36 Drying of Wood

AirCirculation fanWeightsCondenserCompressorAccumulatorStickersTimberLiquidwaterEvaporatorExpansionvalveSubcoolerFIGURE 36.48 A typical configuration for a heat-pump dehumidifying kiln. (Adapted from Davis, C.P., Drying Pinusradiata boards in dehumidifier conditions, Ph.D. thesis, Otago University, New Zealand, 2001.)thumb, these high-frequency heating methods becomeeconomically attractive for new kilns if the dryingrate is increased fourfold over that for conventionaldrying. In general, the use of dielectric and microwaveheating may become attractive for the small-scaledrying of high-value hardwood species that are difficultto dry by conventional means. For example, Smithand Smith (1994) report the use of radio-frequencyheating for the drying of oakwood in a small vacuumkiln of 23-m 3 capacity, which had a lower capital costbut higher energy costs than a conventional dryerfor the same duty. For very small power requirements,microwave heating is more attractive; whenthe power requirement exceeds 50 kW, however,economics favor the higher-power tubes in theradio-frequency range. In one Canadian system, radiofrequencydrying is used to finish the seasoning ofconventionally dried lumber that has not met targetmoisture content.Heating is generated in the dipolar rotation ofwater molecules as they try to orient themselves inthe rapidly changing polarity of the applied electricalfield. The power developed per unit volume is given byP ¼ kE 2 f « 0 tan d (36:24)where k is the dielectric constant, E is the electricfield strength, f is the field’s frequency, «’ is the relativepermeability, and tan d is the loss tangent ordissipation factor. The field’s strength and its frequencyare fixed by the equipment, whereas theother parameters are material-dependent. As the dielectricconstant of water is over an order of magnitudegreater than the woody materials, moisture ispreferentially heated, a process that leads to a moreuniformly moist product with time. This feature isone of the attractions of the technique, for example,in moisture leveling in the manufacture of plywood toavoid delamination during subsequent hot pressing(Schiffmann, 1995).There is also a contribution due to ionic conductionbecause of the presence of ions in the sap. Thismode of heating is not significantly dependent oneither the temperature or the frequency of the appliedfield, but is directly dependent on the charge densityand mobility of the ions.Because the heating is internally generated, ratherthan convectively warmed at the exposed surface of theboards, high and damaging internal pressures can becreated in the process. For example, internal overpressuresof 60 kPa have been reported by Antti (1992) forpower inputs of 1.25 kW on drying 100 50 1660-mm boards. Under vacuum drying, such overpressuresbecome less damaging. Thus, high-frequency heatinghas been advocated for use with vacuum drying becauseof the difficulty in achieving adequate convective heatingunder vacuum, and a summary of its historic developmentis given by Resch and Gautsch (2001). Thisß 2006 by Taylor & Francis Group, LLC.