2934 Chapter 24 



have been clean, simple, and very responsive for good moisture control. Uncer- 

 tainties about supply and cost of these fuels, however, has made it necessary to 

 investigate alternative methods of heating. 



If the plant where the dryer is to be located has a steam generator, several 

 alternatives are available, as follows: 



• Flue gases from a steam generator will usually be at 400 to 600°F in the 

 stack breaching and have substantial volume. These gases can be ducted 

 directly to the dryer furnace. Control of temperature is accomplished by 

 tempering with fresh air, adding additional heat with oil or gas, or a 

 combination of both. 



• Such flue gas for direct firing must be essentially free of combustible 

 materials or they will contaminate the product. If the boiler does not 

 have clean flue gas, then an air-to-gas heat exchanger can be used. This 

 arrangement limits maximum air temperature to about 150°F lower than 

 the flue gas supply temperature. 



• Steam-to-air exchangers are widely used and are more compact than air- 

 to-gas exchangers, for the high latent heat of the steam is available. 

 High-pressure steam is required in the 600 psi (gauge pressure) range to 

 attain air temperatures in the mid 300°F range. Application of steam-to- 

 air exchangers is limited, therefore, to drying of low-moisture-content 

 materials or to multi-stage drying. 



Fuels for steam generators can, of course, be wood wastes available from 

 forestry, lumber, woodworking, and board operation^; there are many such 

 installations, and well-proven equipment is available. (See ch. 26.) Also, both 

 dry and green wood residues can be direct-fired (figs. 26-18 and 26-21) so that 

 steam generation is not a necessary intermediate step. 



SCREENING 



Dry flakes, after passing a fire dump, are discharged from the dryer through 

 an airlock and conveyed to a rotary-drum screen (fig. 24-16). Apertures in 

 screens for face flakes are typically larger (e.g., 5/16-inch) than those in core- 

 flake screens (e.g., 1/4-inch). Wire diameters are typically about 0.054 inch in 

 screens for face flakes and 0.047 inch in screens for core flakes; for long screen 

 life the wire must resist abrasion by the flakes. The screens tumble the flakes and 

 remove excessively strandy material from face flakes (for use in cores), and 

 fines from core flakes (for use as fuel). Typically each screen is revolved slowly 

 by a 10 hp motor with screening capacities and dimensions about as follows: 



Drum diameter 

 and length Hourly capacity 



Feet Tons, ovendry 



6 by 20 7.5 



8 by 20 10.0 



10 by 25 12.5 



As described by Maloney (1977, chapter 10) there are many alternative 

 methods of screening particles, but rotary-drum screens are dominant in the 

 flakeboard industry. 



