Experimental Technique for Tracking the Evolution of Local Moisture Nonuniformity in Moist Paper from Wet to Dry

Local nonuniformity of moisture content, a basic characteristic of moist paper, affects efficiency and hence cost of paper drying, and may influence product quality. Such local nonuniformity may become even more of a problem with the current interest in combining higher intensity air convection drying and cylinder drying to produce the required higher capacity hybrid dryer sections of the future. Direct determination of local sheet moisture content under dynamic conditions during drying is unacceptable because the measuring instrument presence would change local moisture content. A novel indirect technique was developed for quantitative, precise determination of local nonuniformity of moist paper by monitoring continuously the local exit pore air temperature at many positions immediately below a moist sheet subjected to air through flow. This technique was used to investigate local non-uniformity for moist machine-formed papers of grammage 19–55 g/m2, and 20–100 g/m2 handsheets of variable formation. The effect of formation and basis weight on local nonuniformity was quantitatively documented. Formation was characterized using the new method of partitioning formation nonuniformity into its components as a function of scale of formation. The results provide some evidence that it is the components of formation nonuniformity in the range of larger scale of formation, 8 to 37 mm, which most affect moisture local nonuniformity while the formation components at 0.8 to 3 mm scale of formation appear less important. Such knowledge is relevant to the development of the improved drying processes of the future.