36 Drying of Wood

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.