A Hybrid Drying Process: Cylinder Drying with Through Air After-drying

Abstract

BACKGROUND Cylinder drying and air drying of paperHigh paper machine capital costs provide a permanent incentive for increased productivity. Pikulik et al. [1] discussed the evolution of machine speed for printing grades to about 2000m/min. Karlsson [2] considered potential developments for the all too common bottleneck of cylinder drying. Extensive studies at McGill examined the possible integration of impingement air drying with cylinder drying, providing one source concerning multiple technique drying, Bond et al. [3], in hybrid dryer sections, Hashemi and Douglas [4,5]. Hybrid drying for printing paper recently became an industrial reality with a dryer section consisting of alternating cylinder and high intensity impingement air drying at the Nordland Papier mill in Germany, Sundqvist and Anderson [6]. Thus, although impingement air drying was long considered applicable only to tissue and toweling in Yankee dryers, machine builders now propose the integration of these two techniques to produce hybrid dryer sections for heavier grades.The other basic type of air drying, i.e. through air drying, is distinguished by the manner of air -sheet contacting. In through drying the moisture removal is obtained by drawing hot unsaturated air or combustion gas through the permeable web. For the heat and mass transfer of drying, this intimate contact between the drying medium and wet fibres decreases the distance for transport from the thickness of the sheet for cylinder or