Mass and Heat Transport in Dryer Fabrics - Experiments and Modelling

Abstract: A dryer fabric is used in the multi-cylinder dryer in order to improve the runnability of the web and to establish proper contact between the web and the hot cylinder, thus improving the heat transport. The dryer fabric separates the hot and moist paper from the ambient drying air and significantly influences the mass and heat transport. The fabric mass transfer resistance was determined experimentally. The fabric mass transfer coefficient varied in the range of 0.0034 to 0.011 m/s and the tortuosity factor in the range of 1.00 to 1.59. It was also shown that these properties and the effective thermal conductivity may be determined numerically by evaluating three-dimensional fabric models; something that will strongly facilitate the design of dryer fabrics. Fabric parameters of effective diffusivity, porosity, tortuosity, calliper, effective permeability, effective thermal conductivity, and basis weight were implemented while developing a dynamical model for mass and heat transport in dryer fabrics. Results show that the mass transport in the fabric is mainly diffusive, and that the heat transport is mainly caused by the diffusive mass transport. When the fabric is cooled below the dew point of the air-vapour mixture, water vapour will condense in the dryer fabric structure and strongly enhance the evaporation due to decrease in water vapour concentration in the fabric. The cooling technique is favourable for webs of basis weight higher than 200 g/m2, i.e. board qualities, in the multi-cylinder dryer; it was shown that the evaporation increased by 22% for the particular case studied having a basis weight of 400 g/m2. A dryer fabric with a lower mass transfer resistance may also be used to increase the humidity in the hood, preserving the machine capacity and thus increasing the potential for energy recovery.