GB2079120A - Device for carrying electrical resistance heating elements - Google Patents

Abstract

Apparatus for replaceably carrying an electrical resistance heating element 2 in a furnace having a compartment comprises a block 1 of a ceramic fibrous material and at least one plate-shaped carrying element 4A, 4B mechanically fastened, e.g. by rods 6A, 6B, in the block 1 and including a portion protruding from the block 1 into a recess 5, the protruding portion including at least one recess into which the resistance element 2 is inserted. The element 1 may be removed together with plug 3 by moving it to the right. <IMAGE>

Description

SPECIFICATION Device for carrying electrical resistance elements The present invention relates to an apparatus for carrying electrical resistance elements for furnaces. The apparatus is so designed that the elements can be replaced. The device also comprises a block of fibrous, ceramic material which simultaneously supports the resistance element and functions as insulation for the furnace or the like wherein the element is positioned.

Electrical resistance elements in some applications, operate at very high temperatures and can then become so soft that they are deformed by their own weight and also are affected by magnetic forces from neighbouring elements. The elements are also susceptible to mechanical forces since they are brittle at lower temperatures.

Devices for carrying these kinds of resistance elements are known, see for example German Patent No. 1,942,645 and U.S. Patents Nos. 4,154,975 and 4,088,825. In German Patent No. 1,942,645 the resistance elements, usually U-shaped or hairpin-shaped, are horizontally insulated and placed upon a number of omega-shaped elements which, in turn, are carried by a carrying element, i.e., a tube of silicon carbide. The carrying element is then fastened onto a brick in the furnace wall. To replace an element, the entire carrying device assembly must be removed from the furnace.

The resistance elements may also be serpentine in shape with two or more shanks, wherein each single shank must be supported.

It is important that the furnaces at the high temperatures, up to 1600 C, which may occur, are well insulated. Therefore, many modern furnace designs use insulation made from ceramic fibers e.g. a material marketed under the trademark FIBROTHAL.

The present invention is for a device for carrying electrical resistance elements in furnaces of different types insulated by fibrous materials so that the elements can be replaced when the furnace is being used and at high temperatures. The device can be used with elements which are either rods, U-shaped or serpentine in shape and it also enables the positioning of the resistance elements so that they are well protected against mechanical forces. The invention further combines the device carrying the resistance elements with an insulation material made from ceramic fibers. The invention can be used for horizontally as well as vertically mounted resistance elements.The invention makes it possible to achieve a very high surface load of more than 50kW installed power per square meter of fibrous material block surface exposed to the interior of the furnace, measured at working temperatures.

Accordingly, the present invention provides an apparatus for replaceably carrying an electrical resistance element in a furnace having a compartment, the apparatus comprising a block of a ceramic fibrous material and at least one plate-shaped carrying element mechanically fastened in the block and including a portion protruding from the block, the protruding portion including at least one throughpassage into which the resistance element is inserted.

The carrying elements are preferably made of a refractory ceramic material. Preferably, the protruding parts are wholly or partly positioned in cavities in the fibrous material, whereby this material will partly surround the resistance elements and so protect them from mechanical forces. The ceramic carrying elements are mechanically fastened into the fibrous material. This may be achieved by the geometrical shape of the carrying elements as well as by having one or more holes in those parts of the carrying elements which are inside the fibrous material. Each one of these holes has a rod passing through it at right angles to the plate-shaped carrying element.

Preferably, the length of the rods is such that each rod, while entirely embedded in the fibrous material, passes through a hole in each carrying element. The plate-shaped carrying elements protrude from the fibrous material and are of a shape able to carry one or more resistance elements which are rodshaped, U-shaped or serpentine in shape. As distinct from hitherto known designs, the devices accoding to the invention, allow the resistance elements to be removed from the furnace without changing the position of the carrying elements.

It is also known, in the construction of furnaces from fibrous insulating material, to make use of blocks of fibrous material having electrical resistance elements incorporated therein. This arrangement, however, possesses several disadvantages. One does not have a free choice of material from which to form the resistance elements, e.g., elements made from molybdenum disulfide cannot be used since they will react with the fibrous material, which cannot resist the high temperatures obtained with such elements. Compared to furnace temperatures, the temperatures of the elements tend to increase due to the insulating property of the fibrous material.

It may finally be mentioned that the elements in these prior art blocks cannot be replaced, but rather the complete fibrous block with the element has to be replaced if an element fails to function satisfactorily.

The present invention will be further described with reference to the drawings, in which: Figure 1 shows in perspective a block of ceramic fibrous material holding a resistance element with two serpentine shanks. The element is positioned in the block by two plateshaped carrying elements of high temperature-resistant refractory material. These are mechanically fastened into the fibrous block by two through-passing rods.The element can be removed from the block together with a plug of ceramic fibrous material; Figure 2 is a cross-sectional view of the block depicted in Fig. 1; Figure 3 is a cross-sectional view at a right angle relative to that of Fig. 2; Figures 4, 5, 6 and 7 are side elevational views of different types of ceramic carrying elements; Figure 8 shows in perspective an embodiment of the invention where the same element passes through several fibrous blocks; Figure 9A shows in side view an embodiment where the terminals are at a 90 angle relative to the glowing zone of the element; Figure 9B is a front view of the embodiment depicted in Fig. 9A; Figure 10 is a side elevational view of another form of carrying element which can be employed in accordance with the present invention;; Figure ii is a front view of a modified form of block, carrying element, and resistance element in accordance with the present invention; Figure 12 is a longitudinal sectional view taken along line XII-XII in Fig. 11, Figure 13 is a side elevational view of a carrying element of the type depicted in Fig.

11; Figure 14 is a front view of still another preferred embodiment in accordance with the present invention; and Figure 15 is a horizontal sectional view taken along a horizontal plane through the assembly depicted in Fig. 14.

Fig. 1 shows a block of ceramic fibrous material 1 having a recess 5. Carrying elements 4A, 4B of high temperature resistant refractory material are fastened within the ceramic fibrous material and protrude therefrom into the recess 5. The portions of the carrying elements which are disposed in the fibrous material having holes 7 through which are positioned rods 6A, 6B. Those rods are embedded within the block 1. The carrying elements carry a dual-shank serpentine-shaped resistance element 2, preferably made from molybdenum disulfide. The terminals 8A, 8B of the element 2 extend through a plug 3 which is made of the same ceramic fibrous material as the block 1, and which plug 3 fits with a slight friction fit in an enlargement of the recess 5. If the element 2 is to be replaced, it can be removed from the block together with the plug.In the embodiment of Fig. 1, the element 2 and plug 3 would be moved to the right relative to the carrying elements, i.e., perpendicular to the plane of such elements.

Fig. 2 is a cross-sectional view of the unit taken shown in Fig. 1. Fig. 2 better shows how the shanks of the resistance element are positioned in cut-outs 9 in the ceramic carrying elements 4A, 4B.

Fig. 3 is a cross-sectional view of the unit, but at a right angle relative to that of Fig. 2.

In Fig. 3, the plug 3 has been removed for clarity.

Figs. 4 to 7 show, respectively, different designs of the carrying elements. Fig. 4 shows a carrying element 1 4 of the kind which is used in the units of Figs. 1 to 3.

That carrying element is fastened in the ce- ramic fibrous material because of its shape (i.e., its enlarged mounting portion) as well as by rods which can be inserted through the holes 1 7. Carrying elements of this kind can, of course, be designed for resistance elements 12 having different numbers of shanks. For each shank there is one recess or cut-out 1 9 in the carrying element.

Fig. 5 shows a carrying element 24 intended for a straght, rod-shaped element 22 to be positioned in a cut-out 29 in the form of a circular hole. This carrying element, like the one shown in Fig. 6, is intended to be mechanically fastened in the ceramic fibrous material by rods passing through the holes 27.

Fig. 6 shows a carrying element 34 intended for a U-shaped or hairpin-shaped resistance element 32, disposed in a cut-out 39.

Retaining rods are to pass through the holes 37.

In Fig. 7 there is shown a carrying element 44 intended to carry two resistance elements 42A, 42B in a cut-out 49. It is, of course, possible to design the carrying elements correspondingly to carry three or more resistance elements but such designs are thought to be of little interest in practice. Retaining rods would pass through the holes 47.

Fig. 8 shows a device according to the invention where a single resistance element 52 is carried by four separate blocks of ceramic fibrous material having carrying elements of high temperature resistant refractory ceramic fastened in the blocks. In the embodiment shown in the figure, each of the four blocks 51A-51 D has one carrying element 54A-54D. Of course, each block may have several such carrying elements. The two middle blocks are identical. The first block 51A is equipped with a removable plug 53 to make possible replacement of the element as previously described. The last block 51 D has a recess which does not extend for the full length of the block.

The dimensions of devices according to the invention may vary, depending upon the particular application. However, the length of a single fiber block taken in a direction parallel to that of the resistance elements will, as a rule, be 0.3-1.0 meter. The diameter of the resistance element is mostly in the range of 3-15 mm and their length may vary from 0.1 m, up to several meters. Serpentineshaped elements may have different numbers of serpentine shanks.

Figs. 9A and 9B show a device in which a resistance element 62 is located in a recess formed in a ceramic block 61 fitted with a removable plug 63. The resistance element 62 is supported by carrying elements 64 which are fastened to the ceramic block 61.

The resistance element 62 terminates in terminals 65 which are at right angles to the zone of the element contained within the recess.

In Fig. 10 an alternative form of carrying element 74 is shown wherein a throughpassage 79 is provided for receiving a Ushaped resistance element 72. An enlarged portion 74A of the carrying element would be embedded within a ceramic block (not shown).

In Figs. 11 and 12, there is depicted in front view, a ceramic block 81 in which a pair of carrying elements 84A, 84B are embedded, the carrying elements being depicted in Fig. 1 3. A single resistance element 82 is insertable and removable relative to circular openings 89 within the carrying elements.

The carrying elements are embedded within the block 81 without the need for rods of the type previously disclosed.

In Fig. 1 4 there is depicted a further embodiment of the invention wherein a serpentine type of resistance element 92 is vertically insertable into a recess 95 in a fibrous ceramic block 91. Ceramic plugs 93 are mounted at the ends of the resistance elements and are received within corresponding openings in the block 91.

Projecting horizontally from the block and into the recess 95 are a plurality of ceramic support members 100. The support members 100 are spaced by a distance corresponding to the spacing between adjacent peaks of the serpentine resistance element, such that when the resistance element is inserted vertically into the recess 95, the valleys of the resistance element pass between the supports 100 and the peak portions of the resistance element rest against the support. The supports 100 are carried at the end of threaded mounting shanks 102 (Fig. 1 5) which are threadedly inserted within the block. The block includes an adapter plate 104 having a dovetail connection with the block. The mounting shanks are threaded through the adaptor plate 104 and into the block 91. An outer wall 108 of the adapter plate forms the inner wall of the recess 95.

By virtue of the manner in which the resistance element is retained in accordance with the present invention, the heating power which can be given to the resistance element is substantially greater than that previously possible. Former limits of about 2W/cm2 of block surface have now been extended to 7W/cm2 of block surface as a result of the present invention.

Claims (14)

1. Apparatus for replaceably carrying an electrical resistance element in a furnace having a compartment, the apparatus comprising a block of a ceramic fibrous material and at least one plate-shaped carrying element mechanically fastened in the block and including a portion protruding from the block, the protruding portion including at least one throughpassage into which the resistance element is inserted.

2. Apparatus as claimed in claim 1 wherein the block includes a cavity and the protruding portions extends into the cavity.

3. Apparatus as claimed in claim 2 wherein a plate containing the resistance element is disposed within the cavity.

4. Apparatus as claimed in any one of the preceding claims wherein the plate-shaped carrying element includes a mounting portion disposed within the block and containing at least one hole.

5. Apparatus as claimed in claim 4 which includes a plurality of rods disposed within the block and extending through the holes of the carrying element at a right angles to the plane of the carrying element.

6. Apparatus as claimed in claim 5 wherein there are a plurality of carrying elements, each of the rods extending through all of the carrying elements.

7. Apparatus as claimed in any one of the preceding claims wherein there are a plurality of carrying elements arranged to carry at least one straight portion of a resistance element.

8. Apparatus as claimed in any one of the preceding claims wherein the resistance element includes a plug formed of ceramic fibrous material and received within an opening within said ceramic block.

9. Apparatus as claimed in any one of the preceding claims wherein the carrying element includes a mounting portion which is flared outwardly relative to the projecting portion and which is embedded within the block.

1 0. Apparatus as claimed in any one of the preceding claims wherein the carrying element is threadedly mounted within the block.

11. Apparatus as claimed in claim 10 wherein the block includes an adapter plate mounted on a main portion of the block by a dove-tail connection, the carrying elements being threadedly mounted through the adaptor plate and the main portion of the block.

1 2. Apparatus as claimed in claim 1 substantially as hereinbefore described with reference to and as illustrated in any one of the Figs. of the accompanying drawings.

1 3. A furnace having an apparatus as claimed in any one of claims 1 to 1 2 incorpo rated therein.

14. A furnace as claimed in claim 1 3 wherein the heating power is at least 50 kW/cm2 of fibrous block surface exposed to the furnace compartment.