Title:
Ultra high frequency dielectric heater
United States Patent 2467230


Abstract:
Our invention relates to ultra high frequency dielectric heaters, more particularly to ultra high frequency dielectric heaters utilizing a chamber formed by walls made of electrically conducting material, in which chamber electromagnetic fields are produced for heating a material in the chamber,...



Inventors:
Earl, Revercomb Henry
Watts, Donald E.
Application Number:
US77148047A
Publication Date:
04/12/1949
Filing Date:
08/30/1947
Assignee:
GEN ELECTRIC
Primary Class:
Other Classes:
219/700, 219/752
International Classes:
H05B6/76
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US Patent References:



Description:

Our invention relates to ultra high frequency dielectric heaters, more particularly to ultra high frequency dielectric heaters utilizing a chamber formed by walls made of electrically conducting material, in which chamber electromagnetic fields are produced for heating a material in the chamber, and has for its object a continuous or conveyor type heater of this type in which the material is heated uniformly in passing through the chamber and enters and leaves the chamber 1 without substantial loss of energy.

In carrying out our invention in one form we provide a chamber of rectangular cross section and of such size in a transverse direction relative to the frequency of the supply source as to have a transverse electric field mode produced in it. We provide inlet and outlet openings in opposite ends of the chamber and in opposite ends of the transverse dimension of the chamber, together with conveyor means for passing the material to be heated through the chamber between the openings in an oblique or up and down manner so that the material is exposed at all points to the same field heating effect whereby the material is heated uniformly.

We also provide means for preventing the radiation and loss of energy through the entrance and exit openings. In one form we provide each opening with vestibule walls constructed to act like a wave guide with extremely high ettenuation so that no appreciable energy is dissipated through the openings. In another form we provide a pair of doors for each opening made of electrically conducting material spaced apart preferably one-half wave length in the chamber so that one or the other of the doors may be closed or opened without disturbing the field pattern in the heating chamber.

For a more complete understanding of our invention reference should be had to the accompanying drawing, Fig. 1 of which is a diagrammatic view in perspective of an ultra high frequency heater embodying our invention, Fig. 2 is a fragmentary view showing a modified form of our invention, while Fig. 3 is -a view similar to Fig. 1 showing improved means for obtaining uniform heating and for preventing the loss of energy through the entrance and exit openings of the heating cavity.

Referring to the drawing, we have shown our invention in Fig. 1 as applied to a rectangular chamber or cavity I having transverse and lengthwise dimensions of such length with respect to the frequency of the ultra high frequency supply source 2 that a transverse electric field mode Hm,n,p is produced in the chamber, i. e., an electric field whose vector lies transversely to the chamber and to the direction of wave propagation in the chamber. In this mode the letters mr, n, and p specify the number of half-sinusoidal electric field variations in the three dimensions of the chamber indicated by these letters in Fig.

1. For resonance of the chamber these letters m, n and p must be represented by whole numbers.

More specifically, the chamber-the walls of which are made of electrically conducting material such as copper-is preferably constructed for the Ho,2,p mode in which "o" specifies a vertical transverse dimension or side m less than onehalf of the wave length in the chamber during the heating operation when the chamber contains the articles or material being heated, hereinafter referred to as a loaded chamber, the figure "2" specifies a horizontal transverse dimension or side n somewhat over one wave length and less than three half-wave lengths in the loaded chamber, and in which the length p is several wave lengths in the loaded chamber. Standing electromagnetic field waves are therefore produced in this chamber. With this length of the dimension or side n two maximum values of the electric field appear in moving across this side, i. e., two half waves, while along the narrow transverse dimension the magnetic field is constant.

For instance, with a supply source 2 supplying power at a frequency of 1050 megacycles and having a wave length in air of about eleven and oneeighth inches, the dimension m for the wave length in the chamber is less than about five and one-half inches, the transverse dimension n is over eleven and one-eighth inches, and the length p is several times eleven and one-eighth inches. These dimensions, however, are given without regard to the fact that the material being heated ordinarily has the effect of shortening the waves very considerably. Actually, the wave lengths existing in the loaded chamber are equal to the wave length of the supply source divided by the square root of the equivalent dielectric constant of the material to be heated and air in the chamber.

In its left-hand end, as shown, the chamber is provided with an entrance or charging opening 6 enclosed by rectangular vestibule walls 7, this opening extending from the top to the bottom of the chamber but extending only over substantially less than one-half wave length in the horizontal transverse dimension n of the chamber.

Moreover, the opening 6 is located in the righthand crosswise end of the chamber. A similai outlet opening enclosed by vestibule walls 8 is provided in the opposite end of the chamber, this opening, however, being in the left-hand transverse end of the chamber, i. e., diagonally opposite the opening 6. A suitable endless conveyor, such as a belt 9, is provided having its upper length extending through the chamber between the openings so as to convey a series of articles or material 10 to be heated diagonally through the chamber. Our heater is especially adapted by reason of its ultra high frequency for use in the heating of dielectric materials, such as cellulose materials, frozen foods, textiles, rubber, etc.

The supply source 2 is connected through a coaxial line I I (or waveguide) to the chamber I at one corner as shown, the outer cylindrical conductor 12 of the line being electrically connected to the upper longer transverse wall of the chamber, while the inner conductor or probe 13 extends downward into the chamber. The conductors 12 and 13 are electrically insulated from each other, perpendicular to the longer side wall of the chamber and the probe 13 is parallel with the electric field produced in the chamber.

With the generator 2 in operation and maintaining in the chamber the electric field mode previously described, the material 10 in passing through the chamber passes at all points through a plurality of electric and magnetic field strengths in moving lengthwise of the chamber, and in moving crosswise of the chamber, because of its oblique direction of movement, it passes through two points of maximum electric field strength. This serves to produce a uniform interception of both the electric and magnetic fields by the material 10 in horizontal planes.

The vestibule walls 7 and 8 are provided for the purpose of minimizing the dissipation of energy from the chamber through the entrance and exit openings. The vestibules have lengths each of at least substantially one-quarter wave length, but preferably several quarter wave lengths or several wave lengths, and transverse dimensions each less than one-half wave length.

The wave length in each case is that existing in the vestibule while the material to be heated is in the vestibule, i. e. the wave length in the loaded vestibule. In other words, this vestibule acts like a wave guide with extremely high attenuation.

As a further explanation of the entrance and exit vestibule openings below cut-off, we preferably construct each vestibule with transverse dimensions in dependence upon the dielectric characteristics existing in the vestibule as the result of the combined dielectric constants of the air and the material to be heated, as well as the frequency of the supply source. It is assumed that the material being heated has a dielectric constant at least as great as air. Ordinarily, in the heating of dielectric materials, the material will have a dielectric constant several times greater than that of air. Accordingly, we construct the vestibule walls with transverse dimensions each of which is less than the value X 2VE in which X equals the wave length of the supply source and E equals the equivalent dielectric constant of the material to be heated and the air in the vestibule section. If a cylindrical vestibule 13' is used as shown in Fig. 2 which has a circular cross section, its diameter should be less than s 0.585x S5 E In the modified form of our invention shown in Fig. 3, we provide a pair of sheet metal doors, preferably made of the same material as the chamber walls, in the vestibule around the entrance and exit openings for preventing the dissipation of energy through these openings. The doors of each pair, 15, 19 and 21, 22, are spaced apart a distance along the path of movement of the material to be heated approximately equal to one-half of the wave length in the loaded chamber 14 so that the standing wave pattern in the chamber remains the same as long as one of the doors is closed. The inner doors 19 and 21 when closed form continuations of the respective end walls of the chamber 14.

In order to insert a quantity of material 16 into the heating chamber, the door 15 is first raised and the material 16 pushed under the door from an outer conveyor 17 onto a short intermediate conveyor 13. Then the door 15 is closed, as shown in the drawing, the door 19 opened, the material 16 pushed onto the conveyor 20 and the door 19 closed. The conveyor 20 carries the material through the heating chamber. At the ops0 posite end of the chamber the material is removed by successively opening and closing the doors 21 and 22. High frequency power is supplied to the chamber 14 by means of a coaxial line or wave guide 23 at any suitable point for S5 the transfer of power to the chamber.

When the doors are used, the entrance and exit openings need not be restricted to transverse dimensions giving high power attenuation as disclosed in Fig. 1. We contemplate, also, that the doors may be formed of metal screen material and, also, we contemplate that a choke joint may be used in place of the doors.

In Fig. 3 we have also shown a conveyor 20 for raising and lowering the material in its passage through the chamber, as well as moving the material diagonally through the chamber as described in connection with Fig. 1. The upper length of the conveyor is provided with a central supporting roller 24 which elevates it to a point Ssubstantially midway of the height of the chamber. Consequently, the material to be heated is first raised up to the roller 24 and then lowered in its passage through the chamber.

The arrangement of Fig. 3 may be utilized where the material or articles to be heated are large as compared with the wave length in the loaded chamber. We contemplate that in the arrangement of Fig. 3 the material 16 will be caused to travel approximately one-half wave length in both the horizontal and vertical directions. The roller 24 elevates the center of the conveyor substantially one-half wave length so as to provide this vertical movement.

What we claim as new and desire to secure by Letters Patent of the United States is: 1. An ultra high frequency heater comprising walls made of an electrically conducting material forming a chamber provided with diagonally opposite openings in its ends, means for passing a material to be heated diagonally through said chamber between said openings, and a high frequency supply source connected to said chamber for producing a transverse electric field mode in said chamber thereby to heat the material uni15 formly as it passes through said chamber.

2. An ultra high frequency heater comprising walls made of an electrically conducting material forming an elongated chamber provided with diagonally opposite openings in its ends, means for passing a material to be heated diagonally in a predetermined plane through said chamber between said openings and at the same time moving said material in a plane perpendicular to said first plane, and a high frequency supply source connected to said chamber for producing a transverse field mode in said chamber thereby to heat the material uniformly as it passes through said chamber.

3. An ultra high frequency heater comprising walls made of an electrically conducting material forming an elongated chamber provided with diagonally opposite openings in its ends, means for passing a material to be heated diagonally through said chamber between said openings and at the same time moving said material in a vertical direction, and a high frequency supply source connected to said chamber for producing a transverse electric field mode in said chamber thereby to heat the material uniformly as it passes through said chamber.

4. An ultra high frequency heater comprising walls made of an electrically conducting material forming a chamber provided with an opening through which a material to be heated may be pasesd into said chamber, a high frequency supply source connected to said chamber for producing standing electromagnetic waves in said chamber thereby to heat the material in said chamber, a vestibule wall connected to said chamber surrounding said opening, and a pair of doors in said vestibule wall made of an electrically conducting material, said doors being spaced apart a distance at least substantially one-half of the wave length in said chamber when the chamber is loaded so that the material to be heated can be passed into said chamber by successively opening and lclosing said doors without substantial loss of energy through said opening.

5. An ultra high frequency heater comprising walls made of an electrically conducting material forming a chamber provided with an opening through which a material to be heated is passed into said chamber, a high frequency supply source connected to said chamber for producing standing electromagnetic waves in said chamber thereby to heat the material in said chamber, a vestibule wall connected to said chamber surrounding said opening, and a pair of doors in said vestibule wall made of an electrically conducting material, the innermost one of said doors when closed forming a continuation of a wall of said chambers and said doors being spaced apart along the path of movement of the material a distance at least substantially one-half of the wave length in said chamber when the chamber is loaded so that the material to be heated can be passed into said chamber by successively opening and closing said doors without a substantial loss of energy through said opening.

6. An ultra high frequency heater comprising walls made of electrically conducting material forming a chamber provided with diagonally opposite openings in its ends, means for passing a material to be heated diagonally through said chamber between said openings, a high frequency supply source connected to said chamber for producing a transverse electric field mode in said chamber thereby to heat the material uniformly as it passes through said chamber, and an energy blocking vestibule wall connected to said chamber surrounding each of said openings.

7. An ultra high frequency heater comprising walls made of electrically conducting material forming a chamber provided with diagonally opposite openings in its ends, means for passing a material to be heated diagonally through said chamber between said openings, a high frequency supply source connected to said chamber for producing a transverse electric field mode in said chamber thereby to heat the material uniformly as it passes through said chamber, and an outwardly extending vestibule wall connected to said chamber surrounding each of said openings, said vestibule walls having lengths each of at least substantially one-quarter of the wave length in said vestibule when loaded.

40 HENRY EARL REVERCOMB.

DONALD E. WATTS.

REFERENCES CITED 45 The following references are of record in the file of this patent: UNITED STATES PATENTS Number 2,197,123 2,364,526 2,407,690 Name Date King ------------- Apr. 16, 1940 Hansell ------------ Dec. 5, 1944 Southworth ________ Sept. 17, 1946