Modelling of mass and heat transport in dryer fabric structures Part 1. Theoretical model

The dryer fabric influences the mass and heat transfer, separating the hot, moist paper from the ambient drying air. A dynamic model is presented describing this mass and heat transfer in a dryer fabric based on the fabric parameters: effective diffusivity, porosity, tortuosity, calliper, effective permeability, thermal conductivity and basis weight. The model predicts moisture, temperature and pressure profiles. The results show that the mass transfer in the fabric is mainly diffusive and the heat transfer is mainly caused by the diffusive mass transfer. The mass transfer resistance of the fabric is strongly influenced by the effective diffusivity and the fabric calliper. The evaporation from the moist paper increased by 73% when the effective diffusivity was increased three times, compared with the reference case. When the fabric calliper was decreased to half, compared with the reference case, the evaporation increased by 30%. The effective permeability was, however, shown not to significantly influence the evaporation. Including the heat exchange of the dryer fabric in the model facilitates the calculation of condensation/evaporation phenomena in the fabric, such as those occurring in the Condebelt process.