3112 Chapter 25 



Reference Subject 



Britt (1978) Effect of turbulence before and after chemical addition on 



drainage and polymeric retention during sheet forming of 

 a 1 : 1 blend of bleached and jointly beaten hardwood and 

 softwood kraft pulps. 



Ershov (1978) Sheet formation of blends comprised of 40- and 60- percent 



hardwood kraft pulp with sulfite pulp. 



Higgins (1969) Advantages and disadvantages, with respect to sheet forma- 

 tion, drainage, and sheet properties of hardwood and 

 softwood pulps. 



Nakata et al. (1978) High-consistency refining of mixed hardwood and softwood 



unbleached kraft pulps related to sheet formation and 

 properties. 



Przybysz and Czechowski (1977) . . Strength of consolidating web during sheet formation of 



hardwood and softwood kraft pulps. 



SHEET DRYING (CONVENTIONAL) 



As paper is formed on a moving wire screen by deposition of pulp fibers 

 suspended in water, most of the water drains through the screen — often aided by 

 suction boxes. The density of the fiber mat deposited on the screen varies locally 

 to some degree, but excessive variation causes uneven drying and a poor quaUty 

 sheet. The mat is further dewatered to near the fiber saturation point by its 

 passage between pairs of nip rolls that mechanically press or wring out most of 

 the free water. 



The sheet is then further dried by heat conduction as it passes over a series of 

 large (e.g. , 6 feet in diameter) drying rolls. Target final moisture content may be 

 from 2 to 9 percent depending on product. The drying rolls or cylinders may 

 have surface temperatures as low as 190°F or as high as 290°F; they are often in 

 the range of 260 to 280°F (Gardner 1967). The paper probably does not attain 

 intimate contact with the dryer rolls; except where thick spots in the mat touch 

 the rolls, a very small airgap generally separates the paper from the roll. 



Dryer rolls are mounted in multiples, the paper threading through them; felts 

 or synthetic screens are commonly used to back up the paper sheet for closer 

 contact between the paper and the hot surface of the roll (fig. 25-17 top). 



The extreme width of modem paper machines (up to 30 feet) aggravates 

 uneven drying across the width of the sheet (fig. 25-17) resulting from moist 

 dead air in pockets between rolls. Metcalfe (1968) described remedies applica- 

 ble only to paperboard, whereby dry air at jet velocities from 3,000 to 10,000 

 feet per minute blows either vertically against the full width of the moving sheet, 

 or parallel to the roll axis. Also effective, and applicable also to thin papers, is 

 pocket ventilation with dry air blown through the more or less permeable backup 

 felts; moist air is exhausted laterally out the front and back sides of the machine 

 (fig. 27-17 top, inset). 



