EP0374520A1

EP0374520A1 - A microwave oven optionally also having thermal operation

Info

Publication number: EP0374520A1
Application number: EP89121703A
Authority: EP
Inventors: Flavio Gatta; Arturo Negretti
Original assignee: Industrie Zanussi SpA
Current assignee: Industrie Zanussi SpA
Priority date: 1988-12-19
Filing date: 1989-11-24
Publication date: 1990-06-27
Also published as: IT1225856B; IT8845772A0

Abstract

The present invention relates to a microwave oven (1) provided with a space (7) between the continuous front edge of the wall (4) of the cooking cavity and the edge (5) of the frame (3) surrounding the entrance to the cavity, that is equipped with a means (8) for shielding the electromagnetic radiation. The space (7) is usually filled with a thermal seal (6) made of a thermally and electrically nonconductive material.

The means (8) comprises a structural shape of metal having a substantially U-shaped cross section and bent according to a rectangular development having dimensions hardly greater than the outside dimensions of the cavity.

The means (8), mounted about the cavity in a position behind the front frame (3), constitutes with its U shape an efficient trap that prevents the microwaves from escaping from the aforesaid space (7) due to the different electrical potential between the cavity and the frame (3) of the oven.

Description

The present invention relates to a microwave oven optionally also having thermal operation and equipped with a means for shielding high frequency electromagnetic radiation (RF).
It is known that microwave ovens cook food using high frequency electromagnetic waves (2450 MHz) generated by an emitter valve, called a "magnetron", which converts electrical energy into electromagnetic energy.
The waves are conveyed by the magnetron inside the cooking cavity made of metal or another thermally and electrically conductive material into which the food to be treated (thawed, cooked, reheated) has been introduced.
When the radiation thus generated strikes the food or in any case the substances to be treated, this produces the well-known and desired heating effect due to the conversion of the electromagnetic energy into thermal energy.
However, part of the electromagnetic energy is not immediately absorbed but is reflected repeatedly by the walls of the cavity in order to penetrate the food from all directions so as to heat the food as evenly as possible.
For this reason, if the cavity or the door closing the cavity have openings or passages large enough, the radiation may spread through the openings and thus be emitted to the outside.
It has been ascertained that this radiation, if absorbed beyond certain levels of power and duration, becomes dangerous to one's health and there are therefore precise norms (which microwave oven producers must respect) prohibiting the production of ovens that emit to the outside radiation with an intensity exceeding rigorous safety limits.
In order to limit the escape of microwaves, a great many devices have been proposed; the large number of patents filed for on this subject testifies to the attention devoted to this problem.
However, the known technology in this sector has been developed with consideration of a particular feature of ovens using exclusively the microwave cooking method. That is, the cavity and the front metal frame surrounding the entrance to the cavity are closely linked (and sometimes made of the same piece of sheet metal or steel) so that it is not possible for radiation to escape between these two parts of the oven since they are at the same electrical potential.
Consequently, the known technology has been developed taking into consideration the other great possible path of escape for radiation, i.e. the space defined by the cavity and frame, on the one hand, and by the inside surface of the door, on the other hand.
However, today one witnesses a progressively increasing use of the two methods of cooking, the microwave method and the conventional thermal method, inside the same cavity, which raises new problems which the known technology has not yet solved.
Since enormous investments are normally required for designing a new structure that accommodates both operating modes, producers prefer to try using an already existing structure and modifying it as required with the purpose of maintaining these modifications and the consequent costs at the lowest possible level.
The following description will relate to the particular case of wanting to utilize a conventional thermal oven and provide it also with microwave operation.
When designing the conversion of a conventional oven into an oven combining conventional operation with microwave operation, and facing the problem of shielding the microwaves from the door, one can use various solutions known in technology, for example as described in European patents nos. 0049817 and 0122647.
However, some conventional thermal ovens have a constructional detail which makes these solutions difficult to realize.
There are thermal ovens, particularly for domestic use, that have the cavity proper separate from other parts of the oven; in particular there is a space left between the cavity and the peripheral frame surrounding the entire entrance to the oven and functioning as an abutment for the inside part of the door.
The purpose of this space is to create a thermal "jump" between the inside wall of the cavity and the frame so as to increase the thermal insulation of the cavity, thereby increasing its power efficiency, performance and safety.
The space is usually occupied by a rubber seal (with special features). Even if of small thickness, it interrupts the thermal flow of conduction from the interior of the cavity toward the frame, and thus toward the outside of the oven. The rubber seal is utilized also to constitute a barrier (with its protruding part toward the outside and in abutment with the inside face of the door when the latter is closed) for the passage of hot air from the inside of the oven to the outside.
Thus, if one wants to convert this type of oven by adding microwave operation to it, one faces the obvious problem that the aforesaid space, due to its dimensions and position, becomes a ready path of escape for the radiation since it establishes a difference in electrical potential between the oven cavity and the frame, even if both these parts are made of metal.
This disadvantage can be overcome by resorting to the solutions known from the prior art, such as for example redesigning the inside part of the door to make it fit deeply within the cavity, thus creating an efficient trap that averts and blocks the radiation before it reaches the described "jump".
However, this solution presents the great problem that the door must be completely redesigned, which involves high investments and also significantly increases the production costs of the oven. Furthermore, this known solution has the disadvantages of making the door heavier (resulting in greater stress on the hinges) and reducing the inside volume that can be utilized in the oven.
The present invention is intended to realize a means for shielding the microwave radiation in an oven having a space, and thus a difference in electrical potential, between the cavity and the frame surrounding the entrance to the oven.
This means substantially comprises a strip of metal material formed in a U shape in cross section and bent into a substantially rectangular ring having dimensions and a shape similar to the outside transverse periphery of the cavity, and with the open part of the U turned toward the plane of the ring.
This means is mounted about the cavity in its front zone so that the U appears exactly in correspondence with the above-described space.
This creates behind the space a most efficient trap for blocking the radiation, without having to modify the structure of the oven and with relatively limited modifications of the door. Due to the simple design and the possibility of using crude metal materials (for example zinc-coated sheet), the production of the ring-shaped means is possible at extremely low cost.
The present invention will be described better by an exemplary embodiment thereof, provided by way of a nonrestrictive example in connection with the adjoined drawings, in which

Fig. 1 shows a perspective front view of a conventionally operating oven;
Figs. 2a, 2b, 2c show the means of the present invention;
Fig. 3 shows an enlarged horizontal section of the zone of a front vertical corner of the oven with the means mounted.
Fig. 4 shows a view from behind of the means mounted about the cavity;
Fig. 5 shows the development on one plane of the working of one of the corners of the means;
Figs. 6a, 6b, 6c, 6d schematically show variations of the described means.

Referring to Fig. 1, one can see oven 1 with open front door 2, frame 3 surrounding the entrance to the cavity, inside wall 4 of the cavity of the oven, edge 5 delimiting the inside wall of the frame and rubber seal 6 interposed between edge 5 and inside wall 4 along the entire periphery of edge 5.
Referring to Figs. 2a, 2b and 2c, one can see the three orthogonal projections of the means.
One can see in particular that the four corners of the means have notches along the front edge so as to permit an easy and regular bending of the corner without overstressing and without curling of the front edge.
Referring to Fig. 3, one can see in particular the section of the vertical wall of cavity 4. With its outside part seal 6 tightly fits the inside face of the door, thus creating an optimal seal against the loss of heated air from the inside of the oven, while with its inside part it closes space 7 between edge 5 of frame 3 and wall 4.
This Fig. 3 illustrates means 8 which hermetically seals space 7 (in terms of the escape of radiation), its flat end 9 lying adjacent the wall of cavity 4 and its other flat end 10 adjacent the front frame.
Referring to Fig. 4, one can see the assembly plan of closed rectangular ring-shaped means 8 about the cavity and closely fitting frame 3. One can also see at 11 a means for connecting mechanically and electri cally the two opposite ends 12 and 13 of means 8.
Referring to Fig. 5, one can see that means 8 has in each of its corners a series of notches 15 for permitting means 8 to be bent at right angles.
Figs. 6a, 6b, 6c and 6d show schematic views of four constructional variations of the described means. They differ in that ends 9 and 10 are differently bent and realized on different profiles of means 8, that nonetheless fall within the scope of the invention.
The function of the means will be obvious to an expert in the field. Means 8 when installed in the oven as illustrated in the above figures creates a completely closed annular chamber 14 (see Fig. 3) about space 7 since sides 9 and 10 of means 8 closely fit the corresponding flat parts of the oven and thus do not permit any passage of radiation.
Obviously, many variations of the present invention are possible without going beyond the scope of protection of the present patent.
For example, instead of facing the inside of the cavity, space 7 can be disposed on the outside face. In this case the constructional modifications should be obvious that are necessary for realizing a means that closes the passage of radiation and thus falls within the scope of protection of the present patent.
Again, the means can be obtained by different techniques - cutting, bending, welding of various pieces, etc. - evidently without going beyond the scope of the invention.

Claims

1. An oven with microwave operation and optionally also with conventional operation having one cooking cavity (4) ending at the front with a continuous edge and a substantially frontal metal frame (3) surrounding the outside of the entrance to the cavity, wherein the continuous front edge of the cavity and the inside edge of the frame (5) are disposed in such a way as to realize a peripheral space (7) which does not electrically connect the parts (3, 5), the oven being equipped with a door (2) for closing the entrance to the cavity, the door being disposed, when closed, with the outside edge of its inside face in front of the frame (3), characterized in that a conductive means (8) having a U-shaped cross section and bent into a substantially rectangular ring surrounds the outside periphery of the cavity in correspondence with the space, the means (8) having its open side turned toward this space.

2. The microwave oven of the above claim, characterized in that the space (7) separating the cavity and the frame is filled with insulating material (6) along the entire peripheral development of this space.

3. The microwave oven of the above claim, characterized in that the insulating material (6) separating the cavity from the frame protrudes into the front zone with a regular and continuous edge and that the door (2) is supported substantially on the entire periphery of this edge.

4. The microwave oven of the above claims, characterized in that it is realized for the specified purposes according to what is described and illustrated in the adjoined figures.