Experimental and computational evaluation of mass transfer resistance of paper dryer fabrics

The mass transfer aspects of synthetic dryer fabrics used in multi-cylinder paper dryers were investigated both experimentally and computationally. Experiments were performed in a convective dryer to determine which characteristics of the fabric are instrumental in controlling the drying rate. Results obtained from twelve different fabrics showed that neither the permeability nor the porosity of the fabrics can be used alone to quantify the reduction in mass transfer (evaporation) from a completely moist surface. It was indicated that the microscopic structure of the fabric plays a major role. The fabrics reduced the mass transfer coefficient from the wet surface by about 30-50 %. The reduction is quantified by a new parameter called the Fabric Reduction Factor or FRF. Calculations indicated the reduction in the corresponding heat transfer coefficient to be practically negligible. Computer simulations of the multi-cylinder dryer section were also performed. In the simulations the geometry of the paper machine was varied. The results showed that, regardless of the geometry, the value of the FRF had only a slight influence on the total evaporation occurring in the dryer section.