Abstract:
An oven muffle has mutually spaced apart inner and outer insulating walls. An insulation layer composed of particulate material is disposed in an interstice between the inner and the outer insulating walls. At least one of the insulating walls is pretensioned toward the respective other insulating wall in order to exert a permanent pressure on the insulation layer.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to an oven muffle having an insulation layer contained in an interstice provided between two insulating walls. 
     In the following, the term oven muffle is to be generally understood as a muffle of any household appliance which requires insulation. For thermal insulation of oven muffles, glass wool or mineral wool fibers are conventionally used. However, these glass wool or mineral wool fibers are increasingly being replaced for health and safety reasons. Granulates, as used for instance in accordance with Published German Patent Applications DE 40 28 743 A1 or DE 195 29 040 A1 in oven muffles of the type defined above, have the tendency of being compressed or reduced in size during use of the oven muffle. This is caused by motions of the muffle wall and the outer wall defining the muffle wall. These motions are due to temperature changes and cause a loss of volume of the insulation material which results in poor insulating properties. 
     As alternatives, organic binding agents, the use of which should however be avoided, or the use of mats or slabs of composite material, which may include foils, foam and the like, has been suggested for instance in accordance with Published German Patent Application DE 44 38 294 A1. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a thermal insulation for an oven muffle which overcomes the above-mentioned disadvantages of the heretofore-known thermal insulations of this general type and which is fiber-free and free of binding agents and has good insulation properties. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, an oven muffle, comprising an inner insulating wall; an outer insulating wall spaced apart from the inner insulating wall for forming an interstice between the inner and outer insulating walls; at least a part of at least one of the insulating walls being pretensioned toward a respective other one of the insulating walls; and an insulation layer formed of particulate material disposed in the interstice. 
     In accordance with another feature of the invention, the outer insulating wall at least partly encloses an oven space and is pretensioned in a direction toward the oven space. 
     In the oven muffle of the invention, the insulation layer is thus composed of particulate material and is contained in an interstice provided between two insulating walls. One or both insulating walls are prestressed or pretensioned toward the respective other insulating wall. 
     This provision according to the invention has the effect that the particles of the insulation layer are kept in their position by the inner and/or outer insulating wall even in the event of temperature changes, because of the imposition of pressure. Furthermore, an unhindered expansion of the muffle walls is still guaranteed, so that there are no increased mechanical strains on the housing. In this way, it is possible to achieve that the volume of the insulation layer is maintained, and the insulation properties are maintained on all sides. 
     The pretensioning of the outer insulating wall and/or the inner insulating wall is advantageously achieved by providing it with shaped impressions in its surfaces. These shaped impressions allow a slaved motion of the insulating wall in response to temperature changes, but at the same time cause a slight pressure to be exerted on the thermal insulating material located in the interior. 
     Further, the outer insulating wall and/or the inner insulating wall can comprise outward- or inward-bowed plates, which makes production especially simple. 
     In accordance with another feature of the invention, the inner and/or outer insulating walls have a concave or convex shape for generating a pressure on the material of the insulation layer disposed in the spacing between the inner and outer insulating walls. 
     It is in particular expedient to provide the inner insulating wall also as a muffle wall, so that no additional housing except for the outer insulating wall has to be provided. The bottom heating heater unit with its associated outer wall or its housing drawer is expediently disposed directly below the muffle bottom, in other words above the lower insulation layer. 
     In the case of a pyrolysis appliance, the two insulating walls according to the invention form their own internal housing, such that even at maximum expansion of the muffle wall in response to temperature, no warping occurs. The bottom heating heater unit with the associated outer wall or housing is in this case expediently inserted between the muffle bottom and the inner insulating wall. It is understood that the necessary leadthroughs for the bottom heating heater unit, top heating heater unit, lighting, etc. are provided. 
     Examples of materials that can be used for insulation are in particular perlites, diatomaceous earth, swelling clay, etc. However, this list is not exhaustive. Regarding the shapes of the filling materials, balls or beads are especially suitable, but other forms of granulate, such as coarse powder, are also possible. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in an oven muffle with thermal insulation, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of an oven muffle with thermal insulation for a pyrolysis appliance in accordance with a first exemplary embodiment of the invention; 
     FIG. 2 is a cross-sectional view of an oven muffle with thermal insulation in accordance with a second exemplary embodiment of the invention; 
     FIG. 3 is a cross-sectional view of an oven muffle with thermal insulation for a pyrolysis appliance in accordance with a third exemplary embodiment of the invention; and 
     FIG. 4 is a cross-sectional view of an oven muffle with thermal insulation in accordance with a fourth exemplary embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawings in detail and first, particularly, to FIG. 1 thereof, there is shown an oven muffle 2 with a muffle wall 4. The muffle wall 4 is bent away (see reference numeral 4a) on the front of the oven to form the front wall. The muffle wall 4 has typical leadthroughs or ducts. In FIG. 1, one leadthrough 6 is shown for a top heating heater unit 8 in the back wall 4b of the muffle. A bottom heating heater unit 10 with an associated bottom heating outer wall 12, which rests with its front wall 12a on the muffle bottom 4c, is disposed under the muffle bottom 4c. 
     The muffle wall 4 is surrounded by a double insulating wall 14, 16. An inner insulating wall 14 with leadthroughs 18, 20, 22 for the top heating heater unit, bottom heating heater unit and bottom heating outer wall, surrounds the muffle wall 4 with an essentially constant spacing, in such a way that the muffle wall can expand in response to temperature. An outer insulating wall 16 is formed, in the present exemplary embodiment, in multiple parts of various plates, of which in FIG. 1 the top wall plate 16a, back wall plate 16b and bottom wall plate 16c are shown. The outer plates 16a, 16b, and 16c all have upward- or downward-pointing peripheral flanges or regions 16d for mounting purposes. In the exemplary embodiment shown, the outer plates 16a, 16b and 16c are pretensioned inward, and specifically are bowed in the direction of the oven muffle 2. Thus, because of their shape and tension, the plates exert a constant inward-oriented pressure, which is essentially independent of any occurring temperature changes or temperature fluctuations. Between the inner and outer insulating walls 14, 16, there is an insulation layer 24, which, in the exemplary embodiment, is composed of perlite beads. Due to the pressure generated by the outer plates 16a, 16b, and 16c, the perlite beads are kept in their position, even when temperature changes occur, specifically with only a slight but constantly exerted pressure. Even when this perlite bead structure ages and if there are any reductions in size, disintegration, crumbling, or, in other words when the perlite beads settle, these beads are always kept fixed as a consequence of the pressure imposed by the outer insulating wall 16, so that even over relatively long times in service, the insulating properties can be maintained on all sides. 
     In FIG. 1, the layer thickness of the insulation layer is reduced in the upper region, which is made possible, in the present construction, by cooling air channels above the insulation layer. Of course the layer thickness in the upper region may also be the same as in the other regions. 
     FIG. 2 shows a second exemplary embodiment of the oven muffle of the invention. The oven muffle shown in FIG. 2 is for an oven without a pyrolysis mode. Elements of the oven muffle in FIG. 2 that are the same as those of the exemplary embodiment described above are designated by the same reference numerals and will not be described again. The construction of the oven muffle 2 and its insulation are substantially as described above. However, here the two insulating walls do not form a separate housing, but instead the muffle wall 4 forms at the same time the inner insulating wall. The outer insulating wall 16 again comprises individual plates 16a, 16b, and 16c, which are again pretensioned inward by being bowed, in order to provide a permanent compensation for compaction and a loss of volume of the insulation layer 24. 
     The third exemplary embodiment shown in FIG. 3 of the oven muffle of the invention again pertains to an appliance with a pyrolysis mode. It differs substantially from the exemplary embodiment of FIG. 1 in the outwardly bulged curvature of the outer plates 161a, 161b, 161c, wherein this outward bulging or bowing again exerts an inward-oriented pressure on the insulation layer 24. 
     In FIG. 4, a fourth exemplary embodiment of an oven muffle of the invention for an appliance without pyrolysis is shown. Like the third exemplary embodiment, it has an outer insulating wall 161 with outward-bowed plates 161a, 161b, and 161c, which exert an inward-oriented pressure on the insulation layer 24. Instead of this bowing of the kind shown in the above-described four exemplary embodiments, the inner and/or the outer insulating wall can alternatively or additionally have shaped impressions or embossed features of arbitrary type. Through the use of such a shaping, the necessary pressure is then exerted upon the insulation layer or its particles.