Through drying of paper

The first comprehensive set of measurements is obtained for rates of through drying paper with all drying conditions measured and controlled. Complete pressure drop-drying rate curves are determined for 235 handsheets of kraft paper of basis weights 25-150g/m$ sp{ rm 2}$ dried in air over a wide range of temperatures and throughflow rates.The previous procedure of calculating permeability by Darcy's law is substantially in error at relevant throughflow rates. A new characteristic dimension for flow through both dry and moist paper is determined by application of fundamental momentum transport principles. During through drying the value of the d$ sb{ rm p}$ drops substantially from an upper to a lower asymptotic limit, shown to correspond to known water-fibre relations. A theoretically based Re-f-d$ sb{ rm p}$ treatment is shown to be a more powerful method than those used previously.A comprehensive, tested correlation for through drying rates in the constant rate period is reported. Through drying is demonstrated to start with an increasing rate period during which, for industrial conditions, about half the water is removed. An original treatment, "drying period diagrams", shows the extent of the increasing, constant and falling rate periods as a function of drying conditions. The concept of an increasing-constant-falling rate triple point, X$ sb{ rm ICF}$, shows that typically there will be no constant rate period for industrial conditions of through drying light weight paper of low moisture content.Sherwood number is calculated for the constant rate period of drying using the characteristic dimension and interfacial transfer area for moist paper determined by momentum transfer analysis. Sherwood number is related to drying conditions and to transport phenomena within the sheet.Through drying rates are successfully predicted by a model which treats the process as three drying periods, i.e. increasing, constant and falling rate, with a variable specific surface of paper