36 Drying of Wood

36Drying of Wood: Principlesand PracticesPatrick Perré and Roger B. KeeyCONTENTS36.1 Structure of Wood ............................................................................................................................... 82236.1.1 Formation of Wood in Trees .................................................................................................. 82236.1.1.1 Knots ...................................................................................................................... 82236.1.1.2 Tissue and Cellular Structure of Wood .................................................................. 82236.1.1.3 Heartwood Formation............................................................................................ 82536.1.2 Chemical Composition ............................................................................................................ 82636.1.3 Reaction Wood and Juvenile Wood........................................................................................ 82736.1.3.1 Compression Wood ................................................................................................ 82836.1.3.2 Tension Wood......................................................................................................... 82836.1.3.3 Juvenile Wood ........................................................................................................ 82836.1.4 Implications for the Drying Process........................................................................................ 82936.2 Board Scale .......................................................................................................................................... 83036.2.1 Water in Wood: Sorbed and Capillary Water, and Shrinkage................................................ 83036.2.1.1 Moisture Content of Wood .................................................................................... 83036.2.1.2 Free Water .............................................................................................................. 83036.2.1.3 Bound Water........................................................................................................... 83136.2.1.4 Differential Heat of Sorption.................................................................................. 83136.2.1.5 Shrinkage ................................................................................................................ 83236.2.2 Heat and Mass Transfer in Wood........................................................................................... 83336.2.2.1 Fluid Migration in Wood: Single-Phase Flow ........................................................ 83336.2.2.2 Generalized Darcy’s Law: Multiphase Flow........................................................... 83536.2.2.3 Capillary Pressure ................................................................................................... 83636.2.2.4 Bound-Water Diffusion .......................................................................................... 83736.2.2.5 Physical Formulation.............................................................................................. 83936.2.3 Process of Drying .................................................................................................................... 84036.2.3.1 Low-Temperature Convective Drying .................................................................... 84036.2.3.2 Drying at High Temperature: The Effect of Internal Pressure on Mass Transfer.. 84136.2.3.3 Typical Drying Behavior: Difference between Sapwood and Heartwood .............. 84236.2.4 Mechanical Aspects of Wood Drying ..................................................................................... 84536.2.4.1 Mechanical Behavior of Wood ............................................................................... 84536.2.4.2 Drying Stress Formulation ..................................................................................... 84636.2.4.3 Memory Effect........................................................................................................ 84736.2.4.4 Stress Development during Drying: Some Examples.............................................. 85036.2.5 Drying Quality ........................................................................................................................ 85536.2.5.1 Factors Affecting the Drying Duration .................................................................. 85536.2.5.2 Factors Affecting the Drying Quality ..................................................................... 85636.2.5.3 Criteria for Obtaining a Fast and Good Drying Process........................................ 85636.3 Kiln Scale ............................................................................................................................................. 85736.3.1 Lumber Quality....................................................................................................................... 85736.3.1.1 Gross Features of Wood......................................................................................... 85836.3.1.2 Intrinsic Features of Wood..................................................................................... 86036.3.1.3 Sawmilling Strategies .............................................................................................. 862ß 2006 by Taylor & Francis Group, LLC.

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.

Primary growthSecondannual ringFirstannual ringPith1 mmThickening of the structure by secondary growthFIGURE 36.1 Formation of wood in trees: the pith originates from the primary growth whereas the wood material is added,along the years, by the secondary growth. (Microphotograph: polished disc of Douglas-fir (Pseudotsuga menziesii), LER-MAB–ENGREF.)Age of wood cellsPithAge of treeBarkHeartwoodformationCambial activitydivision,elongation,differentiation,lignificationLTRFIGURE 36.2 Cross section of a tree showing the internal structure of the stem. Growth rings can also be observed: light partsare earlywood and dark parts are latewood. Due to this stem geometry, three material directions: longitudinal (L), radial (R),and tangential (T), can be defined at each location. (Photograph: Yew (Taxus baccata L.), LERMAB–ENGREF.)ß 2006 by Taylor & Francis Group, LLC.

Clearwood formedafter pruningDeath of thebranchNonadhesive partof the knotAdhesive part ofthe knotPithFIGURE 36.3 The existence of knots in wood comes from the interaction between primary growth, responsible for thebranch formation, and secondary growth, responsible for thickening of stem and branches. (Photograph: Scots pine (Pinussylvestris), LERMAB–ENGREF.)weeks; the last development stage, namely lignification,induces the death of these cells. Only parenchymacells can live for years and they are responsible for thedevelopment of heartwood.In most species in temperate climates, the differencebetween woods that are formed early in a growing season(earlywood) and that formed later (latewood) issufficient to produce concentric contours in a crosssection (Figure 36.2). These rings are known as growthrings. Each increment in size in the branch or trunkdiameter can be observed in these growth rings thatremain unchanged once formed. Provided no falserings exist (due to an interruption of the growth indiameter by drought or defoliation by insects), the ageat any cross section of the trunk may be determined bycounting these rings. Obviously, this simple rule doesnot apply for tropical species for which growth may bepractically continuous throughout the year and no welldefinedgrowth rings are formed, or for which growthringsare the result of the individual rhythmofeachtree.Wood cells are of various sizes and shapes. Theyare cemented together to form the structural woodmaterial. The majority of wood cells are elongatedand pointed at the ends. The types and dimensionsof wood cells depend strongly on the species.In softwoods, woods formed by cone-bearing trees(e.g., fir, pine, and spruce) with naked seeds, the xylemcontains mainly tracheids (90%). Tracheids are considerablyelongated cells (around 40 mm in diameterand between 2 and 8 mm in length), which ensure bothsap flow, by means of numerous bordered pits situatedon the radial cell walls, and mechanical strength.In softwoods, the earlywood is characterized by cellswith large radial diameters and thin walls, and hencerelatively large cavities. Latewood cells have a muchsmaller radial diameter and thicker walls, which resultin much smaller cavities (Figure 36.4). In addition,some softwoods have resin canals. Parenchyma cellssurround these canals and actively secret resin into thecanals, and ultimately into the heartwood.Hardwood is the common name for the wood ofspecies whose seeds are enclosed in ovaries. Thesespecies are more advanced than softwoods in termsof biological evolution; consequently, they produce amore sophisticated anatomical pattern, with cellsmuch more adapted to meet specific requirements inrelation to water transport, food storage, and mechanicalsupport:. Fibersareusuallyrelativelythick-walled,sparselypitted, and about 1 mm in length. Different fibercells exist, but the tracheid fibers having borderedpits are generally the most abundant. Althoughfibers may have a certain role in sap conduction,theybasicallyfunctionasthemechanicalsupport,making the wood usually stronger, denser, andmore durable than softwoods.. Vessels of relatively large diameter are alsoknown as pores. These cells form the main conduitsfor sap flow. A vessel is built up by severalvessel elements, with more or less open endplates, aligned in the longitudinal direction.ß 2006 by Taylor & Francis Group, LLC.

FIGURE 36.4 Wood is formed by cell division in the cambium zone, hence the radial cell lines appear clearly in this figure, inspite of the large variation of radial diameter between earlywood and latewood. Some bordered pits allowing sap flow fromone tracheid to the other can also be observed on radial cell walls. (ESEM Photograph: Norway spruce (Picea abies),LERMAB–ENGREF.). Longitudinal or axial parenchyma cells functionmainly in the storage of food.Several thousands of hardwood species exist, andeach one has its own anatomical pattern. The density,for example, ranges from less than 100kg m 3 (i.e.,balsa wood) up to more than 1200kg m 3 (i.e., ebonywood). They are usually divided into ring-porous anddiffuse-porous types, though all intermediate typescan be found:. A ring-porous species produces very large vessels(up to 500 mm in diameter) in earlywood.Simple perforation plates allow the vessel cellsto communicate easily.. Diffuse-porous species have smaller vessels (50to 100 mm in diameter) of almost uniform sizeand distribution throughout the growth ring. Indiffuse-porous species, vessel cells are usuallyconnected by scalariform (‘‘ladderlike’’) perforationplates.Figure 36.5 depicts the typical anatomical patternsencountered in temperate species.Both hardwoods and softwoods have cells (usuallygrouped into structures or tissues) that areoriented horizontally in the radial direction andwhich are called rays. The rays, composed of parenchymawith lignified cell walls or sclerenchyma, connectvarious layers from pith to bark for storage andtransfer of food. In softwoods, rays are one-cell thick.In hardwoods, they vary in size from one-cell wideand a few cells high to more than 15-cell wide andseveral centimeters high. Rays represent planes ofweakness along which drying checks develop easily.36.1.1 .3 Hear twoo d F ormationJust few weeks after the formation of xylem, it containsmostlydeadcells,butcontinuestoplayaroleofutmostimportance for the plant: the transport of sap. Hencethevascularsystemsoproducediscalledsapwood.Thisactive zone may vary in thickness and number ofgrowth rings, commonly up to about 15 years, whichrepresents several centimeters in radial thickness. As arule, the more vigorously growing trees have more extensivesapwood. In sapwood, parenchyma cells stayalive and function primarily in the storage of food.ß 2006 by Taylor & Francis Group, LLC.

(a) Norway spruce (Picea abies) (b) Pedunculate oak (Quercus rubra) (c) European beech (Fagus sylvatica)FIGURE 36.5 Typical anatomical patterns encountered in temperate species: (a) softwood, (b) ring-porous species, and (c)diffuse-porous species. The height of these images represents about 2 mm. (Microphotographs: J.C. Mosnier, LERMAB–ENGREF.)After some years, an intense biological activity ofthese parenchyma cells gives rise to heartwood formation.Metabolites are deposited in the heartwoodand the tree uses the heartwood as a place to storewaste products that are collectively known as extractives.These include resins, gums, oils, and tanninsthat stop up the vessels (or the tracheids in softwood)and clog the wood. In heartwood, all the cells aredead and inactive; they do not function in eitherwater conduction or food storage. The heartwood isoften darker, slightly denser, and more durable (resistanceto fungi or insect attack) than the sapwoodand plays an important role in supporting the tree.However, numerous species do not have dark heartwood(e.g., spruce, fir, and beech) and no correlationexists between heartwood color and durability.In some species, such as certain oaks, the vesselsbecome plugged with the development of cellularmembranes known as tyloses, which enter the vesselsfrom adjacent parenchyma cells (Figure 36.6).36.1.2 CHEMICAL C OMPOSITIONWood is a typical organic material made up of threemain elements: carbon, oxygen, and hydrogen. Becausevery small variations between different wood speciesare observed, the numbers depicted in Table 36.1 arebroadly general but can be higher for some tropicalspecies. Nitrogen as well as some additional inorganicelements (sodium, potassium, calcium, magnesium, andsilicon) are also essential compounds, which are mostlyinvolved in the metabolism of living cells during woodFibersRay cellsTyloses invesselsRTTLFIGURE 36.6 Example of tyloses development in the heartwood of Pedunculate oak (Quercus rubra L.). (ESEM photographs:Patrick Perré and Riad Bakour, LERMAB–ENGREF.)ß 2006 by Taylor & Francis Group, LLC.

TABLE 36.1Elementary Composition of WoodElement Content (%)Carbon 49–50Hydrogen 6Oxygen 43–44Nitrogen

softwoods, compression wood is found in the lowerpart. However, the compression wood can also befound in some primitive groups of hardwoods (Carlquist,2001).36.1.3 .1 Com pression Wo odCompared with normal wood, this tissue is characterizedby shorter tracheids, higher lignin and hemicellulosecontent, and lower cellulose content. Thisreaction wood is easily identified on smooth surfaces,in particular in a transverse view. When compressionwood is formed, the growth rings appear darker,reddish brown, and often wider than on the oppositeside. Therefore, when compression wood develops inthe same side for several years, the cross section of thestem tends to be oval with an eccentric pith in thecore; this is typical of branches or stem of bent trees.At the anatomical level the cells observed in crosssection are more rounded than rectangular, showinglarge intercellular spaces (Figure 36.7). The cell wallconsists only of ML, P, S 1 , and S 2 layers. Once dry,the cell wall shows deep, helically arranged checksfrom the lumen. The latter is rather thick and itsmicrofibril angle is much larger than in normalwood (about 45 8 from the axial direction). In consequence,the density is higher, the longitudinal shrinkageis increased to some percentage (compared with0.1 to 0.2% for normal wood), and, in spite of thehigher density, the longitudinal mechanical propertiesare less than in normal wood. Due to the large microfibrilangle in the S 2 layer, the effect is opposite in thetransverse plane: lower shrinkage and higher stiffness.36.1.3 .2 Te nsion Wo odTension wood is characterized by increased cellulosecontent and increased density. Sawn surfaces appearwoolly and rough. Strength properties are reduced.Longitudinal shrinkage can be more than 1%. Nosignificant coloration marks out tension-wood zones.In addition, the regulation of hardwood seems to besubtler than in softwoods, sometimes leading to verylocalized presence of reaction wood (thin layers in theradial direction and variable angular position fromone year to the other). Therefore, the macroscopicfeatures are not very reliable.At the microscopic level, tension wood is mucheasier to identify when it is fully developed. Fiber cellwalls are much thicker than normal, enclosing verysmall lumens. Secondary walls are loosely attached tothe primary wall and thus are responsible for some ofthe differing mechanical properties. The thick secondarywall of tension-wood fibers is significantly lesserlignified; it consists of almost pure cellulose. Due tothis consistency, this layer is termed gelatinous orG layer (Figure 36.8). The microfibrils are almostparallel to the grain. The reason for tension-woodshrinkage is not well understood. The loose contactbetween the G layer and the remaining cell wall,which does not restrain the outer cell region (i.e.,primary layer) and from contraction during drying,is one possible explanation. However, a recent worktends to prove that the G layer itself has a highlongitudinal shrinkage, which is not consistent withthe small microfibril angle (Clair, 2001). It may benoted that intermediate indications of tension woodexist without the G layer.36.1.3.3 Juvenile WoodAll trees during their growth produce juvenile wood,i.e., the inner core of xylem surrounding the pith.The time during which juvenile wood is formed variesamong individuals, with species, and with environmentalconditions. It is often recognized that juvenileRTNormal woodCompression woodFIGURE 36.7 Compression wood compared with normal wood. (ESEM photographs: White fir (Abies alba), LERMAB–ENGREF.)ß 2006 by Taylor & Francis Group, LLC.

FIGURE 36.8 Tension wood in Pedunculate oak (Quercus rubra); note the existence of the G layer. (ESEM photograph:LERMAB–ENGREF.)wood formation lasts as long as living branches existat the corresponding height of the tree. The transitionfrom juvenile to mature wood takes place gradually.No clear demarcation exists between juvenile andadult wood.In juvenile wood the cells are smaller than those ofthe mature xylem. Particular differences exist in thelength of the cells as well as in the structure of thelayered cell wall. The microfibril angle in the S 2 layeris greater than in cells of the mature tissue. As forcompression wood, this causes a higher value of longitudinalshrinkage and a reduced tensile strength. Inaddition, the spiral grain (angle between the stem axisand the fiber orientation) is often large in the juvenilewood. Together with the high longitudinal shrinkage,this explains why the warping of timber containingjuvenile wood may be dramatic after drying. Juvenilewood is a major problem in processing wood fromplantations of fast-growing species (eucalyptus,radiata pine, etc.) that produce logs made up mostlyof juvenile wood.36.1.4 IMPLICATIONS FOR THE DRYING P ROCESSToinducetheascentofsapintree,themeniscipresentinthe leaf stomata pull up water (Zimmerman 1983).Because most trees are more than 10 m high, one candeduce that the absolute liquid pressure in the sapcolumn is negative. No gaseous phase can exist in suchconditions. The vascular system developed in trees hasmany other implications for the drying process:. Because the system is designed for longitudinalsap flow from the roots to the canopy, the woodmaterial is strongly anisotropic.. Because of negative pressure, the vascular systemmust be able to support a gas invasion dueto injury or cavitation. This is the role of borderedpits or vessel-to-vessel pits. These anatomicalfeatures may dramatically inhibit thefluid migration in the wood.. In heartwood, due to metabolite deposition, aspirationor closure of bordered pits, or tylosedevelopment, the permeability is often reducedby one or several orders of magnitude.. The wood is fully saturated in the sapwood partof logs (an air-free sap column is required toobtain negative pressures), whereas the heartwoodzone is generally only partly saturated.Table 36.3 indicates some orders of magnitudegenerally observed for the moisture content of greenwood.Indeed, because the sapwood part is fully saturated,the maximum moisture content in this zonecan be calculated by assuming that the entire porevolume is filled with water:X ¼ fr ‘(1 f)r swith f ¼ 1r 0r s(36:1)where f is the porosity, r 0 is the basic density (ovendrymass/green volume), r s is the density of the cellß 2006 by Taylor & Francis Group, LLC.

TABLE 36.3Typical Values of Moisture Content Found inGreenwoodWood Type Moisture Content Dry Basis (%)wall substance (r s ffi 1530 kg m 3 ), and r ‘ is the sapdensity (r ‘ ¼ 1000 kg m 3 ).To highlight that the development of wood intrees leads to a very anisotropic material, Table 36.4indicates some order of magnitude for dimensionlessanisotropy ratios found in wood for the most importantproperties involved in drying. The ease of fluidmigration in wood (i.e., the permeability) is by far theproperty that presents the highest anisotropy ratio.The reduction in wood permeability from sapwood toheartwood affects particularly the longitudinal directionin hardwoods (especially for ring-porous speciesdeveloping tyloses) and all directions in softwoods.36.2 BOARD SCALE36.2.1 WATER IN WOOD: SORBED AND CAPILLARYWATER, AND SHRINKAGE36.2.1.1 Moisture Content of WoodThe moisture content of wood (dry basis) is definedby the following mass ratio:X ¼Sapwoodmass of wateroven-drymassHeartwoodSoftwoods 150–200 40–80Hardwoods 80–120 60–100(36:2)Moisture in wood is distinguished between the liquidsap present unattached in the cell lumens (free water)and the water molecules held in the cell walls (boundwater), with an activity less than 1.36.2.1.2 Free WaterThe sap flows in wood through the vascular systemproduced by the cambium. This liquid, present in thecell cavities, is usually referred to as ‘‘free water’’ justbecause its properties are very close to those of liquidwater: density, viscosity, saturated vapor pressure, etc.However,onehastokeepinmindthatthiswateristied to the solid matrix through capillary forces. Due tothe surface energy of the interface between liquid andgas (the surface tension), together with the contact anglebetween this interface and the woody matrix, a pressuredifference exists between liquid and gaseous phases.This pressure difference, which obeys Laplace’s law,increases with decreasing pore diameter. Because liquidis the wetting phase in the case of water and wood, theliquid pressure is less than the gaseous one.In addition, due to the curvature of the interface,a deviation of the saturated vapor pressure exists.This deviation can be calculated from the definitionand properties of the Gibbs free energy and leads toKelvin equation (Dullien, 1992):ln w ¼ lnP v¼ s 1 þ 1 Mv(36:3)P vs r 1 r 2 r ‘ RTIn Equation 36.3, P vs is the saturated vapor pressure, P vis the equilibrium vapor pressure at the curved interface,s is the surface tension, r 1 and r 2 are the twoprincipal radii of the surface, and M v is the molarmass of water. The quantity w is known as the relativehumidity. However, a very small pore radius is requiredfor the deviation to become significant (Table 36.5).TABLE 36.5TABLE 36.4Order of Magnitude of the DimensionlessAnisotropy Ratios Encountered inWood Relative to Tangential ValuePropertyDirectionRadius of theCapillaryT R LStiffness 1 2 20Shrinkage 1 0.5 0Thermal conductivity 1 1.5 2Mass diffusivity 1 1–2 20Pressure Difference DP and Relative Humidity wat the Surface for Different Radii Values (ValuesCalculated at 208C, for a Perfectly Wetting Liquidand Cylindrical Tubes)DP (Equivalent WaterColumn)w ¼ P v /P vs1 mm 0.146 kPa (14.9 mm) 0.999999100 mm 1.46 kPa (0.149 m) 0.99998910 mm 14.6 kPa (1.49 m) 0.999891 mm 0.146 MPa (14.9 m) 0.99890.1 mm 1.46 MPa (149 m) 0.9890.01 mm 14.6 MPa (1.49 km) 0.898ß 2006 by Taylor & Francis Group, LLC.

36.2.1.3 Bound WaterBound moisture is associated with the hygroscopicnature of the woody components. There are someuncertainties about the limits of hygroscopic behavior,particularly with woods of high extractives content;but it is useful to define a maximum sorptivemoisture content, called the fiber saturation point(FSP). If the capillary condensation effects in poresgreater than 0.1 mm in equivalent cylindrical diameterare ignored, FSP of the wood may be defined as theequilibrium moisture content (EMC) in an environmentof 99% relative humidity. This yields a value of30 to 32% for most commercial species (Keey et al.,2000) at room temperature. FSP falls with increasingtemperature. For a softwood such as Sitka spruce(Picea sitchensis), FSP falls from about 31% at 258Cto 23% at 1008C (Stamm, 1964).As the relative proportions of the woody componentsvary only within narrow ranges for commoncommercial species, the EMCs at a given relativehumidity and temperature are closely similar forthese woods. However, at high relative humiditiesdeviations from mean values can appear. Shubin’sdata (1990) show, for instance, that at 95% relativehumidity the EMC at 42.48C ranges from 22% for apine to 33% for an oak. Hoadley (1980) notes that inspecies with a high extractives content, such as redwood(Sequoia sempervirens) and mahogany (Swieteniamahogani), the fibers remain saturated at 22 to24% moisture content, whereas birch (Betula spp.)may have a moisture content up to 35% at fibersaturation.Because of sorption hysteresis, the EMC at agiven relative humidity is higher in drying (desorption)from greenwood than in wetting (adsorption)from perfectly dried wood. The ratio of values normallyranges between 0.75 and 0.88 (Schniewind,1989). However, in previously dried wood subject toenvironmental swings in relative humidity, the isothermfor adsorption and desorption becomes moresimilar. Thus, for practical proposes, the hygroscopicityof wood is sufficiently similar for most commercialspecies for generalized sorption data to be useful.The curves depicted in Figure 36.9 represent an averagebetween desorption and adsorption. They wereplotted using the mathematical correlation used in thenumerical code ‘‘Transpore.’’ The parameters of thisexpression have been fitted from various data availablein the literature: those published by Rasmussenin 1961 (Siau, 1984) and those of Loughborough onSitka spruce published by Hawley in 1931 and arrangedby Keylwerth in 1949 (Kollmann and Côté,1968; Joly and More-Chevalier, 1980).36.2.1.4 Differential Heat of SorptionSorbed water in the cell wall has a lower enthalpythan liquid water. However, contrary to other formsof water, such as solid, the enthalpy of bound waterincreases with increasing moisture content up to FSP.Above this value, the enthalpy of water in wood isessentially the same as that of liquid water.The value of the differential heat of sorption canbe calculated from the sorption isotherms using theClausius–Clapeyron equation (Skaar, 1988):35Equilibrium moisture content (%)3025201510520C60C100C00 20 40 60 80 100Relative humidity (%)FIGURE 36.9 Sorption isotherms calculated by a mathematical expression fitted from published data.ß 2006 by Taylor & Francis Group, LLC.

Pure waterWater in woodVaporL vΔh sL v−L fFiber saturation pointIceLiquidBound waterMoisture content≈ −L v /2FIGURE 36.10 Differential heat of sorption versus the moisture content. (Adapted from Skaar, C., Wood–Water Relations,Springer, Berlin, 1988.)Dh s ¼d ln P vR P vsM vd1 (36:4)TDimensionLThis heat of sorption is slightly higher than 1000 kJkg 1 for oven-dry wood and decreases rapidly withmoisture content to reach zero, with a horizontalslope at the FSP (Figure 36.10).RT36.2.1 .5 Sh rinkageIn trees, the cell walls of wood are in a fully swollencondition. However, when sorbed water is removedfrom the cell wall, new hydrogen bonds form betweenthe hydroxyl groups of the molecular chains andreduce the distance between the chains. This variationaffects the dimension in a direction normal to themicrofibril direction. This fact explains why the longitudinalshrinkage is usually negligible. In the transversedirections, the dimensional variations plottedagainst bound water content are very close to astraight line (Figure 36.11). Shrinkage can then bedefined by the total shrinkage (variation of dimensionbetween the green state and the oven-dry condition)or in terms of a shrinkage coefficient (variation ofdimension divided by the variation in moisture content).For most species, the tangential shrinkage isabout twice the radial shrinkage. Several featurescan explain this observation: the cell arrangement oftissues, the difference between earlywood and latewood,and the presence of ray cells. The extrapolationof length values against moisture content to the originalunshrunken length yields the so-called shrinkageintersection point (SIP) (Figure 36.11). Normally, thismoisture content is a little higher than the FSP, butoften the two moisture content values are assumed tobe same.FSP SIPMoisture contentFIGURE 36.11 Dimensional variations of a typical woodsample vs. moisture content.The shrinkage values tend to increase with basicdensity, but values vary strongly between and withinspecies. Table 36.6 indicates the order of magnitudevalues encountered in some species.In the case of compression wood, the microfibrilangle in the S 2 layer is large, which results in a lowertransverse shrinkage and a significant longitudinalshrinkage. In spite of the very low value of themicrofibril angle in the G layer, tension wood ofhardwood has also an abnormally high longitudinalshrinkage.As logs are sawn in green state, shrinkage occursafter sawing. The size of each section is reduced byshrinkage and also the shape of the section changescaused by the tangential or radial anisotropy inshrinkage (Figure 36.12):a. Only the quartersawn sections without the pithkeep their rectangular shapeb. Flatsawn sections cup into a trough-like shapeß 2006 by Taylor & Francis Group, LLC.

TABLE 36.6Order of Magnitude of Shrinkage Values in theTransverse Plane for Some Species. FSP is Assumedto be Equal to 30% for All SpeciesSpecies Radial Shrinkage Tangential ShrinkageTotal(%)Coefficient(%/%)Total(%)Coefficient(%/%)Spruce, pine 5.0 0.17 9.0 0.30Oak 6.0 0.20 11.0 0.37Beech 6.5 0.22 12.0 0.40Balsa 3.0 0.10 6.0 0.20Teak 3.0 0.10 5.0 0.17Okoumé 4.0 0.13 6.5 0.22Azobé 8.0 0.27 11.0 0.37Source: Kollmann, F.P. and Côté, W.A., Principles of WoodScience and Technology, Solid Wood, Vol. 1, Springer, Berlin,1968; Aléon, D., Chanrion, P., Négrié, G., and Perré, P.,FormaXylos 4—Le séchage (Training in Wood Science: Drying,Vol. 4), CD-Rom français/English, CTBA, Paris, 2003.c. Quartersawn sections, which include the pith,present a nonuniform thickness once dried;near the pith, the thickness shrinks along theradial direction whereas the tangential shrinkageis involved elsewhered. A squared section cut along the grain directionbecomes rectangular after dryinge. A diamond shape is obtained if the grain directionis along the diagonal36.2.2 HEAT AND MASS T RANSFER IN WOOD36.2.2 .1 Flui d Migration in Wood:Singl e-Phase FlowThe understanding of the fluid migration in wood isof utmost importance to understand the drying process.The differences in drying behavior between speciesand within the log primarily come from thepermeability value and from the initial moisture saturation.When only one fluid phase is present, theclassical Darcy’s law applies:v ¼ K r(P) (36:5)mwhere v is the apparent velocity of the fluid throughthe specimen (m s 1 ), K is the permeability (m 2 ), andm is the dynamic viscosity of the fluid (Pa s 1 ).Fluid migration in wood uses the vascular systemdeveloped by trees for their physiological requirements.For this reason, wood has several specificfeatures concerning permeability among which themost important are the anisotropy ratios. Wood hasdramatic anisotropy ratios: the longitudinal permeabilitycan be 1000 times greater than the transversepermeability for softwoods and more than a millionfoldfor hardwoods (Banks, 1968).Table 36.7 summarizes some values of directionalpermeability available in the literature for differentspecies. Depending on the experimental apparatusand the protocol used by the authors, some data aremissing in the papers. For example, it is not alwaysFlatsawn(d)(b)(e)Quartersawn(a)(c)FIGURE 36.12 Section deformations depending on the sawing pattern. The shape after drying results from the anisotropyratio between radial and tangential shrinkage. These deformations exist even when the equilibrium is achieved and with auniform moisture content throughout the section. (Adapted from Aléon, D., Chanrion, P., Négrié, G., and Perré, P.,FormaXylos 4—Le séchage (Training in Wood Science: Drying, Vol. 4), CD-Rom français/English, CTBA, Paris, 2003.)ß 2006 by Taylor & Francis Group, LLC.

TABLE 36.7Order of Magnitude of Permeability for Some Species along the Three Material Directions of WoodReferences Species Notes Permeability (m 2 ) Anisotropy RatioL · 10 12 R · 10 16 T · 10 16 K L /K R K L /K T K R /K TChoong and Kimbler, 1971 Populus sp. S. l. a 0.59 — 0.44 — 13,000 —Populus sp. H. l. a 0.61 0.15 0.18 40,000 33,000 0.835Alnus rubra S. l. a 1.9 0.15 0.12 125,000 166,000 1.327Liquidambar sp. S. l. a 2.7 0.19 0.69 145,000 39,000 0.273Liriodendron sp. H. l. a 5.4 — 0.80 — 67,000 —Sequoia sp. S. l. a 4.9 11.2 19.4 4,000 2,000 0.580Sequoia sp. H. l. a 0.25 0.112 0.88 23,000 3,000 0.127Pseudotsuga sp. H. l. a 0.026 0.000 0.000 — — —Pseudotsuga sp. S. l. a 0.049 — 0.37 — 1,300 —Pinus sp. H. l. a 0.0026 0.17 — 153 —Choong. et al., 1974 Acer rubrum S. g. o 10.3 — — 8,300 11,400 1.4Acer rubrum H. g. o 7.4 — —Liriodendron sp. S. g. o 28.9 — — 1,450 1,600 1.1Liriodendron sp. H. g. o 1.87 — —Liquidambar sp. S. g.o 13.9 — — 1,300 2,750 2.1Liquidambar sp. H. g. o 15.3 — —Quercus rubra S g 62.0 — — 13,000 18,000 1.4Quercus rubra H g 56.0 — —Quercus falcata S g 69.0 — — 110,000 143,000 1.3Quercus falcata H g 13.0 — —Chen et al., 1998 Liriodendron sp. S l 26.0 4.5 — 57,000 — —Liriodendron sp. H l 0.1 — — — — —Juglans nigra S l 27.0 0.08 — 3 10 6 — —Juglans nigra H l 0.0052 — — — — —Quercus rubra S l 61.0 0.68 — 900,000 — —Quercus rubra H l 45.0 — — — — —Perré, 1992 and 2002 Picea sp. S. g. a 0.2 — — 700 — —Fagus sylvatica S g. a 3.8 — — 3,000 65,000 21.0Fagus sylvatica H g. a 1.4 — — 3,000 — —Populus sp. H g. s 0.03 — — 10,000 — —L, longitudinal; R, radial; T, tangential; S, sapwood; H, heartwood; l, permeability to liquid; g, permeability to gas; a, air-dried sample; o,oven-dried sample.easy to calculate the permeability ratios from permeabilityvalue, or vice versa. Choong et al. (1974), forexample, have reported the permeability values forsapwood and heartwood for the longitudinal direction,but not for the transverse directions. Only the meananisotropy ratio is available in this paper. Perré (1992)and Perré et al. (2002) have used an experimentalprocedure to determine the longitudinal permeabilityand the anisotropy ratio on the same sample. In theseinstances, they just obtained the ratios and decided notto calculate the transverse permeability accordingly.The variability is impressive for both the permeabilityvalues and the anisotropy ratios. In spite of thescatter in the data, general trends are exhibited for thelongitudinal permeability:. The most permeable species in the longitudinaldirection are among the ring-porous species(Quercus spp.) that have very large vessels (upto 500 mm in diameter).. The diffuse-porous hardwoods are fairly permeabletoo; probably, the large number ofvessels can offset their smaller diameter (around50 mm).. Softwood species are generally less permeable;these trees have no specific elements for sapflow, so the fluid has to pass through the smallopenings, the bordered pits, at the end of eachtracheid along the path, which is 1 to 2 mmeach. Certain softwood species, such as Pinusradiata, however, are very permeable.ß 2006 by Taylor & Francis Group, LLC.

The transverse permeability and the anisotropyratios are very variable. The heartwood part of logsis usually much less permeable than the sapwood part(Comstock, 1967). This is due to tyloses developmentand extractives deposition (tannins, gums, etc.) inhardwoods and due to the aspiration of borderedpits in softwoods.36.2.2.1.1 Pit AspirationIn softwoods, the tracheids have considerable pittingin their radial walls. These bordered pits are specializedvalves to seal and isolate tracheids if they becomedamaged or embolized, but remain open for sap flow.At the pit location, the double cell wall takes theshape of the external part of a torus. The externaldiameter of this torus is in the range 10 to 20 mm,depending on the position in the annual growth ring(the diameter is smaller in latewood). The torus issuspended by the margo, a net of radially orientedmicrofibrils, with openings up to some micrometerswide. In normal operation, the sap flows from onetracheid to the other simply by using these smallopenings (Figure 36.13a).When gas invades one tracheid, the gas–liquidinterface is blocked in the margo due to capillaryforces. These forces press the torus against the oppositeborder of the pit (Figure 36.13b). Then, hydrogenbonds keep the torus in this position; the pit is nowimpermeable (Figure 36.13c). Such sealing of a pit isknown as aspiration.This subtle mechanism is vital for trees, but causessome difficulties in wood drying. Indeed, pit aspirationoccurs as soon as water is removed from the wood,sometimes even during the heartwood formation. Inparticular, it is impossible to avoid pit aspiration whendrying softwoods under normal conditions.Only freeze-drying, or changing the liquid phase fora solvent with low surface tension before drying, allowsair-dried samples without pit aspiration to be obtained(Comstock and Côté, 1968; Meyer, 1971; Bolton andPetty, 1978; Fumoto et al., 1984). The permeabilityvalues depend on the species and on the author,but all these data show that air-dried samples, withaspirated pits, have permeability values considerablysmaller than unaspirated samples (typically rangingfrom 1 to 10%). Due to thicker cell walls, smaller pitradii, and more rigid structures, the percentage of aspiratedpits is much less in the latewood part of samples(Siau, 1984).36.2.2.2 Generalized Darcy’s Law:Multiphase FlowWhen two phases coexist, the generalized Darcy’s lawmust be used. The volumetric flow rate of each phaseis considered to be proportional to the pressure gradientof the corresponding phase. The phenomenologicalcoefficient is the product of the permeabilityK by a function of saturation called ‘‘relative permeability’’to the considered phase.Middle lamellaCapillary forcesMargo≅10 µmTorusGasLiquidPosition of theair/liquidmeniscusGasGasLiquidLiquidSecondary wallConductive bordered pitPit aspirationAspirated pitFIGURE 36.13 The mechanism of pit aspiration: a clever strategy to limit the damage caused by any gas invasion due toinjury or cavitation of the sap column. (Adapted from Siau, J.F., Transport Processes in Wood, Springer, Berlin, 1984.)ß 2006 by Taylor & Francis Group, LLC.

For the gaseous phasev g ¼For the liquid phasev ‘ ¼Kk rg(S)m grP g (36 :6)Kk r‘(S)m ‘rP ‘ (36 :7)The liquid pressure is related to the gaseous pressurethrough the capillary pressure function:P ‘ ¼ P g P c (S) (36 :8)Equation 36.6 and Equation 36.7 must be consistentwith Darcy’s law (Equation 36.5) when one singlefluidphase occupies the porous medium. Consequently,the relative permeability functions fulfill thefollowing conditions:Gas only Liquid onlyk r‘ (0) ¼ 0, k r‘ (1) ¼ 1k rg (0) ¼ 1, k rg (1) ¼ 0(36 :9)Although depending on both initial and boundaryconditions, the relative permeability values are usuallysupposed to be the function of saturation only.Even with this simple assumption, their experimentaldetermination remains very challenging.Very few results are available in the literature forwood. Some data have been published by Tesoro et al.(1972, 1974). A recent paper by Tremblay et al. (2000)describes an indirect method to measure this property.Based on the geometrical model of tracheids proposedby Comstock (1970), Spolek and Plumb (1980)derived an expression for the relative permeability insoftwoods. This model assumes that all tracheids areexactly similar and that the wetting-phase distributionis ideal. Their final expression involves an irreduciblesaturation, below which no liquid flux is possible.This concept of irreducible saturation has beenwidely discussed, especially in the context of petroleumproduction (Dullien, 1992). Depending on theboundary conditions, one part of the fluid phasethat occupied the medium at the beginning of theexperiment remains in the medium, even though itsflow rate has vanished. This part seems to be trappedin the medium in what is called a ‘‘pendular’’ state.The amount of the residual part increases with theimbibition or drainage velocity. In addition, theamount of trapped phase after the experiment reduceswith time; surface spreading in the solid phase and theedge-capillary pressure are the two mechanisms thatcan explain this observation (Dullien, 1992). Due tothe microporosity that exists in the cell walls, suchmechanisms are likely to exist in wood also. However,the concept of irreducible saturation has led to unrealisticcomputed moisture content profiles duringdrying (Perré , 1987), though later modeling of softwooddrying incorporates a similar concept successfullyas a mean to account for pit aspiration (Nijdamet al., 2000). In this case, ‘‘irreducible saturation’’corresponds to the condition when pit aspiration issufficiently advanced such that the remaining freemoisture is immobilized in isolated pockets in thewood.Based on the measurements in the longitudinaldirection reported by Tesoro et al. (1972), on thecurve computed from the tracheid model (Spolekand Plumb, 1980), and on those considerationsregarding the concept of irreducible saturation, thefollowing functions have been proposed for softwoodsby Perré et al. (1993) (Figure 36.14):In the transverse direction (radial or tangential)k T rg ¼ 1 þ (2S 3)S2 and k T r‘ ¼ S3 (36:10)In the longitudinal directionk L rg ¼ 1 þ (4S 5)S4 and k L r‘ ¼ S8 (36:11)36.2.2.3 Capillary PressureSome works can be found on the determination ofcapillary pressure functions in wood. Mercury porosimetryexhibits a dramatic effect of the sample thicknessin the longitudinal direction (Trénard, 1980). Forshort samples, the cell lumens are directly accessible tothe mercury; whereas for longer samples, the liquid hasto pass through the small openings, the pits that existbetween cells, clearly illustrating their bottleneckingeffect. The centrifuge method has been used successfullyby Spolek and Plumb (1981) on softwoods and byChoong and Tesoro (1989) on various species. Todetermine the moisture content–water potential relationshipof wood, Cloutier and Fortin (1991) used atension plate and a pressure plate. This is a drainagemethod and their results could easily be converted intoa classic capillary pressure function.Because certain anatomical features govern themorphology of wood pores, particular methods canbe applied to wood:. To compute a capillary pressure curve, Spolekand Plumb (1981) have developed the geometricalß 2006 by Taylor & Francis Group, LLC.

FIGURE 36.14 Relative permeability curves calculated using equations (36.10) and (36.11).10.8TransverseLongitudinalRelative permeability0.60.40.200 0.2 0.4 0.6 0.8Saturation1model of the tracheid shape proposed by Comstock(1970). Although it may be simplistic toassume that all tracheids have exactly the sameshape,theyobtainedagoodtrendforthecapillarypressure function by this means.. Because the longitudinal direction of wood isvery marked, it is quite simple to obtain thethree-dimensional structure of the materialfrom a cross section. Figure 36.15 depicts theexamples of capillary pressure curves calculatedfrom microscopic images of cross sections ofwood. In this case, the pore-size distributionhas been calculated using image processing(Perré , 1997; Perré and Turner, 2002).36.2.2 .4 Bound -Water Diffusio nMacroscopic bound-water diffusion results fromtransport mechanisms that take place at the microscopicscale, i.e., diffusion of the bound waterthrough the cell walls and vapor diffusion due toFick’s law. At the microscopic scale, these two fluxescan be expressed as follows:f b ¼ r s D b rX b (Bound-water flux) (36 :12)f v ¼ r g D v rv v (Vapor flux) (36 :13)In Equation 36.12 and Equation 36.13, D b and D vrepresent the microscopic bound-liquid and vapordiffusivities, respectively having units m 2 s 1 and v vis the mass fraction of vapor in the gaseous phase.By using the bound-liquid diffusivity data ofStamm (1963), it is possible to obtain the followingleast-squares, best-fit correlation for D b :4300D b ¼ exp 12 :82 þ 10 :90 X b (36:14)Twhere T is the temperature in Kelvin.On assuming isothermal conditions and constanttotal pressure, the microscopic vapor flux can be expressedwith the gradient of the bound-water contentas the driving force by Equation 36.13. M vf v ¼RT D @P vv rX b (36:15)@X bWithin the anatomical structure of wood, anycombination in series or parallel of vapor diffusion(in lumen and pits) and bound-water diffusion (in thecell walls) is a possible pathway to drive water fromhigh to low moisture content regions (Figure 36.16).Because Equation 36.12 and Equation 36.15 use thesame driving force, the expressions for the macroscopicbound-water diffusivity in the radial and tangentialdirectionscanbecalculatedevenusinghomogenizationtechniques according to these microscopic properties,together with the pore morphology (Perré and Turner,2002). Equation 36.14, derived from specific experimentalmeasurements, exhibits a dramatic increase ofß 2006 by Taylor & Francis Group, LLC.

0.4Capillary pressure (P c /P atm )0.30.20.1458 kg m 3 641 kg m 3Late woodMiddle woodEarly wood349 kg m 30(a) 0 0.2 0.4 0.6 0.8 1.0Saturation0.8Capillary pressure (P c /P atm )0.60.40.2656 kg m 3543 kg m 3SaturationLate woodEarly wood(b)00 0.2 0.4 0.6 0.8 1.0FIGURE 36.15 Capillary-function curves determined using image analysis. (a) spruce (Picea abies): the cells have thickerwalls and smaller radial extension in latewood part, hence the highest value of the capillary-pressure curve. One also has to beaware that full saturation is obtained with a lower amount of water in latewood (the porosity of this part is very small); (b)beech (Fagus sylvatica): because beech is a pore diffuse-porous hardwood species, no significant difference is observedbetween these parts. The low capillary pressure obtained for saturation values above 0.2 corresponds to the meniscus radiilocated in the vessel elements. The dramatic increase for low saturation values is due to the small lumen diameters of theparenchyma and fiber cells.the bound-water diffusivity as the bound moisture contentincreases. For this reason, the same trend is predictedfrom the calculations or measurements at themacroscopic scale; bound-water diffusion is alwayseasier for higher bound-water contents.When using the gradient of bound water as adriving force, the macroscopic flux readsf b ¼ r 0 D b rX b (36:16)This expression is consistent with the derivation of thesecond Fick’s law by using the mass balance equation:@X b@t¼r(D b rX b ) (36:17)For the sake of simplicity, the foregoing expressionshave always assumed isothermal conditions. However,this assumption fails for certain drying processesß 2006 by Taylor & Francis Group, LLC.

Low moisture contentHigh moisture contentFIGURE 36.16 Moisture diffusion in the hygroscopicrange: any combination in series or parallel of vapor diffusion(in lumen and pits) and bound-water diffusion (in thecell walls) is able to drive water from high to low moisturecontent regions.(e.g., impingement drying, radio-frequency, or microwaveheating). The problem of mass migration due to athermal gradient is a matter of scientific debate. Siau(1984, 1995) gives a good review of possible formulationsthat can be used in nonisothermal conditions.36.2.2.5 Physical FormulationSeveral sets of macroscopic equations are proposed inthe literature for the simulation of the drying process.The first fundamental difference between them lies inthe number of state variables used to describe theprocess:. One: moisture content (or an equivalent variable:saturation, water potential, etc.). Two: moisture content (or equivalent) and temperatureT (or an equivalent variable: enthalpyetc.). Three: moisture content (or equivalent), T (orequivalent), and gaseous pressure P g (or anequivalent variable: air density, intrinsic airdensity, etc.)Perré (1999) gives a critical review of the possibilitiesand limitations given by these different sets ofequations. The use of moisture content alone is thebasis of many correlations of lumber-drying rates(see Keey et al., 2000). Doe et al. (1996a) have foundthat, at relatively low temperatures (

Water Conservation@@t « wr w þ « g r v þ r bþr rw v w þ r v v g þ r b v b¼rðrg¼Deff rv v ÞðT1ÞEnergy Conservation@@t « wr w h w þ « g ðr v h v þ r a h a Þþr bhb þ r o h s « g P g þr rw h w v w þ ðr v h v þ r a h a Þv g þ h b r b v b¼¼r r gDeff ðh v rv v þ h a rv a Þþl eff rT þ FðT2ÞAir Conservation@@t « ¼gr a þr ra v g ¼r rgDeff rv a(T3)where the gas- and liquid-phase velocities are given by the generalized Darcy’s law:v ‘ ¼¼K ‘¼kr‘m ‘rw ‘ , rw ‘ ¼rP ‘ r ‘ grx (T4)where ‘ is w, g, the quantity w is known as the phase potential, and x is the depth scalar. All other symbolshave their usual meaning.Boundary ConditionsFor the external drying surfaces of the sample, the boundary conditions are assumed to be of the following form: 1 x 1J w j x¼0 þ ^n ¼ h m cM v ln1 x v j x¼0(T5)J e j x¼0 þ ^n ¼ h(Tj x¼0 T 1 )P g j x¼0 þ ¼ P atmwhere J w and J e represent the fluxes of total moisture and total enthalpy at the boundary, respectively, and xdenotes the normal position from the boundary in the external medium.36.2.3 PROCESS OF DRYING36.2.3 .1 Lo w-Temper ature Convecti ve Dry ingLow-temperature convective drying is the most widespreadindustrial process for seasoning wood in kilns.In this case, the role of internal gaseous pressure isalmost negligible and transfer occurs mainly in thedirection of the board thickness. Two periods of dryingmay be distinguished: (1) a constant drying-rateperiod and (2) a decreasing drying-rate period.36.2.3 .1.1 The Cons tant Drying- Rate Peri odThis stage is very common for certain porous media,but is rarely seen with wood. However, it exists almostalways for fresh boards consisting of sapwoodthat are dried under moderate conditions (Perré et al.,1993; Perré and Martin, 1994). During this period,the exposed surface of the board is still above theFSP. As a result, the vapor pressure at the surface isequal to the saturated vapor pressure, and is a functionof the surface temperature only.Coupled heat and vapor transfer occur across inthe boundary layer (Figure 36.17). The heat flux suppliedby the airflow is used solely for transforming theliquid water into vapor. During this stage, the dryingrate is constant and depends only on the externalconditions (temperature, relative humidity, velocity,and flow configuration). The temperature at the surfaceis equal to the wet-bulb temperature. Moreover,because no energy transfer occurs within the mediumduring this period, the whole temperature of theboard remains at the wet-bulb temperature.ß 2006 by Taylor & Francis Group, LLC.

Boundary layersExternal flowTP vHeatVaporCapillary migrationLow moisturecontent=small radiusLiquid flowWoodHigh moisturecontent=large radiusFIGURE 36.17 Constant drying-rate period: the moisture migrates inside the medium mostly by capillary forces; evaporationoccurs at the exchange surface with a dynamical equilibrium within the boundary layers between the heat and the vaporflows. (Adapted from Perré, P., The numerical modeling of physical and mechanical phenomena involved in wood drying: anexcellent tool for assisting with the study of new processes, Tutorial, Proceedings of the Fifth International IUFRO WoodDrying Conference, Québec, Canada, 1996, 9–38.)The exposed surface is supplied with liquid watercoming from the inside of the board by capillaryaction; the liquid migrates from regions with highmoisture content (liquid–gas interfaces within largepores) toward regions with low moisture content(liquid–gas interfaces within small pores).The constant drying-rate period lasts as long as thesurface is supplied with liquid. Its duration dependsstrongly on the drying conditions (magnitude of theexternal flux) and on the medium properties. The liquidflow inside the medium is expressed by Darcy’slaw (permeability gradient of liquid pressure).36.2.3 .1.2 The Decr easing Drying- Rate Pe riodOnce the surface attained the hygroscopic range,the vapor pressure becomes smaller than the saturatedvapor pressure (Figure 36.18). Consequently, theexternal vapor flux is reduced and the heat flux suppliedto the medium is temporarily greater than whatis necessary for liquid evaporation. The energy inexcess is used to heat the board, the surface at firstand then the inner part by conduction. A new, moresubtle, dynamic equilibrium takes place. The surfacevaporpressure,hencetheexternalvaporflow,dependson both temperature and moisture content. To maintainthe energy balance, the surface temperature increasesas the surface moisture content decreases. Thisleads to a decreasing drying rate (the heat supplied bythe airflow becomes smaller and smaller).A two-zone process develops inside the wood: (1)an inner zone, where liquid migration prevails, and(2) a surface zone, where both bound-water andwater-vapor diffusion take place. During this period,a conductive heat flux must exist inside the board toincrease the temperature and to evaporate the liquiddriven by gaseous diffusion. The region of liquidmigration naturally reduces as the drying progressesand finally disappears. The process is finished whenthe temperature and the moisture content attain theoutside air temperature and the EMC, respectively.36.2.3 .2 Dry ing at High Temper ature: The Effectof Inter nal Pressur e on Mass Tr ansferTo reduce the drying time without decreasing thequality of the dried product, the drying conditionsmust be such that the temperature of the product isabove the boiling point of water. Such conditionsensure that an overpressure exists within the material,which implies that a pressure gradient drives themoisture (liquid or vapor) toward the exchange surfaces(Lowery, 1979; Kamke and Casey, 1988).At normal atmospheric pressure, the boiling pointof water equals 1008C. Consequently, in order toß 2006 by Taylor & Francis Group, LLC.

External flowDiffusion of vaporand bound waterFew vapormoleculesHeatVaporBound waterand vaporCapillary migrationMany vapormoleculesLow moisturecontent=small radiusLiquid flowHigh moisturecontent=large radiusFIGURE 36.18 Second drying period: a region in the hygroscopic range develops from the exposed surface. In that region,both vapor diffusion and bound-water diffusion act. Evaporation takes place partly inside the medium. Consequently, a heatflux has to be driven toward the inner part of the board by conduction. (Adapted from Perré , P., The numerical modeling ofphysical and mechanical phenomena involved in wood drying: an excellent tool for assisting with the study of new processes,Tutorial, Proceedings of the Fifth International IUFRO Wood Drying Conference, Québec, Canada, 1996, 9–38.)obtain an internal overpressure, the temperature ofthe porous medium must be above that level during atleast one part of the process. This is the aim of convectivedrying at high temperature (moist air orsuperheated steam) and a possible aim of contactdrying or drying with an electromagnetic field (microwaveor radio frequency).However, as shown in Figure 36.19, it is possibleto reduce the boiling point of water by decreasing theexternal pressure and, consequently, to obtain a hightemperatureeffect with relatively moderate dryingconditions. This is the principle of vacuum drying,particularly useful for lumber that would be damagedby high temperature levels.Whenever an overpressure exists inside a board,the large anisotropy ratios imply intricate transfermechanisms. Heat is often supplied in the thicknessdirection while, in spite of the length, the effect ofthe pressure gradient on gaseous (important for lowmoisture content) or liquid migration (important forhigh moisture content) takes place in the longitudinaldirection (Figure 36.20). This is a result of the anatomicalfeatures of wood. In the case of very intensiveinternal transfer, the end piece can be fully saturatedand, sometimes, moisture can leave the sample in theliquid state. (This is quite easy to observe duringmicrowave heating.)Pressure (kPa)10050Atmospheric pressureExternal pressureBoiling temperature00 20 40 60 80 100 120Temperature (C)Saturated vaporFIGURE 36.19 Vacuum drying seeks to reduce the boilingpoint of water in order to obtain a high-temperature effectwith moderate drying conditions. (saturated vapor pressurevalues from Lide 1995.)36.2.3 .3 Ty pical Drying Behavior : Differenc ebe tween Sapwood and Hear twoodIn a tree, freshly cut down and sawn, it is easy todistinguish sapwood from heartwood (by touch or bysight). But a few days later, the loss of surface moisturecontent makes it impossible to do that. Nevertheless, inß 2006 by Taylor & Francis Group, LLC.

HeatVaporVessel ortracheidLiquidPitsEndpiecefully saturatedOverpressureLiquid evacuationpossible inmicrowave heatingFIGURE 36.20 Drying at high temperature (second drying period): a high-temperature regime means that an overpressuredevelops inside the medium. Depending on the moisture content, this overpressure induces liquid or gaseous flow; inaddition, as wood is strongly anisotropic, the most part of the flow occurs in the longitudinal direction (see the magnifiedviews). (Adapted from Perré, P., The numerical modeling of physical and mechanical phenomena involved in wood drying:an excellent tool for assisting with the study of new processes, Tutorial, Proceedings of the Fifth International IUFRO WoodDrying Conference, Québec, Canada, 1996, 9–38.)the case of high-temperature drying, the increase ofinternal pressure gives rise to longitudinal migrationof liquid toward the end pieces, provided that thepermeability and the moisture content are highenough. This is a good way to spot sapwood after afew hours of drying (Figure 36.21b). This phenomenoncan be observed in industrial kilns (Figure 36.21c).To illustrate the effect of these differences on thedrying process, Figure 36.22 depicts drying experimentscarried out with superheated steam at 150 8Con both sapwood and heartwood of Norway spruce(Picea abies). They are representative of the trendsobserved by different authors (Salin, 1989; Pang et al.,1994; Perré and Martin, 1994).After the initial transient period, the constant drying-rateperiod takes place for the sapwood board.During this period, which lasts several hours, alltemperatures are equal to the wet-bulb temperatureand the overpressure remains very small. At the beginningof the second drying period (around 350min), an important overpressure develops due tothe temperature increase. It disappears only when the(a) A stack at the beginning of the dryingHeartwoodSapwoodSapwood Heartwood(b) The same stack after a few hours of dryingat high temperature(c)FIGURE 36.21 A stack of boards during high-temperature drying (shaded areas indicate wet zones).ß 2006 by Taylor & Francis Group, LLC.

Air flow041 381.0 m65Different locationsin the sample30 mm1501.0T 11300.8Temperature (˚C)11090T 8T 5T 3P 3P 50.60.4Overpressure (P atm )(a)70500100 200 300 400 500 600 700 0 0.2Time (min)1501.25T 1T 8T 5T 3P 3P 51301.00Temperature (˚C)110900.750.50Overpressure (P atm)(b)700.25500 100 200 300 400 500 0Time (min)FIGURE 36.22 Experiment on spruce (Picea abies) dried with superheated steam at 1508C. Temperature and internalpressure at different locations. Note the difference between sapwood (a) and heartwood (b).entire board enters the hygroscopic range. At thismoment, all temperatures approach the dry-bulbtemperature.The results obtained for heartwood are quite different.No constant drying-rate period can be observed.Just a short plateau at the boiling pointß 2006 by Taylor & Francis Group, LLC.

Moisture content (%)15010050SapwoodHeartwood00 200 400 600Time (min)FIGURE 36.23 Moisture content loss obtained for sapwoodand for heartwood (same experiments as in Figure 36.22).is detectable at the rear end of the board (T 8 ).Consequently, the overpressure remains high (especiallyfor the center pressure P 3 ) up to the end of thedrying. The maximum pressure is higher for heartwoodthan for sapwood. The differences in dryingkinetics are of great interest. In spite of the high initialmoisture content of sapwood (170% against 60%), thepermeability of heartwood is so low that the curvescross each other at 450 min of drying (Figure 36.23).This is consistent with the observations on entirestacks (Salin, 1989).The strategy of simulating the differences betweenheartwoodandsapwoodliesinonlytwosetsofparameters:the permeability and the initial moisture content(fortheseexperiments,180%forsapwoodand70%forheartwood). The values of permeability used to differentiatesapwood from heartwood (Table 36.8) arebased on the considerations concerning pit aspiration.By using only these differences, Perré and Turner(1996) found that all the trends observed for sapwoodand heartwood were found in the simulated results.The most spectacular effect is the longitudinal flowdue to the overpressure (Figure 36.24). In the case ofhigh-temperature convective drying, the sapwoodboard delivers a large supply of water to the endpiece after 5 h, while the heartwood end piece isalready within the hygroscopic range. These carpetplots should be compared with Figure 36.21.36.2.4 MECHANICAL A SPECTS OF WOOD DRYING36.2.4 .1 Mech anical Behavior of W oodIndustrial wood drying consists of not only removingmoisture from greenwood but also ensuringthat its quality (fitness for purpose) is adequatein end use. Because wood shrinks during drying,deformations and stresses develop that can lead toTABLE 36.8Intrinsic Permeabilities Used in the Drying ModelingDirection Heartwood SapwoodLongitudinalGas 10 13 m 2 2.10 13 m 2Liquid 10 13 m 2 10 12 m 2TransverseGas 10 16 m 2 2.10 16 m 2Liquid 10 16 m 2 10 15 m 2unusable products (Figure 36.25). The understandingof these aspects must account for the complex mechanicalbehavior of wood, including its memory effect.Shrinkage is the ‘‘driving’’ force for drying stress,i.e., without shrinkage, no drying stresses would develop.Figure 36.26 exhibits the dimensional variationof an unladen sample with the moisture content(the latter is assumed to be uniform). Under normalconditions, the dimensions do not change until themoisture content attains a moisture content close tothe acknowledged FSP. This condition is sometimescalled SIP. Then, the dimension variations are almostproportional to the change in moisture content. Thisstrain field is called free shrinkage.A sample subjected to tensile or compressive stress(Figure 36.27) exhibits the instantaneous deformation(elastic part) at first, which then increases with time(viscoelastic creep). After cycling the moisture contentto and from a higher moisture content, the creephas been significantly greater due to mechanosorptiveaction.Thus, a sample subjected to a compressive stress(as shown in case 1, Figure 36.28) exhibits a smallerlength at the end of drying than an unloaded specimen(case 2), which itself has a smaller length thanthe sample subjected to a tensile stress (case 3). Inthis experiment, the viscoelastic behavior acts becauseof time and the mechanosorptive behavioracts because of the removal of water molecules dueto drying.These ideas can now be applied to the drying oflumber boards, which is assumed to be stress-free atthe beginning. At the beginning of drying (constantdrying-rate period), sap throughout the entire boardremains free. No shrinkage occurs; hence, stressbuildup is absent.At the beginning of the second drying period,shrinkage exists close to the exposed surfaces(Figure 36.29a). At this moment, if the section wascut into slices, the outer slices would have a shorterlength than the inner ones (Figure 36.29b). This displacementfield is not compatible and induces, in theß 2006 by Taylor & Francis Group, LLC.

504030Length (cm)2010Moisture content1.00.50.00 1 20Width (cm)50403020Length (cm)100Moisture content1.00.50.00 1 2Width (cm)504030Length (cm)2010150125100755000 12TemperatureWidth (cm)504030Length (cm)2010015012510075500 1 2TemperatureWidth (cm)50(a)4030Length (cm)20100Pressure1.81.61.41.21.00 1 2Width (cm)50(b)4030Length (cm)20100Pressure1.81.61.41.21.00 1 2Width (cm)FIGURE 36.24 High-temperature drying (140/858C). Carpet plot after 5 h of drying. Internal overpressure, resaturation ofthe end piece, thermal conduction along the thickness, and end piece close to the wet-bulb temperature are evident on theseplots. Note the high value of internal pressure and the absence of end-piece resaturation obtained for heartwood (b).actual section, a tensile stress in the surface layers and(because of equilibrium conditions) a counteractingcompressive stress in the core layers (Figure 36.29c).During this period, surface checking is possible.From this point onward, the wood layers dry underload.As the drying proceeds, viscoelastic creep develops,together with mechanosorptive creep. The outer slicesappear similar in configuration to that exhibited forslice n83inFigure36.28,whiletheinternalslicesresembleslice n 81. Consequently, in spite of the flat moisturecontent profile, slicing the section at the end of thedrying wouldgivepictureFigure36.30b; the coreslices,dried under compression, are smaller than the outerones, dried under tension. In the actual section, compressivestress exists in the inner part (Figure 36.30c).This phenomenon is known as stress reversal or casehardening.The residual stress level depends on manyparameters (growth history, sawing pattern, dryingconditions, species, thickness, etc.), which provide mostof the problems of drying optimization. In addition,one must keep in mind that gradients of moisturecontent, strain, and stress exist along the thickness.This explains the curvature of the slices observed inprong test or cup method commonly used in industryto assess stress levels (Figure 36.31). When the innertensile stress is too high, internal checking occurs(Figure 36.25). An interesting simulation of this testcan be found in Dahlblom et al. (1994).36.2.4 .2 Dry ing Stres s Fo rmulationDuring drying, shrinkage appears in all parts of theboard for which the moisture content X is within thehygroscopic range. The shrinkage strain is proportionalto the difference between the local moisture content andthelocalvalueofthemoisturecontentatfibersaturationat the same temperature. A deformation field noted, « sh ,is defined in the material’s axes by Equation S1.If this deformation field does not fulfill the geometricalcompatibility, a strain tensor « mec related tostress is generated. The constitutive equation, whichrepresents the mechanical behavior of the material,relates this strain tensor « mec and the stress tensor.Due to the memory effect of wood, this tensor « mechas to be divided into two parts: (1) an elastic strain,« elas , connected to the actual stress tensor and (2) amemory strain, « mem , which includes all the strain dueß 2006 by Taylor & Francis Group, LLC.

to the history of that point (« mem can deal with plasticity,creep, mechanosorption, etc.).The geometrical compatibility applies to thetotal strain field « tot . When solving the mechanicalproblems in terms of displacement, the total straintensor is deduced from the displacement field and thisgeometrical condition is automatically fulfilled withinthe domain. The stress field must satisfy the local mechanicalequilibrium and the boundary conditions.Finally, the complete formulation of the stress problemis given by Equation S1 through Equation S4.36.2.4.3 Memory EffectWhile describing the strain field, « mem , lies the entireproblem of developing a constitutive model forwood, which requires both theoretical and numericalwork. Comprehensive formulations are also verydifficult to characterize (Ranta-Maunus, 1975). Theproblem lies in the fact that the memory effect ofwood depends not only on the temperature and moisturecontent values but also on their variations in timeand on the history of their variations in time. This2 3A 0 0« sh ¼ H(~x) 4 0 B 0 5 (S1)0 0 CwithH(~x) ¼ 0 X(~x) X fsp if X (~x) $ X fspif X (~x) # X fsp8« tot« mec ¼ « elas þ « mem (S2)ij ¼ 1 2 (u i, j þ u j,i) over V>< s ij, j þ rf i ¼ 0 over Vs ij ¼ a ijkl (« totkl« 0 kl ) over V with «0 = « sh + « mem and 8i , G Di G Ti = Gs ij n j ¼ T ion G >:Tiu i ¼ D i ¼ 0 on G Di(S3)Remarks:. This static formulation requires that boundary and volumetric forces satisfy the global equilibrium.. G is the surface surrounding the domain V. G Di refers to the subdomain of G where the i componentof the displacement is known and G Ti to the subdomain of G where the i component of the tractionforce is known. In order to ensure the uniqueness of the solution, additional conditions are requiredon the boundary conditions: 8i, mes(G Di ) > 0. Otherwise, the solution is defined within a rigid bodymotion.. As wood is orthotropic, each behavior law involves nine independent terms. In fact, it is morecommon to define the inverse of a ijkl that, for the case of linear elasticity, leads to the generalizedHooke’s law:223« LL« RR« TT6 2« LR ¼ g LR¼74 2« LT ¼ g LT52« RT ¼ g RT641E L LR RLE R1E L LTE LE R TL RT 1E R0 0 0E T TR0 0 0E T0 0 0E T10 0 00 0G LR10 0 0 0 0G LT0 0 0 0 01G RT326475s LLs RRs TTs LRs LTs RT375(S4)ß 2006 by Taylor & Francis Group, LLC.

FIGURE 36.25 Two examples of mechanical degrade during wood drying: board deformation and internal checking.GreenwoodSIPOven-driedLength0SIPMoisture contentFIGURE 36.26 Wood shrinkage: the shrinkage intersection point (SIP), often close to 30%, depends on species andtemperature.ß 2006 by Taylor & Francis Group, LLC.

ConstantmoisturecontentTime t = 0 +LoadTime tLoadAfter cycles ofmoisture contentLoadMoisturecontentLoadTimeTimeLengthvariationViscoelasticMechanosorptiveTimeFIGURE 36.27 Viscoelastic and mechanosorptive behavior of wood. (Adapted from Perré, P., The numerical modeling ofphysical and mechanical phenomena involved in wood drying: an excellent tool for assisting with the study of new processes,Tutorial, Proceedings of the Fifth International IUFRO Wood Drying Conference, Québec, Canada, 1996, 9–38.)subjectremainsamatterofsomescientificdebate(Keeyet al., 2000).Nevertheless, in the case of drying, the moisturecontent only decreases and some simplificationsapply. Here, only the most common way to expresscreep and mechanosorptive effect will be presented.The general formulation of the time dependency ofthe creep property involves the whole stress history:« ij ¼ð t 1J ijkl (t t 0 ) d s kldt 0 dt0 (36 :18)where J ijkl (t) is the creep compliance tensor and t isthe actual time. The experimental creep function isoften analyzed as a number of Kelvin elements inseries (Figure 36.32), each having the property ofa spring and dashpot in parallel (Genevaux, 1989;Martensson, 1992; Mohager and Toratti, 1993; Hanhijärvi, 1999 Passard and Perre, 2005). In the case ofuniaxial load, this leads toJ(t) ¼ J 0 1 þ XNa n (1 et t!n ) (36:19)n ¼1The temperature and moisture dependency of thatfunction can be expressed using a material time orchanging the characteristic time t n . The thermal activation,for example, is often expressed with the aid ofan Arrhenius law: t n ¼t 1 n exp DW n(36:20)RTTime t = 0 + highmoisture contentTime t lowmoisture contentLoad1 2 3 Drying1 2 3LoadFIGURE 36.28 Dimension changes of a specimen loaded during drying. (Adapted from Perré, P., The numerical modeling ofphysical and mechanical phenomena involved in wood drying: an excellent tool for assisting with the study of new processes,Tutorial, Proceedings of the Fifth International IUFRO Wood Drying Conference, Québec, Canada, 1996, 9–38.)ß 2006 by Taylor & Francis Group, LLC.

Second drying periodSecond drying periodEnd of dryingFSPTensilestressCompressivestressProngtest(a)(b)FIGURE 36.29 Appearance of drying stresses followingshrinkage. (Adapted from Perré , P., The numerical modelingof physical and mechanical phenomena involved inwood drying: an excellent tool for assisting with the studyof new processes, Tutorial, Proceedings of the Fifth InternationalIUFRO Wood Drying Conference, Qué bec, Canada,1996, 9–38.)where DW n is the activation energy associated toelement n.The mechanosorptive effect occurs as soon as themoisture content changes during load. The simplerway to express this effect consists in assuming thatthe strain rate depends linearly on both the stress fieldand the time derivative of the moisture content ḣ :_« msij ¼ m ijkl s kl (sign ḣ)ḣ (36:21)The mechanosorptive strain rate is always in the directionof the stress field, hence the factor sign _u inEquation 36.21.A three-dimensional resolution is very costly interms of calculation time and computer memoryspace. The need for such a cost is justified wheneverthe objective of the calculation lies in evaluating theoverall deformation of the board. In this way, theeffect of reaction wood, fiber angle, and propertyFSPX eq(a)End of drying(b)(c)(c)TensilestressCompressivestressFIGURE 36.30 Stress reversal due to the memory effect ofwood. (Adapted from Perré, P., The numerical modeling ofphysical and mechanical phenomena involved in wooddrying: an excellent tool for assisting with the studyof new processes, Tutorial, Proceedings of the Fifth InternationalIUFRO Wood Drying Conference, Québec,Canada, 1996, 9–38.)CupmethodFIGURE 36.31 Two experimental methods used to assessdrying stresses.variations can be analyzed. Good examples of thesepossibilities can be found in the literature (Dahlblomet al., 1994, 1996; Ormarsson, 1999). Nevertheless, tostudy the stress development within a section farfrom the ends of the board, a two-dimensional simulationis sufficient. A ‘‘planar displacement’’ formulationhas to be used in this case, assuming smalldisplacements (Perré and Passard, 1995; Chen et al.,1997a)orlargedisplacements(MaugetandPerré ,1999).36.2.4 .4 Stres s Deve lopm ent dur ing Dry ing:So me Exampl esAll examples presented in this section, except the nonsymmetriccase, refer to a flatsawn board of heartwood,20-mm thick and 40-mm wide. They have beencomputed using the computer code Transpore (Perréand Turner, 1996b and c, 1999; Mauget and Perré ,1999; Perré and Passard, 2002;). Figure 36.33 depictsthe moisture and stress fields calculated at differentdrying stages for quite severe conditions at a mediumtemperature level (T d ¼ 80 8C, T w ¼ 60 8C). After 6 hof drying, the external part of the section is within thehygroscopicrange,whichgivesrisetotensilestressduetoshrinkageinthezonesclosetotheexchangesurface.As a consequence of the mechanical equilibrium, internalzones undergo compressive stress. A negativeshear stress exists close to the edge (the right angleshavedecreased).Attheendofthedryingprocess(60h)the moisture content is almost equal to the EMC (7%)throughout the section. The stress reversal due to thememory effect of wood is clearly exhibited. It can benoted that the shear stress also changes in sign.When a face of a board is insulated, the dryingconditions are not symmetrical anymore andß 2006 by Taylor & Francis Group, LLC.

Element 1 Element 2 Element NElement 0FIGURE 36.32 Modeling the viscoelastic behavior of wood with the number of Kelvin elements in series.significant section deformations can be observed experimentally(Brandã o and Perré , 1996). Figure 36.34is an example of nonsymmetrical drying. In order toincrease the section deformation, a thin quartersawnboard has been simulated, here 5 mm by 80 mm. Thelarge displacement formulation is essential in thiscase. In this configuration, one part of the dryingstress is transformed into section deformation; hencethe stress-reversal phenomenon induces a negativefinal curvature.Thickness (cm)1.00.80.60.40.2Moisture content0.650.580.510.440.370.290.220.15Sigma xx0.00.00.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0Width (cm)Width (cm)Thickness (cm)1.00.80.60.40.21.541.240.930.630.320.01−0.29−0.60Thickness (cm)1.00.80.60.40.2Sigma xy0.00.0 0.5 1.0 1.5 2.0(a)Width (cm)0.150.090.03−0.03−0.09−0.15−0.21−0.28Thickness (cm)1.00.80.60.40.2Sigma yy0.00.0 0.5 1.0 1.5 2.0Width (cm)1.291.040.790.530.280.03−0.22−0.47Thickness (cm)Moisture content1.00.80.60.40.20.00.0 0.5 1.0 1.5 2.0Width (cm)0.080.080.070.070.070.070.070.07Thickness (cm)1.00.80.60.40.2Sigma xx0.00.0 0.5 1.0 1.5 2.0Width (cm)0.12−0.29−0.70−1.11−1.52−1.93−2.34−2.75aThickness (cm)1.00.80.60.40.2Sigma xy0.240.210.180.150.120.090.060.03Thickness (cm)1.00.80.60.40.2Sigma yy−0.10−0.35−0.60−0.86−1.11−1.36−1.62−1.870.00.0 0.5 1.0 1.5 2.0(b)Width (cm)0.00.0 0.5 1.0 1.5 2.0Width (cm)FIGURE 36.33 Moisture content and stress fields after (a) 6 h and (b) 60 h (T d ¼ 808C, T w ¼ 608C).ß 2006 by Taylor & Francis Group, LLC.

7 hMoisture: 0.05 0.09 0.14 0.18 0.22 0.26 0.31 0.3530 hFIGURE 36.34 Example of nonsymmetrical drying: moisture content and section deformation (80/608C).The next example illustrates three different constantdrying conditions chosen to analyze the possibilitiesof prediction (T d stands for dry-bulb temperatureand T w for wet-bulb temperature):1. T d ¼ 40 8C, T w ¼ 358C; mild conditions at lowtemperature (EMC ¼ 15%)2. T d ¼ 80 8C, T w ¼ 60 8C; rather severe conditionsat medium temperature (EMC ¼ 7%)3. T d ¼ 80 8C, T w ¼ 76 8C; mild conditions atmedium temperature (EMC ¼ 14%)Figure 36.35 depicts the variations of the averagedmoisture content and the stress level (direction parallelto the exchange surface) at different positions vs.time. All tests show a first drying period, withoutdrying stress, then a stage with tensile stress in theperipheral zones, and finally the last drying stage thatexhibits the stress reversal. However, the duration ofeach stage and the stress level depend strongly on thedrying conditions.For the mild drying conditions at low temperature(T d ¼ 408C, T w ¼ 35 8C), the drying time is ratherimportant.Thefirstdryingperiodlastsforaround10hand the stress level is high for both the second dryingstage and the final drying stage. The drying conditionsare mild concerning heat and mass transfer,while the temperature level is not high enough forthe creep field to relax the stress field; the final stresslevel reveals the importance of the memory effect.As a consequence of the low relative humidity ofair, the second test (T d ¼ 80 8C, T w ¼ 60 8C) is veryfast. However, both the maximum tensile stress leveland the final stress reversal are important; the rapidexternal transfer imposes a high moisture contentgradient within the board. In addition, the viscoelasticcreep is not sufficient to cancel the memory effect.At the beginning of drying, the board temperature isclose to the wet-bulb temperature, which is below theglass-transition zone (Geoffray 1984, Goreng 1963,Salmen 1984, Ostberg et al. 1990, Passard and Perre2001). At the end of drying, the temperature level issufficient for greenwood, but not for the dry part ofthe board to activate the viscoelastic behavior. Consequently,the outer parts, which are close to EMC,are below the glass-transition zone.In the third test (T d ¼ 80 8C, T w ¼ 76 8C), thedifference in moisture content between surface andcore remains low. The first drying period lasts animportant part of the total drying time. Due to thehigh value of EMC, the board temperature is alwaysabove the softening zone; consequently, all stresslevels remain very low. These conditions allow woodof good quality to be obtained relatively free of stressreversal with a moderate drying time (less than 150 hagainst 400 h for the low-temperature test).These simulations are in good agreement withnonsymmetrical drying experiments performed onoak (Quercus rubra) boards using the same dryingconditions (Figure 36.36; Perré , 2001). However, aß 2006 by Taylor & Francis Group, LLC.

Averaged moisture content (%)907560453015MoisturecontentSurface5 mm10 mmCenter1.51.00.50−0.5−1.0s xx (MPa)00 100 200 300 400 −1.5(a)Time (h)Averaged moisture content (%)907560453015MoisturecontentSurface5 mm10 mmCenter1.51.00.50−0.5−1.0s xx (MPa)0(b) 0 50 100 150 −1.5Time (h)Averaged moisture content (%)907560453015MoisturecontentSurface5 mm10 mmCenter1.51.00.50−0.5−1.0s xx (MPa)(c)00 50 100 150 200 −1.5Time (h)FIGURE 36.35 Averaged moisture content and s xx vs. time: (a) T d ¼ 408C, T w ¼ 358C; (b) T d ¼ 808C, T w ¼ 608C; and(c) T d ¼ 808C, T w ¼ 768C.closely similar schedule for another hardwood(Nothofagus truncata) resulted in gross deformationsand thermal degradation due to the high extractivescontent of the wood (Grace, 1996).Based on this reasoning, new drying procedureshave been devised and tested on different tropicalspecies, including numerous tests in industrial kiln of100-m 3 capacity (Aguiar and Perré , 2000b). The productquality was always very good, often with very littlechecking and rather less deformation than withconventional drying. Most importantly, this methodneeds only one half to one third of the time requiredfor drying according to conventional schedules.The code can also be used to test different dryingschedules (Figure 36.37). The first one (Schedule A) isrecommended for softwoods while the second oneß 2006 by Taylor & Francis Group, LLC.

FIGURE 36.36 Final section curvature of oak samples dried on their upper face with conditions n 2 (T d ¼ 808C, T w ¼608C) and n3 (T d ¼ 808C, T w ¼ 768C), respectively.(Schedule B) is recommended for hardwoods. ScheduleB proposes lower temperature levels and higherrelative humidity values. In this drying schedule,EMC decreases significantly only at the end of theprocess, when the board is supposed to be dry with alow moisture content gradient. As the first consequenceof these drying conditions, one can notice amuch longer drying time for Schedule B (130 hagainst 50 h). The first drying period lasts also for alonger time for the second procedure (30 h instead of10 h). It may be noted that the first drying–periodduration represents about the same percentage of thetotal drying time for both schedules. This remark stillstands for the stress level. One can easily consider arelative time (current time over total drying time) atwhich all curves have the same shape and the samestress magnitude.In the first example, the advantages to be gainedfrom using a high relative humidity level (low moisturecontent gradient, high hygroactivation of theviscoelastic behavior) hardly offset the negative effectof the low temperature levels (slow moisturemigration and low thermoactivation of the viscoelasticbehavior). A careful analysis of these approachesis very promising. New rules can bederived to improve existing drying schedules or todevise innovative drying procedures.Schedule AAverageMoistureContent (%)Dry-BulbTemperature(8C)Wet-BulbTemperature(8C)RelativeHumidity(%)Green 71 66 8050 76.5 68.5 7030 82 70.5 6020 88 67.5 40Schedule BAverageMoistureContent (%)Dry-BulbTemperature(8C)Wet-BulbTemperature(8C)RelativeHumidity(%)Green 40.5 38 8560 40.5 37 8040 43.5 39 7535 43.5 38 7030 46 39.5 6525 51.5 43 6020 60 47.5 5015 65.5 49 40ß 2006 by Taylor & Francis Group, LLC.

Moisture content (%)12010080604020Average moisturecontent(%)Green503020Dry-bulbtemperature(8C)7176.58288Wet-bulbtemperature(8C)6668.570.567.5Temperature edgeTemperature centerMC edgeMC centerMC surfaceMC averageRelativehumidity(%)80706040908070605040Temperature (8C)Moisture content (%)10080604020Average moisturecontent(%)Green60403530252015Dry-bulbtemperature(8C)40.540.543.543.54651.56065.5Wet-bulbtemperature(8C)3837393839.54347.549Relativehumidity(%)858075706560504070Temperature edgeTemperature centerMC edgeMC centerMC surfaceMC average60504030Temperature (8C)00 20Time (h)40 60 302Surface5 mm110 mmCenter00 50Time (h)100 150 202Surface5 mm110 mmCenters xx (MPa)0s xx (MPa)0−1−1−20 20 40 60Time (h)Schedule A−20 25 50 75 100 125 150Time (h)Schedule BFIGURE 36.37 Simulation of two different drying schedules: moisture content, temperature, and stress level at differentpositions vs. time.An alternative procedure in improving kiln scheduleshas beenthe estimationof strainlevels toprovideasafe envelope of dry- and wet-bulb temperatures in kilnoperation. One industrial method uses acoustic emissionson sample boards to determine a stress thresholdto keep the surface strain under 50 to 75% of the estimatedultimate value (Doe et al., 1996b). Later optimizedschedules have been developed by Langrish et al.(1997) using a model predictive control technique tokeep within the strain criterion. The technique reducedthe number of small- and medium-sized cracks, bothinternallyandatthesurface,toless than one quarter ofthose observed in the original conventional schedule.36.2.5 DRYING QUALITY36.2.5 .1 F actors Affecti ng the Dry ing Dura tionLet us assume that the duration of drying is the singleimportant factor. In this case, heat and mass transferonly are to be considered (Figure 36.38). Some parametersdepending on the load are important, but they arenot under control:. The thickness is a very important parameter(roughly speaking, the drying time increases asthe thickness doubled). The transfer properties of the wood (diffusivity,permeability, capillary pressure, thermal conductivity,etc.)In conventional drying, the controlled parametersare the dry- and wet-bulb temperatures as well as thevelocity of the airflow. These three parameters determinethe external heat- and mass-transfer rates:. The ‘‘drying potential’’ of the air flow is theheat-transfer coefficient (which increases withthe air velocity) times the difference betweendry- and wet-bulb temperatures.ß 2006 by Taylor & Francis Group, LLC.

ParametersLow valueHigh valueExternalVelocityDrying potential(dry bulb−wet bulb)ThicknessInternalMass diffusivityThermal diffusivityTemperatureFIGURE 36.38 Guidelines on how to obtain a fast drying operation (from good to excellent and from poor todisastrous). (Adapted from Perré, P., The drying of wood: the benefit of fundamental research to shift fromimprovement to innovation, Proceedings of the Seventh International IUFRO Wood Drying Conference, Tokyo, Japan, 2001,2–13.). The air velocity also plays an important role inuniform drying within the stack. However, itseffect becomes less important as the drying progresseswhen internal transfer mainly controlsthe moisture migration.In addition, we have to keep in mind some moresubtle effects:. The internal transfer (diffusion, liquid migration)becomes easier when the temperaturelevel increases.. Above the boiling point of water, an additionaldriving force, the gradient of total pressure, actswith a dramatic effect (Perré , 1995). Dryingby internal vaporization takes place in suchconditions.. The internal transfer rates depend on the localmoisturecontent(liquidmigrationisusuallymuchmore effective than bound or vapor diffusion).In addition, diffusion becomes very slow whenthe bound-water content decreases toward zero.In general, the drying time is reduced when thevelocity and the temperature of air are high and itsrelative humidity is low. However, an excessively lowrelative humidity may produce a surface zone with lowmoisture content, thus reducing moisture migrationclose to the surface. All high-temperature arrangements(convective drying at high temperature, vacuumdrying, contact drying, etc.) are processes that accelerateinternal moisture migration due to the overpressuresgenerated within the product.Finally, drying with electromagnetic heating(microwave or radio frequency) offers an entirelynew possibility: any internal temperature can beattained without resorting to a heating medium suchas a hot gas.36.2.5 .2 Fac tors Affecti ng the Dry ing Qual ityDrying stresses originate from shrinkage; as soon asthe shrinkage field within the board is not geometricallycompatible, a stress field develops in the material,which is responsible for mechanical degradation. Inorder to reduce the stress level throughout the process,and thereby the surface checking, the internalchecking, and the residual stress, several conditionsshould be fulfilled (Figure 36.39):. Low shrinkage coefficients, not under control. Small thickness, not under control. Low moisture content values between surfaceand core. Retaining important possibilities of viscoelasticcreep (mechanosorptive creep is always a sourceof stress reversal); such an effect is obtained athigh temperatures, provided the moisture contentis sufficiently high (Irvine, 1984)It may be noted that a low-temperature level is sometimesdesired (for example to avoid collapse), becausea high-temperature level may produce thermal degradationor discoloration.36.2.5 .3 Cr iteria for Obtaini ng a Fast and GoodDry ing Pro cessA fast and good drying process should incorporate thecriteria listed in Section 36.2.5.1 and Section 36.2.5.2,ß 2006 by Taylor & Francis Group, LLC.

Second drying periodTensionParametersLow valueHigh valueCompressionThicknessTensionEnd of dryingCompressionMoisture content(bound) gradient(center−surface)ShrinkageTensionCompressionSurface moisturecontentTemperatureFIGURE 36.39 Guidelines on how to obtain a good-quality product (from good to excellent and from poor todisastrous). (Adapted from Perré, P., The drying of wood: the benefit of fundamental research to shift fromimprovement to innovation, Proceedings of the Seventh International IUFRO Wood Drying Conference, Tokyo, Japan, 2001,2–13.)which are, to a large extent, contradictory. Numerousmechanisms involved during drying have to be considered(Figure 36.40).Because of this complexity, compromises have tobe found. Nevertheless, some general rules can belisted (Figure 36.41):. Rule 1: high relative humidit To y. ensure a lowmoisture content gradient, one way is to reducethe drying potential (wet-bulb depression) asmuch as possible. In addition, this conditionimposes a relatively high value of EMC (onlyone part of shrinkage is effected and the influenceof temperature on the viscoelastic creep isnot inhibited by a relatively low moisture contentlevel). However, a high relative humidityvalue can activate the development of fungi.. Rule 2: high tem perature. A high value of temperatureis most often a positive factor. Thisaccelerates the internal moisture transfer andactivates the viscoelastic creep. However, careshould be taken with sensitive species; high temperaturelevels can increase the risk of collapse,problems of color, or even thermal degradationof the wood constituents.. Rule 3: high air velocity. A high air velocitypromotes good uniformity of drying throughoutthe stack. However, a higher velocity increasesthe electricity consumption and may produce,by the heat-transfer coefficient, an excessivelyhigh external transfer flux, which is opposite tothe effect intended in Rule 1.Concerning moisture transfer, Rule 1 and Rule 2meanthatinternaltransferhastobeincreasedwhereasexternal transfer should be reduced. Exceeding theboiling point of water is decisive for internal transfer.However, these rules also require the temperature tobe high with a high value of relative humidity. Suchconditions may be difficult to ensure for certaindryers. Innovative drying procedures may need newdryer designs. Finally, too often, the effect of temperatureand moisture content on the viscoelasticbehavior is disregarded in the optimization of dryingschedules. The situation strongly differs from onespecies to the other. Usually, softwood species arequite easily dried. On the other hand, hardwoods areoften intractable because of their low permeability.36.3 KILN SCALE36.3.1 LUMBER QUALITYThe ultimate fitness for the purpose of dried lumberdepends not only on the chosen drying conditions butalso on the lumber quality itself. This quality may bethoughtofin terms of gross defects suchas knotsaswellasintrinsicwoodpropertiessuchasthedegreeofanisotropy.Drying,whichcausesanisotropicshrinkage,interactswith various wood features in various ways. Theobjective of kiln seasoning, then, is to acknowledge thisinteraction by setting process conditions that yield driedlumber to the specifications in terms of a grade for anend use. There is a world of difference between dryingdecorative hardwoods and drying structural softwoods.Increasingly, kiln operators are drying wood fromever younger, fast-growing stands rather than from mature,old-growth forest. The drying behavior of this newkind of wood is requiring operators to adapt traditionalprocesses on the basis of better understanding of thedrying mechanism, as outlined in the previous sections.ß 2006 by Taylor & Francis Group, LLC.

Transition zone0.55Relative density at breast height0.500.450.400.350.300.25Douglas-firWestern larchWestern hemlockYellow - cypressLodgepole pineSitka spruceInterior spruceSubalpine firWestern red cedar0.200Juvenile wood10 20 30Age (y)Mature wood40 50 60FIGURE 36.43 Trends in basic density at breast height for commercial second-growth softwoods in British Columbia.(Adapted from Josza, L.A. and Middleton, G.R., A discussion of wood quality attributes and their practical implications,Forintek Canada Special Publ. SP-34, 1994.)In practice, however, it is not easy to distinguish theeffect of density with that of moisture content.Within-ring density variations can cause problemsbecause of the differential shrinkage at the ringboundary. Subsequently, severe drying stresses maycause deformation and internal checking (Booker,1994). Further, a low absolute density in earlywoodcan result in collapse on drying, particularly underhigh-temperature conditions (Booker, 1996).Within-tree variations in density can be highlysignificant. Cown and McConchie (1983) show thatthe density in a 24-year-old radiata pine tree can varyfrom 300 kg m 3 in the top log to greater than 450 kgm 3 in the outer wood of the butt log. Consequently,the drying kinetics of boards taken from the same logmay be markedly different (Davis, 2001). Trends inbasic density for a number of second-growth softwoodsare illustrated in Figure 36.43.The situation is more complex with hardwoods.The growth rate has little effect on the wood propertiesof diffuse-porous hardwoods, but has a markedimpact on the density of ring-porous hardwoods. Unlikesoftwoods, these produce denser wood when fastgrown.Regardless of the species or where the forests areestablished, the variation in wood properties betweentrees is very great and can be great even in boardssawn from the same tree. In particular, the socialstatus of the tree in the stand (whether dominatedor dominant trees) has a great effect on the growthrate in diameter and the occurrence of reactionwood. Table 36.10 lists some of the characteristics ofTABLE 36.10Characteristics and Properties of Reaction Wood Compared with Corresponding Normal WoodFeatures Compression Wood Tension WoodPhysical characteristics Darker in color and very hard Darker in color and silvery sheen in mosttemperate hardwoodsDensity 10–100% greater 10–30% greaterLongitudinal shrinkage Order of magnitude greater (up to several fold) About fivefold greaterWarp on drying Liable to warp badly Can warp and is liable to collapseStrength Comparable strength, does not reflect higher density Superior strengthSource: Keey, R.B., Langrish, T.A.L., and Walker, J.C.F., The Kiln-Drying of Lumber, Springer, Berlin, 2000.ß 2006 by Taylor & Francis Group, LLC.

eaction wood produced by environmental factors inthe forest. Compression wood has been found to havea significant effect on drying, with lower drying ratesover the moisture content range of 40 to 100% forboards of P. radiata (Davis et al., 2002). This reductionis attributed to the lower permeability of thecompression-wood zone, with its denser wood andmore thick-walled cells.Large microfibril angles are found in both compressionwood and normal wood near the pith, inducinglarger than normal longitudinal shrinkage and agreater tendency to warp. The longitudinal shrinkageof tension wood, although larger than normal wood,is less than in compression wood.36.3.1 .3 Sa wmilling Strategi esTimell (1986) describes sawmilling strategies to reducewarp in juvenile lumber and compressionwood. The cutting pattern influences the quality ofdrying either by releasing stresses on sawing or thecutting induces a stress pattern in the board that canbe balanced on drying. Warp has become more of aproblem with harvesting from second-growth forestsof short rotation, which contain proportionatelymore juvenile wood than lumber from old-growthforests. Crook and bow are most severe in the corewood of the butt log and where compression wood isencountered; whereas twist is most severe in the corewood of the upper logs where spiral-grain angles arelarge and changing rapidly. The variation of propertiesabout the mean is the critical factor with core wood.Vá zquez (2001) examines silvicultural practiceand sawmilling strategies to counteract the effect ofgrowth stresses in fast-grown Iberian eucalypts (Eucalyptusglobulus). A model of the stress distributionthat enables to determine the deformations on sawingis given. The appearance of checks and warps can belimited by the choice of a suitable sawing pattern, asshown in Figure 36.44. Pang and Haslett (2002) notethat the residual drying stresses in quartersawnP. radiata boards are less than flatsawn boards, particularlyunder low-temperature drying conditionswhen the effect of mechanosorptive stress relief isrelatively minor. The difference in behavior is attributedto the lesser shrinkage in the width direction withquatersawn boards.36.3.2 KILN DESIGNAlthough there are differences in detail betweenmanufacturers, a lumber kiln is essentially a specialpurposeroom fitted with overhead fans for circulatingthe drying air and heating coils for maintainingthe air (and thus the wood) temperature at the setlevels. The moisture in the air is controlled by meansof opening the vents in the kiln’s roof, thus governingthe amount of moist air that returns to the fan whichis to be mixed with the fresh air drawn in. Althoughmany kilns are operated batchwise for ease of controllingthe drying conditions, which may changethrough out the drying schedule, a kiln can be continuouslyworked by arranging the lumber to beslowly railed through the chamber. In this lattercase, the drying schedule is maintained by varying thetemperature and humidity settings along the length ofthe kiln. There is some increase in interest in continuouskilns, which might provide energy savings andbetter quality control (McLean, 2003).The lumber is stacked externally in a rectangularpile on a low, flat-bed trolley, with rows of boardsseparated by wooden stickers of uniform thickness toprovide duct-like spaces between the boards for thekiln air to flow through. The boards may be stackedFIGURE 36.44 Sawing pattern to limit the appearance of checks. (Adapted from Vázquez, M.C.T., Tensiones de crecimiento enEucalyptus globulus de Galicia (España). Influencia de la silvicultura y estrategias de aserrado (Growth stresses in E. globulusfrom Galicia (Spain). Influence of silviculture and sawing strategies), Maderas: Ciencia Tecnologia, 2(1), 68–89, 2001.)ß 2006 by Taylor & Francis Group, LLC.

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.

where the boards butt up due to shrinkage on dryingor unevenness in thickness and there may be gaps dueto the presence of boards with uneven length. Fluiddynamicsimulation of the flow over inline slabs(Langrish et al., 1993) has suggested that gaps assmall as 1 mm might be sufficient to disrupt theflow, with circulation within the gaps themselves.The magnitude of the side gaps in the board influencesboth the drying rates and the development ofdrying stresses (Langrish, 1999), so that experimentson the drying of single boards (as often done) mayyield uncertain information about the drying of a loadof the same wood in a kiln. With regard to variationsin board thickness, Haslett (1998) recommends thatthe coefficient of variation for the board thicknessmust be under 0.04 for successful high-temperaturedrying.Whenever kiln stacks are built from randomlengthlumber so that every second board is flush ateach end of the stack, variations in openness of thestack result. This gives two different zones: (1) acentral zone in which all the available space is filledand (2) two end zones where alternate boards aremissing (Salin, 2001). This arrangement results inhigher within-stack velocities (about 30% higher) inthe center than in the end zones, with correspondingimplications in the variation in drying behavior.36.3.2.2 Moisture-Evaporation ConsiderationsThe airflow through the stack influences the magnitudeof the local airside mass-transfer coefficient, andthus the evaporation into the airstream. Particularly,at the higher air velocities used in high-temperaturedrying, any variations in these transfer coefficientshave a significant effect on the uniformity of dryingthroughout the stack.The air-inlet face of the lumber stack presents a setof blunt edges to the incident airflow, resulting in anenhancement of the mass-transfer coefficients nearthe leading edges (Kho et al., 1990). Computationalstudies (Sun, 2001) of the flow over a series of slabswith inline gaps suggest that for gaps greater thanabout 2 mm there will be similar, but lesser, enhancementsat subsequent boards downstream.With Scandinavian stacking practice, the endzones of the stack dry faster than the central, fullyfilled part (Salin and Öhman, 1998). The lower airvelocity in the ends is more than compensated byhigher heat-transfer coefficients associated with theflow disturbance and smaller wood volume. (Thereis a smaller decrease in temperature and increase inhumidity along the stack in the airflow direction.) Ingeneral, it is expected that the local transfer coefficientsdiminish with distance in the airflow directiondue to a thickening of the boundary layer. This variationand the downwind accumulation of moisture inthe airstream result in the maximum possible evaporationrate dwindling with distance from the air inlet tothe air outlet from the stack.Traditionally, the variation of evaporative ratesacross the stack has been counteracted by the installationof bidirectional fans and by periodically reversingthe airflow direction through the stack. Thispolicy has minimal effect on the drying rates in thecenter of the stack, but reduces the variation in behaviorbetween the two end zones. If only moisturecontent variations are considered, many reversals arenot needed to achieve this equalization (Pang et al.,1995; Nijdam and Keey, 1996; Wagner et al., 1996).However, if stress development in the surface layerwith the likelihood of checking is taken into account,then the flow reversals for a timber such as Pinussylvestris should be less than 2 h apart (Salin andÖhman, 1998). A period of 4 h is a common industrialpractice for permeable softwoods such as P. radiata.36.3.3 KILN OPERATIONTo understand kiln-wide behavior, it is useful to invokethe concept of the characteristic drying curve(van Meel, 1958; Keey, 1978). The concept reducesthe drying kinetics for a specific material of specificgeometry to a single function of the local averagedmoisture content. The concept when applied to thekiln drying of lumber boards is rough, not only due tovariations in drying behavior between boards (Daviset al., 2001) but also due to embedded assumptions inthe concept itself. Nevertheless, it is a sufficient representationof drying behavior to determine the effectof kiln parameters on the course of drying. Thesethings, such as the uniformity of the airflow, thenumber of airflow reversals, the velocity, temperature,and humidity settings, are all those under thecontrol of the kiln operator.The concept of a characteristic drying curve leads tothe following expression for the moisture-evaporationrate per unit of exposed board surface:N v ¼ f bf(Y W Y G ) (36:22)where f is the evaporation rate relative to that at agiven moisture content (either the initial or some criticalvalue of transition from unhindered drying), and isa unique function of the mean free moisture content;b is the external (airside) mass-transfer coefficient; f isthe humidity-potential coefficient, which takes a constantvalue when the wet-bulb temperature remains thesame throughout the kiln; Y W is the saturation humidityat the wet-bulb temperature; and Y G is the bulk-airß 2006 by Taylor & Francis Group, LLC.

humidity between the boards. Although the values ofthe parameter f depend on the extent of drying that hastaken place and the nature of the wood itself, the otherparameters are under the control of the kiln operatorby varying either the stack extent, the kiln-air velocity,or the dry- and wet-bulb temperatures.Consideration (Keey et al., 2000) of the moisturetransfer over a small zone in the kiln leads to the equations,which can be conveniently expressed in nondimensionalform as follows:∂Φ∂qInletOutletz@F@u ¼ @P ¼ f P (36:23)@zwhere F is the moisture content relative to unit valuewhen f is 1, P is the humidity potential (Y W Y G )relative to unit value at the air inlet, z is a nondimensionalextent of the kiln in the airflow direction andis a weak function of the kiln-air velocity, and u isthe relative time of drying which itself depends uponthe value of z for the kiln stack and the capacity of theair to pick up moisture. These equations can be solvedif the parameter f is known as a function of themoisture content (averaged over the board thickness).They imply that the rate of change of moisture contentwith time (i.e., the drying rate) directly dependson the rate at which the bulk air humidifies in itspassage through the kiln. The drying rate is alsodirectly dependent upon the humidity potential (thedriving force for the evaporation) and the parameterf, which reflects the ease of moisture movementthrough the wood.These equations have been used to examine theinfluence of kiln variables on the course of drying,including the impact of exhaust-air recycle and theswitching of airflow direction (e.g., Tetzlaff, 1967;Ashworth, 1977; Ashworth and Keey, 1979; Keeyand Pang, 1994; Nijdam and Keey, 1996). A summaryof this work is given by Keey et al. (2000).Figure 36.46 shows the variation in the dimensionlessdrying rate, @F/ @u, as a function of the normalizedmoisture content F for one-way flow througha single-tracked kiln, for which the nondimensionalextent z in the airflow direction is 1. In the case of atimber, whose initial green moisture content is equalto the critical point of transition between unhinderedand hindered drying by the rate of moisture movementthrough the wood, the drying rate falls monotonicallywith both time and distance in the airflowdirection. The effect with time is due to the intrinsicdrying rate as the wood dries out, whereas that withdistance is due to the progressive humidification inthe kiln. Whenever there is free moisture content inthe wood above the critical point, the drying-rateprofiles are more complex. As soon as the critical(a)∂Φ∂q(b)0InletΦOutlet0 Φ1point is reached at the air-inlet face of the stack, theintrinsic drying rate falls there, resulting in less progressivehumidification in the kiln; the downstreamdrying rates can now rise until the local critical pointis attained. The greatest effect is seen at the outlet faceof the stack.Figure 36.47 shows the effect of reversing the airflowdirection through the stack. On switching over theflow, what was once the ‘‘inlet’’ face now becomes the‘‘outlet’’ face, and vice versa, giving a temporary boostto the drying rate at the former outlet and a moderationto that at the former inlet. The rates within the centerof the kiln are essentially unaffected. With flowswitchovers, the leaflike moisture content profiles becomemore pinched with a lesser variation in moisturecontent across the kiln.z1 = Φ 0Φ 0FIGURE 36.46 Normalized drying rates in a kiln with anextent z of 1 and one-way airflow as a function of boardaveragedmoisture contents: (a) an indicative hardwood withf ¼ F, (b) an indicative sapwood of a softwood with f ¼ F 0.5 .(Adapted from Keey, R.B., Langrish, T.A.L., and Walker,J.C.F., The Kiln-Drying of Lumber, Springer, Berlin, 2000.)ß 2006 by Taylor & Francis Group, LLC.

1.2Normalized moisture content (Φ)1.00.80.60.40.2Air-outlet faceMiddleAir-inlet faceNormalized drying rate (dΦ/dq)1.0Air-inlet face0.8Middle0.6Air-outlet face0.40.2(a)000100 200 300 400 500 0 100 200 300 400 500Normalized time (q)Normalized moisture content (Φ)1.21.00.80.60.40.200(b)OutletMiddleInletNormalized drying rate (dΦ/dq)1.00.80.60.40.2InletMiddleOutletInletOutlet0100 200 300 400 500 0 100 200 300 400 500Normalized time (q)FIGURE 36.47 Board-average moisture contents and normalized drying rates as a function of time and distance in theairflow direction for a twin-stack kiln without reversals in the schedule (a) and for two flow reversals in the schedule (b). Theprofiles represent the drying of 100 50-mm sapwood Pinus radiata dried at 77/65.58C and an air velocity of 2.5 m s 1 .(Adapted from Tetzlaff, A.R., An investigation of drying schedules when kiln-drying radiata pine, B.E. Report, University ofCanterbury, New Zealand, 1967.)Typically, lumber kilns operate at very high ratiosof recycled air to that discharged through the vent tothe outside air to maintain the wet-bulb temperatureat the scheduled values. For that reason, commercialkilns appear to operate under very steamy conditionsto the casual observer. The high degree of air recyclemeans that small deviations in evaporation are fedback to the air-inlet face of the stack, disturbing theconditions there. This disturbance is then propagatedthrough the stack. In theory, with high recycle ratiosand extensive dryers (z > 1), it is possible, once thewood has reached the critical moisture content at theair-inlet face, for internal drying rates to rise abovethose for very wet greenwood at the air-inlet face ofthe stack initially and even to exceed them (Keey,1968). Although the necessary combination of factorsto get substantial rate enhancements is unlikely in thekiln drying of most lumber species, the potential forincreases in drying rate (and strain development)should be borne in mind.36.3.4 PRACTICAL CONSIDERATIONSVarious works give a detailed overview of kilnpractice. Such overviews include those by Pratt andTurner (1986), Boone et al. (1988), Hilderbrand (1989),Mackay and Oliveira (1989), Simpson (1992), andHaslett (1998).ß 2006 by Taylor & Francis Group, LLC.

36.3.4.1 Schedule DevelopmentKiln schedules are of two kinds. A tropical forest containsmany species, often diffusely spread, so that individualidentification of species and separate drying israrely economic or feasible. Economic drying of mixedspecies involves the grouping of these according to somecriteria. Simpson and Baah (1989), for example, choosebasic density and initial moisture content, on thegrounds that high-density, moist lumber is the mostsusceptible to drying defects and requires the longestdrying times. Using this system, Hidayat and Simpson(1994) were about to define drying-time factors for 12species groups. On the other hand, temperate productionforests may be confined to a single kind of tree or alimited range of species, and comprehensive speciesspecificschedules can be developed for these. For example,Haslett (1998) specifies various schedules for asingle species, P. radiata, according to the grade andend use of the wood.Traditionally, schedules have been developed on acautious, trial-and-error basis from small-scale testson the woods of interest. Pandey (2001) notes that theseven standard, empirically derived schedules fornearly 150 Indian woods could be fitted to a diffusionalmodel. Brandaõ and Perré (1996) propose adrying test at 908C on small boards to provide informationon drying rates and deformation criteria. Thisleads to an alternative species-grouping approach tothat put forward by Hidayat and Simpson (1994).Increasingly, however, detailed simulation of themass-transfer and strain-development behavior isused as a basis for determining appropriate kilntemperaturesettings (e.g., Langrish et al., 1997; Aguiarand Perré, 2000a; Nijdam et al., 2000).Kiln drying involves both the transfer of moisturethrough the boards and the evaporation from theexposed surfaces into the air circulating through thekiln. These processes must be kept in balance tomaintain the fastest possible drying rate. With impermeabletimbers, the transfer processes within thewood are rate-limiting. The wood temperature is theimportant variable. With permeable timbers, particularlyunder high-temperature conditions, externaltransfer mechanisms become important and highkiln-air velocities (7 m s 1 and higher) are used toenhance the heat- and mass-transfer coefficients.To sustain the rate of drying as the wood driesout, either the dry-bulb temperature or the wet-bulbdepression may be raised, thus providing a greateroverall driving force for the drying process. However,faster drying rates can lead to steeper moisture contentgradients with the risk of excessive strain developmentand checking. Thus most schedules specifyrelatively small wet-bulb depressions, correspondingto relatively high EMC at kiln temperatures, producingmodest surface shrinkage. In a conventionalschedule the wet-bulb depression is typically about5 to 108C, whereas for high-temperature dryingwet-bulb depressions can be 508C or more.Whenever it is important to avoid significant colordevelopment in the wood on drying, low kiln temperatureshave to be used. Even though in New Zealandwhere accelerated conventional schedules (with dry/wet-bulb temperatures of 90/608C) are used to dryappearance-grade timbers, commercially, kiln temperaturesnot greater than 508C are employed to getvery pale wood (Keey, 2003).Although higher airflows seem justifiable at thestart of a schedule when the evaporation from theexposed surface is more controlling, there is somedoubt whether there is an economical advantage indesigning kilns with variable-speed fans (Riley andHaslett, 1996). However, one manufacturer claims significantfan-energy savings by reducing the rate ofcirculating air toward the end of the schedule in thedrying of a softwood without causing any significantextension of drying time (Fogarty, Priv. Comm., 2002).36.3.4.2 Kiln ControlAs the basic control of the drying process dependsupon the wet- and dry-bulb readings, control of thesetemperatures has been the normal method of maintainingthe drying schedule. Most single-zone, steamheatedkilns mount split bulbs under the overheadheating coils, one in each plenum some distance offthe floor (Culpepper, 1990). In kilns divided into twozones along their length, the bulbs are placed midwayin each zone. The thermometer bulbs must be protectedfrom radiation from hot surfaces and have anadequate air circulation over them. Careful locationof the bulbs is important, as significant air-temperaturegradients can exist in the plenum spaces.Simple lumber-drying installations having one ortwo kilns might have simple electronic-control systemswith discrete programmable controls, timers,and chart recorders for temperature and steamingcontrol. A more sophisticated system for a mediumsizedfaculty might have a computer-based systemrunning on windows-type software to give a visualreadout of kiln conditions during the schedule (Keeyet al., 2000). Endpoint determination can be based onthe temperature drop across the load (TDAL), whichis a function of the extent of drying (Taylor andLandoch, 1990; Martin et al., 1995).As pointed out by Morén (2001), TDAL reflectsthe average drying rate in the kiln directly. He suggeststhat monitoring of the TDAL provides amethod of ‘‘adaptive kiln control,’’ with the TDALß 2006 by Taylor & Francis Group, LLC.

set at a constant value at constant wet-bulb temperatureafter an initialization period, followed by aperiod at constant dry-bulb temperature.The measurement of the boards’ moisture content,at the end of drying, has frequently been done withhand-held resistivity meters, often complementedwith more time-consuming gravimetric measurementswith sample boards (EN 13183-1). The Standard EN13183-2, Round and Sawn Lumber—Procedure toMeasure the Moisture Content, employs a hammerprobe with insulated measuring pins, which areinserted to a depth of approximately one third ofthe board’s thickness, and is valid over the moisturecontent range from about 7 to 30%. This techniquehas been adapted by inserting probes in the kilnstacks to give a continuous reading of the moisturecontent in the later stages of drying. Nonpenetrativesystems are available in which the resistivity probesare aluminum fillets placed along the length of thelumber charge to measure the resistance of all theboards that the probes are in contact with. All resistivemethods, however, are limited to measuring moisturecontents from just above fiber saturation andbelow, with one system having a claimed repeatabilityof +2% when the average moisture content is lessthan 16% (Furniss, Priv. Comm., 2002).The alternative use of microwave-sensing technologiesis attractive in enabling a wider range ofmoisture contents to be determined from green todry (Holmes and Riley, 1996; Riley and Holmes,2001). The first prototype tested used a waveguide,40 20 1.2 mm, with angled slots in the long face,connected by a high-temperature coaxial cable to theoscillator and placed in a stack so that the slots werein contact with the lower face of a board. A laterdevelopment enabled the possibility of the stack’smoisture distribution to be determined. The experimentshave demonstrated the potential for the manufactureof a rugged industrial system. However, unlessthe timber is very uniform in properties, this techniqueis uncertain as the relationship between themicrowave signal and the moisture content becomesfuzzy because of factors such as variations in wooddensity among the boards.An online contact-free method of measuring themoisture content of wood, which involves traversingthe boards on a band between sensor heads, could bedeveloped based on techniques used in papermaking.The method would be suitable for sorting boardsafter drying. At least one such method is commerciallyavailable, but the measurement principle hasnot been divulged (Smith, Priv. Comm., 2002).The benefits of even a relatively simple kilncontrolsystem are illustrated in an example quotedby Culpepper (1990). The incorporation of aprogrammable logic controller at one site increasedthe overall grade recovery from 70.7 to 81.9%, with areduction in energy costs of 43% for steam and 10%for electrical power. The total saving represented a1.2-y payback on the investment.36.3.4.3 Volatile EmissionsAcidic corrosion is a well-recognized feature of kilndrying, particularly with certain species such as oaks.Packman (1960) notes that, of some 150 species studiesof both hardwoods and softwoods, the majority(80%) has a pH between 4 and 6 and only one had apH consistently above 7. In an extreme case, with ahardwood of high extractives content, sufficiently severerusting of steel trolley wheels and other ferrousfittings in a pilot-plant kiln required their replacementafter a 2-week period. Extensive use of aluminumlinings with plastic piping in modern kilns has largelyavoided the problems of corrosion.The corrosive nature of the volatile substances releasedduring kiln drying is derived from the presence offree acetic acid in the wood and from the hydrolysis ofthe various acetyl groups attached to the hemicelluloses.Softwoods generally yield greater emissions thanhardwoods, because the latter are normally dried atlower temperatures and have lower resin content.Some volatile substances, such as formaldehyde, comefrom the thermal degradation of the hemicelluloses andlignin, whereas green pines release terpenes and theirderivatives. Table 36.11 lists the concentrations of volatileemissions from two high-temperature schedules.Milota (2001) compared the emissions from variousNorth American lumber species in a small-scale kilnTABLE 36.11Concentrations of Volatile Emissions Arisingfrom Two High-Temperature Kiln Schedulesfor Pinus radiataCompoundConcentration, g m 3 at dry/wet-bulb temperatures (8C)120/70 140/90Formaldehyde 19.5 31.0Acetic acid 21.7 38.2Monoterpenes 34.8 66.4Hydroxylated monoterpenes 12.6 10.7Condenser residues(resins and fatty acids)16.4 16.4Source: McDonald, K.A. and Wastney, S., Analysis of volatileemission from kiln drying of radiata pine, Proceedings of theEighth International Symposium on Wood Pulp Chemistry, Vol. 3,431–436, 1995.ß 2006 by Taylor & Francis Group, LLC.

with those from the same wood in a commercial kiln.The laboratory kiln functioned like the mill kiln withrespect to venting characteristics, temperature, andhumidity, but it dried the wood faster, possibly becausethe load is narrower. In general, the small-scale methodwas fairly repeatable for the determination of volatileorganic chemicals, with a standard deviation of 8 to16% of the mean value, but the repeatability was poorerfor methanol and formaldehyde.36.3.4.4 Equalization and Stress ReliefAt the end of kiln drying there is always some board-toboardvariations in moisture content, arising from theinherent differences in drying characteristics of individualboards and the progressive humidification of thecirculating air. Because kiln drying is basically a processin which the boards are forced to reach a new equilibrium,moisture content profiles also exist within eachpiece of wood. With moisture-based schedules, the kilnconditions are reset at the end of the schedule to give anEMC that is 2 to 3% below the desired value. Thisstrategy prevents overdrying of the drier boards whileallowing the wetter boards to dry further toward thetarget end moisture content. Some authorities (e.g.,Haslett, 1998) recommend that the softwood lumbershould be slightly overdried to ensure that the moisturecontent variation both within and between boards becomessmall. Any subsequent steaming, which is undertakenfor stress relief, will raise the moisture content ofthe overdried load toward the specified value.Kiln schedules are designed to dry the lumber asfast as possible without causing unacceptable defectsto appear due to excessive strain development. Residualstresses in the wood must be relieved if thelumber is to be further processed. Steaming is a commonmethod of doing this.In smaller installations, steaming may be done inthe kiln; but, in larger installations, a separate chambersupplied with low-pressure, saturated steam isused for this purpose. Such chambers are not normallysupplied with fans, and stratification of thesteam and air can be a problem. Lumber that ishigh-temperature dried should be cooled so that thewood temperature falls to about 70 to 958C to enablethe wood to pick up moisture. The steaming induces areversal of the moisture content profile through thewood, with concomitant reduction in the residualstresses.Chen et al. (1997b) examined various stress-reliefstrategies for sapwood boards of softwoods and havefound that other procedures are suitable in additionto final cooling and steaming. These include simplecooling under cover or the use of a schedule consistingof intermittent drying and conditioning cycles.36.3.5 LESS-COMMON DRYING METHODSThe majority of lumber-drying installations are convectivedrying chambers worked at atmospheric conditionsand temperatures not greatly above ambient. Forstructural-grade, permeable softwoods, however, whencolor development is not a prime concern, elevatedtemperatures can be used to get very fast drying processes.Designs under consideration include kilns beingworked to temperatures of 2008C, with air velocitiesbetween the boards up to 15 m s 1 . Continuouslyworked kilns then become attractive with very fastdrying. Such designs will require particular attentionto thermal expansion, reliable heat-exchange equipment,and venting of moisture vapor.Many of the advantages of high-temperature dryingcan be obtained by working under vacuum. Inparticular, such a process gives rise to an internaloverpressure and an additional and efficient drivingforce for internal moisture migration. The lower operatingtemperatures are an advantage in drying heatsensitivewoods and in minimizing color developmentwhen pale products are required.Similar advantages of lower working temperaturesare obtained in the use of dehumidifying heatpumpsystems, with the added bonus of lower thermalenergyuse to compensate for extended drying times.The drying principle of these kilns, however, is thesame as vented conventional kilns.Most kilns, either direct-fired or steam-heated, usewastewood as the primary fuel. Other heating arrangementshave been advocated such as the use of microwave,radio frequency, and solar energy, with the latterbeing attractive in remote locations for small kilns.36.3.5.1 Vacuum DryingVacuum dryers have been commercially available formany years and their use is regarded as a standardpractice in Europe for the drying of high-qualityhardwoods economically, which would otherwise bedifficult to dry (Hilderbrand, 1989). Descriptions ofvacuum drying are given by Ressel (1994), Audebertand Temmar (1997), and Jomaa and Baixeras (1997).Because of the enhanced internal moisture migrationunder vacuum, the rate of drying can be as rapid asthat at a much higher temperature at atmospheric pressure.However, the higher specific volume of vapor associatedwith the reduced pressure is a severe limitation forheat transport by convection (Perré et al., 1995).Several industrial solutions have been proposed:. The use of plates heated by electrical resistanceor circulation of heated water are placed betweeneach layer of boards; heat is supplied toß 2006 by Taylor & Francis Group, LLC.

oards by conduction whereas the vacuum levelis used to enhance the internal mass transfer.. Discontinuously operating kilns, with twoperiods of about 1 h alternate: a heating periodat atmospheric pressure and a drying period atreduced pressure.. Finally, the most recent ‘‘high-vac’’ kilns use aslightly higher pressure level (more than 100mbar), together a very high linear air velocity(10 m s 1 or more), to compensate for the loss ofthermal capacity of the air; this method hasproved to be very effective.In the latter method, uniformity of the air distributionthrough the load is important to ensure evenness ofdrying, with regions of low velocities resulting inhigher final moisture contents (Ledig and Militzer,1999). The positions of fans and heating coils havean important bearing on the temperature and on thefinal moisture content of the load (Hedlund, 1996).Vacuum dryers with overhead fans provide a fairlycontrolled airflow path through the load, but otherfan locations can result in ill-defined pressure andsuction sides. An overhead-fan dryer, however, wasfound to yield a systematic variation in temperaturebetween the door and the other end of the dryer,which might have been reduced by dividing the unitinto separate temperature-control zones. Techniquesto overcome the inherent poor heat transfer invacuum dryers include the use of heated plates betweenthe boards or intermittent heating with superheatedsteam. Another suggested technique employs aheating cycle at atmospheric pressure, when the heattransfer is better, followed by a vacuum-drying cycle(Guilmain et al., 1996). Tests on drying oakwood atpilot and industrial scales showed that the discontinuousprocess was faster, with less susceptibility tomechanical damage of the wood, but the thermalconsumption was higher than under continuousvacuum conditions.Behnke and Militzer (1996) have produced avacuum-dryer model for design and process-controlpurposes based on a characteristic drying curve for thewood’sdryingbehavior.Hilderbrand(1989)claimsthatcommercialdryingtimeinvacuumkilnsvariesbetweenone half and one third of those found in conventionalconvective kilns under atmospheric pressure.36.3.5 .2 Dehum idifi er Kiln sDrying at low temperatures, which is a feature ofseasoning refractory timbers, is energy-inefficient.A dehumidifier kiln reduces the thermal-energy consumptionby incorporating an air-conditioning unitthat recovers heat by cooling the kiln air below itsdew point and, in effect, recycling the latent heat ofcondensation. As moisture is removed as condensedliquid rather than vapor in warm discharged air, theassociated thermal loss is avoided. However, a smallamount of venting is needed for humidity-controlpurposes. Volatile organic chemicals normally removedwith the vented moist air now appear in thecondensate stream, which potentially could be sent toa separate unit for chemical recovery.Figure 36.48 shows the layout of a heat-pumpdehumidifying kiln. Moist air is drawn over the evaporatorand condenser consecutively in a Rankine cycleheat pump. Besides these basic elements such as anevaporator, a condenser with its associated compressor,and expansion valve, there is an accumulator thatprevents the refrigerant from entering and damagingthe compressor and a subcooling heat exchanger toenhance the effectiveness of the heat pump.The performance of dehumidifier kilns is normallyexpressed in terms of the specific moisture-extractionrate (SMER), which is the amount of moistureextracted per unit energy input. Two such ratiosmay be defined: one representing how effectively thedehumidifier extracts moisture from the air as condensateand the other (the kiln SMER) representinghow efficiently the kiln removes moisture from thelumber including the condensate and venting. Thekiln SMER for convective kilns is limited to about0.8 to 0.9 kg kWh 1 , compared with values in therange of 1.5 to 2.5 kg kWh 1 for commercial dehumidifierkilns (Davis, 2001). Some of the lower valuesmay reflect the poor insulation of the tested kilnsrather than a defect in the process.Dehumidifying kilns are limited in operating temperatureby the working limits of the compressor(

AirCirculation fanWeightsCondenserCompressorAccumulatorStickersTimberLiquidwaterEvaporatorExpansionvalveSubcoolerFIGURE 36.48 A typical configuration for a heat-pump dehumidifying kiln. (Adapted from Davis, C.P., Drying Pinusradiata boards in dehumidifier conditions, Ph.D. thesis, Otago University, New Zealand, 2001.)thumb, these high-frequency heating methods becomeeconomically attractive for new kilns if the dryingrate is increased fourfold over that for conventionaldrying. In general, the use of dielectric and microwaveheating may become attractive for the small-scaledrying of high-value hardwood species that are difficultto dry by conventional means. For example, Smithand Smith (1994) report the use of radio-frequencyheating for the drying of oakwood in a small vacuumkiln of 23-m 3 capacity, which had a lower capital costbut higher energy costs than a conventional dryerfor the same duty. For very small power requirements,microwave heating is more attractive; whenthe power requirement exceeds 50 kW, however,economics favor the higher-power tubes in theradio-frequency range. In one Canadian system, radiofrequencydrying is used to finish the seasoning ofconventionally dried lumber that has not met targetmoisture content.Heating is generated in the dipolar rotation ofwater molecules as they try to orient themselves inthe rapidly changing polarity of the applied electricalfield. The power developed per unit volume is given byP ¼ kE 2 f « 0 tan d (36:24)where k is the dielectric constant, E is the electricfield strength, f is the field’s frequency, «’ is the relativepermeability, and tan d is the loss tangent ordissipation factor. The field’s strength and its frequencyare fixed by the equipment, whereas theother parameters are material-dependent. As the dielectricconstant of water is over an order of magnitudegreater than the woody materials, moisture ispreferentially heated, a process that leads to a moreuniformly moist product with time. This feature isone of the attractions of the technique, for example,in moisture leveling in the manufacture of plywood toavoid delamination during subsequent hot pressing(Schiffmann, 1995).There is also a contribution due to ionic conductionbecause of the presence of ions in the sap. Thismode of heating is not significantly dependent oneither the temperature or the frequency of the appliedfield, but is directly dependent on the charge densityand mobility of the ions.Because the heating is internally generated, ratherthan convectively warmed at the exposed surface of theboards, high and damaging internal pressures can becreated in the process. For example, internal overpressuresof 60 kPa have been reported by Antti (1992) forpower inputs of 1.25 kW on drying 100 50 1660-mm boards. Under vacuum drying, such overpressuresbecome less damaging. Thus, high-frequency heatinghas been advocated for use with vacuum drying becauseof the difficulty in achieving adequate convective heatingunder vacuum, and a summary of its historic developmentis given by Resch and Gautsch (2001). Thisß 2006 by Taylor & Francis Group, LLC.

KilnSolarcollectorCondenserControlvalveAirflowFanRefrigerantdischarge lineWoodstackCompressorBlowerMotorDamperEvaporatorRefrigerantsuction lineFIGURE 36.49 A solar-dehumidifier dryer. (Adapted from Chen, P.Y.S., Helmer, W.A., and Rosen, H.N., Experimentalsolar-dehumidifier kiln for drying lumber, Forest Prod. J., 32(9): 35–41, 1982.)technique is attractive for beech and oak timbers in theEuropean market because of the retention of their naturallight color with low-temperature drying.Perré and Turner (1997, 1999a) have described anumerical model of microwave drying of softwoodwith an oversized waveguide. In this work, internaloverpressure reaching two to three times the atmosphericpressure has been reported both in experimentaland numerical results.36.3.5.4 Solar DryingSolar drying of lumber has attractions in remotelocations with favorable climates because of the‘‘free’’ nature of the energy source. Imré (1995) hasclassified solar-heated dryers into three main groups:1. Solar natural dryers that use only the sun2. Semiartificial solar dryers with a fan to supply acontinuous flow of air through the load3. Solar-assisted artificial dryers, which may use anauxiliary energy source for boosting the heatingrateMixed types include a solar-dehumidifier dryer withforced-air recirculation, as shown in Figure 36.49.Many solar dryers described in the literature aresimple greenhouse kilns (e.g., Langrish and Keey,1992). In these units, the solar collector is fitted withinthe structure that holds the load and the airflow ismaintained by fans. The solar energy, in other cases,is collected externally in heat-storage systems orpanels, as illustrated in Figure 36.49. Greenhousekilns have attractions in simplicity of constructionand operation.The daily world-average solar radiation on a horizontalsurface is 3.82 kWh m 2 (McDaniels, 1984),with values in tropical countries being higher (up to7.15 kWh m 2 ) (Imré, 1995). However, Plumptre(1989), reported by Keey et al. (2000) on reviewing35 solar kiln designs, notes that the location of thesewas spread almost uniformly over the range in latitudefrom 0 to 508.Langrish and Keey (1992) observed one operationalfeature of the use of a greenhouse kiln. Withthe kiln’s vents shut overnight and with the drop inambient temperature, the relative humidity in the kilnwould rise sufficiently for moisture to condense on thewood’s surface. This provided a degree of conditioning,which prevented the development of excessivechecking in a refractory hardwood being dried.REFERENCESAguiar, O. and Perré, P., 2000a. The ‘‘flying wood’’ testused to study the variability of drying behaviour ofoak, in Quality Drying of Hardwood. 2nd Workshopof COST Action E15, Sopron, Hungary, 10 pp.Aguiar, O. and Perré, P., 2000b. Pocesso de secagem aceleradade madeira baseado nas suas propriedadesreológicas (Accelerated drying process for woodbased on its rheological properties)—Industrialß 2006 by Taylor & Francis Group, LLC.

patten N 1265, Instituto nacional de proteçãoindustrial, Brazil.Aléon, D., Chanrion, P., Négrié, G., and Perré, P., 2003.FormaXylos 4—Le séchage (Training in Wood Science:Drying, Vol. 4), CD-Rom français/English,CTBA, Paris.Antti, A.L., 1992. Microwave drying of hardwood: simultaneousmeasurements of pressure temperature andweight reduction, Forest Prod. J., 42(6): 49–54.Ashworth, J.C., 1977. The mathematical simulation ofbatch-drying of softwood timber, Ph.D. thesis, Universityof Canterbury, New Zealand.Ashworth, J.C. and Keey, R.B., 1979. The kiln seasoning ofsoftwood boards, Chem. Eng., 347(8): 593–598, 607.Audebert, P. and Temmar, A., 1997. Vacuum drying ofoakwood: moisture strains and drying process, DryingTechnol., 15: 2281–2302.Banks, W.B., 1968. A technique for measuring the lateralpermeability of wood, J. Inst. Wood Sci., 4(2): 35–41.Behnke, C. and Militzer, K.-E., 1996. 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