EP0362022B1 - Composite cooking oven - Google Patents

Abstract

A cooking oven having both conventional heating as well as microwave heating. The oven muffle includes a metal distribution sheet along a back wall, forming a cavity with the back wall. A bladed turbine fan is included within the cavity, driven by an electric motor. Microwave energy is introduced into the cavity through a waveguide having an exit iris in the cavity. Microwave energy entering the cavity exits through openings in the metallic distribution sheet, as well as past the rotating turbine blade through additional holes in the distribution sheet. The microwave energy exiting the distribution plate provides for a better cooking energy distribution throughout the oven.

Description

The present invention relates to a microwave oven and more particularly relates to a combination oven combining heating by microwave energy with a traditional calorific contribution by heating resistors and convection of the internal atmosphere of the oven.

Already known in the art, for example from patents FR-A-2104482, EP-A-0121892 or JP-A-57120035, so-called microwave ovens, where the electromagnetic waves supplied by an appropriate source and channeled in a guide opening into the oven cavity, are distributed in the latter according to various methods. One of these consists in placing in this cavity or muffle of the furnace, in the vicinity of the iris at the outlet of the waves outside the guide, a propeller with metal blades driven in rotation and that the microwaves coming from the guide come strike by changing phase and then reflecting directions which vary at every moment due to the relative inclination of the blades and their drive. This results in a random distribution of the field in the oven cavity, avoiding the presence of respectively cold and hot regions generated by the nodes and the bellies of the electric field and producing a more uniform distribution of energy. However, this solution, which requires the provision of a specially adapted motor and propeller with the furnace, significantly increases the cost price of the assembly. In addition, the metallic material of the propeller, usually aluminum sheet, undergoes significant deformations at high temperature, which can lead to the formation of electric arcs or also cause the blocking of the propeller, this in particular in the so-called ovens pyrolysis where the temperature can locally reach 500 ° C or more.

To reduce the cost of such ovens, provision has also been made to eliminate the propeller drive motor and to control only the rotation thereof by a jet of air of suitable direction, coming from outside the furnace. . However, this solution is also not well suited since the air intake thus produced immediately degrades the quality of the temperature distribution, in particular in the case of a combi steamer with traditional cooking, by appreciable loss of calories. from the oven as soon as the drive air is introduced.

Another conventional solution also consists in providing the oven muffle with a turntable, supporting the load to be heated. The waves which are established in the oven in a non-uniform way with a succession of hot and cold points, realize in combination with the rotation of the plate a continuous variation of the position of the maxima and minima of energy within the load which, overall, receives roughly uniform energy. But here again, the device requires an additional drive motor for the plate, with mounting on the axis of the latter of a wave trap to avoid the harmful output of a fraction of the microwave energy along this axis. In addition, there is a risk of deformation of the plate and its drive axis at high temperature while the presence of this plate anyway limits the useful volume of the muffle, reduced to that of the cylinder whose radius is equal to the distance from the center of rotation of the plate to the nearest internal wall of the muffle.

Finally, there are known embodiments where the distribution of the flow of electromagnetic waves in the cavity of the oven is carried out by means of a rotating antenna, mounted at the level where the guide opens into the muffle, so as to capture the energy at the outlet. of it and to the radiate in the oven cavity with a more uniform distribution. But there still remain drawbacks, resulting from the need to provide a rotary connection between the cavity and the guide, with the use of an axis of dielectric material, driven by a motor external to the guide, the antenna being formed of a bent metal tube, associated with a ceramic or similar seal to avoid the loss of waves at the outlet of this guide. In addition, if this device is well suited when the oven operates exclusively with microwave energy, it is not suitable for a combi-oven, in particular in which the problems of expansion between the metal and ceramic parts, in particular during pyrolysis phases, raise difficulties, serious and delicate to overcome. In addition, in such ovens, the rotating antenna is generally located in the roof of the muffle in the vicinity of the grill element, which further increases the problem of temperature resistance. Finally, this device is expensive and, once again, significantly reduces the useful volume of the muffle.

The present invention relates to a microwave heating oven combined with traditional heating, which overcomes the disadvantages of solutions known in the art, by allowing a uniform distribution of the electromagnetic field in the muffle of the oven, without reducing the useful volume of the latter and without disturbing the path of the convection currents which provide permanent air circulation in the oven to distribute the calories supplied by the heating resistors and by the grill resistance which is usually provided with the upper part of the muffle from the oven.

To this end, the oven in question, comprising a muffle with preferably insulated walls, a door on the front face, side walls, a bottom hearth and an upper vault, and a bottom wall, a distribution sheet delimiting between it and the bottom wall a cavity in which is mounted the blade turbine of a fan secured to an axis passing through the bottom wall and driven by an electric motor disposed on the opposite side of the cavity relative to this wall, this sheet comprising orifices distributed so as to allow the forced convection by the turbine of a current of air, preferably sucked in by the central part of the sheet and returned in the vicinity of its lateral sides, and possibly towards the bottom and the arch of the muffle, and at least one source of microwave waves, located outside the muffle and associated with a waveguide opening into the muffle through at least one exit iris, is characterized in that the wave exit iris is located in the wall bottom of the muffle opposite a passage provided in the distribution sheet, so that the wave flux is divided between a first fraction entering the muffle through this passage and a second fraction distributing in the cavity behind the sheet metal where the fan turbine is located by striking its blades, the distribution sheet being pierced with holes each forming an elementary iris for output of the waves towards the interior of the muffle.

The invention thus makes it possible, in a traditional baking oven, comprising a ventilation assembly creating a forced convection in the atmosphere of the muffle, to carry out additional heating by microwave energy, by ensuring an improved distribution of the distribution of the field in the muffle thanks to elements which already exist in the oven cavity, in particular by means of the blades of the ventilation turbine.

The invention consists notably, in contrast to conventional combi ovens which completely separate the outlet orifice of the waveguide, vis-à-vis the region of the oven in which the fan is located carrying out the forced convection of the atmosphere of the oven, to make these elements cooperate here by directly using blades of this fan to improve the spatial distribution of the microwaves.

According to a particular characteristic of the oven considered, the holes made in the distribution sheet for the distribution of microwaves towards the interior of the muffle have an elongated shape, in particular oblong, arranged to produce an electric field as they pass through them. quadrature with respect to the orientation of the latter, in line with the guide exit iris and the opposite passage provided in the distribution sheet.

According to various alternative embodiments, the holes of the distribution sheet can have an oblong shape with their large vertical axis, perpendicular to the horizontal transverse direction of the largest dimension of the guide, be arranged in radii regularly distributed around the center of the sheet distribution or present themselves with an inclination tangent to concentric circles, arranged around the center of the sheet. According to yet another variant, the holes formed in the distribution sheet can be inclined on the vertical and arranged in staggered rows.

In another embodiment, the distribution sheet has a single central hole, associated with holes arranged in the lateral sides and possibly the lower side of the sheet.

According to another characteristic of the invention, the iris leaving the guide and the opposite passage provided in the distribution sheet, are closed by plates of a material transparent to microwaves, sealing the cavity delimited between the bottom wall and the sheet. Advantageously, the material of the wafers is mica or consists of an analogous material, having equivalent dielectric properties.

According to yet another characteristic, the exit iris and the passage opposite have relative dimensions determined to ensure an adaptation of appropriate impedance and are preferably arranged in the vicinity of the arch of the muffle, in the central part thereof. .

The oven in question preferably comprises a source of microwave waves, in particular constituted by a magnetron mounted in the thickness of the wall of the muffle. Advantageously also, the axis of the turbine driven by the motor is associated with a quarter-trap. waves, preventing energy leaks along this axis outside the muffle.

• ― La Figure 1 est une vue schématique, en coupe transversale, d'un four mixte établi conformément à l'invention.
• ― La Figure 2 est une vue en perspective, partiellement arrachée du moufle du four de la figure 1.
• ― La figure 3 est une vue en coupe à plus grande échelle d'un détail du four considéré.
• ― La Figure 4 illustre à encore plus grande échelle un autre détail du four.
• ― Les Figures 5A à 5B d'une part, 5C et 5D d'autre part, sont des demi-vues de face de la tôle de répartition du moufle du four, illustrant diverses variantes de réalisation des trous ménagés à travers cette tôle.

Other characteristics of a traditional microwave and heating combi oven established in accordance with the invention will become more apparent from the following description of an embodiment and various variants thereof, given by way of indication and not limiting, with reference to the appended drawings in which:

• - Figure 1 is a schematic view, in cross section, of a combi oven established in accordance with the invention.
• - Figure 2 is a perspective view, partially cut away from the muffle of the oven of Figure 1.
• - Figure 3 is a sectional view on a larger scale of a detail of the furnace considered.
• - Figure 4 illustrates on an even larger scale another detail of the oven.
• - Figures 5A to 5B on the one hand, 5C and 5D on the other hand, are half-front views of the distribution sheet of the oven muffle, illustrating various alternative embodiments of the holes formed through this sheet.

As can be seen in FIGS. 1 and 2 in particular, the furnace considered designated as a whole under the reference 1, mainly consists of an external enclosure 2, usually provided at its lower end with support studs 3. A l 'inside the enclosure 2 is mounted a muffle 4, internally delimiting a cavity 5 where the food is cooked. The muffle 4, generally produced by means of a stamped sheet, comprises an upper wall or arch 6 and a lower wall or sole 7 as well as side walls 8, and finally a bottom wall 9. The lower and side walls of the muffle are associated, outside the cavity 5, with electrical resistors 10 allowing, in conventional cooking, to ensure the heating of the internal atmosphere of this cavity and in particular, by means of a circulation maintained as it is will see below, to realize in the cavity a phenomenon called hot air. In addition and in a manner also known, the muffle has inside the cavity 5 and housed under the upper arch 6, another resistance 11 or grill resistance, of high power. The muffle 4 is surrounded externally by a thickness 12 of an appropriate heat-insulating material, the enclosure 2 moreover, articulated on hinges 13 disposed in front of the enclosure 2, a door 14, closing the muffle 4 in period d use of the oven or allowing, in the open position, free access to the interior thereof. The door 14 generally comprises a window 15 allowing the user a direct view of the interior of the muffle and of the food being cooked.

On the internal face of the door 14 are provided one or more stops 16 actuating contacts not shown located on either side of the muffle in order to disconnect the supply of microwave energy when the door is open. In the side walls 8 of the muffle 4 are also arranged raised portions 17 forming slides for guiding, inside the cavity 5, a tray 18 for supporting a cooking utensil 19.

The bottom wall 9 of the muffle 4 is associated with a protective and distribution plate 20, extending parallel to this wall and disposed inside the cavity 5 slightly forward in the muffle, so as to delimit with the wall 9 a space 21. The distribution sheet 20 can extend from the upper wall 6 of the muffle 4 to its lower wall 7 or else, as in the example shown, stop at a certain distance from the latter to define a slot 22 for circulation of the muffle atmosphere.

In particular in order to achieve this circulation, the oven comprises in known manner, at the rear of the enclosure 2, a fixing device 23 of an electric motor 24 whose axis 25 passes through the bottom wall 9 of the muffle and supports at its end a turbine 26 of a fan whose blades 27 are distributed around the axis 25. The turbine 26 rotates on itself in the space 21 while being protected by the sheet 20 and is arranged so that according to the meaning of its rotation and orientation of the blades 27, it sucks the air contained in the cavity 5 of the muffle 4, in particular through orifices 28 formed in the distribution sheet 20 at the center of the latter. This air is then discharged into the cavity through the slot 22 provided at the base of the sheet 20, and by other holes 29, arranged on the sides of this sheet, in the vicinity of the side walls 8. A lighting lamp 30 schematically shown in Figure 2 is mounted in the bottom 9 of the muffle 4 and preferably in a corner thereof, this lamp to see the evolution of the cooking of food in the oven through the glass 15 of the door .

The oven 1 briefly described above is actually a combi oven, which combines the heating of food to be cooked by convection of the calories supplied by the heating resistors 10, if necessary by the grill resistance 11, according to a circulation maintained by the turbine 26, microwave heating, by establishing in the cavity an appropriate electromagnetic field.

To this end, the enclosure 2 of the oven comprises, outside the muffle 4, a housing 31 in which is mounted a source of high frequency electromagnetic waves, in this case a magnetron 32, discharging in a guide 33. The latter is suitably bent and dimensioned, so that the wave flux coming from the magnetron 32 is delivered into the muffle 4 through an iris 34, provided at the outlet of the guide, this iris 34 being disposed, in accordance with the invention in the space 21 and containing the turbine 26 of the muffle atmosphere circulation fan.

The iris 34 is in particular placed in the central part of the muffle and preferably in the vicinity of the vault thereof, so that the flow of microwaves, coming from the guide 33 entering the cavity 5, is in fact, when crossing the space 21, it splits into two secondary flows, one of which is directly transmitted into the cavity through a passage opening 35, arranged opposite in the distribution sheet 20 while the other s 'introduced into space 21, coming to meet the blades 27 of the turbine 26. The latter being driven in rotation, the relative orientation of the blades is constantly changed; the wave flow thus sees its characteristics change at each instant, by performing an efficient mixing not only in the space 21 but also in the whole of the cavity 5.

In accordance with another important characteristic of the invention, the holes 28 and 29 formed in the distribution sheet 20, respectively in the center and on the sides of the latter, are arranged to also act on the flow of microwaves as well amended. In particular, each hole can be considered as an elementary iris by which the waves penetrate into the cavity 5 through the sheet 20. Under these conditions, they each constitute a separate source whose characteristics are a function of the shape of these holes, which influences directly the propagation of the waves in the cavity. When these holes pass, the electric field is oriented according to the particular shape of the latter. In particular, in order to obtain an electric field in quadrature with respect to the orientation which it presents at the outlet of the guide 33, advantageously, the holes provided in the center of the distribution sheet 20 are given and, if necessary, the lateral holes 29, an oblong or elongated shape, the dimensions of which can be determined on a case-by-case basis, according to each application and in particular the power of the microwave source and the volume of the oven cavity. FIGS. 5A and 5D thus show various alternative embodiments among others, in which the holes 28 and / or 29 are, as the case may be, distributed vertically (FIG. 5A), along radii from a central point of the sheet 20 (Figure 5B), have an inclination tangent to concentric circles whose center is substantially in the middle of the distribution sheet (Figure 5C), or are staggered or with variable and opposite orientations in successive and superimposed rows , these holes having an appropriate inclination on the vertical (FIG. 5D). Also, provision could be made in the sheet metal for a single central hole of larger dimensions and smaller holes, distributed in the lateral sides of the sheet metal. The shape and distribution of the holes in the sheet 20 actually vary each time, depending on the dosage to be carried out between the part of the flow coming from the guide 33 which penetrates directly into the cavity 5 of the muffle and that which is stirred by the blades of the fan in space 21.

FIG. 3 also illustrates other arrangements advantageously implemented whatever the embodiment adopted for the distribution of the holes 28 and 29 in the sheet 20 in order to avoid that the part of the flux collected in the space 21 does not escapes to the outside, in particular through the passage of the axis 25 of the motor 24 driving the turbine 26, between this axis and a seal 36 providing a suitable seal against the gases and circuit vapors, this joint being provided in the bottom wall 9. For this purpose, a wave trap 37 of the quarter-wave trap type 37 is mounted around the axis 25, comprising a sheet metal element 38 attached to the wall and the edge 39 is applied against it. The sheet 38 is shaped so as to define with the bottom wall 9 of the muffle an auxiliary cavity 40 (Figure 4) whose length shown diagrammatically in the drawing by the reference 1 is equal to λ / 4. In this cavity, the incident wave coming from space 21 and the wave reflected by the walls of this cavity are in phase opposition. This results in a cancellation of the electric field in the region 41 at the outlet of the cavity 40, preventing the electromagnetic waves propagating along the axis 25 towards the outside of the enclosure 2. Preferably also, the distribution sheet 20 comprises in the iris 34 and the passage opposite 35, plates 42 and 43 respectively, of mica or of another dielectric material, transparent to microwaves but which close the guide and the space avoiding projection in these elements from products placed in the oven cavity to perform the cooking.

Thus, in accordance with the invention, a combi steamer is produced, combining in a traditional muffle with fan and distribution plate, a conventional fan-assisted heating device and a microwave system, the fan turbine simultaneously achieving homogeneous dispersion. of these microwaves in the cavity without any particular adaptation. The output of the guide joined to the source of microwave waves is arranged so that it is located here, no longer in the muffle ceiling as is generally the case. the case in classic designs, but behind the distribution sheet. The turbine used can in particular be of a standard model, the only precaution to be taken being that it does not have sharp edges, this to avoid the creation of parasitic and dangerous electric arcs. The useful volume of the oven remains unchanged while the efficiency of microwave heating is considerably improved by the homogeneous distribution of the electric field, this being directly produced from already existing parts and components, and without at the same time modifying the performances. carried out in traditional cooking which can on request be implemented in a complementary manner or on the contrary separately.

The temperature reliability of the materials used is not changed, as is their resistance to shocks and vibrations. The fan whose role remains essentially the distribution of temperatures during normal cooking also ensures the balanced distribution of electromagnetic energy during microwave cooking by stirring the fraction thereof derived in the space where this fan before returning to the muffle volume.

The distribution sheet can be made of materials with low dielectric loss and causing limited reflections of the electromagnetic waves, these reflections taking place mainly on the blades of the fan.

Claims (13)

1. Combined oven, combining heating for the traditional method of cooking by revolving heat and microwave heating, comprising a muffle (1) having walls which are preferably insulated, a door (14) in the front face, lateral walls, a lower floor (7) and an upper roof (4), and a rear wall (9), a distributing plate (20) defining between itself and the rear wall a cavity (21) in which there is mounted the bladed turbine (26) of a fan integral with a shaft (25) traversing the rear wall and driven by an electric motor (24) arranged on the opposite side of the cavity relative to this wall (9), this plate (20) containing orifices (28, 29) distributed so as to allow the forced convection by the turbine of an air stream preferably aspirated through the central portion of the plate (20) and returned to the vicinity of its lateral sides, and optionally toward the floor and the roof of the muffle, and at least one source of hyperfrequency waves (32) situated at the exterior of the muffle and associated with a wave guide (33) opening into the muffle through at least one outlet iris (34), characterised in that the outlet iris (34) for the waves is situated in the rear wall (9) of the muffle (4) opposite a passage (35) provided in the distributing plate (20) such that the flux of waves is divided between a first fraction penetrating the muffle through this passage (35) and a second fraction distributed in the cavity behind the plate where the turbine (26) of the fan is situated, while striking its blades (27), the distributing plate (20) being perforated by holes (28, 29) each forming an elementary outlet iris for the waves toward the interior of the muffle.

2. Combined oven according to claim 1, characterised in that the holes (28-29) provided in the distributing plate (20) for the distribution of the microwaves toward the interior of the muffle (4) have an elongate, in particular oblong, shape arranged so as to produce, when traversed by the waves, a field in quadrature with the orientation thereof in line with the outlet iris (34) of the guide (33) and of the opposing passage (35) provided in the distributing plate.

3. Combined oven according to claim 2, characterised in that the holes (28-29) of the distributing plate (20) have an oblong shape with their large vertical axis perpendicular to the greatest horizontal transverse direction of the guide.

4. Combined oven according to claim 2, characterised in that the holes (28-29) of the distributing plate (20) are arranged as radii evenly distributed round the centre of this plate.

5. Combined oven according to claim 2, characterised in that the holes (28-29) of the distributing plate (20) are inclined at a tangent to concentric circles arranged round the centre of the plate.

6. Combined oven according to claim 2, characterised in that the holes (28-29) of the distributing plate (20) are inclined to the vertical and are arranged alternately.

7. Combined oven according to claim 2, characterised in that the holes (28-29) of the distributing plate (20) consist of a single central hole connected to bores arranged in the lateral sides and, optionally, the lower side of the plate.

8. Combined oven according to one of claims 1 to 7, characterised in that the outlet iris (34) of the guide (33) and the opposite passage (35) provided in the distributing plate (20) are closed by panels (42-43) of a material which is transparent to microwaves, blocking the cavity defined between the rear wall and the plate.

9. Combined oven according to claim 8, characterised in that the material of the panels (42-43) is mica or is composed of a material having equivalent dielectric properties.

10. Combined oven according to one of claims 8 or 9, characterised in that the outlet iris (34) and the opposing passage (35) have relative dimensions determined to ensure suitable impedance adaptation and are preferably arranged in the vicinity of the roof (6) of the muffle (4) in the central portion thereof.

11. Combined oven according to one of claims 1 to 10, characterised in that it comprises a source of hyperfrequency waves consisting of a magnetron (32) mounted in the thickness of the wall of the muffle (4).

12. Combined oven according to any one of claims 1 to 11, characterised in that the shaft (25) of the turbine (26) driven by the motor (24) is connected to a quarter wave trap (37) preventing energy leakage along this shaft to the exterior of the muffle (4).

13. Combined oven according to any one of claims 1 to 12, characterised in that the distributing plate (20) is produced from a material exhibiting low dielectric losses such that the reflections of electromagnetic waves are limited, these reflections occurring mainly on the blades (27) of the fan.

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