ABSTRACT
The liquid phase in falling film evaporators represents the main transfer resistance, even if there are several components in the gas phase. For optimal dimensioning of such evaporators it is necessary to describe correctly transfer processes in the falling film which often shows a turbulent flow pattern.
Investigations show that a correct calculation for soluble mixtures depends on the knowledge of heat transfer and also on mass transfer. Great deviations occur in comparison with conventional dimensioning methods which are based on heat transfer controlled processes. Therefore the falling evaporators are often underdimensioned especially for mixtures with high Prandtl numbers using these conventional methods.
Our investigations yield that the discrepancy cited is not caused by mass transfer as proposed above. In fact the heat transfer coefficient at high Prandtl numbers for turbulent falling films is much lower then mentioned previously in literature. Based on a “Three–Zone–Model”, a new Nusselt equation for turbulent film flow is derived and compared with measurements in a wide range of Prandtl numbers. Using this equation, it is shown that mass transfer in the liquid mixture is mostly negligible. Additionally, significant flow of the gas phase increases mass transfer which is very good anyhow. Therefore the mass transfer resistance can usually be disregarded for practical dimensioning of falling film evaporators.
