COMBINED IMPINGEMENT AND THROUGH AIR-DRYING OF PAPER

For this combined drying process the fundamental characteristics were determined using a unique facility in which air convection drying of p aper could be investigated over the full range from pure impingement t o pure through drying. As well as light weight paper the program teste d heavier sheets, of printing paper grammage, which are not currently dried industrially by either impingement air or through air drying. An alysis is complicated by the two component processes becoming nonadiab atic when carried out simultaneously. Relative to pure impingement dry ing, for the combined process the water removal rates in the impingeme nt exhaust are sensitively reduced by through flow. Through flow exhau st water removal rate curves are strikingly different from pure throug h air drying by the addition of a new rate period, termed the secondar y increasing rate period, when the paper temperature is driven up by t he falling rate period of impingement water removal. Parameters of the models for the combined process were related to process conditions. P redictions from this model are consistent with the earlier limited pil ot plant results. Relative to pure impingement drying, the removal of even a small fraction of the drying air as through flow exhaust result s in substantial reduction in drying time for the process of combined impingement and through air drying of paper.