Through drying of paper

Through drying of paper of basis weight 25 to 50 g/m2was studied through 89 experiments with 22 to 90 °C air at mass flow rates from 0.125 to 1.45 kg/m2s. The complete drying rate curves are represented with five quantitative parameters: moisture contents at the end of the increasing and constant rate periods, the constant drying rate, and exponents for the increasing and falling rate period relations. The constant drying rate period disappears at higher drying intensities leaving the increasing rate period accounting for as much as 37% of the drying. The constant drying rate may be controlled by thermodynamics or by transport phenomena. The critical moisture content depends on effects from the intrinsic local nonuniformity of through drying as well as on sheet average conditions. A universal set of relations was obtained for representation of the complete drying rate curve and was verified for prediction of drying rate and drying time. Through drying rates cannot be increased significantly by providing the drying air as an impingement flow.