High-frequency dielectric heating in a resonant chamber

March 14, 1950

E. C. HANSON ET AL

HIGH-FREQUENCY DIELECTRIC HEATING IN A RESONANT CHAMBER Filed June 1, 1946

2,500,752

<img file="US2500752A_D0001.tif" />

ί

HIGH FREQUENCY SUPPLY SOURCE

<img file="US2500752A_D0002.tif" />

Patented Mar. 14, 1950

2,560,752

UNITED STATES PATENT OFFICE

2,500,752

HIGH-FREQUENCY DIELECTRIC HEATING IN A RESONANT CHAMBER

Earl C. Hansen, Cleveland, Ohio, and Donald E. Watts, De Witt, N. Y., assignors to General Electric 'Company, a corporation of New York

Application June 1, 19-16, Serial No. ,673,724

Claims. (&lt;

Our invention relates to high frequency electric heating apparatus utilizing a chamber in which oscillating electromagnetic fields are produced, more particularly to apparatus for heating non-metallic or dielectric materials by electro- j magnetic fields and has for its object a simple, reliable and efficient apparatus for heating articles and for heating material in the form of a continuous thread or strip or articles on a conveyor. H

In carrying out our invention in one form we provide a chamber which may be resonant or nonresonant and which has walls made of .electrically conducting material, together with means for passing the material to be heated through 1 the chamber. All dielectric materials, both organic and inorganic, may be heated for drying, hardening or other purposes, such as paper, fabrics, rubber, rayon, cellulose or other synthetic materials, food products, tobacco, ceramic mate- 2 rials, etc.

For a more complete understanding of our invention reference should be had to the accompanying drawing, Fig. 1 of which is a view in perspective of high frequency heating apparatus &lt;2 embodying our invention, Fig. 2 is a view in perspective of a simplified form of our invention, Fig. 3 is a view in perspective of a modified form of our invention, while Fig. 4 is a sectional view of a modified form of our invention. S

Referring to Fig. 1 of the drawing we have shown our invention in one form as applied to the heating of a strip I of flexible dielectric material, such as damp paper to be dried, which is moved horizontally and continuously at a pre- F determined speed along a predetermined path through the heating apparatus 2 by suitable feed and winding rolls and driving means (not shown), the strip i passing over guide rollers 3 and 4.

The heating apparatus 2 comprises a closed 4 rectangular container 5 formed by walls of electrically conducting material, preferably copper, and provided with a slot 6 in opposite side walls near the bottom wall 7 through which slots the strip I passes through the chamber for heating 1 inside the chamber. Electric power at a suitable high frequency is applied from, electric supply means 8, which preferably is an electronic oscillation generator, by a magnetic coupling or hairpin device 9 to a wave guide 19 having walls made 5 of an electrically conducting material such as copper by means of gradually tapered walls to the upper wall of the chamber 5. As shown the cou□I. 219—47) pling device S is connected to the source 8 by coaxial conductors the center one of which passes through an electric insulator in the wall of the wave guide. By means of the coupling device 9 1 high frequency oscillating electromagnetic fields are set up in the chamber, which fields are intercepted by the strip I with the generation of heat in the strip.

In order to provide for the propagation of elec0 trie waves in the chamber and energy oscillation back and forth from electric to magnetic fields at the frequency of the supply source, at least one dimension of the wave guide and the chamber is selected greater than one half of the wave length 5 of the generator S. For example, with power supplied by the generator 8 at a frequency of 2000 megacycles, whose wave length is 15 centimeters, at least one transverse inside dimension of the wave guide and chamber must be 7.5 centimeters ,0 or greater. The wave guide may have one transverse dimension of substantially 7.5 centimeters and a length of many times this value. The chamber 5, which may be considered also as a wave guide, will probably be larger than the wave guide 15 in a transverse or horizontal direction as shown, although its dimensions in a vertical direction may be less than the length of the wave guide. For example, the chamber 5 may be three feet square transversely and one foot high. The coupling device 9 may be located one-half wave length from the upper end of the wave guide.

For effective and efficient heating of the strip 1 the slots S are positioned a distance from the bottom wall 7 where rapid and efficient heating will !5 be obtained, such as approximately one-fourth wave length, i. e. 3.75 centimeters for a 2000 megacycle supply source. The strip I is supported in the chamber on a plate i! made of low loss material as compared with the material being heated, 10 such as glass, secured to the side walls of the chamber. If desired, separate sheets of material or other objects may be heated by placing them on the low loss plate i I.

Metal plates 12 and 13 are provided at the slot ¹⁵ S respectively above and below the strip. These plates are secured to the container 5 and serve as a capacitor choke to prevent energy radiation from the chamber.

It will be understood that the strip I is heated by heat generated in the strip itself by the electromagnetic fields in the chamber 5.

The strip 1 may consist of a plurality of parallel lengths or threads. Latex in strip or cord

2,600,752 form may be passed through the chamber and heated therein to vulcanize it.

In Fig. 3 we have shown our invention as applied to the heating of dielectric articles 14 on a conveyor belt 15 moved continuously by suitable means (not shown) over supporting rollers 16 made of electrically insulating material having a relatively low power factor. The belt 15 is also made of a dielectric material having a low power factor such as dry cotton fabric.

In this application of our invention the resonant chamber 5 is open at one side, as shown the bottom side and suitably supported just above the conveyor belt. A bottom wall plate 18 for the chamber is provided below the rollers 16, the spacing of the plate 18 from the belt 15 and articles to be heated being such as to provide rapid and efficient heating preferably a distance of at least substantially one-fourth of the wave length of the supnly source. As shown, the plate 18 is provided with an electric ground connection 19 for safety reasons. This form of our invention is adapted for the heating of coffee beans for roasting purposes, the drying of tobacco and ceramic materials, and, in fact, the heating of all dielectric materials having a power factor high enough for effective heating by the high frequency used.

In the modified form of onr invention shown in Fig. 4, we have shown our invention as anplied to the vulcanization of rubber, especially automobile tires. The rubber t⁵re 20 which is to be vulcanized is carried by a forming core 21 and enclosed between the annular mold sections 22 and 23. The core and mold sections are preferably made of a dielectric material such as low power factor glass, or other material having a low power factor as compared with rubber so that the generation of heat is confined substantially in the tire. It will be understood that the mold sections are suitably detachably secured together so that they can be readilv separated for the insertion and removal of the tire. For the generation of heat in the tire the mold and tire are placed in a container 24 such as disclosed in Fig. 1 except that the side walls are imperforate and the bottom wall 25 is removable for the insertion and removal of the mold and tire. The tire is positioned a distance from the bottom plate 25 such as to provide rapid and efficient heating.

If desired, the source of high frequency supply can be connected directly to the heating chamber through which the material to be heated is passed or In which the material is placed, as shown in Fig. 2, without using the wave guide of smaller cross section leading into the heating chamber as shown in Figs. 1, 3 and 4. As shown in Fig. 2, the container 26 made of eletrically conducting material such as copper is provided with slots 27 and 28 in its top and bottom walls respectively through which the material to be heated is passed through the chamber.

The high frequency supply source (not shown) is connected to the chamber by means of a coaxial line consisting of an outer cylinder 29 having its end electrically connected to the top wall of the container, while its inner conductor 39 extends downward through an aperture in the wall of the container and is electrically connected to the bottom of the container. One inside dimension of the container, as shown the side 31, is at least as great as one-half the wave length of the supply source in order to assure the propagation of the electric waves in the chamber required for efficient heating.

Obviously, when the heating chamber is filled with material, tuning of the chamber with the supply source will be broad and a resonant condition may not exist.

While we have shown a particular embodiment of our invention, it will be understood, of course, that we do not wish to be limited thereto since many modifications may be made and we therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of our invention.