EP0624051A1 - Microwave oven in particular for rapid heating at high temperatures - Google Patents

Abstract

A microwave oven, in particular for rapid heating at high temperature, characterised in that it includes: - a metal box (10) having the shape of a cylinder which is open at one (12) of its ends, in the cylindrical side wall (16) of which cylinder at least one waveguide (18) emerges, which waveguide is coupled to a microwave generator (20), - a first outer layer (26) of a thermally insulating material which is transparent to microwaves, applied against the inner surface of the cylindrical side wall (16) of the box (10) and having its lower surface resting on - a second inner layer (28) of thermally insulating material which is transparent to microwaves, having a central opening (30) capable of receiving - a layer (32) which is made of a material which absorbs microwaves and constitutes the heating enclosure of the oven, as well as - a metal plate (38) for closing off the open end of the said metal box (10). <IMAGE>

Description

The present invention relates to a microwave oven, in particular for ensuring rapid heating and at high temperature of different types of products, for example for ensuring operations of melting, cooking, mineralization, calcination, heat treatment of metals and annealed materials. , aging tests, thermal shock tests, analytical work, drying at high temperature, ash reduction for chemical tests, and more generally all endothermic chemical reactions.

In fact, the furnace which is the subject of the present invention was designed to replace laboratory ovens of the electric resistance heating type. These traditional ovens of the state of the art have a certain number of drawbacks which the present invention aims precisely to eliminate, among which there may be mentioned a very high thermal inertia, a limited maximum temperature, significant risks of degradation of the constituent parts. high cost price and high energy consumption to reach high temperatures.

• un boîtier métallique affectant la forme d'un cylindre ouvert à l'une de ses extrémités et dans la paroi latérale cylindrique duquel débouche au moins un guide d'ondes couplé à un générateur de micro-ondes
• une première couche extérieure d'un matériau thermiquement isolant et transparent aux micro-ondes, appliquée contre la surface intérieure de la paroi latérale cylindrique du boîtier et dont la surface intérieure repose sur
• un manchon métallique muni d'une pluralité de fentes de couplage et contre la surface interne duquel vient s'appliquer
• une seconde couche intérieure de matériau thermiquement isolant et transparent aux micro-ondes, présentant une ouverture centrale apte à recevoir une couche, se présentant par exemple sous la forme d'un moufle qui est réalisé en un matériau absorbant les micro-ondes, qui est ouvert du même côté que ledit boîtier métallique et qui constitue l'enceinte de chauffage du four, ainsi qu'
• une plaque métallique d'obturation de l'extrémité ouverte dudit boîtier métallique.

The oven according to the present invention is characterized in that it comprises:

• a metal case in the shape of a cylinder open at one of its ends and in the cylindrical side wall from which at least one waveguide opens coupled to a microwave generator
• a first outer layer of a thermally insulating material transparent to microwaves, applied against the inner surface of the cylindrical side wall of the housing and the inner surface of which rests on
• a metal sleeve provided with a plurality of coupling slots and against the internal surface of which is applied
• a second inner layer of thermally insulating and transparent material to microwaves, having a central opening capable of receiving a layer, being for example in the form of a muffle which is made of a material absorbing microwaves, which is open on the same side as said metal case and which constitutes the oven heating enclosure, as well as
• a metal plate for closing the open end of said metal housing.

According to an alternative embodiment according to the invention, a metal sleeve provided with a plurality of coupling slots is interposed between the first outer layer and the second inner layer.

• la figure 1 représente, de façon schématique, une vue éclatée d'un mode de réalisation du four selon l'invention, et
• la figure 2 représente une vue du même four, à l'état monté, avec arrachement faisant apparaître la présence d'un manchon de couplage;
• et la figure 3 représente une vue latérale en coupe d'un four selon l'invention équipé d'un système d'évacuation de fumées et d'entrée de gaz dans l'enceinte de chauffage.

A certain number of other characteristics and advantages of the subject of the present invention will appear on reading the description given below with regard to particular embodiments of ovens and with reference to the appended drawings in which:

• FIG. 1 schematically represents an exploded view of an embodiment of the oven according to the invention, and
• 2 shows a view of the same oven, in the assembled state, with cutaway showing the presence of a coupling sleeve;
• and Figure 3 shows a side sectional view of an oven according to the invention equipped with a smoke evacuation system and gas inlet into the heating enclosure.

In the various appended figures, identical elements will be designated by the same references.

The microwave oven according to the invention firstly comprises a metal casing 10 having the shape of a cylinder open at its front end 12 and comprising a bottom 14 as well as a cylindrical side wall 16. All of the parts metal of this oven, and in particular the housing 10 can advantageously be made of stainless steel, or even of a steel with high temperature resistance.

The cylindrical side wall 16 of this metal housing 10 comprises at least one waveguide 18 coupled to a microwave or magnetron generator 20, the antenna of which opens precisely into the waveguide 18. In the illustrated embodiment in Figure 1, the cylindrical side wall 16 is equipped with two microwave generators arranged diametrically opposite. In the case where the microwave oven according to the present invention comprises more than two microwave generators, these will advantageously be arranged at the periphery of the metal case 10, for example with a constant mutual angular offset.

According to an advantageous variant of the invention, on the bottom 14 of the metal housing 10 is mounted a metal sleeve 22 provided with a plurality of coupling slots 24. This sleeve will advantageously be made of the same metal as that of the housing 10 and will be fixed by an appropriate means capable of withstanding the temperatures to which these parts of the microwave oven will be brought. The coupling slots 24 which are provided on the metal sleeve 22 are advantageously distributed equal to the circumference of said sleeve 22, preferably while respecting a spacing close to a guided half-wavelength. These coupling slots 24 are advantageously provided on the sleeve 22 while respecting an inclination relative to the circumferential direction of said sleeve. According to an alternative embodiment of this sleeve, all of the coupling slots may have the same inclination. According to another variant, some of the coupling slots may have a certain inclination in the opposite direction, relative to the circumferential direction of the sleeve.
Between the inner surface of the cylindrical side wall 16 of the housing 10 and the outer surface of the metal sleeve 22 is interposed a first outer layer 26 of a thermally insulating material and transparent to microwaves. This material must of course be chosen so as to be able to transmit the microwaves, so as to have a low thermal conductivity, and so as to resist ambient heat. In practice, a material based on mineral fibers, in particular fibers or foam of porous ceramic, for example fibers based on alumina or zirconia, will advantageously be used. Such a material which can, for example, consist of a stack of layers of non-woven ceramic fibers, will rest on the external surface of the metal sleeve 22.

According to the alternative embodiment of the oven which does not include a metal sleeve 22, it will be readily understood that the two layers 26 and 28 will be placed in intimate contact with one another. It is clear that while in the variant comprising the metal sleeve 22, the latter participates in strengthening the cohesion of the entire oven, in the case where this sleeve 22 is eliminated, it is the inherent resistance of the insulating layers 26 and 28 which will mainly ensure the mechanical strength of the entire oven.

It is clear that in both variant embodiments, it will of course be possible, without departing from the scope of the present invention, to have recourse not only to two layers of insulating material, but to a succession of a larger number of layers, preferably respecting a temperature resistance gradient as one approaches the core of the oven.

Against the inner surface of the metal sleeve 22 is applied a second inner layer of thermally insulating and transparent material to microwaves. This second layer referenced 28 will be made of a material of the same kind as that of layer 26, but advantageously having better temperature resistance than that of layer 26. The second inner layer 28 has a central opening 30 capable of receiving a muffle 32 made of a microwave absorbing material. Such a material can advantageously be chosen from different types of materials based on silicon carbide, which can be in the solid state, sintered, vitrified, and / or combined with other materials such as silicon nitride. It can also be chosen from ferrites and garnets as well as any composite material of these aforementioned materials. It will be specified here that garnets are minerals corresponding to the general formula R₂ R'₃ (Si O₄) ₃, in which R represents Al, Fe, Cr, Ti, etc.
and R 'represents Ca, Mg, Fe, Mn, etc ...
According to an alternative embodiment, the muffle 32 intended to be inserted into the central opening 30 of the insulating inner layer 28, is advantageously in the form of a cylinder closed at its rear end 34 and open at its front end 36. In practice, such a type of muffle has been produced in the general form of a tube with a square cross-section 10 cm in side and having a wall thickness of the order of 2 mm. Of course, various forms can also be envisaged and depending on the use for which the oven is intended, it is also possible to considerably modify the dimensions and the proportions of the various parts of the oven and in particular of this muffle 32 constituting the oven heating enclosure.
Alternatively, this muffle 32 can also be simply produced in the form of a layer of microwave absorbing material, which can be applied by any suitable means against the inner surface of the central opening 30 formed in the second inner layer 28. This layer can therefore be in the form of a simple coating applied by coating, spraying or any other suitable technique.

The oven according to the invention also includes a metal shutter plate 38 intended to close the opening 12 of the metal housing 10. Again, the metal used to make this shutter plate 38 will advantageously be chosen identical to that of the metal housing 10 and the fixing will be ensured by any appropriate means capable of withstanding the thermal stresses to which these parts are intended to be subjected.
As illustrated in the accompanying drawings, this metal closure plate 38 has in its center an opening 40 intended to allow the introduction of the muffle 32 which, if necessary, must be replaced in the event of wear. Under these conditions, the metal shutter plate 38 also includes a door 42 allowing access to the heating enclosure. This door 42 must of course allow the oven to be closed in a leak-proof manner, and it is thus advantageously equipped with an additional layer of thermally insulating material. Sealing against electromagnetic leaks can be conventionally obtained by equipping door 42 with a quarter-wave trap.

The oven according to the invention will advantageously be equipped with a temperature measurement system, not shown in the accompanying drawings. The temperature which it is important to determine, is of course that prevailing in the heating enclosure, that is to say within the muffle 32. To do this, the temperature measurement system could for example be constituted by an infrared pyrometer, disposed opposite an opening in the door 42 for access to the heating enclosure. This opening may be provided with a shielded glass intended to avoid electromagnetic leaks and of course also thermal losses.
When manufacturing the oven, it is necessary to ensure good contact between the different metal parts 10, 18, 22, 38 and 42 so as to avoid electromagnetic leaks. The assembly of these different metal parts will therefore be done while ensuring continuity of the metallic material between metallic parts, for example by welding, screwing, riveting or the like.

When the oven is used at temperatures which remain substantially below 1500 degrees Celsius, it will also be possible to produce the temperature measurement system in the form of a thermocouple which will be embedded in the absorbent material of the muffle 32.

The magnetrons 20 can advantageously be controlled by an electromagnetic card further comprising the microwave power control functions, the shaping of the signal supplied by the temperature measurement device, the temperature control and the communication with a programmer. . It is thus easy to ensure a power regulation of the microwave generators, in order to carry out controlled temperature cycles in the oven enclosure, these cycles being able to be memorized beforehand in the programmer.

In the various embodiments illustrated in the accompanying drawings, it is observed that the metal housing 10 has an axis of symmetry which, in this case, is parallel to the direction of the generatrices of the cylinder defining said housing. In the embodiment described, it can also be seen that the various main constituent parts of the oven, that is to say the metal casing 10, the metal sleeve 22 as well as the layers of insulating material 26 and 28 are arranged in a configuration coaxial. Such an arrangement is important for ensuring good distribution of the microwave radiation within the oven and of course in the muffle 32. It should however be noted that the circular shape of the cross section of all of these parts is not essential. It is perfectly possible to also consider a coaxial configuration of all these parts having a cross section of regular polygonal shape.
The oven according to the invention being intended to be used for very high temperature heating up to 1800 degrees Celsius, it is advantageous to arrange the whole of this oven at the heart of a structure for maintaining and protecting against overheating and electromagnetic leaks, referenced 44 in the accompanying drawings. It is in fact an additional box, in which the oven described above is housed and containing an additional lining material 46, consisting of a thermally insulating material and / or a heat exchange material. Rock wool or silica wool can be used for this purpose.

The oven object of the present invention is intended to carry products introduced into the heart of the muffle 32, at very high temperatures. When heating these products, for example in the context of a mineralization reaction, or else in the preparation of sulfuric ash, it is clear that fumes or other reaction gases can and must be able to escape from the enclosure of heating. Thus, in the embodiment illustrated in FIG. 3, the heating enclosure has at its rear part, and preferably in the high position, a vent 48 allowing the evacuation of the fumes produced in the oven, by circulation through 'an exhaust duct 50 opening into the open air.
In certain cases, the furnace according to the invention will be used to conduct reactions requiring the presence of a particular gaseous atmosphere, for example an inert atmosphere of nitrogen or on the contrary, an oxidizing atmosphere provided by a renewal of air. In this case, the heating enclosure also includes a gas introduction orifice 52, preferably also arranged at the rear part of the muffle 32, but advantageously in its lower part. A supply duct 54 therefore allows the introduction or insufflation of a gas through this orifice 52.
Given the significant temperature differences that may exist between the gas introduced through line 54 and the fumes discharged through line 50, it has proven advantageous in practice to couple these two lines 50, 54 within an exchanger heat, blade type, fins or the like, so as to take advantage of the heat of the exhaust fumes to ensure preheating of the air or neutral gas introduced into the heating chamber. This arrangement considerably increases the thermal efficiency of the oven. Of course, this heat exchanger structure will preferably be embedded in the thermally insulating material 46 lining the holding structure 44 of the oven.

By its very structure, it is clear that the microwave oven in the The present invention is independent of the dielectric characteristics of the material to be heated. In other words, the microwave energy is not concentrated in the sample itself, but it is actually the muffle 32, which concentrates the microwave energy, heats up and causes l heating by radiation of the product placed in the heating enclosure. In this way, the muffle of a traditional heating oven is reconstituted.
The main interest of the microwave oven according to the present invention lies in the fact that it no longer comprises any heating element foreign to the muffle 32. It therefore no longer comprises any heating resistor or other external heating source which is usually susceptible to deteriorate over time, especially when trying to reach high temperatures. In addition, as appears from the examination of the appended drawings, the muffle made of absorbent material 32 can be completely thermally isolated from the rest of the apparatus, which allows conversion of electrical power to thermal power with excellent efficiency and very low thermal inertia. Such an oven therefore allows a high speed of temperature rise and cooling. Thus with two magnetrons each producing a power of 1000 Watts, it is possible to reach an interior temperature of the oven of 1500 degrees Celsius in about fifteen minutes. It should be noted that to achieve such performance with a traditional oven, it would be necessary to use an energy source of 8 to 10,000 Watts.
Conversely, during cooling, the oven according to the invention has practically no thermal inertia, since in fifteen minutes after the oven has stopped, it is possible to reach a temperature below 200 degrees Celsius. With a traditional oven, it is necessary to wait several hours to obtain such cooling.
The oven object of the present invention also has the decisive advantage of a low cost of manufacture and use. In the prior art, to reach temperatures above 1200 degrees Celsius, the ovens necessarily had to use very expensive refractory resistors and moreover very fragile. It has previously appeared, however, that in the case of the oven according to the invention, no fragile part was used, no wear of the heating element could be observed, and in the event of an incident, the muffle 32 could be easily interchanged.
The oven according to the invention also makes it possible to achieve significant energy savings due to the good power conversion efficiency and the low thermal inertia. Indeed, by its very design, the oven does not present significant radiation to the outside, as is necessarily the case when using heating resistors.
Finally, it will be observed that all the usual problems encountered in traditional ovens, originating from ruptures or poor contacts of the wires of connections to the heating resistors, necessarily disappear with the oven according to the invention.

Claims (20)

Microwave oven, in particular for rapid heating at high temperature, characterized in that it comprises: - a metal housing (10) affecting the shape of a cylinder open at one (12) of its ends and in the cylindrical side wall (16) from which opens at least one waveguide (18) coupled to a generator microwave (20), - A first outer layer (26) of a thermally insulating and transparent material to microwaves, applied against the inner surface of the cylindrical side wall (16) of the housing (10) and whose inner surface rests on - a second inner layer (28) of thermally insulating material and transparent to microwaves, having a central opening (30) capable of receiving - a layer (32) which is made of a microwave absorbing material, and which constitutes the oven heating enclosure, as well as - a metal plate (38) for closing the open end of said metal housing (10). Oven according to claim 1, characterized in that a metal sleeve (22) provided with a plurality of coupling slots (24) is interposed between the first outer layer (26) and the second inner layer (28). Oven according to one of claims 1 and 2, characterized in that the layer (32) is produced in the form of a muffle which is open on the same side as said metal housing (10). Oven according to one of claims 2 and 3, characterized in that the metal housing, the metal sleeve and the layers of insulating material (26, 28) are arranged in a coaxial configuration. Oven according to one of claims 2 to 4, characterized in that the metal housing (10), the metal sleeve (22) and the layers of insulating material (26, 28) have a cross section of regular polygonal shape. Oven according to one of claims 2 to 4, characterized in that the metal housing (10), the metal sleeve (22) and the layers of insulating material (26, 28) have a cross section of circular shape. Oven according to one of claims 2 to 4, characterized in that the coupling slots (24) provided in the metal sleeve (10) are distributed equally around the circumference of the latter, preferably with a spacing close to half a - guided wavelength. Oven according to one of claims 2 to 7, characterized in that the coupling slots (24) provided in the metal sleeve (22) have an inclination, regular or alternating, relative to the circumferential direction of said sleeve. Oven according to one of Claims 1 to 8, characterized in that the layers (26, 28) of thermally insulating and microwave-transparent material are ceramic-based, preferably in the form of fibers or ceramic foam porous. Oven according to one of claims 3 to 9, characterized in that said muffle (32) is made of a microwave absorbing material, chosen from materials based on solid silicon carbide, sintered, vitrified and / or combined with silicon nitride, ferrites, garnets and composites of the aforementioned materials. Oven according to one of claims 3 to 10, characterized in that the muffle (32) constituting the enclosure of the oven is in the form of a cylinder closed at its rear end and open at its front end. Oven according to one of claims 1 to 11, characterized in that the metal shutter plate (38) is equipped with a door (42) for access to the heating enclosure, which is sealed against micro leaks. waves, and preferably provided with an additional layer of thermally insulating material. Oven according to one of claims 1 to 12, characterized in that it is equipped with a temperature measurement system prevailing in the heating enclosure. Oven according to claim 13, characterized in that the temperature measurement system is an infrared pyrometer arranged opposite an opening formed on the access door to the heating enclosure. Oven according to claim 13, characterized in that the temperature measurement system is a thermocouple embedded in the absorbent material constituting said muffle. Oven according to one of claims 1 to 15, characterized in that it is arranged at the heart of a structure (44) for holding and protecting against heating and electromagnetic leaks, said structure being lined with a thermally insulating material (46) and / or a heat exchange material. Oven according to one of claims 1 to 16, characterized in that it is coupled to an electromagnetic card enabling regulation of the power of the microwaves and therefore of the temperature in the heating enclosure. Oven according to one of claims 1 to 17, characterized in that the heating enclosure (32) comprises a vent (48) making it possible to evacuate the fumes produced in the oven, through a conduit (50) opening out outside the oven. Oven according to one of claims 1 to 18, characterized in that the heating enclosure (32) has an orifice (52) for introducing a gas through a duct (54) coming from the outside from the oven. Oven according to claims 18 and 19, characterized in that the smoke evacuation pipe (50) and the gas introduction pipe (54) are coupled within a heat exchanger, preferably arranged within the insulating material (46) present in the structure (44) for holding and protecting the oven.

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