EP0209887A2 - Catalyst for an electric baking oven - Google Patents

Abstract

A catalytic afterburner for an electric baking oven has an annular catalyser (13) with, provided in a cage-like housing (14), a catalytic filling (26), in which an electric heating element (29) in the form of a tubular heating element is embedded in such a manner that this heating element (29) on the one hand heats the catalyser both by heat-conducting connection and by convection and on the other hand during the cleaning process brings the air to its necessary working temperature for the respective baking process. By these means a high degree of efficiency is achieved with very compact design. <IMAGE>

Description

Catalytic post-burners for ovens, cookers or the like are used, for example in so-called self-cleaning ovens according to the pyrolytic principle, to oxidize the smoke to largely odorless and pollutant-free exhaust gases or the vapors produced in cooking, baking or roasting processes by oxidation or harmful to take disruptive effects. For this purpose, it is essential to bring the catalytic assembly contained in a catalyst carrier, for example, to a working temperature at which a results in an optimized effect; this working temperature is usually higher than the temperature of the incoming smoke or fumes. The electrical catalytic converter heating offered by the usual electrical operation of ovens is associated with difficulties because, with good efficiency and space-saving arrangement, it must be taken into account that the smoke or vapors to be cleaned often contain fat and similar components which are present when combustion is insufficient Deposits, especially carbon deposits. These deposits endanger safe and trouble-free operation, as well as the precise thermal tuning of the catalytic converter, especially if they reach the area of the electrical heating element directly.

From DE-PS 28 11 650 a catalytic afterburner has become known, which meets the stated requirements for protection of the heating element from deposits, compact design and uniform heating of the catalytic filling quite well. This is achieved in that the heating element is a tubular heating element which has a metallic jacket in which a fleece wire lying in an insulating investment material is arranged, and in that the heating element has at least two helically arranged windings one behind the other in the axial afterburner flow direction. Even with this design, however, it is expedient if the heating element is outside the air or smoke duct of the catalytic converter, so that it is not directly added to impurities. For this purpose, the heating element is arranged in a separate annular space surrounding the actual catalyst.

A catalytic afterburner has become known from US Pat. No. 3,785,778, in which electrical heating wires engage in cells of a catalyst block which is coated with the catalytic. This requires a very complex catalyst carrier, which takes up a lot of space. A very large number of heating elements are required for uniform heating of the catalytic converter, which results in complex and fault-prone wiring.

The invention has for its object to provide a catalytic afterburner of the type mentioned in the preamble of claim 1, which ensures high efficiency with a simple and compact structure.

This object is achieved in that the heating element is combined with the catalyst to form a unit.

If it is at least partially within the space accommodating the catalytic body, the outer surface of the heating element, expediently a tubular heater, can thereby be in direct contact with the catalyst, so that the preferably spherical, filled catalytic body is both by heat conduction and by gases around the heating element are heated indirectly by the heating element. The immediate proximity of the surface of the heating element to the catalytic filling ensures that this surface is kept very effectively clean, so that deposits on the heating element are avoided despite the direct contact with the contaminants.

The effects described can be achieved particularly well if the heating element is essentially completely embedded in the catalytic filling or is arranged within its receiving space and is preferably distributed approximately uniformly at least over a cross section of this space, for example in such a way that adjacent cross sections of heating elements are free. In contrast to a coating, a filling of the receiving space with a grain, granules of any shape or the like is particularly suitable as a catalytic filling, so that the catalyst contained in a flowable bed can surround the heating element in a particularly narrow and evenly contacting manner. It is conceivable to make this catalytic filling self-supporting, for example by sinter-like caking, so that it is possible to dispense with a receiving space surrounding the catalytic filling as a catalyst support; However, an increased air permeability of the catalytic converter can be achieved if the individual particles of the catalytic fill, without adhering to one another and adjacent to the heating element, fill a receiving space which is designed like a housing and the walls of which are air-permeable on the one hand and impermeable to the particles of the filling on the other hand .

So that the contaminated air can be brought to the working temperature particularly quickly at the beginning of entry into the catalytic converter, the heating element is closer in cross section through the catalytic filling, in particular immediately adjacent, to its air inlet side than to its air outlet side, whereby that Heating element or the Heating elements can extend over a zone which corresponds to approximately one third of the distance between the air inlet side and the air outlet side, such that - measured in the air flow direction - the area free of heating elements is larger than the area in which heating elements or parts of a heating element are arranged.

A particularly advantageous development of the subject matter of the invention consists in the fact that the heating element forms the, preferably only, main heating of a circulating air heating of the baking chamber. As a result, the catalytic afterburner according to the invention is particularly suitable for so-called convection bathtubs, in which the baking, cooking, thawing and similar processes are carried out with convection heating, which circulates the baking space with heated air, since in this case the heating element is provided both as an oven heater and as a catalyst heater and only a single heating element is required for this. Nevertheless, it is conceivable to provide the oven with additional heating elements, for example a grill heater, for special preparations.

According to a further feature of the invention, the catalytic filling is arranged in a catalytic converter housing, which is preferably through at least one window-like air passage opening, which is larger than the grain size of the catalytic filling, is flat in processing and in the area of the passage opening on the inside Formed from a grid covered sheet metal or the like. Is formed. It is achieved with a stable design and easy manufacture of the catalyst that the mostly sharp edges of the grid on the inside of the Ge are covered and cannot cause injuries either during assembly or when cleaning the oven or the catalytic afterburner.

The efficiency of the catalyst can be significantly improved in a compact design in that the heating element and / or the catalyst are arranged in a ring around a central suction axis of the circulating air conveyor, so that both the catalyst and the heating element over a circumferential angle of at least almost 360 ° are effective and the axial extent of the catalyst and thus its depth can be kept very low. The housing can be assembled in a simple manner from two ring parts, at least one of which is shell-shaped in cross section and which are placed against one another in an annular parting plane and / or with externally exposed flange edges, so that the housing can be easily filled and then, for example, by welding the flange edges close. If only one of the two ring parts is bowl-shaped in cross section, the other ring part can be formed by a simple hoop from a ring-shaped, flat, for example ladder-shaped band, the longitudinal edges of which form the flange edges and which have through openings.

In a further embodiment of the invention, narrow webs, approximately in the width direction, of the shaped strip, which in particular is ladder-shaped, are provided between the through-openings, which preferably extend from the shell bottom to the shell side walls in the respective shell-shaped ring part and over the ends of which the grid extends to the fastening on the inside of the respective ring part is sufficient. Thereby With a bowl-shaped design, both ring parts can be produced in a simple manner by sections of the same, for example ladder-shaped band material. This band material is first produced, for example, by a punching process, after which the grid is fastened to its inside, for example by spot welding. The strip material can then be brought into its shell-shaped cross-sectional shape by embossing, rolling or the like, which is particularly simple if only the conductor webs need to be angled. Then or at the same time, the strip material thus profiled can be brought into the ring shape of the catalytic converter by means of steering rollers or the like, after which the two ring parts are connected to one another, including the catalytic filling.

A tubular heating element is particularly suitable as the heating element for the catalytic converter, which is preferably bent in a helical shape and the connection ends of which are in particular guided approximately radially outwards through the outer peripheral wall of the housing. This heating element does not require any special attachment, since it is precisely secured in its position by the arrangement within the catalytic converter or by embedding it in the catalytic fill and can therefore be attached indirectly via the attachment of the catalytic converter. The tubular heater, which has a metallic jacket, in which a heating wire enclosed in an insulating investment material is arranged in a contact-free manner compared to this jacket, guarantees, on the one hand, with very good efficiency, direct heating of the catalytic filling through good heat-conducting contact on a large area, and on the other hand, a contact the catalytic fill or the catalyst with live parts of the heating element completely excluded.

The lattice can be formed in a simple manner by a fabric or the like, the lattice meshes or their boundaries expediently lying diagonally to the longitudinal direction of the shaped strip, so that the initially flat lattice strip can be converted very easily both into the described cross-sectional shape and into the ring shape . The grid is expediently made of stainless steel.

It is also advantageously possible to construct the entire housing of the catalyst from wire. Larger wire rings can bring together through the cross section of the overall toroidal housing individual wire struts, which are connected for example by spot welding, to form a housing in the form of a ring cage. The wires assembled in this way can themselves form a grid which holds the catalytic bodies together, or can additionally be surrounded by a grid.

Due to the design according to the invention, it is also advantageously possible to arrange the heating element within the catalytic filling around a feed wheel of an axial fan with a radial outlet, so that the circulating air delivery device is directly surrounded by the air inlet side of the catalyst and itself result in very short flow paths. Since the heating element lies within the catalytic converter, the delivery wheel can extend radially very close to the air inlet side of the catalytic converter; in any case, it is expedient if the fan outlet runs directly through the catalytic converter. A further improvement of the air circulation is achieved if the lleizelement and the catalyst are arranged between the back wall of the baking chamber and a suction and air guide orifice, through which the air is expediently sucked in axially and in the area of the outer edges of the cleaned and heated air circulation in a second air flow resigned.

In a further preferred embodiment, the catalytic converter consists of annular lamellae coated with catalytic material, which are preferably profiled in a wave shape. They can be stacked to form an annular body, forming air channels between them, by stacking the lamellae, for example, which are corrugated at an angle to the radius, in the opposite position. This relatively compact body ensures a large surface and best heat distribution from the inside out. If the tubular heating elements are arranged on the inside, the heat is distributed relatively evenly over the entire catalytic body.

The ring body can be enclosed between two cover rings. which are held together by connecting elements and which protrude somewhat over the inner circumference of the catalyst body. The tubular heating elements can lie in the space formed in this way and can thus be integrated structurally and fluidically in the catalytic converter.

• Fig. 1 einen Ausschnitt eines Backofens mit einem erfindungsgemäßen katalytischen Nachverbrenner in Ansicht,
• Fig. 2 einen Axialschnitt durch die Anordnung gemäß Figur 1 in vergrößerter Darstellung,
• Fig. 3 einen Ausschnitt der Figur 2 in vergrößerter Darstellung,
• Fig. 4 einen Ausschnitt des Katalysators gemäß Figur 1 in Ansicht,
• Fig. 5 das Heizelement des Katalysators in Seitenansicht
• Fig. 6 einen Teil- Querschnitt durch einen Katalysator und
• Fig. 7 eine teilgeschnittene Seitenaussicht eines Ringausschnitts des Katalysators nach Fig.6.

These and further features of preferred developments of the invention also emerge from the description and the drawings, the individual features being able to be implemented individually or in groups in the form of subcombinations in an embodiment of the invention and in other fields. Exemplary embodiments of the invention are shown in the drawings and are explained in more detail below. The drawings show:

• 1 shows a section of an oven with a catalytic afterburner according to the invention in view,
• 2 shows an axial section through the arrangement according to FIG. 1 in an enlarged representation,
• 3 shows a detail of FIG. 2 in an enlarged representation,
• 4 shows a detail of the catalytic converter according to FIG. 1 in view,
• Fig. 5 shows the heating element of the catalyst in side view
• Fig. 6 is a partial cross section through a catalyst and
• 7 is a partially sectioned side view of a ring section of the catalyst according to FIG. 6.

As the figures 1 to 5 show, an electric oven according to the invention in an oven muffle 1 delimiting the baking space 2 on the inside of its upright rear wall 3 has a circulating air conveying device 4 lying approximately in the middle thereof, which is essentially provided by a horizontal or to the rear wall 3 of a right-angled central axis 5, there is formed a fan feed wheel 6 which is fastened on the output shaft 7 of a blower motor, which is not shown in more detail and which is arranged behind the rear wall 3, passing through the rear wall 3. At the front of the feed wheel 6 is a substantially plane or plate-shaped suction and air-guiding orifice 8, which is perpendicular to the central axis 5 or parallel to the rear wall 3, and of all of the walls of the oven sleeve 1 adjoining the rear wall 3 at an angle, i.e. from their side walls, bottom wall and deck wall 9 has approximately the same distance and thereby limits an essentially rectangular annular outlet opening 10 for an envelope current to be conducted into the baking chamber 2. The diaphragm 8, which is fastened, for example, with stud bolts 11 to the rear wall 3, has air inlet openings 12 in the form of downwardly open slots, which are hidden in the view according to FIG Baking chamber 2 draws air in such a way that it reaches the area of the feed wheel 6 mainly parallel to the central axis 5 and is essentially only conveyed radially outward by this through an arc angle of 360 °.

The feed wheel 6 is surrounded by an annular catalytic converter 13, which also lies in the central axis 5 and which projects slightly beyond the feed wheel 6 in its axial direction to the rear and / or front and extends essentially from the rear wall 3 to the diaphragm 8. As a catalyst carrier, the catalyst 13 has, in the manner of an annularly bent tube or cage with a rectangular cross section, a housing 14, the housing cross section of which is larger in the axial direction than in the radial direction. The housing 14 is composed of two ring parts 15, 16 which are essentially the same in cross section according to FIG. 3 and which are produced from sections of the same ladder-shaped shaped strip. The ladder rungs of this shaped strip form narrow, U-shaped webs 18 between through openings 17 lying one behind the other in the circumferential direction, while the transverse parts connecting the U legs lie parallel to the central axis 5. Each shaped strip is bent in a U-shaped cross-section in such a way that the U-transverse web 19 is essentially formed exclusively by the webs 18 which, with their angled ends, extend into the U-legs 20 of the respective ring part 15 and 16 are sufficient. The ends of the U-legs 20 are each angled outwards to form an annular flange edge 21 or 22, the flange edges 21, 22 abutting one another over the entire circumference of the housing 14 and being connected to one another, for example by welding or the like. Arranged on the inside of each ring part 15 or 16 is a cross-sectionally also U-shaped grating 23 or 24 which extends over the entire circumference of the housing 14 and which completely covers the respectively associated passage openings 17 and fits snugly against the inside of the respective ring part 15 or 16 is provided. The legs of each grid 23 and 24 are fastened to the inside of the U-legs 20 of the ring part 15 and 16 in the area outside the webs 18, that is to say in an area which is not interrupted by the passage openings 17; the attachment can be done, for example, by spot welding. The housing 14 delimits an accommodating space 25 lying in a ring around the central axis 5 for a catalytic filling 26 completely filling this space, as shown in FIG. 2, which is not shown in FIG. 3 for the sake of clarity. The catalytic fill 26 consists of a bed of granules, granules or the like, the particles of which are preferably of essentially the same size and, in an advantageous embodiment, are formed by individual, essentially regularly shaped balls. The air inlet side 27 of the catalytic converter lies essentially on the circumference of the catalytic filling 26, while the air outlet side 28 lies essentially on the outer circumference thereof, so that the fan outlet runs directly through the catalytic converter 13.

An electrical heating element 29 in the form of a tubular heating element is arranged in the receiving space 25, which essentially consists of a metallic jacket 30 in which a heating wire 32 lying in an insulating investment material 31 is arranged coaxially. Instead of circular cross sections, as shown, the heating element 29 can also have a cross-sectional shape flattened on one or more sides, for example a triangular cross section. The heating element 29 is bent in several turns, in the illustrated embodiment in two turns 33 in a helical manner around the central axis 5 and arranged so that its turns are evenly distributed over the axial extent of the catalytic converter or the catalytic fill 26. The turns of the heating element 29 are expediently immediately adjacent to the inner circumference of the catalytic filling 26, that is, they essentially adjoin the inside of the radially inner grid 24, a sufficiently large distance being provided for the passage of air between adjacent turns 33. The turns 33 of the heating element 29 are completely embedded in the catalytic fill 26 such that their jacket 30 is in direct heat-conducting contact with the particles of the catalytic fill 26 surrounding this jacket. The two connecting ends 34 of the heating element 29 extending from the end turns penetrate the housing 14 approximately radially to the central axis 5 and are angled outside the housing 14 against the rear wall 3 in such a way that they penetrate it. In the exemplary embodiment shown, the connection ends 34 lie on the outer circumference of the housing 14; by the inventive design, it is also possible to lead the connection ends radially inward out of the housing 14 and lead out of the oven muffle 1 within the space enclosed by the housing 14, so that a particularly protected arrangement is guaranteed. The heating element 29 forms the only heating for the operation of the oven as a convection oven.

The air sucked in axially by the circulating air conveying device 4 from the baking chamber 2 is pressed radially outward through the inlet side 27 into the catalytic filling 26, whereby it is first quickly heated up on the heating element 29 and on the particles of the filling 26 which are heated directly by it and flows through the catalytic filling 26 like a labyrinth. Here, the air is cleaned so that it is cleaned and heated to the temperature set for the respective cooking process from the outlet side 28 at the circumference of the catalyst 13 radially exits and is fed to the baking chamber 2 as a sheath flow after deflection through the outlet opening 10.

6 and 7 show a variant of the catalytic converter in which the catalytic converter annular body 40 is composed of catalytic bodies 26a which have the shape of annular, corrugated sheet metal lamellae. The shafts 41 are, as shown in FIG. 7, generally radial, but directed at an angle to the radial direction, so that, as indicated, two adjacent lamellae, which are stacked on top of one another in an arrangement rotated by 180 °, only touch at the shaft apices and thereby forming air channels 42 through which the air or gases to be cleaned can flow.

The lamellae can preferably consist of aluminum-plated steel sheet with a thickness between 0.2 and 0.6 mm, preferably 0.4 mm, which is coated on the surface with catalytic material. The aluminum plating not only ensures good resistance to corrosion, but also a good connection. between the support and the catalytic layer. During heat treatment, for example during operation, the aluminum surface layer forms an intermetallic connection with the iron and forms a layer enriched with aluminum oxides on the surface, which bonds well with surface layers of all types.

The ring body 40, which is stacked from numerous lamellae, is delimited on both sides by a cover ring 43, which terminate with the outer circumference of the ring body, but whose inner diameter is smaller than that of the lamellae 26a, so that they form an annular space 44 covered on both sides a tubular heater element 29 is arranged in the form of two or three screw turns. Connecting elements 45, 46 run through the space 44 and guide the tubular heating elements from outside and inside between them and connect the cover rings 43 to one another. At the same time, the bolt-shaped outer connecting element 45 guides the ring body 40, in which it bears against the inner circumference thereof.

The connecting elements can also be arranged differently. For example, they could also be formed by bent tabs of the cover rings, which are also bent or riveted on the other side. Tabs could be formed on these connecting elements with which the tubular heating elements are attached to them. In any case, the tubular heaters sc are attached near the inside of the catalyst ring body 40 and can transfer the heat by radiation, convection and in some cases also conduction well to the ring body 40, where the corrugated sheets which pass from the inside to the outer circumference distribute the heat evenly. This creates ideal conditions for the catalytic effect. The arrangement of the tubular heater in Annulus 44 shields them from radiation and flow around, so that the heat remains concentrated on the catalyst. The corrugation of the slats also creates an enlarged surface and good swirling and the necessary spacing between the slats with relatively little air resistance. However, other profiles of the slats or other spacers are also possible.

As FIG. 7 shows, the connection of the tubular heating element can be led to the outside through a cutout in the catalyst ring element if a connection on the outer circumference is desired. Otherwise, the tubular heating elements can be led out to the side directly from the annular space 44. Fastening tabs 48 can be formed on one of the cover rings 43.

Claims (10)

1.Catalyst for an electric oven, especially for baking, roasting, thawing or the like, with a circulating air conveying device connected to the baking chamber, such as a blower, in the airway of which a cleaning catalytic converter (13) with catalytic bodies (26 , 26a) and a heating element are arranged, characterized in that the heating element (29) is combined with the catalyst (13) to form a unit. 2. Catalyst according to claim 1, characterized in that the heating element (29) is at least partially within the catalytic body (26) receiving space (25) and advantageously substantially completely embedded in the preferably spherical catalytic body (26) or within them Receiving space (25) is arranged and is close to the air inlet side (27) and is arranged side by side in several turns. 3. Catalyst according to claim 1, characterized in that the heating element (29), which is preferably formed by the electric tubular heating element, forms the only main heating of a circulating air heating of the baking chamber (2). 4. Catalyst according to one of the preceding claims, characterized in that the catalytic body (26) is arranged in a catalyst housing (14), which is preferably larger by at least one with window-like, compared to the grain size of the catalytic body (26) Air passage openings (17) provided in the development plane and in the area of the passage openings (17) on the inside by a grid (23, 24) covered form strips made of sheet metal or the like. The housing (14) preferably consists of two ring parts (15, 16), at least one of which is cup-shaped in cross-section and which are placed next to one another in an annular parting plane and / or with externally exposed flange edges (21, 22), and in particular between the through openings (17), for example narrow Crosspieces (18) lying in the width direction of the strip, in particular in the form of a ladder, are provided which, in the case of the respective shell-shaped ring part (15, 16), are preferably may extend from the shell bottom (19) to the shell side walls (20) and The grating (23, 24) extends over the ends of the grating until it is fastened to the inside of the respective ring part (15, 16). 5. Catalyst according to one of the preceding claims, characterized in that the heating element (29) and / or the catalyst (13) are arranged in a ring around a central suction axis (5) of the conveyor (4) and of whose air flow is substantially radial be flowed through inside. 6. Catalytic converter according to one of the preceding claims, characterized in that the catalytic bodies (29) are arranged in a catalytic converter housing, the one another and partially made of wire, or. a wire mesh. 7. Catalyst according to one of the preceding claims, characterized in that the catalyst (13) around a feed wheel (6) of an axial fan with a radial outlet and / or between the rear wall (3) of the baking chamber (2) and a suction and Air guide orifice (8) is arranged, the fan outlet running directly through the annular catalyst (13) which forms the fan outlet channel. 8. Catalyst in particular according to one of the preceding claims, characterized in that the catalytic body consists of annular, preferably wave-shaped, lamellae (26a) coated with catalytic material, which form between them from the inside to the outside extending air channels (42) are stacked to form an annular body (40), the wave directions advantageously running at an angle to the ring radius and intersecting at adjacent lamellae (26a). 9. A catalyst in particular according to claim 8, characterized in that the annular body (40) is arranged between two cover rings (43) which are connected to one another via connecting elements (45, 46), in particular bolts. 10. Catalyst in particular according to claim 8 or 9, characterized in that the heating element (29) in the laterally delimited by the cover rings space (44) close to the inside of the ring body (40) adjacent, preferably arranged helically and of connecting elements (45 , 46) is guided from the inside and outside, the outer connecting elements (45) guiding the inside of the lamellae (26a) with their outside.

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