EP0105175A1 - Vacuum-formed electrical heating unit and method of making it - Google Patents

Abstract

The vacuum molding process for the production of an electrical heating device, in which a resistance heating coil (5) is placed on a sieve-like base (1) above a suction box and a silt of ceramic fibers is applied, so that a ceramic fiber layer (4 ), it is proposed according to the invention to partially close the screen bottom (1) in areas under the resistance heating spiral (5), in particular with spacer strips (11), which partially cover the perforation of the screen bottom (1), however so that the impermeable areas of the sieve bottom (1) are narrower than the width dimensions of the heating coil (5). These measures ensure that the interior (8) of the heating coil (5) remains free of fiber material during vacuum forming, so that the temperature difference between the radiating side (5) and the rear side lying in the mass of the fiber block (4) ( 7) the heating spirals (5) is less than in conventional heating devices of this type, in which there is a risk of crystallization of the fibers of the fiber block (4). The use of spacer strips (11) creates retaining webs (12) on the radiating side (9) of the fiber block (4), which ensure secure anchoring of the heating spirals (5).

Description

The invention relates to a vacuum-shaped electrical heating device according to the preamble of claim 1, in which a resistance heating coil is embedded in an insulating body made of ceramic fiber material so that a surface area of the heating coil is exposed on the radiant heating surface. Such a heating device is also referred to as a heating module. The invention also relates primarily to a vacuum molding process for making such an electrical heater.

The basic technique for vacuum forming electrical heating devices referred to here as "heating modules" is described, for example, in US Pat. No. 3,500,444 and in a more modern form in US Pat. No. 4,278,877. In heating modules produced according to these vacuum molding processes, the heating coils or heating spirals are embedded in the ceramic fiber mass in such a way that the interior of the heating spirals is normally filled with fiber material.

• Auf einen siebartigen Boden 1, beispielsweise eine perforierte Platte, wird eine Heizspirale 5 aufgelegt. Unter dem Boden 1 befindet sich ein nicht dargestellter Saugkasten, durch den mittels des allgemein mit dem Bezugshinweis 2 angegebenen Vakuums Flüssigkeit aus einem oberseitig aufgefüllten Schlick 3 abgezogen wird, der aus einer Lösung von keramischen Fasern, Bindemittel und Wasser besteht. Der Flüssiganteil wird durch den siebartigen Boden 1 abgesaugt und es baut sich eine Schicht aus keramischen Fasern auf.

In order to explain the starting point for the invention, the usual vacuum molding process is first explained with reference to FIG. 1:

• A heating coil 5 is placed on a sieve-like base 1, for example a perforated plate. Under the bottom 1 there is a suction box, not shown, through which liquid is drawn off from a top-filled silt 3, which consists of a solution of ceramic fibers, binding agent and water, by means of the vacuum generally indicated with reference 2. The liquid portion is sucked through the sieve-like bottom 1 and a layer of ceramic Fa on.

In this conventional method, the interior 8 of the heating coil 5 is generally also filled with ceramic fibers, specifically the density in this interior 8 will correspond approximately to the density of the remaining mass of the ceramic fiber block 4, which is approximately 200 kgfm ³ .

• Wird der freistrahlende Oberflächenbereich der Heizspirale 6 beispielsweise auf eine Betriebstemperatur von 1150°C gebracht, so wird auf der gegenüberliegenden, von allen Seiten in die keramische Fasermasse eingebetteten Seite (der Rückseite 7) der Heizspirale 5 eine beträchtlich höhere Temperatur auftreten. Dadurch ist es nicht möglich, die Heizspirale 5 auf ihrer freistrahlenden Oberflächenseite 6 bis zu einer maximal erwünschten Betriebstemperatur zu erwärmen, da dann die Rückseite 7 überhitzt werden würde. Ein damit verbundenes Problem beruht auf der maximal möglichen Anwendungs- oder Betriebstemperatur der für die Fasermasse ganz überwiegend verwendeten Aluminiumsilikatfasern, die aus wirtschaftlichen Gründen am häufigsten verwendet werden. Neuere Erkenntnisse haben ergeben, daß die maximal zulässige Betriebstemperatur für solche Aluminiumsilikatfasern bei etwa 1150°C liegen. Oberhalb dieser Temperatur findet eine übermäßige Kristallisation der Faser statt, wodurch die Faser ihre Struktur und erwünschten Eigenschaften völlig verliert. Heizt man nun die Heizspirale 5 an der freistrahlenden Oberflächenseite 6 auf bis zu 1150°C auf, so kann die Rückseite 7 der Heizspirale 5 eine Temperatur von ca. 1250°C erreichen. Diese Temperatur liegt dann um ca. 100°C über der maximal zulässigen Betriebstemperatur der Faser und wird zu einer übermäßig schnellen Kristallisation des Fasermaterials führen. Damit verliert die Heizspirale 5 im überhitzten Teil der Fasermasse ihren Halt und wird sich mehr oder weniger rasch, vor allem bei Deckenelementen in einem Ofenraum, aus der Faser lösen. Die Heizspirale 7 wird dann zunächst an der strahlenden Seite 9 des Faserblocks 4 mehr und mehr hervorstehen und schließlich herausfallen.

The technical difficulties that arise when using such heating modules are described below with reference to FIG. 2:

• If the free-radiating surface area of the heating coil 6 is brought to an operating temperature of 1150 ° C., for example, a considerably higher temperature will occur on the opposite side (the rear side 7) of the heating coil 5 embedded in the ceramic fiber mass from all sides. As a result, it is not possible to heat the heating coil 5 on its freely radiating surface side 6 up to a maximum desired operating temperature, since the rear side 7 would then be overheated. A problem associated with this is based on the maximum possible application or operating temperature of the aluminum silicate fibers which are predominantly used for the fiber mass and which are used most frequently for economic reasons. Recent findings have shown that the maximum permissible operating temperature for such aluminum silicate fibers is around 1150 ° C. Excessive crystallization of the fiber takes place above this temperature, as a result of which the fiber completely loses its structure and desired properties. If you now heat the heating coil 5 on the free-radiating surface side 6 to up to 1150 ° C., the rear side 7 of the heating coil 5 can reach a temperature of approximately 1250 ° C. This temperature is then about 100 ° C above the maximum permissible operating temperature of the fiber and becomes an excessively fast Lead crystallization of the fiber material. The heating coil 5 thus loses its hold in the overheated part of the fiber mass and will detach itself more or less quickly from the fiber, especially in the case of ceiling elements in an oven space. The heating coil 7 will then first protrude more and more on the radiating side 9 of the fiber block 4 and will eventually fall out.

The invention is therefore based on the object to provide heating modules of the type mentioned and a vacuum molding process for their production, by which it is achieved that the heating coil in its anchoring in the aluminum silicate fiber mass does not loosen or dissolve even when the heating coil on an optimal operating temperature is heated such that a temperature of 1150 ° C. occurs on the radiating side of the module, for example.

A heating module according to the invention has the features specified in claim 1.

A vacuum molding process for producing such a heating module is the subject of claim 2.

Advantageous refinements and developments of the method according to the invention are specified in the subclaims.

The invention ensures that the interior of the heating coil remains more or less free of fiber material, so that the temperature difference on the heating coil between the radiating surface of the heating module and the rear is significantly reduced and the heating coil can be operated at a significantly higher operating temperature overall without the risk of a gradual loosening from the anchorage within the Fiber blocks exist.

Because the heating coils are underlaid with underlay elements during vacuum forming, or the perforation in the sieve bottom is left out under the heating coils, i.e. is not available, the support elements or the impermeable areas of the sieve bottom are narrower than the width dimensions of the heating spirals in a plane parallel to the radiating surface or narrower than the diameter of the heating spirals, it is achieved that the heating spirals remain largely free in their interior of fiber material, since the openings of the sieve-like bottom in the longitudinal extension of the heating spirals can be seen partially closed during the vacuum molding process or are not present in these areas.

In a particularly advantageous embodiment of the invention, the heating coils are underlaid during the vacuum molding process by strip-like elements, hereinafter referred to as "spacer strips", so that for reasons explained below, the heating coils are later exposed on the radiating surface of the heating module, but overall are set back into the fiber block by the thickness of the spacer strips, so that optimal anchoring is achieved, but at the same time the interior of the heating spirals remains free of fiber material.

• Fig. 1 und 2 den bereits erläuterten Stand der Technik;
• Fig. 3 ein erstes Ausführungsbeispiel zur Erläuterung des erfindungsgemäßen Vakuum-Formverfahrens;
• Fig. 4 in schematischer Darstellung das Produkt als Ergebnis des Vakuum-Formverfahrens nach Fig. 3;
• Fig. 5 ein zu bevorzugendes Ausführungsbeispiel für ein erfindungsgemäßes Vakuum-Formverfahren; und
• Fig. 6 wiederum in schematischer Darstellung das Produkt des Vakuum-Formverfahrens nach Fig. 5 zur Erläuterung bestimmter vorteilhafter Eigenschaften.

The invention and advantageous details are explained in more detail below with reference to the drawing in exemplary embodiments. Show it:

• 1 and 2 the already explained prior art;
• 3 shows a first exemplary embodiment to explain the vacuum molding method according to the invention;
• 4 shows a schematic representation of the product as a result of the vacuum molding process according to FIG. 3;
• 5 shows a preferred embodiment for a vacuum molding process according to the invention; and
• Fig. 6 again in a schematic representation of the product of the vacuum molding process according to Fig. 5 to explain certain advantageous properties.

Corresponding parts are identified in all figures with the same reference notes.

3 illustrates a first embodiment: on the sieve-like base 1 (the perforated plate), for example, adhesive strips 10 are applied, which cover the perforation in the longitudinal extent of the heating spirals 5, that is to say in the direction perpendicular to the plane of the drawing. These adhesive strips 10 are attached directly under the heating spirals 5 which are subsequently placed on the perforated plate and are slightly fixed. The partial closure of the perforation does not result in a suction effect caused by the vacuum 2 at these points, so that the interior 8 of the heating spirals 5 remains largely free of ceramic fiber material.

FIG. 4 shows the result of the manufacturing method explained with reference to FIG. 3. Similar to the exemplary embodiment in FIG. 2, the heating coil 5 is also flush with the radiating side 9 of the fiber block 4. The interior 8 of the heating coil 5 is now hollow, that is to say free of fiber material, so that the rear side 7 of the heating coil 5 radiates much more freely can. It is thus achieved that the temperature difference on the heating coil between the free-radiating side 6 on the radiating surface 9 and the rear 7 is greatly reduced is, so that undesired overheating in the area of the rear 7 of the heating spirals 5 is avoided.

However, this first principle embodiment of the invention still has the disadvantage that the heating coil 5 is now less overall connected to the ceramic fiber block 4, although the above-explained effect of recrystallization of the fibers due to partial overheating is no longer observed. However, the heating coils 5 are only surrounded by fiber material along their outer circumference and, moreover, they are not held on the free-radiating side 6, as is also the case in the prior art according to FIG. 2. Despite the principle advantage that the crystallization of the fiber material no longer occurs, a difficulty can also arise with this construction, however, in that the heating spirals fall out of the fiber block because of inadequate anchoring, especially when such heating modules are used for ceiling constructions in furnace rooms.

The much improved embodiment of the invention according to FIGS. 5 and 6 is based on the idea of embedding the heating coil 5 on the one hand in the mass of the fiber block 4 in such a way that its interior 8 remains free of ceramic fibers without, on the other hand, the risk that the Heating coils 5 can fall out of the fiber block 4 due to poor adhesion.

The principle of manufacture is first explained with the aid of the schematic sectional illustration in FIG. 5: 5 spacer strips 11 are attached to the sieve-like base 1 below the intended positions of the heating spirals. These spacer strips 11 can e.g. made of metal, wood or plastic. The width of these spacer strips 11 should in any case be somewhat less than the diameter or the width dimension of the heating coil 5 in a plane parallel to the radiating surface side 9 of the fiber block 4; the thickness of the spacer strips 11 should be in the range of at least 0.1 to approximately 30 mm, preferably in the range of 2 to 10 mm. If the silt 3 is now introduced into the frame (not shown in more detail) equipped with the sieve-like base 1 and the liquid portion is drawn off through the sieve-like base 1, the fibers build up in such a way that the spacer strips 11 are enclosed, while the interior 4 of the heating coils 5 largely hollow, ie remains free of fiber deposits.

6 shows a basic sectional illustration of the product result: the free-radiating side 6 of the heating coil 5 is no longer flush with the radiating side 9 of the fiber block 4, but is set back into the fiber block 4 by the thickness of the spacer strips 11. The resulting on the spacers 11 retaining webs 12 partially enclose the exposed side 6 of the heating spirals 5, but without the interior 8 is filled with fibers. Thus, the desired goal was achieved, namely to keep the interior fiber-free, so that the temperature difference between the radiating side 6 and the rear 7 of the heating coils 5 is significantly less than in the conventional technology, in which the heating coil len completely, ie with fiber-filled interior 8 are embedded in the fiber block 4. On the other hand, the heating spirals 5 are held securely by the holding webs 12, so that there is no longer any danger of falling out, even if such a heating module is used as a ceiling element in an oven.

In the described embodiments of the invention, as can be seen from the figures, so-called oval heating spirals or heating coils 5 are provided, as are also described in the above-mentioned US Pat. No. 4,278,877 with the advantages mentioned therein. It is readily apparent to the person skilled in the art that the invention can also be used advantageously for heating spirals with other cross sections, for example with a round cross section or a cross section deformed into a rectangle.

Claims (7)

1. Vacuum-shaped electrical heating device with a resistance heating coil, which is embedded in a ceramic fiber layer so that a surface area of the heating coil is exposed on the radiant heating surface, characterized in that - The heating coil (5) in its interior (8) is essentially free of ceramic fiber materials, and - The exposed on the heating surface areas (6) of the heating coil (5) by a small distance from the outer surface (9) of the ceramic fiber layer (4) are offset inwards. 2. Heating device according to claim 1, characterized in that the distance between the exposed surface areas (6) of the heating spirals (5) from the outer surface (9) of the ceramic fiber layer (4) is 0.1 to 30 mm, preferably 2 to 10 mm. 3. Vacuum molding process for the production of an electrical heating device, in which a resistance heating coil is placed on a sieve-like bottom of a frame above a suction box and a sludge consisting of a slurry of ceramic fibers, binders and water is introduced into the frame, so that a ceramic fiber layer builds up under the suction effect, which is cured and contains the resistance heating coil as an embedded heating element,
characterized in that the surface sections of the sieve-like bottom lying under the resistance heating coil are made impermeable, but these impermeable surface sections are narrower than that largest diameter or width dimension of the heating coil in a plane parallel to the sieve-like bottom. 4. Vacuum molding method according to claim 3, characterized in that the strips are placed on the sieve-like bottom at the positions of the heating coil before inserting the heating coil. 5. Vacuum molding process according to claim 3, characterized in that the strips are previously attached to the heating coil on the side facing the sieve-like bottom in a detachable manner. 6. Vacuum molding process according to one of the preceding claims 3 to 5, characterized in that the strips are designed as spacer strips with a thickness of 0.1 to 30 mm, preferably with a thickness of 2 to 10 mm. 7. Vacuum molding method according to one of the preceding claims 3 to 6, characterized in that the sieve-like bottom is not perforated in the surface sections under the heating coil.

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