Simulation of Paper-Drying Systems With Incorporation of an Experimental Drum/Paper Thermal Contact Conductance Relationship

Abstract

A theoretical model for simulation of conventional steam-heated cylinder dryers is developed by considering the heat and mass transfer in a porous sheet during drying. Expressions for sheet shrinkage as a function of mass of water removed and for reductions in sheet porosity are derived for inclusion in the model. The interface thermal contact conductance of moist paper handsheet/metal interfaces has been experimentally investigated. A resulting empirical correlation, representing the thermal contact conductance between the cast iron dryer surface and paper web, is incorporated into the drying simulation model to reflect reductions in heat input to the sheet during drying. Finite difference techniques are used to obtain the numerical solutions. Average sheet moisture content and temperature along the length of the dryer section as well as average evaporation rates per cylinder are predicted by the model. Consideration of the internal dynamics of the drying process allows profiles of sheet moisture content, temperature, liquid flux, and vapor flux through the sheet thickness to be developed throughout the dryer section. Drying results are consistent with actual production cases. The model can be used to design dryer sections, study changes in operating conditions or in layout of a multi-cylinder dryer, or simulate the application of enhanced drying devices to a conventional drying section. The effect of the drum/paper contact conductance on drying rates and resultant dryer section requirements is provided as an example application of the model.