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 judgements, 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. In combined impingement and through air drying, an adiabatic process, removal of impingement boundary layer humidity by through flow makes both components of the process nonadiabatic. Relative to pure impingement drying, impingement water removal rates are sensitively reduced by through flow. Because critical moisture content is intrinsically lower for through drying than impingement drying, through flow water removal rate curves can be strikingly different from pure through drying, with addition of a unique feature, a secondary increasing rate period when paper temperature is driven up by the falling rate period of impingement water removal. Parameters for quantitative representation were determined, analyzed and related to drying process conditions. Of two prior models for the combined impingement and through air drying of paper, trends assumed by Randall (1984) are now found contrary to actual rates while the Crotogino and Allenger (1979) model has the trends correct but over predicts the drying rates now measured. Results of the pilot plant work of Burgess et al. (1972) are consistent with the comprehensive model now developed to predict drying time and drying rate for combined impingement and through flow air drying of paper.

Douglas, W. J. M. (Supervisor)