Paper Drying: A Strategy for Higher Machine Speed. I. Through Air Drying for Hybrid Dryer Sections

Abstract

For all grades of uncoated paper heavier than tissue, the search for new drying technology has recently become directed towards finding combinations of air drying with cylinder drying which have advantageous techno-economic characteristics. Air drying exists in two forms, through air drying and impingement air drying. An advantage intrinsic to through air drying is exceptionally high drying rates because the distance for heat and mass transport processes is reduced from the thickness of the sheet to the dimensions of pores and fibres. Use of through air drying combined with traditional cylinder drying into multiple technique dryer sections is constrained by two basic characteristics, the cost of the pressure drop for air flow through the sheet, and the tendency for through air drying to produce undesirable local moisture nonuniformity during drying. These two characteristics of through drying are documented and strategies for limiting these disadvantages are identified.

Mechanism of Through Air Drying of Paper: Application in Hybrid Drying

“…Thus an alternative strategy for printing and heavier grades which has finally started to receive consideration is use of some combination of air convection drying with hot surface cylinder drying, as summarized recently by Hashemi et al [1,2] For direct contact between the moist sheet and drying fluid there are two basic types of air drying, distinguished by the manner of air-sheet contacting. In one contacting technique the moist sheet passes under the impingement flow field from a multiple array of air jet nozzles.…”

“…[1,2] As investigating a considerable variety of alternatives for such combinations could be carried out most effectively using a dryer simulator that modeled both drying techniques in a hybrid dryer, it would be useful in future to have a model for through air drying that treated thickness direction gradients of the relevant variables. [1,2] As investigating a considerable variety of alternatives for such combinations could be carried out most effectively using a dryer simulator that modeled both drying techniques in a hybrid dryer, it would be useful in future to have a model for through air drying that treated thickness direction gradients of the relevant variables.…”

Drying of paper: A review 2000–2018

“…McGill University has also been active in developing the TAD technology. [145][146][147][148][149][150] Several studies were focused on the variation in grammage and formation that will lead to uneven flow through the sheet, uneven drying and poor quality parameters. To quantify this a drying nonuniformity index was developed and measured for different parameters.…”

“…[150] One conclusion from these studies was that in order to implement the TAD process and avoid high-pressure drops for drying of heavier grades would be to use the cylinder dryer for the first part of the drying and the TAD-process for the final stages of drying. [145,146] Etemoglu et al [151] developed a model for drying of paper with good agreement for TAD and impingement drying.…”

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

“…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].…”