Aerodynamic Resistance
Thin regions of slow moving fluid form immediately adjacent to the surface of a moving web during high speed coating and drying processes. Proper management of boundary layer airflow dictates the efficiency of solvent evaporation and the uniformity of moisture removal in paper production. This layer acts as a thermal and mass transfer barrier that must be overcome to reach the desired processing speeds.
Mechanical Disturbance
Active disruption of the stagnant air film using high velocity nozzles or turbulence generators increases the rate of drying by bringing fresh, dry air into direct contact with the substrate. Without sufficient control, boundary layer airflow causes uneven temperature profiles across the width of the machine, leading to moisture streaks or cockling in the finished paper. Increasing the nozzle pressure substantially improves the heat transfer coefficient.
The thickness of the layer depends on the surface roughness of the paper and the linear speed of the production line.
Transport Efficiency
Static air carried by the moving web can also introduce contaminants or cause air entrainment at the nip where two surfaces meet. In high speed lamination, boundary layer airflow often traps micro bubbles between the film and the substrate, which results in visible silvering or poor adhesion. Installing air foils or specialized rollers can help strip away this boundary layer before it reaches the critical bonding point.
Successful removal of the trapped air ensures a smooth, void free finish that meets the requirements for high quality graphics. Careful placement of these components reduces the power consumption of the drying system by allowing for lower overall air temperatures.
