Device for adjusting the microwave energy applied to a band or a sheet to be treated in a resonant cavity furnace

[57] ABSTRACT

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

Claims, 8 Drawing Figures

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PATENTEDJAK 1 3^783^21

SMI 1 Of 3

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PATENTEDJAM 1 1974

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PATENTED JAH i 13' ?

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SHEEI 3 OF 3

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DEVICE FOR ADJUSTING THE MICROWAVE ENERGY APPLIED TO A BAND OR A SHEET TO BE TREATED IN A RESONANT CAVITY FURNACE

In microwave heating it is well known to use wave- 5 guide 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 ?. longitudinal slot is generally used, said waveguide being excited on a mode TE 01 by a magnetron ' 10 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 15 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 20 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 25 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 ³⁰folded 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 ³⁵or 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. ⁴⁰

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 ⁴⁵of 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 micro wave energy. 50

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 ⁵⁵

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 _6g

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 ft 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 f₂.

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 λ 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.

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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 nonuniform 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, 9b, 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,, 9b, can be brought in front of slots 9a, 9b or be more or less shifted with respect to said slots with a view towards re- 25 ducing 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 ³⁰metal, 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 IS, 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.

Ako, the resonant cavity 1 can be provided with the said direction, and a plurality of apertures'100^1^additional load 6 which constitutes a further safety de- ⁵⁵ --^;------- ^dleQ lat vice 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 IS 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 con35 seqUently 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.