36 Drying of Wood

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.