Impingement drying of paper

The complete drying history of paper dried under an array of multiple round jets of air was determined. The drying rate - paper moisture content relations, determined as a continuous function by on-line measurements for about 80 sheets, permitted examination of the following parameters: basis weight 20 to 50 g/m2, which covers the tissue to communication paper range; initial moisture content 1.5 to 3 kg water/kg fibre; nozzle to paper spacing of 5, 7.6 & 8.5 nozzle diameters; nozzle plate open area ratio from 1.4 to 3.1%; jet Reynolds number in the range of 450 to 11,100. Three methods of quantifying the complete drying rate curves were tested. Use of the common assumption of drying rate linear with moisture content over the falling rate period gave inadequate representation. Secondly, the general method of Churchill for any transport process that has a transition between two asymptotic relations was applied for the first time to the drying of paper. This approach gives statistically valid fits, but was rejected because the values of the constant drying rate were generally too high and because the method does not give a critical moisture content. A power law treatment of the falling rate period provided a good representation of the complete drying rate curves as well as realistic values of the constant drying rate and critical moisture content. To reduce subjectivity, a statistical program provided for each drying rate curve the best fit values of three parameters, the constant drying rate, critical moisture content and the exponent for the power law falling rate period relationship. With the general correlations of these three parameters for the experimental conditions studied, drying time may be successfully predicted for a wide range of drying conditions.