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Chapter 23 



Figure 23-19. — Asplund pressurized disk refiner. See text for explanation of numbered 

 components. (Drawing courtesy of American Defibrator, Inc.) 



determined by level control, a conveyor screw (7) at the bottom of the preheater 

 removes the softened chips and feeds them into the center of the single-revolv- 

 ing-disk mill (8). A gap, variable from 0.008 to 0.016 inch, is maintained 

 between the stationary disk and the revolving disk. Pulp is discharged from disk 

 periphery to atmospheric pressure through valves or blow pipes that permit only 

 minimal steam loss. Total dwell time in the system depicted (fig. 23-19) is about 

 1 minute for fiber suitable for wet-process hardboard. Basic specifications of 

 such machines are as follows (maximum disk speed is about 1,800 rpm): 

 Disk diameter Minimum motor size Capacity 



Defibrator pulp is only slightly darker than the parent wood, consists of 

 individual fibers with undamaged walls, and is free, springy, and bulky. Defi- 

 brator pulps are used for both wet- and dry-formed hardboard and for medium- 

 density fiberboard. Like Masonite pulp, defibrator pulp cannot be fibrillated and 

 is, therefore, unsuited for paper manufacture unless the equipment is used in 

 conjunction with chemical treatments of wood chips (see sects. 25-5 and 25-6). 



The Sprout- Waldron pressurized refiner (fig. 25-21 bottom), like the As- 

 plund is a single-revolving-disk machine with capacities up to 200 tons per day 



