Impingement and through air drying of paper

Air drying of paper by pure impingement, pure through flow and by combination of impingement and through flow was studied. For the combined process the inlet and both outlet air flow rates were held constant throughout drying, thereby providing results appropriate for analysis of the complex and interacting kinetics. Pure impingement drying rate curves were quantified with three parameters which were related to drying process conditions: constant drying rate, critical moisture content and the exponent for a power law relationship over the falling rate period. The Churchill - Usagi asymptotic approach model was tested and found not applicable. For pure through drying the constant drying rate period disappears at higher drying intensities leaving two drying rate periods, the increasing rate and falling rate periods. A theoretically based exponential relationship was obtained for the increasing rate period, where up to 47% of the drying may occur. Drying rate curves were quantified with no subjective judgments, using five parameters: moisture contents at the end of the increasing rate and constant rate periods, constant drying rate, and exponents for the increasing and falling rate period relations. A universal normalized drying rate curve was obtained. Through drying rates are the same with or without impinging jets.