Nonisothermal Radiofrequency Drying of Red Oak
Keywords:Drying, Soret effect, radiofrequency, convection, red oak, nonisothermal, drying strain
AbstractThe combination of nonisothermal moisture movement and radiofrequency heating has a potential application in lumber drying as it affords rapid heating of wood and provides an additional driving force for the removal of water. This paper describes the experimental setup and presents the results of the nonisothermal radiofrequency (NIRF) drying of red oak (Quercus spp.). NIRF drying is implemented by bulk heating the lumber at a preset temperature using radiofrequency energy while continuously circulating air conditioned at a lower temperature over the wood surface. The temperature at mid-thickness of the lumber is maintained at the dry bulb temperature required by the appropriate kiln-drying schedule, while the air temperature is maintained so as to establish a temperature gradient of 3°C/cm from the mid-thickness to the surface of the material. Excessive checking was observed when green lumber was dried using the NIRF method. The drying defect is due mainly to the high drying rate and steep surface moisture content gradient during the early stages of drying. Nonisothermal radiofrequency drying was successfully implemented when a pre-drying step was incorporated at the start of the process. When red oak was pre-dried to 40% MC, the modified NIRF method required a total of 275 h to dry 25-mm-thick boards from 85% to 12% MC. For lumber pre-dried to 60% MC, the total drying time for the modified NIRF method was 160 h. These drying times are significantly shorter than the 530 h needed to dry red oak over the same moisture content range using the conventional kiln-drying method.
Adesanya, B. A., J. N. Beard, and A. K. Nanda. 1986. Moisture distribution during high temperature drying of yellow poplar. In A. S. Mujumdar, ed. Drying '86. Vol. 1. Hemisphere Publishing Corp., New York, NY.nAvramidis, S., P. Englezos, and T. Papathanasiou. 1992. Dynamic nonisothermal transport in hygroscopic porous media: Moisture diffusion in wood. AIChE J. 38(8): 1279-1287.nBeard, J. N., H. N. Rosen, and B. A. Adesanya. 1985. Temperature distribution in lumber during impingement drying. Wood Sci. Technol. 19:277-286.nBird, R. B., W. E. Stewart, and E. N. Lightfoot. 1960. Transport phenomena. John Wiley & Sons, New York, NY. 780 pp.nBiryukov, V. A. 1961. Dielectric heating and drying of wood. (Translated from Russian). Israel Program for Scientific Translations, Jerusalem. 117 pp.nBoone, R. S., C. J. Kozlic, P. J. Bois, and E. M. Wengert. 1993. Dry kiln schedules for commercial woods: Temperate and tropical. Forest Products Society, Madison, WI. 158 pp.nBrown, G. H., C. N. Hoyler, and R. A. Bierwirth. 1947. Theory and application of radio-frequency heating. D. Van Nostrand Co., New York, NY. 370 pp.nChoong, E. T. 1963. Movement of moisture through a softwood in the hygroscopic range. Forest Prod. J. 13(11):489-498.nCross, A. D., P. L. Jones, and J. Lawton. 1982. Simultaneous energy and mass transfer in radiofrequency fields. I. Validation of the theoretical model. Trans. Inst. Chem. Eng. 60(2):67-74.nJames, W. L. 1975. Dielectric properties of wood and hardboard: Variation with temperature, frequency, moisture content, and grain orientation. USDA Forest Serv. Res. Paper FPL-245. 32 pp.nJost, W. 1960. Diffusion of solids, liquids, and gases. Academic Press, New York, NY. 558 pp.nLin, R. T. 1967. Review of the dielectric properties of wood and cellulose. Forest Prod. J. 17(7):61-66.nMcMillen, J. M. 1963. Stresses in wood during drying. USDA Forest Serv. Rep. No. 1652. Forest Prod. Lab., Madison, WI. 52 pp.nNorimoto, M. 1976. Dielectric properties of wood. Wood Research 59/60:106-152.nPeralta, P. N. 1990. Nonisothermal moisture transport in wood. Ph.D. Dissertation, Virginia Polytechnic Institute and State University, Blacksburg, VA.nPeralta, P. N., and C. Skaar. 1993. Experiments on steady-state nonisothermal moisture movement in wood. Wood Fiber Sci. 25(2): 124-135.nPeralta, P. N., and R. G. Joseph. 1996. Nonisothermal-radiofrequency drying of wood: Experimental setup and preliminary results. Proc. 5th International IUFRO Wood Drying Conference, Quebec City, Canada.nPeralta, P. N., R. G. Joseph, and A. W. C. Lee. 1997. Nonisothermal radiofrequency drying of red oak. Abstracts of Technical Sessions and Technical Forum Presentations, 51st Annual Meeting of the Forest Products Society, Vancouver, B.C.nRice, R. W. 1988. Mass transfer, creep, and stress development during the drying of red oak. Ph.D. dissertation, Virginia Polytechnic Institute and State University, Blacksburg, VA. 152 pp.nSchiffmann, R. F. 1987. Microwave and dielectric heating. Pages 327-356 in A. S. Mujumdar, ed. Handbook of Industrial Drying, Marcel Dekker, New York, NY.nSkaar, C. 1988. Wood-water relations. Springer-Verlag, Berlin, Germany. 283 pp.nVoigt, H., O. Krischer, and H. Schauss. 1940. Die Feuchtigkeitsbewegung bei der Verdunstungstrocknung von Holz. Holz Roh-Werkst. 3:305-321.n
The copyright of an article published in Wood and Fiber Science is transferred to the Society of Wood Science and Technology (for U. S. Government employees: to the extent transferable), effective if and when the article is accepted for publication. This transfer grants the Society of Wood Science and Technology permission to republish all or any part of the article in any form, e.g., reprints for sale, microfiche, proceedings, etc. However, the authors reserve the following as set forth in the Copyright Law:
1. All proprietary rights other than copyright, such as patent rights.
2. The right to grant or refuse permission to third parties to republish all or part of the article or translations thereof. In the case of whole articles, such third parties must obtain Society of Wood Science and Technology written permission as well. However, the Society may grant rights with respect to Journal issues as a whole.
3. The right to use all or part of this article in future works of their own, such as lectures, press releases, reviews, text books, or reprint books.