36 Drying of Wood

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.