The material to be heated is placed as a slab between two metallic electrodes across which high-frequency voltage is applied. To ensure sufficient loss and to give an adequate amount of heating, frequencies between 10 to 20 MHz must be used. The voltage varies from 10 to 20 kV. The necessary high-frequency supply voltage is obtained from a valve oscillator.
The current drawn by the capacitor, when an a.c. supply voltage is applied across its two plates, does not lead the supply voltage by 90° exactly. It means that there is a certain component of the current which is in phase with the voltage. Due to this component of current, heat is produced in dielectric material placed in between the two plates of the capacitor.
This electric energy, dissipated in the form of heat energy in the dielectric material is known as the dielectric loss. The dielectric loss is directly proportional to V 2f. That is why high-frequency voltage is used in dielectric heating.
Generally, a.c. voltage of about 20 kV at a frequency of 10-30 MHz is used. Dielectric heating is also employed for drying of textiles, manufacture of plywood, paper etc. The overall efficiency in case of dielectric heating is about 50%.
Advantages and Disadvantages of Dielectric Heating
- Since the heat is produced throughout the whole mass of material, we get uniform heating. By the conventional method of heating, it is not possible to achieve this.
- Short time is required to complete the process as compared to another method.
- Dielectric heating is suitable for non-conducting materials like wood, plastics and synthetic compounds etc.
- Only those materials can be heated which have the high dielectric loss.
- The cost of equipment required for dielectric heating is so high that it is employed only where other methods are impracticable.
- The overall efficiency of dielectric heating is very low (about 50%).
- High frequencies may cause radio interferance.
Applications of dielectric heating
- It is used in drying tobacco, paper, wood, gluing and bonding of woods.
- Welding of PVC.
- Dielectric heating is used in producing artificial fibers, heating of bones and tissues etc.
- It is used in food processing.