Structural Flakeboards and Composites 



2931 



Figure 24-14. — Rotating three-pass drum dryer. (Bottom) Major components. Infeed of 

 furnish and hot air to inner drum on left; discharge from outer drum to hot cyclone (to 

 separate hot gases from the dry particles) and final cyclone on right. (Top) Flow-path 

 of furnish from hot inner drum to intermediate drum to cooler outer drum. Baffles 

 cause particles to tumble as they pass through the three drums. Outfeed end shown. 

 (Drawings after Stillinger 1967.) 



Typical statistics for a three-pass rotary-drum dryer for green, 0.020-inch- 

 thick southern hardwood flakes vary with throughput as follows (personal corre- 

 spondence May 3, 1982 with B. Emt, MEC Company): 



Statistic 



Inlet gas temperature, °F. 



Drum speed, rpm 



Drum diameter, feet . . . . 



Drum length, feet 



System horsepower 



Throughput of flakes/24 hours (dry-weight 

 basis; 3- to 4-percent moisture content) 



240 tons 



900 



7.5 



12 



48 



250 



325 tons 



1,000 



7.5 



12 



60 



310 



Dryer feed. — Particle size should be reasonably consistent; for example, 

 pulp chips and sawdust should not be mixed. Included with flakes, however, up 

 to 15-percent fines can be dried without difficulty. If woods of different moisture 

 contents are to be mixed before drying, they should be stored separately under 

 roof, and the mix accurately metered and thoroughly blended before admission 

 to the dryer. 



The material to be dried must be admitted to the dryer through a rotary valve 

 or other type of air seal to prevent induction of cold air. Although most infeeding 

 devices such as screw-feed or belt-feed equipment deliver a uniform volume of 

 material, a better system would provide a constant rate of input based on weight. 

 Gradual changes of infeed rate cause no particular problem, but sudden changes 

 cause the final moisture content of the output material to fluctuate. Induced air 

 for combustion and conveying must be clean; induction of dust-laden air pro- 

 motes fines and explosions. 



