Drying of planar films and webs (plastic, paper, printing, ...)

Air duct construction detail in dryer.In the printing, plastic, paper and similar converting industries production speed and output is often limited by drying processes either of the film/web itself or of the various coatings that are used to print and otherwise convert the material. While suitable dryers have been in use throughout the history of manufacturing even in modern systems we often see only a simple volume flow and temperature optimization, often neglecting the possibilities that modern CAE and CFD techniques provide.

Consequently there is a big potential in optimizing these drying processes by maximizing the saturation of the fluid phase to transport as much of solvent (or water) as possible with the least amount of volume flow and heating. It is very difficult for engineers to design their dryers without visualizing the fluid flow and even if the CAD system does allow that, it is still often not clear where the actual evaporation - or depending on the process, condensation - takes place.

Our solver for phase changes (mollierSolver) does exactly that for you. We can calculate evaporation and condensations processes for arbitrary substances (solvents, water, ...) for moving geometries and with full consideration of conjugate heat transfer effects (i.e. when solvents evaporate the surface will cool, just like when you hold a wet finger into the wind).

In a recent project with a leading company in the printing business, we looked for possibilities to speed-up the printing process. We analyzed the dryer together with the leading engineer and quality functions and identified throughput bottlenecks to get a thorough picture of the dryer behavior. When production increases there is a point for every dryer when the drying capacity is exceeded. In this case this occurred when the cool surface of the printed film caused condensation of solvent out of the saturated exhaust air in one part of the dryer. After the crucial issue was identified in the simulation and confirmed by measurements, the dryer geometry was optimized in additional simulations.

CFD simulation of condensation and evaporation in a dryer.

The result was an optimized design for the dryer that ensures that the crucial bottleneck was addressed in the most economic way. Instead of hoping that a certain change in the dryer will improve the performance, engineering can now move forward with confidence that the planned changes will address the key issues effectively and efficiently.