Through drying of machine formed paper and drying nonuniformity

Transport phenomena characteristics were determined for the through air drying of machine-produced printing and heavier papers. The momentum transport is strongly affected by the specifications of machine-formed papers, but of the drying rate characteristics only the critical moisture content varied between the types of paper tested. To determine the impact of individual parameters of commercial papermaking these experiments with machine-formed paper were supplemented by the use of handsheets. The papermaking parameters of beating, wet pressing, formation and pulp blending all strongly affect the momentum transport through the paper but only the quality of sheet formation and composition of TMP-kraft pulp blends influences drying rates measurably.;The general phenomenon of nonuniformity in through drying was examined, including its association with nonuniformity of paper structure, with process conditions affecting drying nonuniformity, and implications for minimizing the cost of providing the flow of drying air through semi-permeable grades of paper. The development and decay of non-uniformity during drying was demonstrated through two techniques, one based on the measurement of local sheet moisture content, the other with continuous monitoring of the local air exhaust temperature from the sheet. Drying nonuniformity is quite sensitive to the quality of formation and to drying intensity, and is less severe at both the higher and lower end of the basis weight range. The biggest effect on drying nonuniformity is the improvement possible by reduction in the initial moisture content. In-plane moisture diffusivity was determined to be a very strong function of sheet moisture content, and is not fast enough to reduce the sheet moisture nonuniformity during drying.;The study provides guidance in the possible future use of through air drying for application to grades of paper heavier than those for which it is now used.