Control of VOC Emissions During Impingement Drying in Web Offset Lithography, 1997 Advanced Coating Fundamentals Symposium Proceedings

At high printing speeds possibly toxic ink solvent vapors may be dragged by the web boundary layer and may escape from the dryer into the press environment. An air-bar design, proposed earlier by Quadracci and Modi [l] for the dryer, that scrubs the boundary layer of solvent gases transported by web motion is examined. The design consists of two symmetrically placed air jets impinging on the web surface. Using computational fluid dynamics(CFD) techniques, a renormalization group theory based k-e turbulence model is used to compute the air flow. The model is first validated against existing slot impingement flow and heat transfer data in the absence of web motion. Subsequently web motion is taken into account, and in particular the interaction of the jet flow with the flow generated by the web motion is determined. For a fixed web speed as the jet speed is increased, there is a strong reduction in the boundary layer gases being dragged along by the web. It is found that a jet speed of at least twice the web speed is needed to largely eliminate any tendency of the web to transport boundary layer gases with it.