Fiberboards 



2763 



Horizontal continuous digesters (fig. 23-15) are typically made in sizes up to 

 40 inches in diameter and 40 feet in length. The dwell (cycle) time is controlled 

 by a variable speed feed screw which moves the chips through the pressure 

 cylinder. Rotary valves charge and discharge chips into and from the digester at 

 constant rate. Steam is admitted to the pressure cylinder to maintain cooking 

 conditions continuously. 



For uniformity of pulp quality refiners should run at full load, force-fed with 

 chips at a constant controllable rate. Single- or double-screw (fig. 23-16) and 

 coaxial (fig. 23-17) force feeders are typically aranged so that they have a 

 constant oversupply of chips; surplus chips are recirculated back to chip bins. 



Pulp quality is difficult to define and often can be expressed only in terms of 

 fiberboard properties. Important parameters include freeness, fiber-length distri- 

 bution, and springiness. The dominant operating variable is throughput rate 

 which affects specific energy, the energy required per unit weight of pulp 

 produced. Figure 23-18 illustrates the interrelationships between specific energy 

 and various pulp and sheet properties. In the experiment from which figure 23- 

 18 was derived, the factors in parentheses were held constant; arrows indicate 

 relationships between variables; e.g., freeness is changed by increasing or 

 decreasing energy input, which in turn affects pulp properties as indicated by the 

 arrows. 



CHIPS 



FEEDER WITH VARIABLE 

 SPEED DRIVE 



DISCHARGE 



Figure 23-15. — Grenco continuous digester. (Drawing after Textor 1957.) 



