Suspension Drying of Sawdust

An experimental suspension dryer was constructed and tested at the Oregon Forest Products Laboratory. Drying variables were studied to obtain design information for a sawdust dryer. Potential applications were considered. Two principal methods are used for drying granular products such as the rotary drum and suspension, entrained, or flash drying. Suspension dryers require less floor space and there is little contact between the material and hot surfaces, thus minimizing scorching. There has been little published information on the performance of suspension dryers for sawdust. Drying principles are explained as they relate to the temperature and humidity of the heated gas, the exposure of the product to drying conditions, and the physical characteristics of the product. Dryer performance with as-received sawdust is presented for 4,180 lb. air/sq. ft./hr. and 6,130 lb. air/sq. ft./hr. Tests at higher air rates resulted in lower efficiencies (600?F. air flowing at 4,180 lb./sq. ft. /hr. gives a superficial velocity of 1,870 ft./min.) It appears that the lowest air rate that will convey the sawdust would give the best performance. Final moisture was lowered as gas temperature was raised, but maximum drying efficiencies did not vary greatly with initial temperature. However, dryer capacity was more than doubled for the same air rate by raising initial gas temperature from 600? to 1,200?F. The effect of initial moisture content of sawdust on dryer performance is given for various sawdust feed rates and two air rates. The effect of particle size on dryer performance is also given for various sawdust feed rates and two air rates. Applications of results to an independent dryer and a dryer associated with a steam plant are discussed.

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