KilnSolarcollectorCondenserControlvalveAirflowFanRefrigerantdischarge line<strong>Wood</strong>stackCompressorBlowerMotorDamperEvaporatorRefrigerantsuction lineFIGURE <strong>36</strong>.49 A solar-dehumidifier dryer. (Adapted from Chen, P.Y.S., Helmer, W.A., and Rosen, H.N., Experimentalsolar-dehumidifier kiln for drying lumber, Forest Prod. J., 32(9): 35–41, 1982.)technique is attractive for beech and oak timbers in theEuropean market because <strong>of</strong> the retention <strong>of</strong> their naturallight color with low-temperature drying.Perré and Turner (1997, 1999a) have described anumerical model <strong>of</strong> microwave drying <strong>of</strong> s<strong>of</strong>twoodwith an oversized waveguide. In this work, internaloverpressure reaching two to three times the atmosphericpressure has been reported both in experimentaland numerical results.<strong>36</strong>.3.5.4 Solar <strong>Drying</strong>Solar drying <strong>of</strong> lumber has attractions in remotelocations with favorable climates because <strong>of</strong> the‘‘free’’ nature <strong>of</strong> the energy source. Imré (1995) hasclassified solar-heated dryers into three main groups:1. Solar natural dryers that use only the sun2. Semiartificial solar dryers with a fan to supply acontinuous flow <strong>of</strong> air through the load3. Solar-assisted artificial dryers, which may use anauxiliary energy source for boosting the heatingrateMixed types include a solar-dehumidifier dryer withforced-air recirculation, as shown in Figure <strong>36</strong>.49.Many solar dryers described in the literature aresimple greenhouse kilns (e.g., Langrish and Keey,1992). In these units, the solar collector is fitted withinthe structure that holds the load and the airflow ismaintained by fans. The solar energy, in other cases,is collected externally in heat-storage systems orpanels, as illustrated in Figure <strong>36</strong>.49. Greenhousekilns have attractions in simplicity <strong>of</strong> constructionand operation.The daily world-average solar radiation on a horizontalsurface is 3.82 kWh m 2 (McDaniels, 1984),with values in tropical countries being higher (up to7.15 kWh m 2 ) (Imré, 1995). However, Plumptre(1989), reported by Keey et al. (2000) on reviewing35 solar kiln designs, notes that the location <strong>of</strong> thesewas spread almost uniformly over the range in latitudefrom 0 to 508.Langrish and Keey (1992) observed one operationalfeature <strong>of</strong> the use <strong>of</strong> a greenhouse kiln. Withthe kiln’s vents shut overnight and with the drop inambient temperature, the relative humidity in the kilnwould rise sufficiently for moisture to condense on thewood’s surface. This provided a degree <strong>of</strong> conditioning,which prevented the development <strong>of</strong> excessivechecking in a refractory hardwood being dried.REFERENCESAguiar, O. and Perré, P., 2000a. The ‘‘flying wood’’ testused to study the variability <strong>of</strong> drying behaviour <strong>of</strong>oak, in Quality <strong>Drying</strong> <strong>of</strong> Hardwood. 2nd Workshop<strong>of</strong> COST Action E15, Sopron, Hungary, 10 pp.Aguiar, O. and Perré, P., 2000b. Pocesso de secagem aceleradade madeira baseado nas suas propriedadesreológicas (Accelerated drying process for woodbased on its rheological properties)—Industrialß 2006 by Taylor & Francis Group, LLC.
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