36 Drying of Wood

Water Conservation@@t « wr w þ « g r v þ r bþr rw v w þ r v v g þ r b v b¼rðrg¼Deff rv v ÞðT1ÞEnergy Conservation@@t « wr w h w þ « g ðr v h v þ r a h a Þþr bhb þ r o h s « g P g þr rw h w v w þ ðr v h v þ r a h a Þv g þ h b r b v b¼¼r r gDeff ðh v rv v þ h a rv a Þþl eff rT þ FðT2ÞAir Conservation@@t « ¼gr a þr ra v g ¼r rgDeff rv a(T3)where the gas- and liquid-phase velocities are given by the generalized Darcy’s law:v ‘ ¼¼K ‘¼kr‘m ‘rw ‘ , rw ‘ ¼rP ‘ r ‘ grx (T4)where ‘ is w, g, the quantity w is known as the phase potential, and x is the depth scalar. All other symbolshave their usual meaning.Boundary ConditionsFor the external drying surfaces of the sample, the boundary conditions are assumed to be of the following form: 1 x 1J w j x¼0 þ ^n ¼ h m cM v ln1 x v j x¼0(T5)J e j x¼0 þ ^n ¼ h(Tj x¼0 T 1 )P g j x¼0 þ ¼ P atmwhere J w and J e represent the fluxes of total moisture and total enthalpy at the boundary, respectively, and xdenotes the normal position from the boundary in the external medium.36.2.3 PROCESS OF DRYING36.2.3 .1 Lo w-Temper ature Convecti ve Dry ingLow-temperature convective drying is the most widespreadindustrial process for seasoning wood in kilns.In this case, the role of internal gaseous pressure isalmost negligible and transfer occurs mainly in thedirection of the board thickness. Two periods of dryingmay be distinguished: (1) a constant drying-rateperiod and (2) a decreasing drying-rate period.36.2.3 .1.1 The Cons tant Drying- Rate Peri odThis stage is very common for certain porous media,but is rarely seen with wood. However, it exists almostalways for fresh boards consisting of sapwoodthat are dried under moderate conditions (Perré et al.,1993; Perré and Martin, 1994). During this period,the exposed surface of the board is still above theFSP. As a result, the vapor pressure at the surface isequal to the saturated vapor pressure, and is a functionof the surface temperature only.Coupled heat and vapor transfer occur across inthe boundary layer (Figure 36.17). The heat flux suppliedby the airflow is used solely for transforming theliquid water into vapor. During this stage, the dryingrate is constant and depends only on the externalconditions (temperature, relative humidity, velocity,and flow configuration). The temperature at the surfaceis equal to the wet-bulb temperature. Moreover,because no energy transfer occurs within the mediumduring this period, the whole temperature of theboard remains at the wet-bulb temperature.ß 2006 by Taylor & Francis Group, LLC.