Dielectric heating
The heating of a nominally electrical insulating material due to its own electrical (dielectric) losses, when the material is placed in a varying electrostatic field.
The material to be heated is placed between two electrodes (which act as capacitor plates) and forms the dielectric component of a capacitor (see illustration). The electrodes are connected to a high-voltage source of 2-90-MHz power, produced by a high-frequency vacuum-tube oscillator.
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The resultant heat is generated within the material, and in homogeneous materials is uniform throughout. Dielectric heating is a rapid method of heating and is not limited by the relatively slow rate of heat diffusion present in conventional heating by external surface contact or by radiant heating.
This technique is widely employed industrially for preheating in the molding of plastics, for quick heating of thermosetting glues in cabinet and furniture making, for accelerated jelling and drying of foam rubber, in foundry core baking, and for drying of paper and textile products. Its advantages over conventional methods are the speed and uniformity of heating, which offset the higher equipment costs. Because of the absence of high thermal gradients, an improved end-product quality is usually obtained.
Dielectric Heating
the heating of dielectrics in an alternating electrical field. Upon application of an alternating electrical field, a displacement current caused by polarization and a conduction current caused by the presence of free, electrically charged particles appear in the dielectric. The flow of the total current causes the liberation of heat. The liberated specific power is proportional to the intensity (E) and frequency (f) of the field, as well as to the dielectric constant (e) and the tangent of the loss angle (tan δ) of the dielectric. At frequencies of 0.3-300 megahertz, dielectric heating occurs in the field of a capacitor (when tube oscillators are used as a power supply); at superhigh frequencies, it occurs in the field of a cavity resonator or radiator (when magnetrons are used as the power supply). The intensity of the electrical field in industrial installations using dielectric heating is 5–3,000 kilovolts per m.
The advantages of dielectric heating equipment are high rate of heating, uniform heating of materials having a low thermal conductivity, and the feasibility of local and selective heating. Dielectric heating is used in drying various materials (wood pulp, paper, and ceramics), heating plastics prior to pressing or molding, welding plastics, and gluing of wood.
REFERENCES
Vysokochastotnyi nagrev dielektrikov i poluprovodnikov, 2nd ed. Moscow-Leningrad, 1959.Britsyn, N. L. Nagrev v elektricheskom pole vysokoi chastoty, 3rd ed. Moscow-Leningrad, 1965.
A. B. KUVALDIN
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