MASS TRANSFER OPERATION CONDENSATION.pptx

OPERATION IN INDUSTRIES

1. In filmwise condensation, the film covering the surface acts as a resistance to heat transfer , while in In dropwise condensation, a large portion of the surface is directly exposed to the vapour . Because of this the heat transfer coefficient (hence the heat transfer rates) in dropwise Condensation is four to eight times larger than those for filmwise condensation. The main difference is the vapours are condensed in the form of droplets on the surface of the condenser. while in filmwise condensation the vapours are condensed in the form of thin-film which wets the surface resulting in a lowering of the rate of heat transfer . A laminar condensation on outside surface of a sphere of diameter D is expressed as LAMINAR FILM CONDENSATION ON A Spherical of Diameter Film condensation on ( a ) a sphere

2. LAMINAR FILM CONDENSATION ON A HORIZONTAL TUBE For film wise condensation on a horizontal tube of outside diameter D, the mean heat transfer coefficient ‘h’ is given as Film condensation on, ( a ) a single horizontal tube, ( b ) a vertical tier of horizontal tubes with a continuous condensate sheet ( a ) ( b ) ( N Re <1800)

3. Calculation of Reynolds Number TURBULENT FILMWISE CONDENSATION

4. DROPWISE CONDENSATION In dropwise condensation, the condenser surface does not wet by condensate, the vapour condenses in drops. These droplets grow and merge into one large drop and falls down under the action of gravity During such condensation process, a large part of condenser surface is not covered by condensate film , therefore, the heat transfer coefficient is four to eight times higher than that results in filmwise condensation. The ratio of condensate mass flux for dropwise condensation , m D on outside surface of tube of diameter D to that for film condensation, m f can be related as However, the dropwise condensation can be expected only under controlled conditions , that cannot be always maintained in practice