36 Drying of Wood

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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.
Page 1 and 2: 36Drying of Wood: Principlesand Pra
Page 3 and 4: Primary growthSecondannual ringFirs
Page 5 and 6: FIGURE 36.4 Wood is formed by cell
Page 7 and 8: TABLE 36.1Elementary Composition of
Page 9 and 10: FIGURE 36.8 Tension wood in Peduncu
Page 11 and 12: 36.2.1.3 Bound WaterBound moisture
Page 13 and 14: TABLE 36.6Order of Magnitude of Shr
Page 15 and 16: The transverse permeability and the
Page 17 and 18: FIGURE 36.14 Relative permeability
Page 19 and 20: Low moisture contentHigh moisture c
Page 21 and 22: Boundary layersExternal flowTP vHea
Page 23 and 24: HeatVaporVessel ortracheidLiquidPit
Page 25 and 26: Moisture content (%)15010050Sapwood
Page 27 and 28: to the history of that point (« me
Page 29 and 30: ConstantmoisturecontentTime t = 0 +
Page 31: Element 1 Element 2 Element NElemen
Page 35 and 36: Moisture content (%)12010080604020A
Page 37: Second drying periodTensionParamete
Page 42 and 43: eaction wood produced by environmen
Page 44 and 45: where the boards butt up due to shr
Page 46 and 47: 1.2Normalized moisture content (Φ)
Page 48 and 49: set at a constant value at constant
Page 50 and 51: oards by conduction whereas the vac
Page 52 and 53: KilnSolarcollectorCondenserControlv
Page 54 and 55: Doe, P.E., Oliver, A.R., and Booker
Page 56 and 57: Perré P., 1992. Transferts couplé

36 Drying of Wood

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