Leakage Radiation And Top Electrode Voltage Determination Of Radio Frequency Heating Systems

Radio frequency(RF) heating is one of dielectric heating methods and widely used in industrial drying, welding and agricultural products processing due to its rapid and volumetric heating. To optimize the RF heating system for industrial applications, objectives of this research were to study the electromagnetic leakage radiations and top electrode voltage of the RF systems.A safe operational environment is critical for industrial pasteurization and disinfestation applications of RF treatments. In this study, electric and magnetic field intensities around inlet and outlet openings for 27.12 MHz Strayfield and Jiyuan RF systems were measured by an electromagnetic detector. Safety level of the RF systems was evaluated based on the results and international limit standards.In developing the effective RF heating technology, computer simulation has always been used as a valuable tool for rapid analysis of RF heating processes. Top electrode voltage of RF heating systems is an important parameter to accurately determine final sample temperatures using computer simulation. Since the top electrode voltage is difficult to accurately estimate, in this study, the correlation between the top electrode voltage and the measured electrical currents was established both by analytical and simulation methods. The major contents and conclusions of studies are as follows:(1) The leakage electric or magnetic field intensity and estimated power density were determined as a function of the distance from the opening, load type and electrode gap with shielding conditions in inlet and outlet openings. Results showed that both electric and magnetic field intensities increased with decreasing distance and electrode gap. With a load of 1 kg water, the RF power increased but the leaked electric or magnetic field intensity decreased, especially for the Strayfield system. After shielding was added around inlet and outlet openings for Jiyuan RF system, the leaked electric field intensity and the estimated power density were significantly reduced at each location under each of the distances and electrode gaps.(2) The averaged electric field intensity was lower than the limited values allowed by International Commission on Non-Ionizing Radiation Protection(ICNIRP) and Institute of Electrical and Electronics Engineers(IEEE) standards. During the daily operations of the RF systems, the measures to reduce operators’ radiation exposure include additional shielding of the interaction area, reducing exposure time, increasing the distance between the operator and the electrode, and avoiding empty-load running.(3) A measuring circuit was designed and developed to directly measure the top electrode voltage in the 27 MHz 6 kW RF unit together with the anode current read from the RF generator. The top electrode voltage was experimentally determined with 3 kg soybeans under five electrode gaps using computer simulation based on the matched temperature profiles in three layers. Analytical methods based on the heating rates measured by infrared camera, fiber optic sensors and thermocouples.(4) Results showed that the electrode voltage decreased with increasing electrode gap or decreasing heating rate. The correlation between the electrode voltage estimated by temperature distributions from three-layer thermal imaging and the electrical currents both from the anode reading and direct measurement was the best one among the estimation methods and could be used for future computer simulations.