SI9200315A - Electric heating conductor for infrared heating element - Google Patents

Abstract

The radiation heating element (11) has a heating conductor (18a,18b) supported by an insulating carrier (14) beneath the glass ceramics surface (12) of the hob. The heating conductor (18a,18b) has a high surface to mass ratio, the mass of the heating conductor (18a,18b) to its electrical power being less than 7.10 power(-3) gamma per Watt. Pref. the heating conductor (18a,18b) comprises a number of individual wires connected in parallel, forming a loose mesh. The individual wires may have a circular, or flat, cross-section.

Description

(57) Radiation-heating conductor, in particular electric radiant heater. The radiant heater 11 is mounted beneath a glass ceramic plate 12. A heating conductor 18a, 18b is provided on its insulating support 14, which consists of more than a cloth or fabric. interlacing, preferably as tubular interlacing of interconnected individual wires 19, which are connected in parallel. The heating conductor has a low mass-to-power ratio of less than 7.10-3 g / W.

Sl 9200315 A

// and «

02. 11. 92

E.G.O. Electro-Gerate Blanc u.Fischer

Radiation-heating conductor, especially electric radiation heater

The invention relates to a radiation-heating conductor, especially an electric radiation heater. Such radiant heaters are used for heating plates, especially glass ceramic plates. They are mounted on an insulating support spaced from the panel.

From DE-A-39 11 761, a radiation heating device has become known, in which the heating conductor is divided into several twisted wires. This wanted to increase the life of the heater because the thermal radiation was distributed over a larger plot. The specific surface load is reduced by increasing the surface area with the same overall cross section of the heating conductor. When twisting or. the rotation of several individual wires, the wires are at least at least opposite to each other facing surfaces, also impeded in their radiation. Further, this file proposes a different cross-section design, e.g. star shaped wires. These, however, are difficult to manufacture and process.

DE-A-35 09 985 also known for this kind of heating conductors.

DE-B-1 094 383 discloses tubular heaters with a non-circular surface.

DE-A-35 45 454 discloses flat heating conductors which are made in thick-layer technique with paste. Similar metal foil-made heating conductors are known from EP-A-0 202 969. However, they are included in the insulation and thus do not radiate freely.

For radiant heaters in which the heating conductor is shielded from the atmosphere by being incorporated into a silica tube, relatively thin heat conductors may be used which occupy very high temperatures and therefore radiate in the field of visible light, especially if, for example, with a halogenating gas atmosphere in the tube, the heating coil is also protected against evaporation. Such halogen lamps are used for cooking, but they are expensive and also cause steering problems.

GB-A-2 074 828 seeks to achieve an improvement in the radiative behavior of a non-circular shaped helix.

It has further become known (DE-A-36 23 130) that the firing time of the heating conductor is shortened by an occasional increase in current, e.g. by parallel coupling of the resistor with cold conductivity to the part of the heating conductor.

It is an object of the invention to create a radiant heater which, despite being exposed to the atmosphere, has a reduced firing time without shortening its life span.

This problem is solved by claim 1.

The low mass-to-power ratio allows the firing time to be reduced significantly. This produces a fast-firing heating conductor which, at substantially the same end surface temperature as a conventional single-wire, circular cross-section heating conductor, has a smaller overall cross-sectional area. It has been found that the ignition delay is caused to a great extent by the thermal inertia of the heating conductor itself if the heat dissipation with the insulating support is maintained to a tolerably low degree. The advantages of the invention are achieved to a particular extent if the heater virtually rests on an insulating carrier with good thermal insulation properties. In spite of the smaller overall cross section, a sufficient lifetime of the heating conductors can be achieved, even if the heating conductors formed by the invention consist of several parallel connected individual conductors. It has even been found that, in other comparable circumstances, the heating conductor lasts longer if thin. This has already been recognized in DE-A-39 11 761, but the specific surface load there has been reduced and further considered the effect as a result of twisting of individual wires. Furthermore, it has been found that iron-alloy aluminum conductors have better values when their aluminum content is relatively high, e.g. over 4 if it is preferably e.g. 5%. Their specific heat capacity value of approximately 0.53 (kJ / kg.K) in the range of 300-1100 K forms the basis for the mass-power ratio value.

The heating conductors can be designed as flat conductors and applied to the insulating support also by injection molding, such as plasma spraying. Even when using hollow conductors, e.g. thin tubes made of resistive material, under certain conditions the division into several individual conductors may be omitted because the surface / cross section ratio is thereby increased. Furthermore, foil fabrication is possible which, by etching, laser cutting and the like, results in a pre-formed pattern. Depending on the starting material, round wires, flat wires, foils of discs and dust are thus possible, ranging from straight wire to simple or double screws, meanders, etc. and foil fabrication can be performed by stamping, laser cutting, eroding, etching or electroplating. In the case of powder starting material, sinter or foil casting may be used in addition to plasma injection. Installation on an insulating body may be carried out by casting, clamping, nailing, clamping, snapping, or other jointing procedures, under given conditions also with a holding structure such as a rod, mesh structure or, preferably, a helix, while layer suitable anchoring with appropriate surface design of the insulating support.

The geometry of mounting the heating conductors on the insulating support (macro - geometry) may cover more than usual round wires, eg. ten parallel conductors arranged in lines in the form of lines, meanders or spirals.

In addition to screwing, the installation could include braided, rope or coaxial geometry. Flat wires are wavy or preferably screwed and may be mounted in a meander, coiled or other configuration when standing or lying down. Grooved or warty shapes are also suitable for foil fabrication and installation as a layer of trap and the like.

In any case, it is preferable to carry out the installation in such a way that the heating conductor is exposed to the least thermal movement or. stretching and that the curvature of the wall of the heating conductor is relatively large to retain the resulting layer of aluminum oxide on the heating conductor. For this reason, it should also be avoided that any contact of the heating conductor with elements that could be scratched by thermal expansion etc. prevent the oxide layer from being damaged. This protects the heating conductor and allows for a long life span.

According to a particularly advantageous feature of the invention, a heating conductor may consist of several individual conductors, which are connected in parallel to one another and interact in the form of a grid. This arrangement allows the cross section of the entire heating conductor to be reduced substantially with respect to the individual wire at the specific surface load of the heating conductor, and thus substantially the same surface temperature. It is also possible that with a relatively increased radiating surface, the surface temperature decreases with the same power transition. The mesh arrangement creates the advantage that on the one hand the individual heating conductors form an interconnected structure, but on the other hand they barely cover each other, because they only touch point and shading each time. With a largely regular grid structure being preferred, it is also ensured that the individual conductors at each intersection point generally have the same potential so that no significant voltage difference occurs between them. Most oxide-coated materials, as well as the life span, are important in the formation of oxide coatings, e.g. made of aluminum oxide, ensures that the wires are virtually electrically isolated from each other at their points of intersection.

The heating conductor may consist of a fabric, interlacing or knitting of any connecting structure. Particularly preferred is a tubular interconnect, the individual conductors of which are interconnected in the form of one another in the opposite direction of the coiled screws. Such a tubular web is quite stable in itself, although it is flexible enough to be mounted on an insulating carrier in the form of a spiral, meander or the like, without the fear of the tube breaking or squeezing. The tubular interconnect may also be formed from groups of parallel conductors. Another advantage is that due to the relatively small wire diameter, the individual conductor holds the oxide coating protecting the heating conductor very well and does not cause abrasion or debris by mechanical or thermal displacement.

Preferably, this heating conductor is also produced with a low total mass / nominal power ratio of less than 7.10 ' ³ g / W, but is achieved with a mesh structure advantageous in other ratios.

These and further features derive from the claims but also from the description and drawings, whereby individual features may be realized in the embodiment of the invention and in other fields each time, individually or separately, in the form of sub-combinations, and may be preferred and self-protective embodiments, which protection is required here. A derivative example of the invention is shown in the drawing and will be explained in further detail.

FIG. 1 shows a schematic, vertical section through a radiant heater with two variants of the heater conductors in the partial figures la and lb; FIG. 2 and 3 show enlarged detailed views in FIG. la and lb shown heating conductors.

The radiant heater 11 is provided for the electric cooking appliance and is pressed against the underside of the ceramic glass plate 12 on which the cooking vessels can stand. The radiant heater 11 contains in the tin cup 13 an insulating body 14 of thermally well insulated and high temperature resistant insulating material, e.g. microporous quartz acid gel, which can be reinforced with ceramic fibers. The heating conductors 18a, 18b are mounted at the bottom 17 in the insulating support 14 of the radiating space 15 formed from its edge 16 surrounded by the radiating space 15.

One or more of the heating conductors shown in FIG. or formed in the form of a tubular interlacing, the mesh surface of which consists of interconnected single wires, respectively. from strands of single wires. In an enlarged detail view of FIG. 2, it can be seen that several strands with two individual wires run parallel to each other in the form of a helix and are interwoven with corresponding strands running in the opposite direction to the helix. The tubular interlacing is relatively slight, forming loops with a free surface of loop 21, which is at least half as large as the corresponding surface, occupying a surface of the individual wires 20, preferably larger than this. This ensures that, from the rear, the radiating or reflected radiation can exit through these free loop spaces 21.

Any weaving, knitting or knitting method that can be used for such tubular knitting is useful. The number of single strands is determined by the number of strands in strands and by the number of strands running parallel to each other in each direction of the helix. This can be, for example, twelve, as schematically indicated in FIG. la. The tubular sheath 18a contacts the bottom 17 of the insulating body 14 and can be fastened there with clips or the like, taking care to minimize thermal contact with the substrate. It is also possible, such as. shown to close the heating conductor I8a into a holding screw 22, which also consists of an electrical resistive material. This helix should have sufficient intermediate spaces to prevent obstruction from free radiating. In its internal diameter, it should be slightly larger than the outer diameter of the heating conductor 18a and with its lower part 23 inserted into the insulating body 14 and thus keeping the heating conductor in a position largely out of contact but well secured.

In the event that the tubular sheath has to be made of relatively thin wires and is therefore labile, especially when heated, the holding or supporting helix 24 may be drawn into the tubular sheath, which is then secured in turn. on insulating carrier 14 with clips or the like. The arrangement of the heating conductors on the insulating support may be in the form of a spiral, meander or the like.

In FIG. Ib illustrates a heating conductor 18b formed in the form of a mesh flat web of individual conductors 20. This may also be a flat strip of which FIG. Ib and 3 show only one side, which on the other side also shows the outer edge 26 of the individual conductor arches 25 so that the individual conductors 20 pass from connector to connector. This strap may be fastened to the insulating support by clips 27.

Each connection is made jointly with one end, so that all the individual conductors are connected in parallel. Since they all lie at the same potential, the contact between the individual conductors does not lead to a short circuit, so that their attachment is no major problem and that they cannot have any significant differential voltage at the junction points.

The individual conductors consist of an iron-chromium-aluminum alloy with an aluminum content of approximately 5% and have a diameter and length such that the total mass of the individual wires of the existing heating conductor from connector to connector is less than 7.10 ' ³ g / W. For the arrangement of ten parallel-bonded individual conductors in one radiator 180 mm in diameter and rated at 1700 watts at 230 V and a surface load of 6 W / cm, the heating conductor shall have a total mass of 9.1 g and a corresponding length of each of the ten individual conductors 5, 12 m, ie the sum of the lengths of single wires 51.2 m with a wire diameter of 0.176 mm.

In contrast, using only one wire with a 0.817 mm wire mass diameter of 41.7 g would give the same diameter to achieve the same surface load, while giving three wires a total mass of 20.16 g at a diameter of about 0.4 mm. Thus, it can be seen that by maintaining the surface temperature of the invention, the weight of the heating conductor can be significantly reduced, but the diameter of the wire is reduced and the overall length of the individual conductors is increased. The length of the heating conductor itself does not increase by looking at the length of the entire conductors. While the ten individual conductors each have a length of 5.12 m, in the same case the individual wire would be 11.03 m long. It is possible, therefore, to use a tubular twist in which the individual helixes do not lie as tightly as they would normally be with a helix, so that it can be relatively light and allow enough space around it to allow it to sift.

An essential advantage of the grid construction of the individual conductors is that the individual conductors may be relatively thin, but with interlocking they gain stability and convenience. Furthermore, they position themselves in their mutual position without being attached to this function by means of fasteners. A particularly significant advantage is the fact that the individual conductors at the intersection point are touching mainly only pointwise, ie that they can radiate freely over the largest part of their entire length and circumference. In contrast to the braid, in which the conductors are very much directed at each other and therefore cannot freely radiate, this is a particularly significant advantage. In FIG. 2 and 3 the embodiments shown are examples only. All knits, knits and fabrics can be used in as many different ways as possible. A particular advantage is the fact that the aluminum content in the alloy of the heating conductors on the individual conductors forms an oxide layer, which not only protects the heating conductor from further oxidation, but acts in isolation in the soot in the voltage region, which can occur at all between the individual conductors, so that no electrical connection normally occurs at the intersection points.

The support and retaining screws 22, 24 may, in each case, also be brought to the same potential each time as the actual heating conductor, are also connected in parallel to the heating conductor and take over e.g. one tenth of the total power. This proportion of the strength of the support or retaining screws may be controlled by the corresponding extension factor of the screws, i.e. more or less tight winding or. through the lower conductivity values and the diameter of these helixes.

For networks of heating conductors, fasteners may also be formed at the same time, eg. provide for looped loops or tips in the edge region which are embossed or inserted into the insulating support material.

For all the values given here, especially the mass / nominal power ratio in g / W, the described iron-chromium-aluminum alloy having a specific thermal capacity of about 0.53 kJ / kg is assumed for the heating conductor. 300 and 1100 K (approx. 20-800 ° C). Since the mass / power ratio depends on the specific value of the heat capacity c, it varies inversely proportional to the change in the heat capacity value (by 1 / c).

The so-formed heating conductor occupies the annealing temperature over a period of about 3 seconds, which is the case at about 1100 K (about 800 ° C), while the maximum temperature is expected to be over 1300 K (about 1000 ° C), e.g. at about 1350 K (1050 ° C).

It should be borne in mind that the mass / power ratio always refers to the rated power of the respective heating conductor, ie not to the power which is reduced or controlled by control or regulation. by special measures increased short-term. An essential advantage of the invention is that, due to the configuration of the heating conductors, electronic or other switching technical measures are not necessary for the occasional change of power. The relatively large surface area of thin, parallel, single conductors, which can be wires as well as flat strips, ensures favorable conditions of irradiation, so that the maximum permissible temperatures are not exceeded and therefore a sufficient lifetime.

When using flat wires or strips, the width / thickness ratio should preferably not be less than 10. They may be made by injection molding, foil or the like. In this embodiment, it is positioned to radiate freely, only about half of the surface, while in the arrangement of the individual wires, as shown in the left half of the drawing, it is by far the larger proportion. When designing as a helix, in given conditions as a multiple helix or as a standing flat wire, this percentage can be increased even further, which has advantages for radiation and therefore for a specific surface load on the surface of the heating conductor.

It should also be noted that in many applications, it is sufficient to produce only a fraction of the total radiant power of the radiant heater. Thus, e.g. a heating conductor according to the invention is installed only in the annular region around another conventional heating conduit. In this case, the output of the rated power only relates to the fast-radiating formed partial power of the radiant heater according to the invention.

For

E.G.O. Electro-Gerate Blanc u.Fischer:

P F! v'4 s;

Claims (16)

1. A radiator-heater conductor, especially of an electrically exposed radiant heater 11, which, in order to heat the plate 12, in particular the glass ceramic plate, must be mounted on an insulating support 14 away from the plate 12, having a substantially free radiant heating conductor according to the configuration of a single circular wire, an increased surface to mass ratio, characterized in that the ratio of the weight of the heating conductor (18a, 18b) to its rated power is less than 7.10 ' ³ grams per watt (g / W). Heating conductor according to claim 1, characterized in that only part of the total radiant heater heating is formed as a low mass / power ratio heating conductor (18a, 18b). Heating conductor according to claim 1 or 2, characterized in that the specific surface load (W / cm ² ) of the heating conductor (18a, 18b) corresponds substantially to the same end temperature of the individual circular section of the individual solid conductor. Heating conductor according to one of the preceding claims, characterized in that the heating conductor (18a, 18b) consists of several parallel connected individual conductors (19) and / or is formed as a flat conductor or. as such. Heating conductor according to one of the preceding claims, characterized in that the majority of the surface of the heating conductor (18a, 18b) is positioned so that it is free to radiate. Heating conductor according to one of the preceding claims, characterized in that the heating conductor is mounted on an insulating support (14) by injection molding, such as plasma or flame spraying and / or formed into a foil supplied in a pre-specified pattern, by a molding process , such as etching, laser cutting or the like. Heating conductor according to one of the preceding claims, characterized in that the heating conductor (18a, 18b) consists of an iron-chromium-aluminum alloy with an aluminum content of more than 4, preferably about 5%. Heating conductor according to one of the preceding claims, characterized in that it has a heating conductor (18a, 18b) which, with respect to the easy fit on the insulating body (14) of 10, is provided with increased heat dissipation against the insulating body, e.g. by partial insertion of each helix, respectively. with other thermal bridges foreseen at short intervals, a mass / nominal power ratio of less than 5.10 ' ³ g / W. The heating conductor according to any one of the preceding claims, characterized in that its annealing temperature is stationary beyond 1300 K (about 1000 ° C), and preferably below 1600 K (about 1300 ° C). The heating conductor according to one of the preceding claims, characterized in that it is formed as a hollow conductor. Heating conductor of an electrically exposed radiant heater 11, in particular according to one of the preceding claims, characterized in that the heating conductor (18a, 18b) consists of a plurality of mesh-mounted individual conductors (20). Heating conductor according to claim 11, characterized in that it consists of interconnecting individual conductors (20) or more parallel conductive individual conductors (20), preferably of tubular interconnecting. Heating conductor according to one of the preceding claims, characterized in that the individual wires (20) are isolated by surface oxidation against each other. Heating conductor according to one of the preceding claims, characterized in that the heating conductor (18a, 18b) is secured by an internal or external fastening structure, such as a helix (22,24), which is anchored in the insulating support (14) under the given conditions. ). Heating conductor according to one of Claims 11 to 14, characterized in that it comprises a loop mesh (21) with a free surface, especially at least half as large, and preferably larger than the individual conductors (20) occupied by the partial surface of the grid . A radiant heater with a heating conductor (18a, 18b) according to one of the preceding claims. 22365-ΧΙ-92 / LJ For