In the manufacture of paper it is necessary to remove all water from the paper before it is rolled up. The majority of water is typically removed by squeezing the paper between large rollers. The remaining moisture can be removed by forcing hot air through the paper to accelerate its evaporation. Using heated air can be an expensive process so there is interest in investigating optimum arrangements for achieving the fastest and least expensive means of removing the residual water from paper. Both the paper and the fabric backing are porous materials. The support blocks may not be porous. It is expected that the permeability of the fabric and paper will be a function of their water content.
This report describes a software development that allows for a realistic treatment of the drying process in porous media. A description is given of the new model, which has been validated with available data. This data does not cover the entire range of moisture content or airflow rates that are typically encountered in practice. Consequently, it may be found necessary to make some small model adjustments. It is hoped that the formulation of the model, which is based on simple physical principles, will be easy to adjust to fit a larger range of observations.
Before describing the new model, a better perspective of its capabilities can be appreciated by noting some of the ways that it differs from previous work on paper drying. Most importantly, the new model considers the paper as having finite thickness and properties such as moisture content, temperature and vapor concentration that vary through the thickness. Thus, the paper may have dried on the upstream side, but still be completely wet on the opposite side. Another difference is that air and water vapor constitute a two-component gas that is compressible. Compressibility means, for example, that the gas velocity in the paper must increase in the direction of flow because of the decrease in pressure through the paper (i.e., density and/or temperature must decrease by expansion to give a pressure decrease). Since flow velocity has an important effect on drying rate, compressibility can influence local drying conditions in the paper. Finally, by computing transient conditions throughout the paper, this model could be used to investigate arbitrary non-uniform initial conditions or paper with a non-uniform thickness and/or porosity distribution.
C.W. Hirt, “Moisture Drying Model,” October 2014, Flow Science Report 05-14, Copyright Flow Science