 

Graham, C. and Griffith, P., Drop Size Distributions and Heat Transfer in Dropwise Condensation, Int. J. Heat Mass Transfer, Vol. 16, p. 337, 1973. 

Hannemann. R. and Mikic, B., An Experimental Investigation into the Effect of Surface Thermal Conductivity on the Rate of Heat Transfer in Dropwise Condensation." Int. J. Heal Mass Transfer, Vol. 19. p. 1309, 1976. 

Consider a vertical flat plate exposed to a condensable vapor. If the temperature of the plate is below the saturation temperature of the vapor, condensate will form on the surface and under the action of gravity will flow down the plate. If the liquid wets the surface, a smooth film is formed, and the process is called film condensation. If the liquid does not wet the surface, droplets are formed which fall down the surface in some random fashion. This process is called dropwise condensation. In the film-condensation process the surface is blanketed by the film, which grows in thickness as it moves down the plate. A temperature gradient exists in the film, and the film represents a thermal resistance to heat transfer. In dropwise condensation a large portion of the area... 

Because of the higher heat-transfer rates, dropwise condensation would be preferred to Him condensation, but it is extremely difficult to maintain since most surfaces become wetted after exposure to a condensing vapor over an extended period of time. Various surface coatings and vapor additives have been used in attempts to maintain dropwise condensation, but these methods have not met with general success to date. Some of the pioneer work on drop condensation was conducted by Schmidt [26] and a good summary of the overall problem is presented in Ref. 27. Measurements of Ref. 35 indicate that the drop conduction is the main resistance to heat flow for atmospheric pressure and above. Nucleation site density on smooth surfaces can be of the order of 10 sites per square centimeter, and heat-transfer coefficients in the range of 170 to 290 kW/m2 °C [30,000 to 50,000 Btu/h ft2 °F] have been reported by a number of investigators. 

Fig. 9-1 Dropwise and filmwise condensation of steam on a copper plate from Westwater [47], The right side of the plate is clean copper and the steam condenses as a continuous film. The left side has a thin coating of cupric oleate which causes dropwise condensation. The heat transfer coefficient for the dropwise condensation is about seven times the value for filmwise condensation The diametei of the horizontal thermocouple probe is 1.7 mm (Photograph courtesy of Professor J W Westwater.)...

Why are higher heat-transfer rates experienced in dropwise condensation than in film condensation ... 

Heat Transfer Correlations for Film Condensation 581 10-6 Film Condensation Inside Horizontal Tubes 591 10-7 Dropwise Condensation 591... 

Dropwise condensation, characterized by countless droplets of varying diameters on the condensing surface instead of a continuous liquid film, is one of the most effective mechanisms of heat transfer, and extremely large heat transfer coefficients can be achieved with this mechanism (Fig. 10-35). 

C What is the difference between film and dropwise condensation Which is a more effective mechanism of heat transfer ... 

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