Microwave heating

Aug. 25, 1953

Τ. P. KINN MICROWAVE HEATING

Filed Oct. 4, 1946

2,650,291

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WITNESSES:

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INVENTOR

TTiepdoreF Η ί'ππ.

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Patented Aug. 25, 1953

2,650,291

UNITED STATES PATENT OFFICE

2,650,291

MICROWAVE HEATING

Theodore P. Kinn, Baltimore, Md., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania

Application October 4,1946, Serial No. 701,305

Claims.

This invention relates to micro-wave heating, and has particular relation to apparatus for heating strips of material with ultra-high frequency energy.

In many industries there are dielectric heating problems involving the drying, curing or setting of resins or glues in continuous processes. In the textile industry, for example, cloth strips are to be dried after dyeing and often on a continuous basis, that is, in an operation in which the strip is continuously moving. Similar drying operations are necessary in the paper industry, where a continuous sheet is moved at a relatively high speed.

At conventional frequencies of electric energy in the usual high frequency heating range, apparatus with enough power for the drying of paper or textiles or other material is available, but frequently difficulty is encountered in getting that power into the work. Referring to the familiar relation for power in a dielectric material, it is evident that power into the work, once the frequency is set, varies only as the square of the voltage across the electrodes through which the power is transferred to the 25 material in conventional high frequency heating apparatus. Thus,

:. 219—47) ing positioned so that the strip is fed therethrough with the plane of the strip substantially parallel to the direction of the electric field within the resonator. However, the path of move⁵ ment of a point on the strip through the resonator is arranged to be angular with respect to the line of the electric field vector, representing the direction of the field, so as to cause any given, point on the strip to progress an even number 10 of half wave length along and within the interior of the resonator in a component direction substantially perpendicular to the electric field vector. By this arrangement rapid and uniform heating of the strip of material is accomplished 1® without danger of flashover.

The features of my invention which are believed to be novel are set forth with more particularity in the accompanying claims. The invention itself together with additional objects and 20 advantages thereof may be better understood from the following description of a specific embodiment when read in connection with the accompanying drawing, in which the single figure is a perspective view of apparatus embodying my invention.

As shown in the drawing, a hollow rectangular envelope forms a cavity resonator 3 which is adapted to have high frequency electromagnetic field oscillations established therein. This 30 energy may be introduced into the resonator by any suitable means as, for example, through an opening 5 in a wall of the resonator through which electromagnetic energy is supplied from a hollow wave guide 7. The wave guide, in turn, 35 may receive energy from any suitable high frequency source and, of course, may be provided with suitable matching apparatus.

The electromagnetic field within the resonator may have an electric field vector extending 40 in one direction as represented by the arrow 9. A pair of slots I ί and 13 are provided in opposite walls of the resonator 3 to receive a strip of dielectric material ί 5. The strip ί 5 is to be continuously moved through the resonator by any suitable driving means (not shown) connected to suitable rollers H. The slots H and 13 are so located that the plane of the strip 8 5 is substantially parallel to the arrow 9 representing the direction of the electric field. In addition, they are preferably located so that the strip is approximately at the positions of maximum intensities of the electric field transversely across the resonator, that is the strip is substantially at the points of maximum intensities found across the resonator in each direction perpendicular to

Power ~ frequency X voltage X loss factor

For thin materials, such as paper and cloth, it is impossible to apply sufficient voltage across the electrodes at conventional frequencies as flashover between the electrodes takes place long before any appreciable amount of power is coupled into the dielectric work material.

It is, accordingly, an object of my invention to provide new and improved apparatus for use in heating a moving strip of material.

Another object of my invention is to provide novel apparatus for heating a continuously moving strip of thin dielectric material.

A further object of my invention is to provide novel apparatus for heating a continuously moving strip of dielectric material in which uniform heating of the material is accomplished with a ^4orelatively small expenditure of energy.

Still another object of my invention is to provide novel apparatus for heating a moving strip of material quite rapidly while avoiding any flashover through the material. ⁵⁰

In accordance with my invention, I propose to heat a moving strip of material with apparatus which comprises a cavity resonator adapted to have high frequency electromagnetic field oscillations established therein, the resonator be- ⁵⁵

2,650,291 the strip. In the embodiment shown, such maximum intensities are found along a center line of the resonator, in the plane of the strip. Thus, as the strip of material passes through the resonator, it is subjected to the strong dielectric field which provides a rapid heating of the strip.

There will, of course, be a lengthwise standing wave effect along the length of the resonator 3 in the plane of the strip. Because of this standing wave effect, the material would be heated unevenly if passed straight through the resonator with the direction of motion parallel to the electric field vector. However, the slots 1 ί and ! 3 have their center lines displaced by approximately n half wave lengths, where n is a whole number, preferably one-half wave length, in a direction substantially perpendicular to the. electric field vector. Consequently, any given point on the strip 15, in passing through the resonator 3, also progresses approximately n half wave lengths along and within the interior of the resonator in a direction substantially perpendicular to the electric field vector. As a result, the standing waves exert a uniform effect over the entire width of the strip (5 with each point of the strip being subjected, to the same integrated heating so that uniform heating is obtained. This uniform heating is obtained by having each moving point of the strip pass through the same range of field strengths between minimum and maximum field intensities of the lengthwise standing wave. A distance or displacement of a quarter-wave length includes this range; but in the preferred embodiment described, each point passes through such range twice (one-half wave) for greater assurance of uniform heating.

Although I have shown and described a preferred embodiment of my invention, I am aware that many modifications thereof may be made without departing from the spirit of the invention. I do not intend, therefore, to limit my invention to the specific embodiment disclosed.