Abstract
In the present work the residence time distribution curves (RTD curves) for a liquid film on an evaporating cylinder are presented for a wiper with cylindrical wiping elements from PTFE having a deep screw thread of a great lead on their surface. The wiping elements bear on the evaporating cylinder due to centrifugal force and roll over it. The analysis of both RTD curves and mean residence times shows that there are three different regimes covering flow of the film over the evaporating cylinder in dependence on the process parameters, like liquid load and the peripheral speed of wiper. Attention is paid to the regime for which the RTD curves have a nonconventional elongated multipeak character with markedly longer mean residence times. A model for liquid transport over the evaporating cylinder is proposed for this regime. The passage channels of the profiled wiper element form the liquid into strands which are then moved down along the screw. Each liquid strand has its elementary distribution curve whereby the number of the elementary curves can be stated from material balance and construction parameters of the wiping element. In summary, the RTD curve is then a linear combination of these elementary distribution curves.