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Chapter 23 



FULL EXHAUST GAS RECIRCULATION LINE 



HIGH -TEMPERATURE 

 EXHAUST GASES 



CYCLONE 

 SEPARATOR 



WET FURNISH 

 NLET 



=^ 



DRY f FURNISH 

 ROTARY DRUM DRYER 



^ 



MATERIAL HANDLING 

 FAN 



Figure 23-56. — Drum dryer with full recirculation of exhaust gas. (Drawing after Junge 

 1977.) 



controlled relatively easily by installation of cyclones, filters, and scrubbers. 

 Control of the blue haze caused by evaporation of volatiles is more difficult. 

 This evaporation could be greatly reduced by lowering drying temperatures, but 

 this severely reduces dryer productivity and efficiency. The only other alterna- 

 tive is complete recirculation of exhaust gases to the dryer combustion chamber. 

 The disadvantage of this system is the higher energy input required because of 

 the high temperature of the exhaust gases vented from the combustion chamber 

 (fig. 23-56). 



Forming. — The basic difference between wet forming in water and dry 

 forming in air is the much lower density of air. Some of the typical difficulties of 

 dry forming arise from the difference; e.g.: 



• Fibers remain in supsension in air only at considerable air velocities and 

 will promptly settle out when the air flow slows down. 



• A fiber- air suspension does not flow laterally on a horizontal support. 



Another important characteristic of dry fiber furnish is its tendency to congre- 

 gate and form lumps as soon as the concentration of the fiber-in-air suspension 

 exceeds certain limit values. 



The technical and patent literature offers many approaches to achieving a mat 

 of uniform density from air-suspended fibers (Swiderski 1963; Sandermann and 

 Kunnemeyer 1957). These devices either deposit the furnish by gravity or filter 

 the fibers out of the fiber-air suspension. There are also examples of combina- 

 tions of these principles. 



The first dry process (actually semi-dry) fiberboard plant in the United States 

 at Anacortes, Wash, employs a gravity forming machine which was developed 

 and patented by the Plywood Research Foundation. This lines does not use a 

 dryer, and the forming machine is a felting box above the forming belt. The 



