US2456611A - High-frequency dielectric heating apparatus - Google Patents

Description

Dec. 21, 1948. R. M. BAKER HIGH-FREQUENCY DIELECTRIC HEATING APPARATUS Filed Sept.

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ATTORN EY Patented Dec. 21, 1948 UNITED STATES PATENT OFFICE HIGH-FREQUENCY DIELECTRIC HEATING APPARATUS Robert M. Baker, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application September 28, 1944, Serial No. 556,141

out the material.

An object of my invention is to provide a dielectric heating system of high frequency, and to utilize such a system in a continuous production line for heating of dielectric material comprising a plurality of separate articles carried in succession into the electrostatic field of the system.

A further object of my invention is to provide a dieletcric heating system which is operated at such high frequencies as to introduce a standingwave voltage pattern on the power generating and transmitting means of the system, but which is, nevertheless, utilizable for satisfactory heating of dielectric material.

In accordance with the preferred form of my invention, I provide a push-pull tube-oscillator generator having a transmission line connected to the tubes of opposite phase. The transmission line, comprising conductors with the dielectric material to'be heat treated therebetween,

is the prime factor in establishing the frequency at which the generator oscillates. As the position the frequency can be permitted to. shift or be changed through a relatively considerable range without ob'jectionably affecting the heat treatment.

Additional objects, features, innovations, methods and combinations of my invention will be discernible from the following description thereof which is to be taken inzconnection with the accompanying diagrammatic drawing. In the drawing, in some-figures of which parts have been omitted for clarity.

Figure 1- is a: view illustrating a; form of my invention with the transmission line portion shown-inaplan;

Fig. 2 is-an elevational view. of a transmission line and a material-feeding meansof the system;

Fig 3 is a modified formof anendof a transmission line; i

Fig. 4 is a curve illustrating a standing-wave voltage patternlonatransmissionline of a quarter wavelength, having. the far end short-circuited;

and

Fig. 5 is a curve illustrating a standing-wave voltage pattern, similar to. Fig. 4, on a transmission line of threeequarters of, aiwavelength.

. In carrying out. my invention, I provldea gen- 2 crating means for generating high frequencies at powers utilizable for large-scale dielectric heating. In the preferred form, frequencies in the order of megacycles and more are recommended, but my invention is not limited to such conveyed from the oscillator tubes and introduced into the dielectric material being heat treated. In order to provide a high Q in these conductors, and to be able to transmit the high power, they comprise a plurality of flat metal members which are substantially parallel and spaced apart a distance sufilcient to receive the dielectric material to be heat treated. The generator means is operated at a frequency high enough to establish a wave-pattern along the conductors, which for practical purposes can be considered to be a standing wave. Conveyor means are provided for moving the material lengthwise between the transmission-line conductors, so that the existence of a standing-wave on the conductors can be tolerated since it would only affect the amount of heat introduced into the material at a particular point alon the length of the conductors. Inasmuch as each piece of material moves along the transmission line for a definite length, the amount of heat which it receives is determined by the speed with which it is conveyed and the fraction of the standing-wave voltage pattern along the conductors with which it is associated during its movement. A related form of invention in which power transmittin conductors are used for a frequency-determining tank circuit of a radio or high frequency generator means, is disclosed and claimed in my copending patent-application Serial No, 556,140 filed concurrently herewith.

Referring to Fig. 1, the generating means, shown in simplified form, comprises a tube 2 having a plate electrode or anode 4, a grid or control electrode 6 and a cathode 8, and a tube In having a plate electrode or anode I2, 2, grid or control electrode l4 and a cathode IS. The elements of the tubes 2 and 4 are connected in push-pull arrangement, the grids 6 and I 4 being interconnected by an inductance coil I8 across which a variable capacitor 20 is connected to provide a tunable grid circuit.

Each of the anodes 4 and I2 have an end of conductors 2-2 and 24, respectively, electrically associated therewith and extending therefrom to provide the equivalent Ul a transmission line, indicated in its entirety by the reference numeral 26. The conductors are preierably copper sheets or similar plate-like material. They are preferably of the same width, and should have a thickness greater than the so-called depth of current penetration. Their far ends, away from the tubes 2 and 4, are short-circuited by a strap 28, or other suitable means, which is grounded as at 30. The cathodes 8 and 46 of the tubes 2 and 4 are grounded at 32 through a by-pass capacitor 34 for completing; the high frequency circuit. Direct current for the plate supply is provided by connecting the positive end of any suitable direct current voltage source to the strap 28 or ground 30, as indicated by the conductor 36 having a plus sign applied thereto. The negative side of the direct current voltage source is connected to the cathodes through connection 38.

The transmission line, comprising the conductors 22 and 24, is approximately a quarter of a wave-length of the operation-frequency of the system. If desired, the length of these conductors can be adjusted for varying the resonant frequency, by providing slidable over-lapping sections, as indicated at 40, the sections being se cured in any suitable manner. The resonant point of the grid circuit can be harmonized to that of the plate oscillatory circuit by the variable capacitor 20.

The conductors 22 and 24 are spaced apart a distance sufiicient to receive the upper run 42 of a belt-conveyor 44 which is wrapped around a plurality of drums 46, 4B and 50, the first two having their upper edge portions inside the space between the conductors, and the latter providing an incline on which blocks of material may be placed so as to be conveyed into the space between the conductors, the conveyor being moved as indicated by the arrow on the driving drum 4B. The drums 46 and 48 and the conveyor are preferably of insulating material slightly less in width than the distance between the conductors. If desired, a gas blast device 52, or other means, may be provided for blowing cooling gas against the lower side of the conveyor. Similar means may be provided for the drums 46 and 48 for keeping them cool, if desired.

In passing between the conductors, the dielectric material becomes heated, and after treatment drops from the end of the belt-conveyor into a hopper 54. or other suitable materiah receiving means. In the event that a straight upper run is desired for the conveyor, the shortcircuiting strap at the end of the transmission line can be turned upwardly as indicated in Fig. 3 in which an arch 56 short-circuits the ends of the transmission line 28.

With a quarter wave length transmission line having its far end or extremity short-circuited so that there is no high frequency voltage thereacross, the standing-wave pattern assumes the form shown in Fig. 4 where the point A represents the point on the conductors substantially the anodes of the generating means, and the point B represents the terminus of the transmission line at the short-circuiting strap 28. Maximum voltage across opposite points of the conductors exists at A and minimum voltage at B. It is preferable to feed the dielectric material into the heating electrostatic field between the conductors 22 and 24, at or near B which may be designated as a voltage nodal point.

In the embodiment shown in Fig. 1, using a quarter wave length line, the heating material will drop from the conveyor near a point of high heat voltage. However, by making the line longer, the discharge of the heat-treated material can also be located at a voltage nodal point. This is indicated in Fig. 5 in which the standing-wave voltage pattern is shown for a transmission line of three quarters of a wave length. The point A corresponds to that of Fig. 4, being close to the source of the high-frequency power at the plateelectrodes of the tubes. The point C represents the far short-circuited extremity of a transmission line about three times the electrical. length of that of Fig. 1. Two voltage nodal points are provided, that at C and another at D. The material to be heat treated can be placed between the conductors at either of these voltage nodal points and discharged at the other so that there is no high-frequency voltage at the places where the material is fed into and removed from the electrostatic heating field between the conductors. It may be desirable to provide a chargc removing and grounding device for the conveyor system. This is indicated in Fig. 1 by a grounded metal brush 58 contacting and spanning the conveyor.

There is a further advantage in using a halfwave of the standing wave pattern, or a multiple thereof, for the heat treatment. While the material is passing along the conductors, one edge will be heated more than the other for one quarter of a wave length, while the reverse condition will take place for the other quarter wave length so that the heating across each article will be averaged. However, irrespective of how much of the field between the conductors is utilized for heating, each article obviously will be subjected to substantially the same heat treatment.

My invention is not limited to any particular dielectric material which can be heated by dielectric means of a type described. Dielectric material has a low power factor so that a high ratio between the circulating kilowatt-amperes in the transmission line and the real power, which is desirable for a tank-circuit, is not too far upset when the material is inserted between the conductors. Rayon yarn cakes can be passed in succession between the conductors, or the pieces of material might be foods of different kinds for precooking. My invention is particularly suited for heating molded plastic preforms in the preliminary state for making shock helmets, hand wheels, or any other object made from a cellulose fiber which is first impregnated with a resin and then lightly pressed into shape. Such a preform contains a certain amount of solvent and can be preheated in the apparatus disclosed for a preliminary removal of the solvent and the raising of its temperature close to the molding value, so that the molding time is considerably decreased.

Satisfactory heating of preforms such as described was obtained at frequencies 01' about 10 megacycles per second, but a higher frequency in the order of 20 to 40 megacycles was more satisfactory. The length of a quarter wave length transmission line will be approximately 10 feet at 20 megacycles or approximately 5 feet at 40 megacycles. One-half and three-quarter wave lengths will obviously be respectively 2 and 3 times these lengths.

The height of each conductor, which is, 0! course, the width thereof, should preferably be at least that of the article passed therebetween and even higher. Wide conductors provide a condition which permits a high reactive kilovolt component in the tank-circuit comprising the conductors, so that the generator means can readily oscillate with power going into the work.

However, the height should be relatively small in said conductors, and portions at the ends of said comparison with the length, about one-fourth or horizontal portion arranged to carry the material less, so that the wave-pattern across the width oi; into and out of the space between said conductors the conductors will have maximum and minimu substantially at said points. values that are close to each other, or substan-I: 5 ROBERT M. BAKER. tially-the same. By having anumber of pieces; of material between the conductors at all timesff the load on the tube-oscillator generator will re; main essentially constant. p

While I have described my invention in forms 10 REFERENCES CITED The following references are of record in the file of this patent:

which I now prefer, it is obvious that the prin- E UNITED STATES PATENTS ciples and teachings thereof have broader appli- Number Name Date cation and can be readily utilized by'those skilledi 1,945,867 Rawls Feb. 6, 1934 in the art for other embodiments and modi'flca-j r 1,972,050 Davis Aug. 28, 1934 tions involving high-frequency heating of the 2,159,782 Conklin et a1. May 23, 1939 type described. 2,239,069 Worden et a1 Apr. 22, 1941 I claim as my invention: 2,304,958 Rouy Dec. 15, 1942 Dielectric heating apparatus for the high-"ire: 2,308,043 Bierwirth Jan. 12, 1943 quency heating of dielectric material, comprising; 2,308,204 Parry Jan. 12, 1943 in combination, high frequency power generating 2,364,526 Hansell Dec. 5, 1944 and transmitting means comprising a plurality 2,370,423 Roberts Feb. 27, 1945 of spaced substantially parallel elongated up- 2,332,435 M n t a1 Aug, 14, 1945 standing plate-like conductors and connections to 2,433,067 Russell Dec. 23, 1947 said conductors for providing a standing-wave voltage pattern lengthwise thereon during opera- OTHER REFERENCES tion of said means, said pattern having a pair of Bierwirth et al.: Radio-Frequency Heating spaced points at substantially ground potential, Applied to Wood Gluing, Proceedings of the Inand conveyor means having a material-carrying stitute of Radio Engineers, October, 1943, pages run having a horizontal portion arranged go-move 529-537, particularly pages 534 and 535.

said material horizontally lengthwise between

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