36 Drying of Wood

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.