EP0627604B1 - Multi hole ceramic plate used as a heat conducting element support for the heating of electric furnace installations - Google Patents

Abstract

A multi-hole ceramic plate used as a heat-conductor support of a heating insert for industrial furnace construction is described, the outer contour of the ceramic plate being given an undulating shape according to the invention, with continuous transitions between the peaks and the valleys of the undulations. In addition, the bearing length of the heating conductors in the guide bores of the ceramic plate is considerably reduced by bevelling the guide bores for the heating conductors. <IMAGE>

Description

The invention relates to a multi-hole ceramic disc as a heat conductor carrier of a heating insert for the electrical heating of industrial furnace systems with a central hole on the axis of symmetry of the ceramic disc for a support element, further with several holes arranged uniformly distributed on at least one coaxial circle for heating conductors, and with several , Recesses evenly distributed over the circumference of the ceramic disc at the edge of the ceramic disc.

With the aid of such ceramic disks, so-called heating plugs are built up, the ceramic disks having the task of holding and positioning the resistance heating elements inserted through the holes mentioned. In such a heating plug, the ceramic disks are spaced apart and arranged parallel to one another, a support element, usually a rod or a tube, being inserted through the central holes in the ceramic disks. This structure is usually pushed into a protective tube made of metal or ceramic, which emits the heat to the outside during operation. Due to the thermal changes in length, there must be some play between the outer circumference of the heating plug and the inner diameter of the protective tube, also called the radiation tube. The heating insert can also be installed and operated as a freely radiating heating element in a thermal system without a protective tube.

The outer circumference of these ceramic disks is usually circular and all holes in the ceramic disk have a constant diameter over their length.

It is, among other things, necessary for these heating elements to function properly essential that the heating conductors are constantly supplied with oxygen. The heat conductors and also the other metal parts of the heating element are made of an aluminum-containing alloy, especially if higher temperatures are to be achieved. In operation, the oxygen combines with the aluminum of the alloy and forms Al₂O₃. The formation of this oxide (like ceramics) is desirable because it significantly increases the life of the heating element. The phenomena described are known.

In the known heating elements mentioned, the metal parts of the heating elements are not supplied with sufficient oxygen during operation. This is due to the fact that each ceramic disc forms a barrier for oxygen (air) flowing through, because all holes of the ceramic disc are almost completely filled with the heating conductors or with the support element and there is also only a very small gap to the outer one on the outer circumference of the ceramic discs Protection tube, the size of which also depends on the respective temperature.

Such heating elements are often provided with thermocouples. These thermocouples are rod-shaped and can extend the length of the heating element. For this purpose, it is known to provide a single recess on the circumference of each ceramic disk and then to insert the thermocouple into the recesses which are in alignment with one another. Here, however, the assembly is cumbersome and complex, because care must always be taken to ensure that the recesses mentioned are aligned with one another.

Another disadvantage of the known construction described is that the heating conductors have contact with the ceramic disks over an annular region with a tangible length. This is due to the holes for the heating conductor, which, as mentioned, have a constant cross-section over their length. After the ceramic washers are poor heat conductors, heat accumulates in this area, which not only deteriorates the efficiency of the heating elements, but in particular the ceramic disks work over a relatively large ring area in operation on each heating conductor and can damage it. This work is due to the often considerable thermal changes in length.

The prior art used so far in the applicant's house has been described above. However, the applicant is also aware of statements by third companies, which will be discussed in more detail below. These versions of multi-hole disks as heating conductor supports are obviously not intended for industrial furnace construction, but for use at lower temperatures.

In a known embodiment of a multi-hole disc, recesses are provided in its edge which are uniformly distributed over the circumference of the disc and are circularly bordered. The recesses are spaced from one another in such a way that a part of the original circular shape of the disk remains between the recesses.

However, this has the disadvantage that the transitions between the approximately semicircular recesses and the original circular shape of the disk are relatively sharp. These sharp edges can flake off during operation and, in particular when the heating elements are arranged horizontally, the sharp edges work on the inner surface of the protective tube and can destroy the oxide layer present there. When the heating elements are arranged horizontally, the ceramic disks either rest with the remaining part of their outer circumference on the inner surface of the protective tube, which results in a disadvantageous single-point support, because there must be some play between the outer diameter of the disks and the inner diameter of the protective tube. Or the ceramic discs lie with two neighboring ones Edges on the inside of the protective tube, whereby the support is defined, but is purchased with the disadvantage of possible damage to the inner wall of the protective tube due to the working of the sharp edges.

In the known embodiment already mentioned, the holes of the ceramic disk carrying the heating conductors are chamfered, apparently in order to prevent damage to the ceramic material when the ceramic disks are being pulled onto the heating conductors. In other words, edge protection of the holes in the ceramic disc is sought because the edges of the ceramic disc are broken by the chamfering. In a known embodiment, the depth of attack is 2 mm with a total length of the bore of 11.2 mm. In another version, the depth of cut is approx. 4 mm with a total hole length of approx. 12 mm.

This prior art was assumed when the main claim was drawn up, although it should be noted that one of the exemplary embodiments shows the mentioned recesses on the edge of the ceramic disk with smooth through holes, while two other exemplary embodiments have the chamfered holes without the mentioned recesses on Circumference of the ceramic disc. Still other exemplary embodiments show ceramic disks without the recesses mentioned and without the chamfers. Another data sheet from a company not known here reveals such multi-hole ceramic disks, in which some also have the chamfer mentioned, in the order of magnitude of 2 × 1 mm with a total length of the bore of 15 mm. Other embodiments have no bevels and none of the ceramic disks shown there has the mentioned recesses on the edge of the disk. Here, too, the chamfers serve only as edge protection for the holes.

Another known embodiment shows such a ceramic disc, also without the mentioned recesses on the edge the disc, and with two bolt circles, of which the outer bolt circle has chamfers, in one embodiment the total thickness of the disc and thus the entire length of the bores is 16 mm and the chamfers are 2 x 5 mm long. In a second embodiment, the disc is 19 mm thick and the chamfers are also 2 x 5 mm long. Although here in the first embodiment mentioned the bevels extend just over half the total length of the bores, the second embodiment shows that emphasis was only placed on edge protection there, so that the construction of this known ceramic disk was also not in the possession of the invention .

Furthermore, it should be noted that in all known multi-hole ceramic discs, the transition between the cylindrical part of the bore and the chamfer is angular, with the disadvantage that the annular edges can damage the surface of the heating conductor when the heating element is working.

US-A-4 016 403 describes a multi-hole ceramic disk as a heat conductor carrier of a heating insert for the electrical heating of industrial furnace systems with a central hole on the axis of symmetry of the ceramic disk for a support element and with a plurality of holes arranged uniformly distributed on at least one coaxial circle for heating conductors. The outer circumference of this known ceramic disc is circularly edged. The holes for the heating conductors, which are arranged coaxially to the central hole, widen outwards from the center, on both sides, in order to protect the edges in question.

In contrast, the invention has for its object to propose a multi-hole ceramic disc with the features of the preamble of claim 1, which in particular causes a noticeably reduced risk of injury in a heating element equipped with such ceramic discs.

To achieve this object, the invention is characterized in that the recesses provided on the circumference of the ceramic disk are wavy-edged with circular shapes of the mountains and valleys of the waveform and with continuous transitions between the circular shapes, the holes of the outer row of holes with respect to the waveform so are arranged so that the distance from these holes to the undulating edge of the disk is as constant as possible.

The holes in question should therefore, seen from the inside of the disk, be located in the radial direction under the "peaks" of the waveform. The distances to the edge of the pane that have been achieved and are as constant as possible thereby reduce the loads in the case of thermal changes in length and therefore increase the service life of the construction.

The features mentioned avoid the sharp-edged transitions in the described prior art with the existing disadvantages.

It is preferred if the diameter of the holes intended for the heating conductors increases outwards at least at one end of the hole in question, in these holes the transition between the cylindrical part of the bore and the outwardly widening part of the bore is rounded and the cylindrical one Part of the bore is less than half the thickness of the ceramic disk, preferably less than a quarter of the disk thickness.

Due to the rounded transition mentioned, the risk of injury to the outside of the heating conductor inserted into the holes is noticeably reduced at this point. In contrast to the edge protection of the holes or bores desired in the prior art described, the length of the bores, by way of which the heating conductors rest against the material of the ceramic disk, is substantially reduced, so that the risk of injury to the heating conductor surface is likewise avoided is reduced because the contact area is noticeably smaller. In addition, there are improved radiation conditions for the heating conductor, combined with a lower risk of overheating. The shorter length of the heating conductors also minimizes oxide abrasion on the surface of the heating conductors.

Instead of specifying the length of the cylindrical part of the bores in relation to the thickness of the disk and thus to the total length of the bores, as has been done above, this can also be done in absolute lengths. Here, too, the length depends on the thickness of the pane. Tests have shown that good results can be achieved if, as is preferred, the cylindrical part of the bore is at most 5 mm long and is preferably between 0.5 mm and 2 mm to 3 mm.

It should be mentioned that in the case of thinner disks the cylindrical part of the bore will be selected higher than that of thicker disks, so that one will not exceed the limit of one quarter of the disk thickness with thicker disks, always in an effort to keep the contact surface so to keep it as small as possible.

• Fig. 1 eine Ansicht einer erfindungsgemäßen Keramikscheibe;
• Fig. 2 einen Schnitt längs der Linie II-II von Fig. 1;
• Fig. 3 vergrößert die Einzelheit Z von Fig. 2.

Als Beispiel ist eine Keramikscheibe mit einer inneren Lochreihe 1 und einer äußeren Lochreihe 2 gezeigt. Wenigstens eine dieser Lochreihen 1,2 muß vorgesehen sein.The invention is explained in more detail below on the basis of an exemplary embodiment from which further important features result. It shows:

• Figure 1 is a view of a ceramic disc according to the invention.
• Fig. 2 is a section along the line II-II of Fig. 1;
• 3 enlarges detail Z of FIG. 2.

A ceramic disk with an inner row of holes 1 and an outer row of holes 2 is shown as an example. At least one of these rows of holes 1, 2 must be provided.

In addition, the ceramic disk has a central hole 3 on its axis of symmetry 4.

The disc is bordered on its circumference by a wavy line 5.

2 and 3 in particular show that the holes 6 forming the rows of holes 1, 2 have a central, relatively narrow web 7. This is followed by rounded

Transitions, as indicated by the radius R in Fig. 3, sections 8, in which the diameter of the holes 6 widens outwards.

The arrangement need not be symmetrical, as shown in Fig. 2; in extreme cases, the coaxial web 7 can be provided at one end of the hole, so that only one of the widening sections 8 is then provided.

Fig. 2 shows, moreover, that the middle hole 3 is chamfered for reasons of edge protection, just as in the prior art described in the introduction, so that from Fig. 2, the qualitative difference in the holes 6 between the prior art and the subject of the invention emerges.

Fig. 1 shows that the waveform 5, seen from the center of the disc, is formed by sections 9 which go out with a predetermined radius, and adjoining sections 10 which have the indentations of the with a predetermined radius Form waveforms. The transitions between these radii or sections 9, 10 are smooth and smooth.

The radii of the two sections 9, 10 do not have to be the same, but they should not be too different.

• Ihr Außendurchmesser liegt zwischen 30 und 500 mm, vorzugsweise zwischen 40 und 200 mm.
• Ihre Dicke liegt zwischen 5 und 30 mm, vorzugsweise zwischen 8 und 20 mm.
• Mindestens eine der Lochreihen 1,2 ist vorgesehen. Die Anzahl der Bohrungen pro Lochreihe liegt zwischen 2 und 70, vorzugsweise zwischen 4 und 24.
• Der Bohrungsdurchmesser liegt zwischen 3 und 30 mm, vorzugsweise zwischen 5 und 15 mm.
• Als Material wird ein geeignetes Keramikmaterial gewählt, das mindestens bis 1.300°C temperaturbeständig ist und das sich durch eine hohe Temperaturwechselbeständigkeit auszeichnet, z.B. ein Keramikmaterial mit einem Anteil von mindestens 70 % an Al₂O₃.
• Durch die gewählte Außenkontur mit der Wellenlinie 5 wird eine verbesserte Konvektion innerhalb des Schutzrohres erreicht. Der damit verbesserte Sauerstoffaustausch verbessert die Oxidation des Heizleiters und des Rohres. Als Material dafür wird FeCrAl bevorzugt.
• Die gleichmäßige Wandstärke zwischen den Löchern der äußeren Reihe 2 und der Wellenlinie 5 ergibt eine höhere Temperaturwechselbeständigkeit der Keramikscheibe.
• Bei horizontal oder geneigt angeordneten Konstruktionen ergibt sich eine Zweipunkt-Linienauflage der Lochscheiben auf der Innenfläche des Schutzrohres, nämlich auf zwei der benachbarten Berge 9. Dadurch ergibt sich eine verbesserte Gewichtsverteilung, verbunden mit einer verringerten Bruchgefahr.
• Auch der Einbau eines Thermoelements wird vereinfacht. Hierzu müssen die Scheiben nicht mehr orientiert eingebaut werden, wie dies bei einer einzigen Aussparung erforderlich wäre. (Die durch die Löcher 6 hindurchgeführten Heizleiter sorgen dafür, daß die Berge und die Täler 9,10 der Scheiben des Bauelements alle miteinander fluchten.)
• Falls eine Zwangsbelüftung erforderlich wird, ergibt sich ein einfacher Einbau von Rohren (Keramikrohren), verbunden mit einer besseren Oxidation der Heizleiter und es ist auch eine Schnellkühlung durch die Zwangsbelüftung möglich.
• Der Winkel der Anfasungen der Heizleiter-Führungsbohrungen 6 soll zwischen etwa 20 und 25° betragen. Der gerundete Übergang R in Fig. 3 soll etwa zwischen 0,75 und 1,25 mm betragen.
• Durch das Anfasen werden scharfe Kanten vermieden, verbunden mit einer geringeren Verletzungsgefahr der Heizleiteroberfläche (z.B. bei einem Heizleiter aus FeCrAl wird das Schutzoxid dieses Materials nicht mehr beschädigt.)
• Wichtig sind auch die verbesserten Abstrahlungsbedingungen für den Heizleiter, weil nur noch der sehr schmale Ringraum im Bereich des Steges 7 vom Keramikmaterial abgedeckt wird. Dadurch ergibt sich eine fühlbar verringerte Überhitzungsgefahr.
• Die verkürzte Auflagefläche im Bereich der Stege 7 minimiert den Oxidabrieb.

Some preferred dimensions for the disc are given below.

• Their outside diameter is between 30 and 500 mm, preferably between 40 and 200 mm.
• Their thickness is between 5 and 30 mm, preferably between 8 and 20 mm.
• At least one of the rows of holes 1, 2 is provided. The number of holes per row of holes is between 2 and 70, preferably between 4 and 24.
• The bore diameter is between 3 and 30 mm, preferably between 5 and 15 mm.
• A suitable ceramic material is selected as the material, which is temperature-resistant up to at least 1,300 ° C and which is characterized by a high resistance to temperature changes, for example a ceramic material with a proportion of at least 70% of Al₂O₃.
• The selected outer contour with the wavy line 5 improves convection within the protective tube. The improved oxygen exchange improves the oxidation of the heating conductor and the pipe. FeCrAl is preferred as the material for this.
• The uniform wall thickness between the holes in the outer row 2 and the wavy line 5 results in a higher thermal shock resistance of the ceramic disk.
• In the case of constructions arranged horizontally or at an incline, there is a two-point line support of the perforated disks on the inner surface of the protective tube, namely on two of the neighboring peaks 9. This results in an improved weight distribution combined with a reduced risk of breakage.
• The installation of a thermocouple is also simplified. For this purpose, the panes no longer have to be installed oriented, as would be the case with a single recess. (The heating conductors passed through the holes 6 ensure that the peaks and valleys 9, 10 of the panes of the component are all aligned.)
• If forced ventilation is required, pipes (ceramic pipes) can be easily installed, combined with better oxidation of the heating conductors, and rapid cooling is also possible thanks to the forced ventilation possible.
• The angle of the chamfers of the heating conductor guide bores 6 should be between approximately 20 and 25 °. The rounded transition R in FIG. 3 should be approximately between 0.75 and 1.25 mm.
• Chamfering avoids sharp edges, combined with a lower risk of injury to the surface of the heating conductor (e.g. with a heating conductor made of FeCrAl, the protective oxide of this material is no longer damaged.)
• The improved radiation conditions for the heating conductor are also important because only the very narrow annular space in the region of the web 7 is covered by the ceramic material. This results in a noticeably reduced risk of overheating.
• The shortened contact surface in the area of the webs 7 minimizes the oxide wear.

Claims (3)

1. Multi hole ceramic plate used as a heat conducting element support for the heating of electric furnace installations with a central hole (3) at the axis of symmetry (4) of the ceramic plate for a supporting member, and additionally with several holes (6) uniformly distributed in at least one coaxial circle (1,2) for heat conductors and with several recesses (10) on the edge (5) of the ceramic plate uniformly distributed around the periphery of the ceramic plate, characterized in that, the recesses provided at the periphery of the ceramic plate are rimmed by a waveform with circularities formed by the peaks (9) and troughs (10) of the waveform (5) and constant transitions between the circularities (9, 10), whereby the holes (6) in the external circle of holes (2) are positioned relative to the waveform (5) in such a way that the distance between these holes (6) and the waveform edge (5) of the plate is as constant as possible.
2. Ceramic plate according to Claim 1, characterised in that, the diameter of the holes (6) provided for the heat conductors is tapered to make it larger on the outside of at least one end of the hole concerned (6), whereby in these holes (6) the transition (R) between the cylindrical part (7) of the bore (6) and the tapered part (8) of the bore (6) is curved and the cylindrical part (7) of the bore (6) is less than half the thickness of the ceramic plate, preferably less than a quarter of the plate thickness.
3. Ceramic plate according to Claim 2, characterized in that, the cylindrical part (7) of the bore is maximum 5 mm long, preferably between 0.5 mm and 2 to 3 mm.

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