Dielectric properties of four softwood species at low-level radio frequencies for optimized heating and drying

Abstract

ABSTRACTDielectric heating and drying are important nontraditional technologies for the phytosanitation and quality dehydration of wood products. Unit operation optimization requires, among others, knowledge of the variable dielectric properties of wood so that an optimum dielectric generator matching wood impedance is maintained throughout the process. For this reason, the dielectric properties of four commercially important west coast softwood species were experimentally obtained at various moisture contents, temperatures, and radio frequencies (RF) between 0.1 and 30 MHz. Measurements were carried out on all-sapwood and all-heartwood specimens in both radial and longitudinal directions. Measurements were also done with specimens that were fully saturated with distilled and seawater.All specimens mostly revealed similar qualitative trends in loss factor changes; that is, increases with moisture content and temperature and decreases with increasing frequency. The measurements carried out in the longitudinal direction showed relatively high loss factor values compared to the radial grain direction. Both distilled and seawater saturated loss factors resulted in similar trends and with absolute values hundreds and in some cases thousand times larger than those for specimens in the hygroscopic range.