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The drying of lumber in a fluidized bed of inert solids Veljkovic, Maja

Abstract

The use of fluidized beds of hot inert solids for drying wood is a relatively new concept. Recent investigations on fluidized bed drying of thin veneer (1,2) have shown that more rapid drying can be achieved by this method than by conventional means. In the present work, blocks of Western Hemlock wood, 2 in. x 4 in. x 1 ft. containing 70% to 100% moisture (dry-basis) were dried in a fluidized bed of -20 +30 mesh sand at four levels of bed temperature (175, 190, 204, and 217°F) and three air velocities. The drying time required to reach 15% moisture content (M.C.) was 14-15 hrs. for lumber dried at 204°F as against two or more days generally taken in Kiln drying. The quality of the wood dried at bed temperatures of 204°F and below was not adversely affected. Bed temperature had a strong inverse effect on drying time, while the fluidizing air flow rate had little effect. The diffusion equation was employed to describe the movement of moisture during the falling-rate period of drying and the heat conduction equation to describe the unsteady-state movement of heat inside the drying block of wood. Mathematically, drying was treated both as a one and a two-dimensional problem. The resulting equations were solved on a digital computer to predict the average moisture content and the average temperature of the drying block of wood, each as a function of time. The distribution of moisture content within the drying block was also computed. The calculated results showed a good agreement with experimental data. The economics of fluidized bed drying were estimated and compared with the cost of Kiln drying. The results showed that the capital cost of the fluidized bed system is considerably lower while the operating cost is similar to that for kiln drying.

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