FR2683420A1 - Device for applying microwaves with a view to treating a material - Google Patents

Abstract

The present invention relates to a device for applying microwaves with a view to heat treatment of a material. This device comprises, in particular, a microwave generator (11), a waveguide (12) and resonant cavity (14) of elongate shape. This cavity is advantageously of circular cross-section and functions in the TEm1p mode (with m odd and m and p positive integers). The cavity according to the invention is designed for treating a material entering the cavity through an access slot 23 and emerging therefrom through an exit slot. Each of these slots has the advantage of having a height of approximately 3.5.10<-2> m without leading to significant microwave leaks. The present invention is intended for heat treatment of material of any kind and in various forms (in sheet form, as a powder, etc.).

Description

DEVICE FOR THE APPLICATION OF MICROWAVE
FOR THE TREATMENT OF A MATERIAL
The present invention relates to a device for the application of microwaves for the treatment of a material. This treatment can be drying, gluing, baking, etc. Hereafter, the term microwaves will be used to designate the waves whose frequencies are between 100 MHz and 100 GHz.

More precisely, the device according to the invention makes it possible to carry out a heat treatment of a material. Preferably, this material is in the form of sheets and can be of any material such as wood, a synthetic material, photographic film, paper, glass, food products, powders, plants, etc. ..

It is already known, in particular from the patent
FR-A-2 523 797, to dry ink deposited on a sheet of paper by passing the latter through a cavity in which there is an electromagnetic field which creates microwaves.

The device according to this document comprises a microwave source, a waveguide and a resonant cavity in which the material to be dried passes. The resonant cavity is of rectangular section and has a longitudinal slot formed on each of two opposite faces.

The material to be treated enters the interior of the cavity through one of these slits and comes out treated through the opposite slit. A means of transport, such as a conveyor belt, supports and drives the material to be treated throughout its journey outside and inside the cavity.

Such a device is suitable for drying the ink deposited on sheets moving at high speed as is the case for the production of a newspaper, for example.

However, such devices have many drawbacks and in particular microwave leaks at the slots. To reduce such leaks, the height of the slots must be limited. Another solution would have been to increase the energy level inside the cavity, so as to compensate for the leaks. This solution is generally not adopted because it leads to exceeding the standards regulating the permissible leaks and can lead to a deformation of the fields and therefore to the homogeneity of the treatment.

The limitation in height of the access and exit slots of the cavity is however a major drawback since in patent FR-A-2 523 797 this height is limited to a maximum of 2.5.10-2 m. Now, it is necessary for these slots to be able to pass not only the material to be treated but also the conveyor belt or any other means of transporting these products to be treated.

In addition, when a resonant cavity is of rectangular section, it is generally necessary to place between the waveguide (which is also of rectangular section but smaller) and the cavity, a transition element of frustoconical shape based on rectangle. This element must gradually flare up to present the section of the resonant cavity. When such an element is omitted, the transfer of the microwaves from the waveguide to the cavity is poorly carried out; part of the energy is reflected back to the generator and the efficiency of the entire device is reduced.

Such an element is however difficult to produce since its shape and dimensions must be observed with precision.

The object of the present invention is to overcome all of these drawbacks and in particular to create a device for the application of microwaves having slots of less limited height in order to be able to process materials of greater thickness than hitherto. .

Likewise, the present invention aims to create a device for the application of microwaves, simple to produce, economical and not requiring the creation of elements with specific shapes.

To this end, the present invention relates to a device for the application of microwaves for the treatment of a material, said device being of the type comprising
- a microwave generator,
- a waveguide, adapted to receive the microwaves created by the generator and to transmit these microwaves through a coupling iris towards a resonant cavity of elongated shape, and
- the resonant cavity in which an electromagnetic field prevails, presenting nodes and bellies of intensity, the cavity being provided with two symmetrical slots located opposite in the elongated direction of the cavity and adapted to allow the material to travel inside the cavity.

The device according to the invention is characterized in that the resonant cavity has a circular section and operates according to the TEm1p mode (with m odd and m and p integers positive).

Such a circular resonant cavity device thus makes it possible to be placed in the operating mode of the TEm1p type with odd m and m and p positive integers, and therefore the current lines developed in the walls of the cavity are parallel to the slots . Such an embodiment has the advantage of allowing a greater height for the slots intended for the passage of the material to be treated since the microwave leaks through these slots are limited compared to the prior art.

The device according to the invention thus makes it possible to process materials of greater thickness which can range up to 3.5 × 10-2 m, which represents a 40% increase in thickness compared with the devices known to date.

In addition, the transmission of microwaves between the rectangular section waveguide and the circular cavity is direct and without loss of energy. Therefore no element of transition to the studied shapes is necessary.

The device according to the invention is thus easy to produce and economical, and it has at least equal performance without increasing the amount of energy involved.

The direction of travel of the product to be treated is preferably perpendicular to the elongated direction of the cavity. However, to increase the passage time in the cavity, that is to say also the treatment capacity of this cavity, the elongated direction of the cavity can be inclined (preferably 450) with respect to the direction of travel. of the material. The time taken for the material to pass through the cavity is thus increased by Wrz.

In order to be able to process any materials, it should be noted that the length of the resonant cavity which is linked to the mode of propagation of the waves inside it is adjustable. Therefore, whatever the material treated, it is possible to adjust the length of the cavity so as to always work in TEm1p mode (with m odd and m and p integers positive). For this purpose, the resonant cavity is provided at its end opposite to that carrying the coupling iris with a disc movable in the elongated direction of the cavity (this sliding s1 takes place without contact with the wall of the cavity in order to avoid electric arcs) and adapted to define the length of this cavity.

To avoid any loss of energy due to the presence of the slots, they are provided on their faces external to the cavity, with an energy absorption means. This means is advantageously produced in the form of a cut-off waveguide.

One of the advantages of the invention is in particular that the device according to the invention can be coupled to one or more other identical devices. This coupling takes place at the level of the resonant cavities through which the material passes. Each resonant cavity is then placed such that material emerging from a first cavity enters a second cavity and so on. Several devices according to the invention can thus be placed in "series"; the microwave generators being able to be placed alternately at one or the other of the ends of the cavities.

The electromagnetic field prevailing inside a cavity presents bellies and nodes of intensity. As a result, the materials moving inside a cavity have areas of stronger and weaker heating. Therefore, the drying of the material may not be uniform.

Advantageously, in the case where two cavities are placed in series, by shifting by> / 4 (S is the internal wavelength of the microwaves) each cavity with respect to the one which follows it, the hot points of this cavity with respect to the following cavity. Thus, it is possible to obtain a homogeneous distribution of heat over the entire width of the material to be treated, when this material has passed through the entire series of cavities.

Of course, this placing in series of several devices according to the invention can be done with a movement of the product perpendicular to the elongated direction of the cavity or in an inclined manner with respect to this direction.

It should be noted that all the metallic materials used for making the cavities or waveguides are non-magnetic.

Other objects, characteristics and advantages of the present invention will emerge from the following description, by way of simple illustration of the invention, with reference to the accompanying drawings in which
- Figure 1 is a perspective view showing a device according to the invention,
- Figure 2 is a vertical sectional view along the line II-II of Figure 1, and
- Figure 3 is a perspective view of three devices according to the invention mounted in series.

According to the embodiment shown in Figures 1 and 2, the device 10 according to the invention comprises a source of microwaves 11 emitting electromagnetic waves for example of 800 W (or of 1200 W, or of 2000 W) restored at 2450 MHz, to a waveguide 12, connected to a coupling device 13 having an iris 30 of diameter d (FIG. 2), directing the microwaves towards the interior of a resonant cavity 14. This resonant cavity 14 has a circular section of constant diameter f and length L, and is made of thin stainless steel (fig. 2) and not magnetic.

Each of the ends of the cavity 14 receives a non-magnetic metal closure disc 15 and 16. The disc 15 is provided with an opening 17 in its center forming the iris 30 and adapted to allow microwaves coming from the source 11 to pass through. The disc 16 is, for its part, provided with a threaded bore 18. at its center and receives in this bore a fixing means 19, for example a threaded rod.

A first end 19a of this rod protrudes towards the outside of the cavity and a second end 19b of this rod is integral with a metallic and non-mangetic tuning disc 20 located inside the cavity 14. The diameter D of this magnetic disc is slightly smaller than the diameter of the cavity, so that this disc slides inside the cavity without contact and without creating an electric arc.

A wheel 21 is attached to the end 19a of the rod.

Two slots 22 and 23 respectively (exit and access) are formed one opposite the other in the cylindrical wall of the resonant cavity 14 and are adapted to allow the passage of a means of transport 24 (visible in FIG. 3) constituted for example by a conveyor belt made of a material marketed under the Teflon brand or of another non-polar material. These slots have a height h (fig. 2). The dimensions of the resonant cavity 14 and of the various elements constituting the device are for example as follows, in the case of a product with low absorbency = = 0.250 mh = 0.035 m
L = 0.970 m (for p = 12) d = 0.060 m
D = 0.249 m.

In this case, the minimum length of the slits lmin is equal to 0.080 m, the maximum size being any.

Preferably, the minimum length lmin of the slits is> / 2 (> is the internal wavelength).

The microwave source has a fixed or variable power, for example 800 W, 1200 W or 2000 W restored at 2450 MHz. For any other frequency, the dimensions would be correlated (homothety).

Each of the access and exit slots (23, 22) is provided with a wave trap 35 disposed at the level of the slots and outside the resonant cavity 14.

With such a device, 100% drying of the ink of a sheet is obtained by scrolling this sheet up to 1 m / s (this in the case of eight 1200 W generators).

The operating mode of such a cavity is a transverse electric mode TEm1p where m is an odd positive integer and p a positive integer.

The fact that m is odd implies that in the walls of the cavity, the current lines develop parallel to the slits made. This prevents leakage of microwaves directly through the slots.

However, if the height of the slits were to become greater (greater than 0.035 m in the example shown), losses similar to those existing with a cavity with a rectangular section could appear.

It should be noted that the material (here a sheet of paper) to be treated passes through the cavity in a direction perpendicular to the elongated direction of the cavity.

As a variant (not shown), it is possible to give the direction of movement of the material an inclination with respect to the elongated direction of the cavity.

Preferably, this inclination is 450, which has the consequence of multiplying by 2 the length of the path of the material through the cavity. The drying capacity of the device is thus increased without varying the running speed, which makes it possible, for a given running speed, to multiply the drying time by u / 2.

The mode of operation of the invention is described below. After excitation of a magnetron used as a microwave generator 11, the waves created are guided in the waveguide 12, then pass into the iris 30 before being distributed in the resonant cavity 14.

There, by moving the tuning disc 20 in the elongated direction of the cavity (by means of the wheel 21 and the threaded rod 19), the length L of the cavity is defined so that the electromagnetic field prevailing in the cavity is established according to the TEm1p mode (with m odd and m and p positive integers). As a result, the electric field which is established in the walls of the cylindrical cavity is such that its field lines are parallel to the slots 23 and 22 at the access and exit of the cavity.

When we operate in the TE1îp pout (m = 1), we are in the fundamental mode of this type of cavity. Microwave leaks through the slots are then limited as much as possible. This operating mode is however very difficult to adjust.

When operating in TE31p mode (m = 3), it is in the no-load operating mode, which cannot be used by its nature, when a material is introduced into the cavity.

The usual operating modes of this type of cavity are the modes where m is greater than or equal to 3.

Once the operating mode has been set and established, the material to be treated passes into the cavity, supported by a conveyor belt 24 of non-polar material (for example of a material marketed under the Teflon brand) or a similar means of transport. During its passage through the cavity, the material thus undergoes suitable heating, for example to dry an ink deposited on the material.

Advantageously, the microwave generator (magnetron for example) is associated with a protection means.

Indeed, when the material to be treated is not very absorbent or is momentarily absent from the cavity, part of the emitted waves can be reflected towards the magnetron and damage it.

To overcome this drawback, the magnetron of the device according to the invention is associated with a protection means continuously measuring the temperature of its body.

As soon as a set temperature value is exceeded, the power supply to the magnetron is cut off by a quick cut-off device. The magnetron is restarted very quickly (a few seconds) because of the proximity of the on / off instructions. In the present embodiment (not shown), the means for measuring the temperature is a thermal sensor with a very low time constant.

Thanks to such a device, it is possible in all cases to keep the reflective metal tuning disc 20, without it being necessary to replace it with a water charge as is usually the case for materials to be treated. not very absorbent. In addition, the dimension of the iris 30 is determined experimentally so that the energy reflected towards the magnetron is on average as low as possible.

Advantageously, as shown in FIG. 3, it is possible to associate a plurality of devices according to the invention.

The material to be treated, carried by its means of transport, in this case penetrates successively inside each cavity. As inside each cavity the electromagnetic field presents nodes and bellies of intensity, it creates more or less heated points.

The distance between two "cold" (or "hot") points corresponds to the internal wavelength of the cavity. By shifting the location of each of these points from one cavity to another by a value) </ 2n (n is the number of devices placed in series), uniform heating and drying of the product is achieved over all its width (when it has passed through the series of devices, of course). Thus, when two cavities are coupled, they are offset transversely with respect to each other by / 4.

It will be noted that in order to further reduce the microwave emissions to the outside of the cavity, in the vicinity of the slits, the latter are fitted with wave traps and / or absorption means. As can be seen in FIG. 3, these absorption means are produced in the form of cut-off waveguides, surrounding each slit.

As a variant (not shown), in the case where devices mounted in series are used, the waveguides at the cut-off of the exit slots of one cavity and of access to a following cavity are combined. This prevents any leakage of microwaves to the outside.

As a variant (not shown), between two successive cavities, there are pressure rollers suitable for pressing the material when it comes out of a cavity in order to stick it for example. These two rollers can be made of metal.

It should be noted that, when the product to be treated has very little absorbency, the conveyor belt is suitable for carrying plates of materials, such as, for example, doped glass, to heat the product to be treated.

Of course, the present invention is not limited to the embodiments described and encompasses any variant within the reach of those skilled in the art. In particular, the materials constituting the various elements of the invention can be modified and the means of transporting the material to be treated is not limited to that described.

Claims (17)

1 / - Device for the application of microwaves for the treatment of a material, said device (10) being of the type comprising - a microwave generator (11), - a waveguide (12), adapted to receive the microwaves created by the generator (11) and to transmit these microwaves through a coupling iris (30) to a resonant cavity (14) of elongated shape , and - the resonant cavity (14) in which an electromagnetic field prevails having nodes and bellies of intensity, the cavity being provided with two symmetrical slots (22, 23) located opposite in the elongated direction of the cavity and adapted to allow movement of the material inside the cavity at a predetermined angle of inclination with respect to the elongated direction of the cavity, said device being characterized in that the resonant cavity (14) has a circular section and works according to the TEm1p mode with odd m and m and p positive integers. 2 / - Device according to claim 1, characterized in that m is greater than or equal to 3. 3 / - Device according to claim 1 or 2, characterized in that the elongated direction of the cavity (14) is perpendicular to the direction of travel of the material. 4 / - Device according to one of the preceding claims, characterized in that the resonant cavity (14) is provided on its side opposite the coupling iris (30) with a tuning disc (20) movable without contact along the elongated direction of the cavity, adapted to define the length L of the cavity. 5 / - Device according to claim 4, characterized in that the disc (20) is provided with an attachment means (19) to a closing disc (16) of the cavity (14). 6 / - Device according to one of the preceding claims, characterized in that the access and exit slots (23, 22) have a maximum height h of 0.035 m and a minimum length lmin of> / 2 (where represents the internal wavelength). 7 / - Device according to one of the preceding claims, characterized in that the access and outlet slots (23, 22) are arranged to be parallel to the current lines developed in the walls of the cavity (14). 8 / - Device according to one of the preceding claims, characterized in that the slots are provided with wave traps and / or absorption means adapted to limit the leakage of microwaves to the outside. 9 / - Device according to one of the preceding claims, characterized in that at least a second device (10) is coupled to the first, so that said material passes successively in at least two successive cavities. 10 / - Device according to claim 9, characterized in that each resonant cavity (14) of each of n devices (10) placed in series, is offset transversely with respect to the direction of travel of the material by a value of r 3K / 2n. 11 / - Device according to one of claims 8 to 10, characterized in that the absorption means are made in the form of waveguides at cut-off and connect to each other the slots of two successive resonant cavities. 12 / - Device according to one of the preceding claims, characterized in that it comprises a material transport means, made of non-polar material. 13 / - Device according to claim 12, characterized in that the material transport means is a conveyor belt of a material marketed under the Teflon brand. 14 / - Device according to one of claims 1 to 11, characterized in that it comprises a material transport means made of an absorbent material suitable for heating by contact of low-polar materials to be treated. 15 / - Device according to one of claims 9 to 11, characterized in that there is, between two successive cavities, the pressure rollers. 16 / - Device according to one of the preceding claims, characterized in that the microwave generator is a magnetron associated with a protection means adapted to protect the magnetron, in the momentary absence of material to be treated in the cavity. 17 / - Device according to claim 16, characterized in that the protection means is a device for rapidly cutting off the power supply to the magnetron adapted to assess the temperature of the body of the magnetron and to immediately reset the power supply to the magnetron, when ' material is present in the cavity.