ABSTRACT
The limitation of relatively low heat-transfer rates in conventional drying with hot air, particularly toward the falling rate period, can be overcome by taking advantage of volumetric heat generation during dielectric (microwave [MW] and radio frequency [RF]) heating. This can be done by combining RF heating with conventional heat pump drying (Marshall and Metaxas, 1998). Similar to MW drying, the RF fi eld generates heat volumetrically within the material wetted with polar molecules such as water by the combined mechanisms of dipole rotation and ionic conduction. Such internal heat generation speeds up the drying process mainly because of unidirectional temperature and moisture concentration gradients, internal pressure buildup, and short time delays. Nonpolar materials such as fat or oil, and dry solids that usually form a skeleton of the material being dried, do not react essentially to the electromagnetic fi eld, and therefore they are not directly heated by RF energy (Clark, 1997).
