Structural Flakeboards and Composites 2929 



They are then hfted by flights within the rotary drum and dropped into the gas 

 stream again and again, each time advancing a distance along the drum axis 

 determined by the particle weight, which decreases as drying progresses. This 

 action allows time for moisture migration from the interior to the exterior of the 

 particle. Heavy particles may take 5 to 6 minutes to travel through the dryer. The 

 material and hot gases are separated by a cyclone collector at the dryer exit. 



Gas temperatures at the furnace outlet where the wood particles enter the gas 

 stream will reach a maximum of 1 ,800°F to dry flakes with 175 percent moisture 

 content (dry basis) and higher. Normally, with wood at 67 to 100 percent 

 moisture content (dry basis), the inlet gas temperature will be between 700 and 

 1 ,200F. The outlet gas temperature at the cyclone collector will be 160 to 250°F 

 when drying flakes. Wood leaving the dryer will have a temperature of only 130 

 to 140°F; consequently there is little or no degradation of the wood. 



Rotary-drum dryers have evaporative capacities up to 30,000 pounds of water 

 per hour and are generally either one-pass or three-pass types. 



One-pass rotary drum-dryers. — One-pass dryers vary in design, but all 

 retain wet, heavy particles for a longer time in the drying zone than drier, lighter 

 particles. In one design (fig. 24-13) a conical gas inlet carries flakes into a 

 flighting system designed to disperse material throughout the dryer cross-sec- 

 tion; in this design, dryer exhaust gases at 200° to 220°F may be recycled. 

 Recycling of exhaust gases is difficult to accomplish without undue fire hazard. 

 Typically, incoming gas temperature is 550° to 750°F; temperatures depend on 

 species dried. Some dryer specialists suggest that the flighting systems, because 

 they act as flow retarders, break the flakes more than three-pass suspension 

 systems, and wear more rapidly. Single-pass dryers have less throughput and 

 less control over dry flake moisture content than three-pass dryers. 



Three-pass rotary-drum dryers. — The three-pass dryer (fig. 24-14), the 

 type most widely used, has a center cylinder (first pass) with a high entrance gas 

 velocity, up to 1,600 fpm, and high temperatures (e.g., 1,200°F). Successive 

 passes through the middle and outer drum or annulus have lower velocities in the 

 640- to 320-fpm range and lower temperatures, depending on evaporation, with 

 temperature at the exit cyclone of about 220° to 260°F to attain 3- to 4-percent 

 moisture content of flakes. A regulator actuated by the outlet temperature of the 

 gases modulates the furnace burner to control the moisture content of the parti- 

 cles leaving the dryer. 



Thermal efficiency of the three-pass dryer is such that materials in the 100- to 

 185-percent range of moisture content (dry basis) require 1 ,500 to 1 ,900 Btu per 

 pound of water evaporated. Materials in the 18- to 33-percent moisture range 

 require 1 ,900 to 2,300 Btu per pound of water evaporated. Generally, the higher 

 the temperature differential across the dryer and the higher the input moisture 

 content, the greater the efficiency and the fewer the Btu required to evaporate a 

 pound of water. 



