Why is Press Drying / Impulse Drying Delayed? A Critical Review 1989, 1990 Papermakers Conference Proceedings

Ersnst L. Back--For a decade, the great potential or press drying/impulse drying has been pointed out with regard to: high outgoing solids content, high strength by paper densification, and high drying efficiency with low investment. These advantages have, however, been exaggerated by: - a comparison with a cold single felted nip, whereas one or several hot and often double felted, extended nips now is the best available technique for comparison. - the use of nip dwell times for which today or even tomorrow machinery is not available at modern machine speeds. - low speed experiments in pilot plant with too slow a deloading of applied pressure and too high an outgoing solids content, in whch experiments thereby the tendency to web delamination and microcracking has been partly overlooked - neglecting the significant loss in brightness with mechanical pulps against a hot surface exceeding 150 to 200 „a C at relevant nip dwell times and the loss in opacity for other printing papers. This critical review approaches these problems with experience from hot wet pressing and from commercial single nip press drying of building board. It specifies conditions in respect to wet z-strength for preventing web delamination. It illustrates means to prevent delamination in fiber building board production. It indicates that with present extended nips and within the temperature limits of delamination, no considerable gain in solids content and densification appears when comparing with hot wet pressing using e.g. steam heating of the web. The proposed interpretation of the press drying mechanism, in which vapour formed at the hot side is said to blow water out of the web into the felt, is questioned. A paper web even at thigh solids is very compressible. This compressibility increases rapidly with increasing temperature. Thereby the pressure sustainable by the fiber network and the mean pore size both fall off with increasing temperature. Thus additional hydrodynamic pressure for dewatering is formed. This is illustrated by experimental data on static wet pressing. Press drying, up to about 70% outgoing solids here is interpreted as a superhot wet pressing with web layers containing superheated water, which water partly evaporates on deloading at nip exit. This approach is similar to that for commercial press drying of hardboard, for which data on heat flux, heat pipe effect and evaporated water are included.