Condensation dropwise

As we have already explained in section 4.1.1., if the condensate does not completely wet the wall, individual liquid droplets form instead of a continuous condensate him. Heat transfer coefficients in dropwise condensation are significantly larger than in film condensation. In the condensation of steam, the heat transfer coefficients measured have been a factor of four to eight times larger. However, it has been shown that all investigated substances, in particular water, which condense on commonly used heating surfaces, will completely wet the surface. This is true as long as the material of the heated surface and the liquid have not been contaminated. This also corresponds to the experience that the formation of a water film is taken to be an indication that laboratory equipment is well cleaned. 

4 Convective heat and mass transfer. Flows with phase change 

Even when the type of condensation is not precisely known, film condensation is assumed for the calculation of condensers, so that the condenser area will be sufficient. 

Experiments on dropwise condensation are difficult as they entail the measurement of temperature differences of 1 K or less for the determination of the heat transfer coefficient. At these small temperature differences the wall temperature fluctuates with time and also locally. The cost of the measuring techniques for the achievement of accurate results is, therefore, considerable. 

Several theories on dropwise condensation have been developed for the calculation of heat transfer. One of the oldest theories, that from Eucken [4.33], starts with the concept that the first droplets arise from an adsorbed monomolecular condensate layer, this layer is favoured by nuclei, and that the new condensate flows to the droplets by means of surface diffusion which primarily occurs at the edge of the drop. This theory was taken on later by other authors [4.34], 

In the discussion so far, it is assumed that the condensing vapour, on coming into contact with the cold surface, wets the tube so that a continuous film of condensate is formed. If the droplets initially formed do not wet the surface, after growing slightly 

In the previous discussion it has been assumed that the vapour is a pure material, such as steam or organic vapour. If it contains a proportion of non-condensable gas and is cooled below its dew point, a layer of condensate is formed on the surface with a mixture of non-condensable gas and vapour above it. The heat flow from the vapour to the surface then takes place in two ways. Firstly, sensible heat is passed to the surface because of the temperature difference. Secondly, since the concentration of vapour in the main stream is greater than that in the gas film at the condensate surface, vapour molecules diffuse to tiie surface and condense there, giving up their latent heat. The actual rate of coi nsation is then determined by the combination of these two effects, and its calculation requires a knowledge of mass transfer by diffusion, as discussed in Chapter 10. 

In the design of a cooler-condenser for a mixture of vapour and a permanent gas, the method of COLBURN and HOUGEN is considered. This requires a point-to-point calculation of the condensate-vapour interface conditions Tc and Pg. A trial and error solution is required of the equation  

To evaluate the required condenser area, point values of the group l/AT as a function of must be determined by a trial and error solution of equation 9.181. Integration of a plot oi qc against I/IJAT will then give the required condenser area. This method takes into account point variations in temperature difference, overall coefficient and mass velocities and consequently produces a reasonably accurate value for the surface area required. 

The individual terms in equation 9.181 are now examined to enable a trial solution to proceed. Values for hg and kg are most conveniently obtained from the Chdlton and 

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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. 

Figure 7.14 As in Fig. 7.13. Note the presence of orange iron oxides and carbonate. Also note how attack starts as small pits due to dropwise condensation.

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. 

Figure 11.2 Surface activity on the waterside of a condensate line, showing filmwise condensation of water on an unprotected area (a) and dropwise condensation on an area protected by a closely packed arrangement of filming amine molecules (b).

Dropwise condensation process Drum plates, inspection of Drumless delivery Drumless delivery service Dry ice blasting Dry lay-up... 

Drew. T. B. 477, 497, 499, 564, 565 Drift factor 578, 580 Drops, mass transfer 651 Dropwise condensation 476 DRTlNA, P. 307.312 Drying calculations, example 749 Duckworth, R. A. 209. 228 Duct, non-circular 86... 

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. 

Merte, Hf, Jr., Yamali, C and Son, S., A Simple Model for Dropwise Condensation Heat Trans r Neglecting Sweeping, Proc. Int. Heat Trans. Conf.. Vol. 4, pp. 1659-1664, San Francisco Hepiisphere Press, New York, 1986. 

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. 

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