2800 



Chapter 23 



tipple is provided at the infeed. This belt conveyor is hinged at one end of the 

 Hne conveyor level while the other end can be raised or lowered to match up with 

 any of the eight dryer decks, each of which is provided with speed-up sections at 

 the infeed end so mats being delivered by the tipple can quickly catch up to those 

 loaded in the previous sequence. Tipple speed matches line speed (about 40 feet/ 

 minute); therefore dryer speed is l/sth this value or about 5 feet/minute. 



Dryer performance. — Three phases of drying can be distinguished. In the 

 first phase water evaporates from the wet surface of the mat and is resupplied by 

 capillary action from the interior. Under constant conditions drying during this 

 phase proceeds at a high but constant rate. The second phase begins when the 

 surface of the mat starts to dry because water from the interior does not rise to the 

 surface as fast as it is being removed. The temperature of the mat rises during 

 this second phase and the rate of drying decreases. In the third stage, during 

 which drying further slows, water from the interior moves to the surface as water 

 vapor, by diffusion. 



Drying must be controlled to prevent too-rapid initial surface drying, which 

 would create an insulating barrier, severely retarding moisture removal from 

 interior board portions during later drying phases. Excessively high surface 

 temperatures must also be avoided to prevent discoloration of surfaces and to 

 reduce fire hazard. Heat energy required per hour is a function of entering and 

 leaving mat moisture content and temperature, and hourly production of dry 

 fiber. Typically roller dryers are 50 to 75 percent efficient. 



Safety. — Danger of explosion and fire is inherent in high temperature drying 

 of combustible material, particularly when the recirculating drying air is in 

 direct contact with open flames. Protective devices such as automatic burner 

 shut-off and water deluge systems triggered by sensors responding to sudden 

 temperature rises within the dryer, are mandatory. 



800 



400 



30 40 50 60 70 



PERCENTAGE OF DRYING PERIOD 



80 



90 



100 



Figure 23-40. — Theoretically ideal and actual temperature distribution for entry zone 1 

 to discharge zone 4 in a four-zone fiberboard dryer. (Drawing after McMahon n.d.) 



