Total and partial condensation of binary mixtures under gravity driven film flow

Laminar film condensation at a vertical flat plate is considered under the aspects of total and partial condensation. With the aid of a perturbation approach the heat and mass transfer correlations for total condensation are derived from the governing balance equations, thus avoiding the complex iterative procedure of numerically solving the balance equations. This approach is particularly valuable during partial condensation where the more volatile component accumulates in the vapor phase. If this vapor is also the lighter component, which is most often the case, buoyancy forces lead then to an upward directed vapor flow, whereas the liquid flows down. Only the buoyancy forces of such vapor mixtures, as for example methanol–water, where the more volatile component is specifically heavier, are downward directed as is the liquid film. Buoyancy forces of the vapor reach a maximum for a given suction rate depending on the relevant properties of the mixture. These effects are illustrated on the basis of practical examples.