Device For Adjusting The Microwave Energy Applied To A Band Or A Sheet To Be Treated In A Resonant Cavity Furnace

Abstract

A wave-guide has a slot whose area increases in the direction of reduction of the energy transmitted by a microwave generator.

Description

In microwave heating it is well known to use waveguide furnaces for the treatment of materials in form of sheets or bands, such as papers, adhesive tapes, or cinematographic films. In the art a rectangular wave-guide having a longitudinal slot is generally used, said wave-guide being excited on a mode TE 01 by a magnetron placed at one of the ends of the guide, and the sheet or band to be treated is moved transversely with respect to the slot, i.e. with respect to the axis of the wave propagation inside the resonant cavity of the wave-guide furnaces. In such furnaces, the attenuation of energy is not linear relative to the material being treated, because the propagation direction of the energy is perpendicular to the travel of the material whereby one side of the latter is heated more than the other side.

This disadvantage is not too serious when the energy applied to the band or sheet to be treated is not important, or when the sheet or band is not wide, as is the case in cinematographic films. On the contrary, however, it prevents the treatment of wide bands or sheets having a width of for example several meters, and also of sheets having a thickness greater than 1 millimeter, or sheets having a high energy absorption coefficient, because then only one portion of the width of the sheet is properly heated.

To cope with this problem, the wave-guide may be folded in serpentine manner in a meander furnace so that the resonant cavity of such wave-guide furnaces delimits several successive windings. However, even in such a case it has not been possible, up to now, to obtain uniform heating over the whole width of the bands or sheets to be treated.

The invention solves the above problem in a simple way and makes possible the treatment of sheets or bands of great width and which travel continuously through the microwave furnace.

According to the invention, there is provided a device for adjusting the microwave energy transmitted to a sheet or band travelling transversely to the axis of propagation of microwave energy in a resonant cavity furnace. The resonant cavity furnace is provided in one of the walls thereof with at least one slot in front of which the sheet or band is made to travel, and the slot area increases in direction of microwave energy attenuation along the axis of propagation of the microwave energy.

Other characteristics of the invention are shown in the following detailed description.

Embodiments of the invention are shown by way of none restrictive examples in the accompanying drawings, in which

FIG. 1 is a sectional perspective view of a microwave furnace according to the invention;

FIG. 2 is a diagrammatic plan view of FIG. 1;

FIGS. 3, 4 and 5 each are plan views similar to FIG. 2 and illustrating miscellaneous modifications;

FIG. 6 is a sectional perspective view similar to FIG. 1 but of the embodiment of FIG. 5;

FIG. 7 is a sectional view showing a further embodiment of the invention; and

FIG. 8 is a diagram of an additional adjusting device.

In the drawings, the microwave furnace comprises a wave-guide excited according to the mode TE; this wave-guide is designated in the following description as a "resonant cavity." In the resonant cavity 1 is located the antenna 2 of a microwave generator 3, a magnetron for example, supplied with electric energy by a source 4. An adjustment or matching device 5 constituted, for example, by one or several rings made of polytetrafluorethylene is placed between antenna 2 and the resonant cavity.

Moreover, an additional absorption load 6 is provided near the end of the resonant cavity to absorb such energy as is not applied to the material to be treated. Said material is shown as a plate or sheet 7 which is continuously moved in the direction indicated by arrow f.sub.1 by means of a driving device which may be constituted by a pair of cylinders 8 or by any other suitable conveyor.

The magneto-electric energy transmitted from the antenna 2 is applied to sheet 7 through one of the walls of the resonant cavity 1, i.e., in the example shown in the drawings, through the upper wall wherein openings are provided.

According to the invention and as particularly shown in FIGS. 2-5, the openings can have various shapes. In fact, it has been noticed that in a resonant cavity of the above described type and of rectangular cross-section, which is excited according to mode TE 01, the attenuation of the energy transmitted through a longitudinal slot is not linear since the direction of energy propagation is perpendicular to the travel direction of the material, that is the direction of advancement of sheet 7. To prevent this disadvantage, FIG. 2 shows that the slot 9, whose length is equal to the width of sheet 7, has an increasing width in the direction of propagation of the waves passing through the resonant cavity 1, that is following the direction of arrow f.sub.2.

Because the energy attenuation in the sheet 7 to be treated varies according to an exponential law, the slot 9 is advantageously so configurated that its width increases inversely to the attenuation. A first embodiment is shown in FIG. 2, for use when the sheets 7 to be treated are always of the same nature and consequently have a known absorption coefficient and also an invariable thickness.

The opening of slot 9 need, however, not be continuous, as is shown in FIG. 3. FIG. 3 shows longitudinal slots 9a arranged in succession, the number or extent thereof increasing in the direction of the microwave energy transmission. It is advantageous, as shown in FIG. 3 that successive slots 9a are alternated or staggered to ensure that the material of sheet 7 will receive energy in all the portions thereof.

A further development of the arrangement shown in FIG. 3 is illustrated in FIG. 4, according to which the successive longitudinally extending slots 9a are completed by transversely extending slots 9b which are separated from each other by an interval approximately equal to .lambda. g/2, which means half of the wave length in the resonant cavity.

Preferably, the transversely extending slots 9b are located laterally of the longitudinal slots 9a, because the magnetic field is at a maximum at the sides of the resonant cavity 1. Thus by arranging the slots 9b in a diverging pattern as shown, the attenuation is compensated according to the longitudinal energy propagation axis, which results in an energy application that is substantially uniform over the entire width of sheet 7 during the advancement of the same.

FIGS. 5 and 6 illustrate another embodiment according to which the wave-guide 1 has an extended slot 9 at the type above described in FIG. 2, as well as transversely extending slots 9b which are located in a divergent arrangement. In addition, a tapered plate or ridge 10 tends in the cavity 1 in front of slot 9; it has an inclined top face 10a, the slope of which is directed towards the antenna 2. Preferably the tapered plate 10 can be adjusted vertically by an adjusting device 11, thus making it possible to concentrate the field towards the slot 9 along the field attenuation into the resonant cavity, whereby practically all the energy transmitted by the antenna 2 is uniformly absorbed by the sheet 7.

When the furnace must be utilized for successive treatment of sheets 7 of various thickness and/or with different absorption coefficients, or also of non-uniform thickness then it is advantageous, as shown in FIG. 7, to place on the wall of the resonant cavity 1, provided with the slots 9 or 9a or 9b above described, a grid 12 with apertures 9a.sub.1 9b.sub.1 similar to said slots 9a, 9b. Said grid 12 is connected to an operating screw or other similar device 13 for imparting an axial sliding motion to the grid, so that the apertures 9a.sub.1, 9b.sub.1 can be brought in front of slots 9a, 9b or be more or less shifted with respect to said slots with a view towards reducing the amount of energy applied to sheet 7. Also preferably a tapered plate 10 for the concentration of the magnetic field is utilized as above described with reference to FIGS. 5 and 6.

It is also possible for the plate 10 which is made of metal, to be associated with a plate 14 of a material pervious to microwaves, for example polytetrafluorethylene, in order to adapt or match the impedance of the resonant cavity according to the adjustment of the magnetic field concentration in said cavity, and consequently to the concentration of the electrical field applied to the absorbing material of sheet 7.

The arrangement of FIG. 7 and also of FIGS. 5 and 6 are particularly suitable for the treatment of materials with a very high absorption coefficient.

FIG. 8 illustrates an additional adjusting device including, in the cavity 1, a detector 15, for example a neon lamp which becomes excited if a sufficient amount of energy is applied to it and which then lights up. A cell 16 is placed in front of the lamp 15 to determine if the lamp is or is not lighted. The output of cell 16 is connected to an amplifier 17 whose output signal 15 is used for the control of a power unit 18 for the magnetron 3. For example, the output signal from amplifier 17 can directly control a servo-motor 19 which adjusts the input voltage of a self-transformer 20, the output voltage of the same directly feeding the magnetron 3.

Also, the resonant cavity 1 can be provided with the additional load 6 which constitutes a further safety device intended to absorb, for short intervals of time, an oversupply of energy produced by the magnetron 3.

As it is evident from the above, if some power is not absorbed by sheet 7, then the lamp 15 lights up, which illumination is detected by the cell 16 the output current of which, after amplification in 17, reduces the feeding voltage of the magnetron 3.

In the above description, it has been considered that detector 15 was constituted by a neon lamp. It is possible to constitute said detector as a thermo-electric probe placed into the additional load 6. Said probe then detects any temperature rise of said load and consequently the presence of an excessive amount of power radiated by the magnetron 3. The output voltage from the thermoelectric probe, after amplification in amplifier 17, operates the servo-motor 19 or any other device for adjusting the input voltage of magnetron 3.

It is also possible to use a ferrite element which becomes saturated if the microwaves are not absorbed. The detection of the saturation then can be used to adjust the magnetron power supply.

The invention is not restricted to the embodiments shown and described in detail, for various modifications thereof can moreover be applied to it without departing from the scope of the invention. Especially in case where the whole power developed by the magnetron would not be absorbed by the wave-guide, then the energy could be sent back into one or several other similar wave-guides through suitable bends.

Claims

I claim:

1. A micro-wave cavity furnace comprising, in combination,

a resonant wave guide cavity having longitudinally spaced end portions and a wall extending from one to the other end thereof;

a source of microwaves arranged in one of said end portions and operative for radiating microwaves whose axis of propagation extends from said one towards the other of said end portions;

advancing means for advancing a band of microwave-absorbing material to be treated over said wall exteriorly of said resonant cavity and transversely of said axis of propagation; and

aperture means provided in said wall extending along said axis and having a cross-sectional area which divergingly increases in direction from said one toward said other end portion, the increase in said cross-sectional area compensating for the microwave energy attenuation which takes place in said direction so as to assure uniform treating of said band as the latter becomes incrementally exposed to the microwave energy through said aperture means.

2. A furnace as defined in claim 1, wherein said aperture means comprises at least one slot which diverges in said direction.

3. A furnace as defined in claim 2, wherein said band has a given width in said direction; and wherein said slot has a length in said direction which corresponds to said given width.

4. A furnace as defined in claim 2, wherein said slot diverges in said direction in inverse ratio to the degree of microwave energy attenuation in said direction.

5. A furnace as defined in claim 1, wherein said aperture means comprises an elongated slot extending in said direction, and a plurality of apertures located laterally adjacent saod slot, and wherein the combined cross-sectional area of said slot and apertures increases in said direction.

6. A furnace as defined in claim 5, wherein said apertures are elongated substantially normal to the elongation of said slot and are spaced from one another in said direction.

7. A furnace as defined in claim 6, wherein the distance between consecutive ones of said slots equals substantially one-half of the length of microwaves radiated by said source.

8. A furnace as defined in claim 1, wherein said aperture means comprises a plurality of slot-shaped apertures all elongated in said direction, and wherein the combined cross-sectional area of said slot-shaped apertures increases in said direction.

9. A furnace as defined in claim 1, wherein said aperture means comprises at least one slot which is elongated in said direction, and a plurality of additional slots located at opposite lateral sides of said one slot and extending transverse to the elongation of the same; and wherein the combined cross-sectional area of said slots increases in said direction.

10. A furnace as defined in claim 1; and further comprising a plate member located in said resonant cavity and having an edge face located inwardly adjacent and facing said aperture means.

11. A furnace as defined in claim 10, wherein said edge face is inclined towards said aperture means in said direction.

12. A furnace as defined in claim 1; and further comprising a grid element juxtaposed with said wall and having additional aperture means configurated in correspondence with the aperture means in said wall; and operating means associated with said grid element for shifting the same relative to said wall so as to obtain varying degrees of registry between said aperture means of said wall and of said grid element.

13. A furnace as defined in claim 1; further comprising a detector component located in said resonant cavity in the region of said other end portion and operative for producing a signal upon detecting microwave energy in excess of a predetermined amount; amplifier means connected with said detector component for amplifying the signal thereof; and a control circuit connected with said amplifier means and said source for receiving the amplified signal from the former and for varying the operation of said source in dependence upon the magnitude of the signal.

14. A furnace as defined in claim 13, wherein said detector component comprises a neon lamp which is energized in the presence of said excess microwave energy, and a photosensitive cell positioned so as to detect energization of said neon lamp and produce said signal.

15. A furnace as defined in claim 13, wherein said detector component comprises a thermo-electric probe.

16. A furnace as defined in claim 13, wherein said detector component comprises a ferrite probe.

17. A micro wave furnace for treating a work piece in the form of an elongated band, comprising

a substantially electrically continuous cavity having a longitudinal axis;

a source of microwave power to energize the cavity for propagating therein microwave energy along said longitudinal axis;

at least one longitudinal aperture provided on the cavity on one of the walls thereof, said at least one aperture having a surface extent increasing along said longitudinal axis;

travelling means for moving said band along its own longitudinal plane and transversely to said longitudinal axis in front of said at least one aperture;

thereby providing a substantially even distribution of the microwave power applied to said band.

18. A furnace as defined in claim 17, wherein at least one longitudinal aperture has a length substantially equal to the width of the band.

19. A furnace as defined in claim 17, wherein the microwave energy propagated along the longitudinal axis is regularly attenuated from the source of microwave power and wherein the at least one aperture has a surface extent increasing along said longitudinal axis in inverted ratio to attenuation of microwave energy along said longitudinal axis.

20. A furnace as defined in claim 17, wherein the at least one longitudinal aperture is delimited by sets of longitudinally extending slots, said slots being in a number and extension increasing along said longitudinal axis, said slots being moreover staggered whereby said travelling means are moving the band in front of at least one slot.

21. A furnace as defined in claim 17, further comprising at least one set of transversely extending slots placed in a fringed arrangement around the at least one longitudinally extending aperture.

22. A furnace as defined in claim 17, wherein two of said transversely extending slots are distant from each other by approximately one-half of the microwave length into the resonant cavity.

23. A furnace as defined in claim 17, further comprising a field concentrating plate, said plate being placed inside the resonant cavity in front of the at least one slot and transversely to a plane limited thereby.

24. A furnace as defined in claim 23, wherein said field concentrating plate is a tapered ridge having a slope increasing from the source.

25. A furnace as defined in claim 17, further comprising a grid having at least one hole of a shape corresponding to the shape of the at least one aperture, said grid being placed in parallel relationship to a plane limited by said at least one aperture, and one operating device being connected to said grid for moving it, whereby controlling the opening surface of the at least one aperture.

26. A furnace as defined in claim 17, further comprising a detector component placed inside the resonant cavity downstream from the band while considering propagation direction of the microwaves, said detector component providing a signal at output thereof when it is energized by microwave energy, said detector component being connected to the source of microwave power through a control circuit whereby the source of microwave power is adjusted by means of the signal from said detector.

27. A furnace as defined in claim 26, wherein the detector component comprises a neon lamp and a cell placed in front of said lamp, said cell detecting lighting up the neon lamp when the same is submitted to a radiation and being connected to input of said control circuit.

28. A furnace as defined in claim 26, wherein the detector component is constituted of a thermoelectric probe.

29. A furnace as defined in claim 26, wherein the detector is constituted of a ferrite probe.

30. A method of uniformly heating sheets and bands, comprising the steps of

radiating microwave energy from a location in a wave-guide which is provided in a boundary wall thereof with aperture means that is elongated along the axis of wave propagation and whose cross-sectional area increases along said axis in direction away from said location; and

advancing a sheet or band to be treated over said aperture means transversely to the elongation thereof and outside said wave-guide, so that the increments of said sheet or band which are exposed through said aperture means to said microwave energy will be uniformly heated despite the attenuation of microwave energy along said axis in said direction away from said location.

31. A method as defined in claim 30, wherein said sheet or band is advanced continuously.

32. A method as defined in claim 30, wherein the width of said sheet or band is at most equal to the length of said aperture means along said axis, so that the entire width of said sheet or band is exposed simultaneously in said aperture means to said microwave energy.

33. A micro-wave cavity furnace comprising, in combination,

a wave-guide cavity having longitudinally spaced end portions and a wall extending from one to the other thereof;

a source of microwaves arranged in one of said end portions and operative for radiating microwaves whose axis of propagation extends from said one towards the other of said end portions, said source having a protruding antenna;

advancing means for advancing a band of microwave-absorbing material to be treated over said wall exteriorly of said resonant cavity and transversely both to said axis of propagation and to said axis of the protruding antenna;

aperture means provided in said wall extending along said axis and axis of the aperture means lying in the same plane with the antenna, said aperture means having a cross-sectional area which divergingly increases in direction from said one toward said other end portion, the increase in said cross-sectional area compensating for the microwave energy attenuation which takes place in said direction so as to assure uniform treating of said band as the latter becomes incrementally exposed to the micro-wave energy through said aperture means.

34. A microwave cavity furnace for treating a workpiece in the form of an elongated band, comprising

a substantially electrically continuous cavity having a longitudinal axis;

a source of microwave power to energize the cavity for propagating therein microwave energy along said longitudinal axis, said source having an antenna protruding in the cavity;

at least one longitudinal aperture provided on the cavity on a wall perpendicular to the protruding antenna, said aperture having an axis lying in the same plane with the antenna and a surface extent increasing along said longitudinal axis;

travelling means for moving said band along its own longitudinal plane and transversely to said longitudinal axis in front of said aperture;

thereby providing a substantially even distribution of the microwave power applied to said band.