IL111058A - Method for producing a carrier member for an electric resistance heating coil and an electric heating element using such a carrier member - Google Patents

Description

A METHOD FOR PRODUCING A CARRIER MEMBER FOR AN ELECTRIC RESISTANCE HEATING COIL AND LECTRIC HEATING ELEMENT USING SUCH A CARRIER MEMBER The present invention relates to a method for producing a refractory carrier member for an electric heating coil, in particular a heavy-duty heating coil as is used in industrial furnaces, and a heating element comprising such a carrier member.

Electrical furnaces for melting, sintering, forging or other high-temperature processes are very common in industry and operate at temperatures of up to 3000°C. High-temperature furnaces are generally of one of the following types : 1) Furnaces heated by fossil fuels; 2) Furnaces heated by electrical energy, converted into heat by ohmic resistance, induction, electric arc, plasma, etc.

The furnaces heated by electrical energy have important advantages over those heated by fossil fuel, inasmuch as they are cleaner, their temperature is controllable to a higher degree of accuracy, and they are easier to operate and service.

Of these furnaces, the ones most frequently used are those heated by electrical resistance elements which, basically, consist of a metallic resistor in the form of wires, strips or rods of a suitable ohmic reistance made of metals or alloys, per se well known, that will withstand very high temperatures (in vacuum or in an inert atmosphere up to 2400 °C) without melting or excessively deforming. This metallic resistor is mounted on, or accommodated in, a carrier member usually made of a refractory ceramic. This carrier member which, obviously, must be heat-resistant and electrically insulating, has the essential task of not only supporting the resistor, but also keeping it in shape and preventing excessive proximity between adjacent windings of the resistance coil, which, because of unrelieved radiation concentrations, are liable to produce hot spots that may cause early failure. The carrier member should also be lightweight, easily manufactured, facilitating simple and rapid replacement of a damaged element in a furnace while the latter is still hot without need for skilled manpower.

In other words, it can be safely stated that the ultimate quality and operational as well as cost effectiveness of the heavy-duty heating elements of industrial furnaces are largely functions of the design and construction of their carrier members.

Known carrier members of electrical heating elements suffer from several disadvantages: Their cross-sectional shape is complex, due to which they must be built up from relatively short, modular units. To keep these units together to form an element of the required length, additional mechanical elements are required, as shown, e.g., in U.S. Patent No. 3,846,621. The above heating elements are consequently very expensive and, considering the length and diameter required for industrial furnaces, very heavy, removal and installation requiring at least two people as well as lifting tackle.

Another drawback of this type of carrier member is demonstrated in Fig. 3 of the above-mentioned patent, where it is seen that the resistance coils are largely enclosed in the ceramic carrier, except for a narrow slot. This impedes free heat radiation of the coil, increasing the temperature difference between the interior of the furnace and the coil. In other words, to reach and maintain a certain furnace temperature, the coil must be heated to a much higher temperature .

A further drawback of the above and similar designs resides in the fact that elements of this type cannot be used in the vertical position, as at least some of the red-hot turns of the coils are liable to collapse under their own weight and to cause short circuits.

It is thus an object of the present invention to provide a method for producing refractory carrier members that are of a simple design and geometry, and are thus relatively inexpensive, and that are relatively lightweight and can easily be replaced or installed by non-specialist manpower while the furnace is still hot, thus avoiding costly furnace shutdowns.

It is a further object of the invention to provide a method for producing a carrier member that, while fully supporting the heating coil, enables most of the coil surface to radiate freely, thus increasing its operational efficiency.

The invention achieves the above objectives by providing a method for producing a refractory carrier member for an electric resistance heating coil, comprising the steps of providing an elongated refractory core of predetermined length and outside diameter, and attaching around and along said refractory core a helical ledge made of a refractory material, said ledge having a predetermined width and thickness and a predetermined pitch, so as to constitute a support for said electric resistance coil and an insulating partition between two adjacent turns of said coil.

The invention further provides a method for producing a refractory carrier member for an electric resistance heating coil, comprising the steps of providing an elongated refractory core of predetermined length and outside diameter; using a refractory material, preparing a plurality of helical segments of a predetermined slope and number of turns, each of said helical segments having an internal diameter fitting said core with clearance; firing said helical segments, and sliding said helical segments one after the other onto said core in such a way as to make these sections constitute a substantially continuous helical ledge around and along said core.

The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.

With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

In the drawings: Fig. 1 is an elevation, in partial cross-section, of a first embodiment of the carrier member according to the invention; Fig. 2 is an elevation of some superposed modular segments of the helical ledge of a second embodiment of the carrier member according to the invention, and Fig. 3 is a top view of the segments of Fig. 2.

Referring now to the drawings, there is seen in Fig. 1 an elongated core 2 made of a refractory ceramic. The core 2 is seen to be tubular. Such tubular ceramics are commercially available in all sizes of interest. Particularly advantageous is the use of porous Silimantine 60R (Haldenwanger , Germany), a ceramic material used, for instance, for tubular conveyor rollers in continuous kilns for the production of ceramic tiles. These rollers come in a range of dimensions suitable for industrial carrier members of all sizes and have all the electrical, mechanical, thermal and chemical properties required for their use as cores for carrier members according to the invention. The porosity of this material not only reduces the weight of the element, but also helps the material to withstand thermal shock caused by sudden changes of temperature occasioned by, e.g., rapid withdrawal, from the hot furnace, of an electric heating element for the replacement, say, of a faulty heating coil.

Further seen is a cord 4 of, in this particular case, a substantially square cross-section, which is helically wound around and along the core 2 at a predetermined pitch and, in this state, provides a helical ledge 6 on which the heating coil 8 rests and by which it is supported. The helical ledge also constitutes an insulating partition between the turns of the coil.

The cord 4 is advantageously cut from a ceramic fiber mat such as DurablanketR. The cord 4 is then wound around and attached to the core 2 by means of an adhesive such as, e.g., QF 180 (Carborundum, U.S.A.), at a steady pitch which will leave a gap G between two adjacent turns which is large enough to accommodate the coil 8 with clearance. The adhesive is then cured by heating. In a further step, the still pliable helical cord 4 is rigidified by application of such rigidifiers as colloidal silica and/or colloidal alumina in liquid form. The cord 4 having been thoroughly wetted with the above substances, the core 2 and cord 4 are now heated until the various binders are either burned or evaporated.

The carrier member is now finished and ready for the mounting of the heating coil.

The heating coil can be made of any of the known metal alloys or pure metals such as Fe/Cr/Al; Cr/Ni; W; Mo and others. The heating coil is preferably of the per se known "coiled coil" type, in which the resistance wire is first wound around a rod of suitable diameter (either with or without application of heat) to form a helical coil, which coil is then wound along the helical groove formed by the cord 4, thus constituting the "coiled coil" 8. Heating the latter in situ will stabilize its shape and prevent "springback" .

The ends of the heating coil 8 are treated in a per se known manner: If the original wire is sufficiently thin and pliable, the ends are twisted to increase their effective diameter to prevent their heating. With heavier wire, a piece of rod of a larger diameter (but preferably of the same metal) is welded to the coil ends.

To attach the coil ends and thus the coil 8 to the carrier member, any of the known, conventional methods can be used, such as using an annular clamp, a length of wire, embedding the ends in ceramic collars, and the like. If the two coil ends should be on the same end of the carrier member (as in the so-called "bayonet" type heating element), one coil end is treated as above, the other is led back through the tubular core 2, care being taken to prevent short circuiting by careful insulation, advantageously using ceramic beads of the known type.

In another embodiment of the carrier member represented in Fig. 2, the helical ledge 6 is produced not by a flexible ceramic cord 4, but by a plurality of interlocking or otherwise mutually supporting helical segments 10 of predetermined pitch or slope. These segments 10 are produced by molding, starting out from a ceramic slick, and are then fired. The internal diameter of each of the segments 10 fits the core 2 (Fig. 1) with clearance and each segment 10 surrounds the core 2 for more than 180° (the number of turns thus being larger than 0.5), so that, once slid onto the core 2, the segments 10 cannot move off sideways. The ends 12 of each segment 10 are stepped in such a way that when, one after the other, they are slid onto the core 2, they interlock as seen in Fig. 2 and form a substantially continuous helical ledge 6 around and along the core 2.

Fig. 3 is a top view of the segments 10 of Fig. 2. Clearly, the angle a with which each of the segments 10 "embraces" the core 2 is >180°. When complex, multicore molds are used, a could be much larger.

An alternate method utilizes the existence of so-called "machineable" ceramics which, in the original or "green" state, can be turned or milled like other materials. A helical groove of the desired dimensions and pitch is cut (either by turning or by milling) into a bar of this material, which bar is then fired and thereby becomes a refractory ceramic.

It will be appreciated that by using the "coiled coil" principle, it is possible to maximize the length of resistance wire used, making it possible to achieve high electrical power density per furnace volume or furnace wall surface area while keeping the surface load of the element as low as possible (W/cm2) and to use full line voltage, rather than to series-connect some heating elements and risk shutdown of whole furnace sections whenever a single heating element has failed.

While the cord 4 of Fig. 1 is seen to have a square cross-section, other cross-sectional shapes are also possible (rectangle, right-angled triangle, rectangle with a slightly raised edge rail, etc.).

Electric resistance heating elements according to the invention should be used in the vertical or slanted position.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims (12)

WHAT IS CLAIMED IS:

1. A method for producing a refractory carrier member for an electric resistance heating coil, comprising the steps of : providing an elongated refractory core of predetermined length and outside diameter, and attaching around and along said refractory core a helical ledge made of a refractory material, said ledge having a predetermined width and thickness and a predetermined pitch, so as to constitute a support for said electric resistance coil and an insulating partition between two adjacent turns of said coil.

2. The method as claimed in claim 1, wherein attaching said helical ledge is effected by the use of an adhesive.

3. The method as claimed in claim 2, wherein, after its application, said adhesive is heat-cured.

4. The method as claimed in claim 1, wherein said core is tubular.

5. The method as claimed in claim 1, comprising the further step of rigidifying said helical ledge after the attachment thereof, said rigidifying being effected by the application of at least one rigidifying agent and a subsequent heat treatment of said carrier member.

6. A method for producing a refractory carrier member for an electric resistance heating coil, comprising the steps of: providing an elongated refractory core of predetermined length and outside diameter; using a refractory material, preparing a plurality of helical segments of a predetermined slope and number of turns, each of said helical segments having an internal diameter fitting said core with clearance; firing said helical segments, and sliding said helical segments one after the other onto said core in such a way as to make these sections constitute a substantially continuous helical ledge around and along said core.

7. The method as claimed in claim 6, wherein said number of turns of each of said helical segments is greater than 0.5.

8. The method as claimed in claim 6, wherein said helical segments are interlocking.

9. A method for producing a refractory carrier member for an electric resistance heating coil, comprising the steps of: providing a carrier member blank of predetermined length consisting of a machineable ceramic and having an outside diameter equal to the desired outside diameter of the carrier member as finished; machining into said blank a helical groove of predetermined pitch and of such a width and depth as to freely accommodate said coil, and firing said blank as machined, with said helical groove constituting a support for said coil, the material remaining between said grooves constituting an electrical insulation between adjacent turns of said coil.

10. An electric heating element whenever produced by the method according to claims 1 or 6 or 9, wherein said electric resistance heating coil is a coiled coil.

11. A method for producing a refractory carrier member for an electric resistance heating coil, substantially as hereinbefore described and with reference to the accompanying drawings.

12. An electric heating element whenever produced by the method according to claims 1 or 6 or 9, substantially as hereinbefore described and with reference to the accompanying drawings.