2798 



Chapter 23 



Figure 23-38. — Disk trimmer cutting traveling mat to length after its emergence from a 

 wet press. (Photo from Suchsland and Woodson 1985.) 



DRYING MATS FOR INSULATION AND S2S BOARDS 



Insulation-board plants have mat dryers but no hot press. S2S wet-process 

 fiberboard plants have dryers followed by a hot press. In both types of plant the 

 dryer must reduce water content of the mat from about 70 percent of wet mat 

 weight to almost ovendry , i.e. , about 2 tons of water must be evaporated per ton 

 of dry board. The technological benefit of this costly effort is the development of 

 hydrogen bonding in the case of insulation board and the possibility of making 

 high-density S2S hardboard without screens. 



Fiberboard dryers are continuous machines. While it is possible to couple a 

 single-deck dryer directly to the wet press of a Fourdrinier forming machine, 

 space required for such an arrangement is excessive. Modem dryers are typically 

 enclosed multi-tiered roller-conveyor decks, perhaps 525 feet long if forming- 

 line speed is 35 feet/minute and drying time is 2 hours (fig. 23-39). 



Dryer construction. — Drying of fiberboard can be accelerated without 

 scorching the board if temperatures in initial zones of the dryer are very hot and 

 those of succeeding sections progressively cooler. Figure 23-40 shows theoreti- 

 cal air temperatures which afford most rapid drying without harming the board, 

 and actual temperatures commonly achieved. Dryer temperature increases to 

 about 825°F in the first zone, gradually decreasing in following zones to about 

 225°F; zones 2, 3, and 4 circulate air across the dryer, but air in zone 1 flows 

 counter to board flow to minimize heat leakage through the dryer entrance. After 



