Combined gas-microwave cooking oven with steam cooking facility

Abstract: Gas-heated microwave oven combination, particularly for cooking food for commercial or institutional catering purposes, comprising a cooking cavity, one or more magnetrons for the generation of the microwaves, a plurality of hollow heating elements (5) arranged inside or on the walls of said cavity, possibly provided with a conduit for conveying steam from an external boiler, wherein the products of the combustion of inflammable gases pass through said heating elements (5), the inside of which is nowhere communicating with the inside of the cooking chamber. Said heating elements are formed by substantially vertical pipes extending at least partially through said cooking cavity, whereas the upper flue-gas outlet ends (11) of said heating elements (5) are closed by a box-like element (12) which sealingly closes in a microwave-tight manner against the upper surface of the top plate (9) and is open upwards through at least an opening (14) to the edges of which there is connected a corresponding second pipe (15) constituting a microwave trap.

Description: The present invention refers to a combination food-cooking oven adapted to perform cooking processes both by means of microwave heating and by the heating-up of the cooking cavity through the combustion of fuel gases, as well as by the combination of both of the afore cited heating modes at the same time, in view of creating particular conditions and effects in cooking said food, wherein all said cooking processes can coexist with and be successfully carried out also in the presence of the cooking process commonly called "steaming", ie. performed by injecting steam into said cooking cavity of the oven. In the following description reference will be made particularly to a food cooking oven of the type used in professional kitchens or food-service operations. It will be anyway appreciated that what is described and claimed hereinafter equally applies to any type of food cooking oven as far as it is provided with both gas heating and microwave generating/utilization facilities. The process of cooking food in an oven is commonly known to be carried out according to widely differing conditions and manners depending on the results and effects that have to be reached. Among these various conditions and manners of cooking food in an oven, oven heating through the combustion, outside of the actual cooking cavity, of fuel gases under subsequent heat-exchange process to transfer heat inside the cooking cavity, and microwave cooking can be cited among the most appreciated methods. The actual peculiarities of such different cooking methods from the point of view of their rapidity, cleanliness, safety, energy consumption, cost-effectiveness, evenness and consistency are well known in the art, so that for reasons of conciseness they will not be dealt with here to any greater extent. In order to be able to aggregate the advantageous features of these two cooking methods, various types of combination gas-microwave ovens have been disclosed, which however turned out to have a number of drawbacks from the point of view of both their construction and their use or operation, as it will be explained below. The US patent specifications no. 4,431,889 and no. 4,430, 541 describe combined microwave-flue gas ovens in which a gas burner arranged outside of the cooking cavity generates hot combustion products, or flue gases, which are drawn in from the combustion zone by the action of a fan that also conveys said combustion products into the cooking cavity through appropriate ports provided in the walls of said cooking cavity. Both the outlet ports from and the inlet ports into the cooking cavity have dimensions that are substantially smaller than half of the wavelength. The flow of the hot flue gases inside the cooking cavity is such as to ensure a substantial uniformity in the circulation of said hot flue gases in said cooking cavity. The US patent no. 4,211,909 describes a combination gas-microwave oven in which it is possible to separately perform cooking by means of heating through the combustion of fuel gas and cooking by means of microwave heating, in order to emphasize particular cooking aspects or results. In this oven, the gas burner is arranged outside of the cooking cavity and the hot combustion products are then circulated by a fan inside the cooking cavity. The European patent 0 349 213 discloses a combination gas-microwave oven in which it is possible to shorten the time needed to heat up the oven through the operation of both cooking methods, ie. gas and microwave heating, at the same time. Such an oven is provided with a gas burner arranged on the outside of the cooking cavity and a fan for circulating in a mechanically forced manner the hot flue gases inside the oven cavity. As far as its basic principle of operation is concerned, this oven does not actually differ to any significant extent from the afore described ones. The patent GB 2179529 discloses again a combination gas-microwave oven in which one or more gas burners are arranged on the outside of the cooking cavity, while the flow of the hot flue gases into the cooking cavity takes place without any assistance by a fan and through perforated box-like elements performing a double task in that in one direction they prevent microwave from leaking outside the cooking cavity and, in the other direction, they protect the burners against soiling by fall-outs of the food being cooked. All such kinds of ovens are effective when their use is to be limited to microwave cooking only, either combined or not combined with gas cooking, ie. flue-gas heating. Present needs, as they are particularly being felt in connection with food cooking ovens for professional kitchens or food-service operations, require however that the possibility exists for the above described cooking methods by microwave and gas heating to be performed at the same time with the cooking method with steam, which is commonly known as "steaming" to those skilled in the art and consists in letting a flow of high-temperature, saturated steam into the cooking cavity. For it to prove effective, such a cooking method requires that said steam be enabled to fill the cooking cavity and surronds the food being cooked with an atmosphere of high-temperature, saturated vapour: it is just these characteristics that make this type of cooking process particularly valued, rapid and cost-effective. However, the presence of high-temperature saturated steam in the cooking cavity requires that the latter be substantially given a sealed, air-tight construction, although not necessarily a pressure-proof one, and such a requirement is obviously fully incompatible with the emission of hot flue gases out of the combustion chamber, which shall be able to freely draw in oxygen from the outside atmosphere, where also flue gases shall ultimately be exhausted. For such a reason, none of the above described ovens is compatible with such a "steaming" mode of operation. In order to realize an oven able to put together the performance of microwave and heat cooking it is known, from the publication of JP-A-54146040, a cooking oven equipped with both an high-frequency oscillator or magnetron, and a heat source, such as a plurality of gas burners; Such kind of solution is able to solve the problem of the combined cooking performance; however such a solution does not solve the problem to implement a high-temperature saturated steam cooking, and even more to have a sealed, air-tight and vapor-tight construction of the cooking cavity. The need therefore arises for an oven to be available, in particular an oven for use in professional kitchens and food-service operations in general, which is capable of performing, both separately and in a combined manner, all such cooking methods by microwave heating, gas-combustion heating and steaming, which has a simple and reliable construction, and which can be manufactured using readily available techniques. It is therefore a purpose of the present invention to improve the construction of said cooking ovens with the features and characteristics as substantially described with particular reference to the appended claims. The present invention will be more clearly understood from the description given below by way of non-limiting example with reference to the accompanying drawings, in which: Figure 1 is a schematical view of a half-height horizontal section of a cooking oven according to the present invention; Figure 2 is vertical cross-sectional view of the oven shown in Figure 1; Figure 3 is a view of a construction detail of the oven appearing in Figure 2; Figure 4 is a view of a second construction detail of the oven according to the present invention; Figure 5 is a view of a variant of the oven shown in Figure 1. With particular reference to Figure 1, this is shown to illustrate the interior of a cooking cavity of a food cooking oven for professional kitchens, comprising a motor-driven fan 1, arranged on the rear side of said cooking cavity, and appropriate devices (not shown) for generating microwaves and propagating them inside said cooking cavity, such as for instance devices of the type described in the EP-A-429822 of the same applicant. Underneath the bottom plate 2, or sole, of the cooking cavity there are arranged one or more gas burners 3 of a traditional type, which are supplied through an appropriate gas inlet conduit 4. Arranged over said burners there are corresponding first vertical pipes 5, which penetrate the cooking cavity 6 through appropriate openings 7 that are provided in said bottom plate, and leave the same cooking cavity through corresponding openings 8 that are provided in the top plate 9 of said cook log cavity, said pipes ending upwards just above said openings made in the top plate of the cooking cavity. The ring-like joints between said pipes and the bottom plate of the cooking cavity are sealed in an air-tight manner, preferably by means of welding. The ring-like joints 10 between said pipes and the top plate 9 of the cooking cavity are on the contrary of the unsealed type in view of enabling said pipes, when subject to thermal expansion, to freely extend longitudinally, ie. in their height direction, without distorting and/or putting excessive stress upon said joints 10. Since the interior of the cooking cavity is exposed to the propagation of microwaves, the latter would tend to leak through said ring-like unsealed joints, thereby giving rise to the well-known problems associated with both health hazards brought about by microwave leakage, and due compliance with corresponding safety standards. In order to solve all such problems, the upper flue-gas outlet end portion 11 of said pipes and said unsealed ring-like joints 10 are enclose' with a box-like element 12, which sealingly closes in a microwave-tight manner against the upper surface of said top plate 9 of the cooking cavity. The upper side 13 of said box-like element is open upwards through at least an opening 14, to the edges of which there is connected a corresponding second vertical pipe 15 which seals against the edges of said opening 14 in such a manner as to eliminate any microwave leakage hazard. The cross-section dimension 16 of said vertical pipes 15 shall be smaller, and their minimum height 18 shall be greater than 1/4 (a fourth) of the wavelength. In this way a heating arrangement has been provided which operates on the basis of a heat exchange from the hot flue gases to the interior of the cooking cavity, wherein said first pipes are allowed to expand due to the rise in temperature without impairing the integrity of the top plate of the cooking cavity, wherein said flue gases can be freely exhausted by flowing from said pipes into the inner space of said box-like element 12, and from here to the outside atmosphere by flowing through said second vertical pipes 15, and wherein any possible microwave leakage through the unsealed ring-like joints existing between said first pipes and said top plate of the cooking cavity is practically arrested by the dimensional characteristics of said second pipes departing from said box-like enclosing element. A variant aimed at making such a construction simpler consists in providing said burner means and said pipes in such a manner that the plurality of straight vertical pipes 5 is replaced by a single pipe 19 formed into a coil, and that the flue-gas inlet and outlet end portions are arranged on a side wall 20 of the cooking cavity, so as this is shown in Figure 5. The zones through which said coil-shaped pipe passes front one side to the other side of said wall are sealed in a microwave-tight manner, for instance by welding and/or flange-fitting. Such a configuration is particularly advantageous since thermal expansion of the coil-shaped pipe can be taken up by the same coil-like form of the pipe, without putting any stress upon the intersecting zones of the pipe through the oven cavity wall, so that it is possible and advantageous to do away with the construction of the afore described box-like enclosing element, while anyway ensuring that the flue gases are exhausted freely and, at the same time, microwaves are effectively prevented from leaking. Microwave ovens usually have their interior lighted by one or more lamps, which are normally arranged behind the inner wall that delimitates the cooking cavity. This has a main drawback in that, when the need arises to reach the lamp, the oven itself has to be disassembled to a considerable extent, as anyone skilled in the art is well aware of. Such a problem is even more so if the oven is a combination gas-microwave oven, for instance such a one of the type described here, since its greater complexity and overall delicacy makes such an operation much more complicated and care-demanding. In order to do away with this oven dismantling problem, a lamp-holding fixture arrangement is proposed in which said lamp 25 is accomodated in a lamp-holder 26 located outside said cooking cavity, but close to the inner wall 27 thereof, in such a manner that it is possible to reach the lamp from the interior of the cooking cavity upon removal of just a few minor component parts, while the structure itself of the oven remains totally unaffected. As illustrated in Figure 4, said lamp-holder arrangement is provided with a peripheral flange 28 which is parallel to the wall 27 of said cooking cavity, said flange being provided with a plurality of openings 29 that are adapted to engage corresponding stud elements 30 applied against a portion of the outer surface of the wall of the cooking cavity with respect to the interior of the same cavity. Such a wall portion is provided in its central zone with at least an opening 31 adapted to enable the light generated by said lamp to pass therethrough. Furthermore, in order to enable the light to propagate from the lamp into the cooking cavity, the side of said opening facing the interior of the cooking cavity is closed by a rigid, transparent element 32, which is locked in place against said wall by an attachment bracket 33 adapted to be applied against said wall. In a preferred manner, between the surface of said wall facing the interior of the cooking cavity and said transparent element 32 there is arranged at least an elastic gasket 34 so as to prevent steam from leaking and also to lower the risk that the pressure exerted by said bracket against said transparent element, which is usually made of light-gauge glass, eventually breaks it. In order to prevent microwaves from leaking through said lamp-holding arrangement, two measures are specifically taken: the first one lies in the fact that the recess of the lamp-holder is constituted by a fully enclosed metal cavity; the second one is due to the fact that sealing means 36, preferably a narrow-meshed metal gauze, are applied between said flange 28 and the corresponding wall 27 of the cooking cavity. The way in which the above described arrangement operates will by now be fully apparent: in order to reach the lamp, it will be necessary to just remove the locking means 35, such as for instance screws or similar fasteners which keep said fastening bracket in place, remove said bracket, the underlying glass plate and the gaskets being possibly associated therewith, in order to be able to directly gain access through the opening 31 to the recess accomodating the lamp-holder and, therefore, to the lamp, wherein the whole procedure can be simply and conveniently performed from the interior of the oven cavity, without any need arising for a structural disassembly of the same oven. The oven is of course equipped with a number of other devices and arrangements which, not being relevant to the purposes of the present invention, are not dealt with here.

Claims: Combined microwave and gas-heated cooking oven, in particular for cooking food for commercial or institutional catering purposes, comprising a cooking cavity (6), one or more magnetrons (4) for the generation of microwaves, and possibly provided with a conduit adapted to convey steam from an external boiler into said cooking cavity, characterized in that a plurality of hollow heating elements (5), whose interior is run by the products of combustion of inflammable gas and separated from the interior of the cooking cavity, are arranged inside or on the walls of said cavity, said heating elements (5) being formed by substantially vertical pipes extending at least partially through said cooking cavity. Combined microwave and gas-heated cooking oven according to claim 1, characterized in that said pipes extend through said cooking cavity from the bottom plate to the top plate thereof. Combined microwave and gas-heated cooking oven according to any of the preceding claims characterized in that underneath the bottom plate (2), or sole, of the cooking cavity there are arranged one or several fuel-gas burners (3) of a traditional type, said burners being arranged below the lower ports of said hollow heating elements (5) which penetrate the cooking cavity (6) through appropriate openings (7) provided in the sole (2) thereof and leave the cooking cavity through corresponding openings (8) provided in the top plate (9) thereof. Combined microwave and gas-heated cooking oven according to claim 3, characterized in that said hollow heating elements end upwards just above said openings (8) provided in said top plate of the cooking cavity. Combined microwave and gas-heated cooking oven according to claim 3 or 4, characterized in that the annular joint between said hollow heating elements and the sole of the cooking cavity is sealed in an air-tight manners, whereas the annular joint (10) between said hollow heating elements (5) and said top plate (9) of the cooking cavity is unsealed and capable of enabling said hollow heating elements to expand vertically. Combined microwave and gas-heated cooking oven according to claim 1, characterized in that said hollow heating elements (5) comprise one or several pipes formed into a coil (19), the inlet and outlet ports (21, 22) of said coil being arranged on one or more side walls (20) of the cooking cavity. Combined microwave and gas-heated cooking oven according to claim 6, characterized in that the zones through which said coiled pipe (19) passes from one side of said side wall (20) to the other one are sealed in a microwave-tight manner. Combined microwave and gas-heated cooking oven according to any of the preceding claims from 1 through to 5, characterized in that the upper flue-gas outlet ends (11) of said heating elements (5) and said unsealed annular joints (10) are closed by a box-like element (12) which sealingly closes in a microwave-tight manner against the upper surface of said top plate (9), the top side (13) of said box-like element being open through at least an opening (14) to the edges of which there is connected a corresponding second pipe (15) closing in a microwave-tight manner against the edges of said opening (14), wherein the cross-section dimension (16) of said vertical pipes (15) is smaller and the minimum height (18) thereof is greater than 1/4 (a fourth) of the wavelength. Combined microwave and gas-heated cooking oven according to any of the preceding claims, which is further provided with a lamp arrangement for lighting the interior of said cooking cavity, characterized in that said lamp (25) is contained in a lamp-holding fixture (26) on the outside of said cooking cavity, said lamp-holding fixture being provided with a peripheral flange (28) arranged parallelly to the surface (27) of the wall of said cooking cavity facing the opposite side thereof, said flange being provided with a plurality of openings (29) adapted to engage corresponding stud elements (30) applied against a portion of the outer surface of the wall of said cooking cavity with respect to the interior thereof, said portion being provided centrally with at least an opening (31) adapted to enable the light generated by said lamp to pass therethrough, the side of said opening facing the interior of the cooking cavity being closed by a rigid transparent element (32), said rigid transparent element being locked in place against said wall by means of a fastening bracket (33) adapted to be applied against said wall. Combined microwave and gas-heated cooking oven according to claim 9, characterized in that between the surface facing the interior of the cooking cavity and said transparent element there is arranged an elastic gasket (34). Combined microwave and gas-heated cooking oven according to claim 9 or 10, characterized in that microwave-tight sealing means (36), preferably comprising a narrow-meshed metal gauze, are applied between said flange (28) and the corresponding surface (27) of said wall facing said lamp-holding fixture.

Inventors: CAPPELLO MASSIMO, MARCHESIN CLAUDIO, CAPPELLO, MASSIMO, MARCHESIN, CLAUDIO
Applicants: ELECTROLUX ZANUSSI GRANDI IMPI, ELECTROLUX ZANUSSI GRANDI IMPIANTI S.P.A.
Application year: 1995
Main classification: F24C7
IPC codes: F24C15, H05B6