Investigation of an Energy-Saving Strategy in a Rotational Paper Dryer by Utilizing Hydrophobic Coating

In the paper industry, superheated steam inside rotating dryer cylinders removes moisture from wet paper wrapping the cylinder’s exterior. The steam condenses into water, forming condensate that is removed using stationary or rotary siphons. The condensate flow inside the rotating cylinder progresses through puddling, cascading, and rimming phases, directly impacting condensate extraction, heat transfer, torque, and power consumption. Heat transfer is highest during puddling but diminishes in the rimming phase, where the condensate forms a layer along the inner wall, introducing thermal resistance and reducing heat transfer. This study aimed to explore the feasibility of implementing a layer of hydrophobic coatings on the inner surfaces of paper dryer cylinders to remove the condensate rimming layer to achieve greater energy efficiency in the drying process. This investigation started by examining the difference in the total thermal resistance by replacing the condensate rimming layer with the hydrophobic coating inside the rotating cylinder. Computational analyses were followed by using a transient multiphase Volume of Fluid (VOF) model to simulate air-water mixtures in a 2D rotating domain. Results showed hydrophobic surfaces delayed rimming onset, keeping the upper cylinder portion dry. Finally, an experimental test rig was developed by utilizing a clear polycarbonate cylinder driven by a variable-speed motor. Different hydrophobic coatings were applied on the inner surface of the polycarbonate cylinder to make it superhydrophobic. The experimental results were exhibited with a simultaneous display of flow behavior inside the cylinder, rotating speeds, and power consumption which successfully demonstrated that the rimming speed with a hydrophobic surface was much delayed, implying that condensate rimming could be prevented in the real operation, and thus heat transfer could be augmented, and drying energy be reduced.