36 Drying of Wood

in separate packages on bearers and loaded into thekiln by a forklift vehicle. The boards are butted up,with their long faces incident to the airflow. The stackis squared off as far as possible to provide a uniformresistance to the airflow, and thus minimize variationsin drying throughout the kiln. In Scandinavian practice,kiln stacks are normally built from boards ofrandom length, so that every second board is placedflush at one end of the stack and the other boards flushat the other end (Salin, 2001). Kilns may be singletrackedwith a 2.4-m wide stack, or twin-tracked withtwo stacks side by side, to yield a double-width stackof 4.8 m. Figure 36.45 illustrates a vertical cross sectionthrough a single-tracked, batch kiln.To obtain a uniform air distribution to the airinletface of the stack as possible, the plenum spacesat each side of the stack must be sufficiently wide(Nijdam and Keey, 2000). An internal ceiling directsthe air through the lumber stack or stacks, with bafflesor curtains to direct the airflow through the lumberpile. Inward-swinging baffles and contoured,right-angled bends to the plenum space from theceiling zone improve the uniformity of this airflow(Nijdam and Keey, 2002). Bypass of air around thestack is minimized by the fitting of side baffles orcurtains. The kiln is designed so that the pressureloss through the heating coils and other ancillaryfittings is small compared with that through thestack of lumber.36.3.2.1 Airflow ConsiderationsKröll (1978) reports the air distribution in a batchdryer fitted with 15 shelves, a heat exchanger, and afan above a false ceiling, which thus resembles a boxtypelumber kiln in a number of respects. In the fifthgap from the top, the air velocity was over twice theaverage, whereas in the top gap there was a smallbackflow with air streaming toward the inlet. Theflow reversal appeared to be the result of a vortexgenerated at the top of inlet plenum below the halfcirclebend out of the ceiling space. Although such airmaldistribution may be extreme in a modern lumberkiln, existing kiln designs may still yield a nonuniformairflow through well-stacked lumber loads, as may beinferred from the kiln audits reported by Nijdam andKeey (1996). Haslett (1998) recommends that the coefficientof variation across the outlet face of the stackshould not exceed 0.12 at velocities through the stackbetween 4.5 and 8 m s 1 when the dry-bulb temperatureis greater than 908C.Industrial rules of thumb generally equate theceiling-space height to the plenum-space width andto the combined sticker-spacing height. Hydraulictests on a model kiln confirm the former rule(Nijdam and Keey, 1999), and the latter is verifiedby a pressure-drop analysis (Nijdam, 1998).The transverse gaps between the boards are notsimple smooth ducts, but there may be irregularitiesRoof ventHeater coilsCeilingspaceReversible fanWeightPlenumchamberPlenumchamberFilletFIGURE 36.45 A vertical cross section through a single-tracked, box kiln. (Adapted from Keey, R.B., Langrish, T.A.L., andWalker J.C.F., The Kiln-Drying of Lumber, Springer, Berlin, 2000.)ß 2006 by Taylor & Francis Group, LLC.