36 Drying of Wood

36.3.2 Kiln Design ............................................................................................................................. 86236.3.2.1 Airflow Considerations........................................................................................... 86336.3.2.2 Moisture-Evaporation Considerations.................................................................... 86436.3.3 Kiln Operation........................................................................................................................ 86436.3.4 Practical Considerations.......................................................................................................... 86636.3.4.1 Schedule Development............................................................................................ 86736.3.4.2 Kiln Control ........................................................................................................... 86736.3.4.3 Volatile Emissions................................................................................................... 86836.3.4.4 Equalization and Stress Relief................................................................................ 86936.3.5 Less-Common Drying Methods.............................................................................................. 86936.3.5.1 Vacuum Drying ...................................................................................................... 86936.3.5.2 Dehumidifier Kilns.................................................................................................. 87036.3.5.3 High-Frequency Electrical Heating......................................................................... 87036.3.5.4 Solar Drying ........................................................................................................... 872References ...................................................................................................................................................... 87236.1 STRUCTURE OF WOOD36.1.1 FORMATION OF WOOD IN T REESWood is produced in the hard stems and branches oftrees and shrubs. In these plants, the primary growthis responsible for the stem and branch elongation,whereas the secondary growth, achieved through thecambium activity, is responsible for the thickening ofelements (Figure 36.1). Wood has evolved to fulfill thebasic needs of these plants during their life: watertransportofwaterbornenutrients;mechanicalstrengthtosupportthephotoactivecanopyofleaves;andresistanceto biological attacks. Appearance and propertiesof this material depend strongly not only on the speciesbut also on the biological diversity and growthconditions of site and climate. Indeed, even for thesame species, wood properties depend on the tree andon the position within the tree. One part of this variabilityis genetically controlled, whereas the otherpart comes from the varying growth conditions (standcharacteristics and silviculture practices). Consequently,wood is a variable material that is extremelydifficult to characterize precisely. Hence wood processing,including drying, is very difficult to optimize.A cross section of a tree (Figure 36.2), from thecore to the outer region, shows the following features:. Pith, a small core of tissue located near themiddle of a tree’s stem or branches, which originatesfrom the primary growth of the plant. Woody material, the most important part of maturetrees, which is differentiated into sapwood(outer region), where the sap migrates from rootsto leaves and heartwood (inner region) that is nolonger used for sap transport, which exists onlywhen the stem, at that height, is old enough. Bark, differentiated into an outer corky deadpart (external part of the stem), whose thicknessvaries greatly with species and age of trees, andan inner thin living part (just near the cambiumzone), which carries food from the leaves to thegrowing elements36.1.1 .1 Knot sAs the tree grows in height (primary growth), branchingis initiated by lateral bud development. Knots arethe bases of branches, which have been covered as thetree grows laterally. After a branch dies, the trunkcontinues to increase in diameter and surrounds thatportion of the branch while the dead branch is stillpresent. This branch has to drop from the tree beforeclearwood can form. If the knot was alive when thetrunk grew around it, the xylem of the trunk and thebranch are continuous and the knot fits tightly intothe wood. If the branch was dead when the trunkgrew around it, no anatomical connection exists betweenthe xylem of the knot and the trunk. The knotis nonadhesive; it may fall out of the wood, leaving aknothole (Figure 36.3).36.1.1 .2 Tiss ue and Cellu lar Structure of Woo dGrowth in thickness of the bark and wood is causedby cell division in the cambium. New wood cells areformed on the inside of the cambium and new barkcells on the outside. In the cambium region, immaturecells differentiate into various kinds of mature xylem(wood) and phloem (inner bark) cells characteristic ofthe species (Panshin and de Zeeuw, 1980). Then enlargement,elongation, and maturation allow thewoody material to be developed (Figure 36.4). Mostof the wood cells stay alive for not more than fewß 2006 by Taylor & Francis Group, LLC.