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.

Keywords:

• Through drying
• Paper
• Multiple technique drying
• Hybrid dryer
• Nonuniformity
• Formation

ACKNOWLEDGMENTS

Although there are only two co-authors of the present paper, this work rests on the results from a sustained effort in this laboratory to understand the fundamentals of the through air drying of paper. The contributions to this continuing program from the theses of former students Drs. Osman Polat and Guohua Chen are recorded in the text. In addition, it is a pleasure to acknowledge the important influence from the participation of Dr. R.H. Crotogino during this substantial period, an influence reflected in his co-authorship of numerous of the publications listed from this laboratory.