ABSTRACT
In contrast to the conventional thermal processes that rely on surface heating by convection, conduction, and radiation with a subsequent diffusion of heat into the material interior, dielectric heating is based on volumetric heat generation where high-frequency electromagnetic energy is transformed into heat practically throughout the whole volume of the material. This heating method is particularly advantageous in thermal drying because the favorable dielectric properties of water and other polar liquids cause heat to be generated in the wet parts of the material. This greatly eliminates thermal lag caused due to heat diffusion and provides some unique features such as (Strumillo and Kudra, 1986; Metaxas and Meredith, 1983; Metaxas, 1996; Sanga et al., 2000)
• Enhanced diffusion of liquid and vapor moisture • Coincidental temperature and mass concentration gradients • Internal pressure gradient as an additional mass-transfer driving
force • Stabilized material temperature at or below the liquid boiling point
Electromagnetic energy is one of the forms of energy that is stored and transmitted by alternating electric and magnetic fi elds. It is propagated in space as an electromagnetic wave, which is characterized by the wavelength λ, frequency f, magnetic fi eld strength H, and electric fi eld strength E. When an alternating electromagnetic fi eld is applied to the special category of materials called lossy dielectrics, the energy is transformed into heat (dielectric heating) due to ionic conduction and dipole oscillations, which attempt to follow the rapid changes in the electric fi eld orientation. Materials with no dipolar electric charge do not heat because the electric fi eld affects only the asymmetrically charged molecules, of which water is by far the most common (Figure 30.1).
