Dielectric heating

Compared with induction heating , used to heat electrically conductive materials with alternating current with a frequency of not more than 30 MHz, dielectric heating is usually carried out using higher frequencies.

Dielectric material (wood, plastic, ceramics) is placed between the plates of the capacitor ^[1] , to which a high-frequency voltage is supplied from an electronic generator on radio tubes. An alternating electric field between the capacitor plates causes polarization of the dielectric and the appearance of a bias current that heats the material.

Advantages of the method: high heating rate; pure non-contact method that allows heating in vacuum, shielding gas, etc .; uniform heating of materials with low thermal conductivity; local and selective heating, etc.

Areas of application of the method: drying of materials (wood, paper, ceramics, etc.); heating plastics before pressing; welding of plastics; drying of glue joints; warming up the soil before excavation; bonding wood, etc.

Microwave heating uses electromagnetic waves with frequencies above 100 MHz. Modern microwave ovens typically use a frequency of 2.45 GHz, although there are devices operating at a frequency of 915 MHz.

When using electromagnetic microwave waves, heating is caused by molecular dipole rotation in a dielectric - a typical water molecule is a dipole molecule. At the same time, magnetron devices are used as a generator.

Forced vibrations of polar molecules under the influence of an external electric field lead to intermolecular friction, as a result heat is released in the entire volume of the dielectric. In non-ideal dielectric materials (partially conducting electric current), additional heating occurs due to conductivity. In dielectrics, in which the process of molecular polarization is insignificant, and the electrical conductivity is extremely small, there will be no heating by an electromagnetic field; such materials: glass, paper, porcelain, earthenware, many polymeric materials, air, etc. ^[2] .

The method is most widely used for defrosting and heating during cooking. Since water in food products contains a large number of different salts, which dissociate into ions that serve as carriers of electric charges and also respond to an alternating electromagnetic field, the heating of products is caused by both the reorientation of polar dipole molecules and ion displacement.

In medicine

The effect of heating a dielectric in an alternating electromagnetic field was first noted by E.V. Siemens in 1864, then in 1886 I.I. Borgman studied the heating of the glass wall of a Leyden jar during charge and discharge. The effect has found application in medicine. In 1891, Nikola Tesla proposed using the thermal effect of the electromagnetic field for the needs of medicine, and d'Arsonval , finding that an alternating electromagnetic field with a frequency above 10 kHz does not cause tissue irritation, but has various physiological effects, including thermal effect, proposed three practical treatment method: using electrodes, capacitive plates and inductors .

In 1899, the Austrian chemist R. von Zaynek determined the rate of heat generation in tissues depending on the frequency and current strength and proposed the use of electromagnetic fields with a frequency of over 200 kHz for deep heating of body tissues and treatment. Since 1906, the method began to spread rapidly and in 1908, a German doctor Karl Franz Nagelschmidt called it diathermy and in 1913 wrote the first textbook on this area of therapy .

Until the 1920s, long-wave diathermy devices with Tesla coils of spark discharge operating at frequencies of 0.1 - 2 MHz were used for therapeutic purposes. In 1925, A. Ezau ( Eng. Abraham Esau ) noticed that the high-power transmitter of the meter range caused a feeling of warmth among the staff, and proposed the use of microwave waves for therapy; Together with E. Schliphake, he conducted tests on animals and humans, and I. Petzold investigated the influence of frequency on the depth of heating. The result was a “short-wave diathermy” using frequencies in the range 10 - 300 MHz.

In Industry

Despite the complexity and high cost of equipment, dielectric heating has found wide application in industry, since it allows heating non-conductive homogeneous materials with high speed and uniformity, and heterogeneous materials selectively, for example, during drying or gluing.

In 1930-1934, N. S. Selyugin in the Leningrad branch of the Central Research Institute of Mechanical Wood Processing developed a technology for drying wood with high-frequency currents ( Selyugin N. S. Drying of wood . - Leningrad: Goslestekhizdat, 1936. - 560 p.; Drying and heating of wood in high-frequency field / N. S. Selyugin, S. N. Abramenko, V. A. Zhilinskaya, G. A. Sofronov; Under the general editorship of Prof. D. F. Shapiro; People's Commissars of the USSR, All-Russian State Trust Sevzaples. ”Central Scientific and Research Laboratory of Mechanical Wood Processing. - Leningrad: Goslestekhizdat, 1938. - 127 pp.). At the same time, A. I. Ioffe received an author's certificate for high-frequency drying of wood. For the first time on an industrial scale, the method was used to dry birch and beech blanks at the Skorokhod shoe factory in Leningrad.

In the 1930s, drying and sterilization of fruits using electromagnetic waves was studied, and P.P. Tarutin at the VNIIZern studied the drying and destruction of grain pests using high-frequency currents ( The use of ultra-short waves for disinsection and thermal effects on wheat. - Gostorgizdat, 1937 - 190 p.).

In the 1940s, the United States developed methods for high-frequency heating of plastics, gluing wood and plywood. In France, A. Ezau was involved in the development of methods for high-frequency drying of textiles and food products, gluing wood and heating plastics before pressing; HF heating of ceramics was developed by M. Deskarsin (1946); rubber vulcanization - LeDuc and Dufour.

Heating using microwave frequencies began to be applied after the invention of the magnetron in the 1940s. In 1947, in the United States, in the restaurant car, the first microwave radar stove was installed, in which 2400 MHz electromagnetic waves were used. Work on the industrial use of heating at microwave frequencies began in the early 1960s: in the United States and Japan, methods of rock destruction were developed; In the United States and Germany, experiments were conducted to obtain a plasma torch.

In the late 1980s, the Austrian company Lynn created a high-temperature (up to 2000 ° C) microwave installation for sintering oxides.

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