Modelling of mass and heat transport in dryer fabric structures Part 2: Model validation and industrial implementation

The effects of dryer fabric cooling were studied. Using a dryer fabric cooled to a temperature below the dew point of the air-vapour mixture resulted in condensation of water vapour in the fabric structure. This condensation process was studied by means of a small-scale dynamic pilot dryer. The novel model on mass and heat transport in dryer fabrics, described in part 1, was validated against experimental data obtained. It was shown that cooling of the fabric led to strongly enhanced evaporation due to condensation. Evaporation rates increased by up to 61% with this cooling technique. In a case study of fabric temperatures in the range from 30 to 75 degrees C, the highest evaporation rate was obtained at 30 degrees C. Further, the change in thermal conductivity of the dryer fabric influenced the evaporation if both air and water were present. Without air, as in Condebelt drying, the evaporation did not change. These observations were attributed to the. differences in molecular and convective mass and in heat transport between the two cases. Finally, when introducing the cooling technique to the multi-cylinder paper dryer, simulations suggested that the machine capacity could be increased by as much as 34%.