Fiberboards 2769 



it allows more water to be squeezed out in the first phase of the press cycle. 

 Forming machine consistency is, of course, the same as with conventional 

 pulping, about 1 to 2 percent. The result is a shorter press cycle because the free 

 pulp drains faster on the Fourdrinier and in the press section. 



The reduced density of the mat is a disadvantage in the wet S2S process since 

 it is more fragile, easier to break, and bums more easily. Here, high consistency 

 refining may be applied to only part of the furnish to free up the stock and to 

 speed up the machine. 



Increasing the consistency in single disk refiners and opening the plate gap, 

 causes a pad of pulp to be formed between the plates, which produces a highly 

 satisfactory furnish at lower power consumption. Under these conditions, chips 

 apparently are defiberized by wood-to- wood contact. If consistencies are too 

 high, steam will develop and rupture the pad between the plates. This will allow 

 chips and shives to leave the refiner which ruins the pulp and endangers person- 

 nel (O. B. Eustis^). 



Pressurized refiners are essentially high-consistency refiners. Fiber for dry- 

 process boards is always refined at high consistencies, with no water added; 

 typical consistency for such pulp, measured at refiner outlet, is about 50 percent 

 when chip moisture is about 100 percent (ovendry- weight basis). 



Plate design, accomplished empirically, affects pulp properties but the con- 

 trolling mechanism is poorly understood. Readers interested in plate design and 

 hydraulic action in refiners are referred to Tappi (1971) and Leider and Rihs 

 (1977). 



Selection of refiners for primary chip reduction is based on needed capacity. 

 For example, a 100-ton-per-day insulation board plant would require atmospher- 

 ic refining capacity of about 2,500 hp (100 tons/day x 25 hp days/ton). This 

 could be handled by one 2,500 hp double-disk refiner (e.g. , a Bauer 412) or by 

 three 1 ,0(X) hp machines (e.g. , Bauer 41 1's); the three smaller refiners might be 

 a better choice, because plate changes or other maintenance would not interrupt 

 all fiber production. Secondary refining in such a plant would require only 200 

 hp (1 or 2 hp days/ton), which could be handled by a 32-inch pump-through 

 refiner (fig. 23-13). Some fiberboard plants operate without secondary refiners, 

 doing the complete fiberizing job with primary refiners. 



PULP WASHERS AND SCREW PRESSES 



Pulp washers and screw presses, used only in wet-process plants, remove 

 dissolved solids — primarily hemicelluloses, which, if retained can cause boards 

 to stick to hot-press platens. To maintain processing consistency, the "contami- 

 nated" water is replaced with fresh water. Dissolved solids are the primary water 

 pollutants from wet-process fiberboard plants, and their removal and treatment 

 is costly. (See sect. 23-12.) 



In vacuum pulp washers (fig. 23-22 top) a wire-mesh-covered cylinder 

 revolves in a vat of diluted pulp. Vacuum applied to the interior of the cylinder 

 causes water from the vat to flow through the wire mesh depositing thickened 

 pulp on it. Fresh water supplied by shower pipes outside the pulp-covered 

 revolving cylinder, drawn through it by the vacuum, completes the washing 



