EP0542128B1 - Electric heating conductor for infrared heating element - Google Patents

Abstract

A radiant heating element (11) is arranged under a glass ceramic panel (12). Arranged on its insulating support (14) is a heating conductor (18a, 18b) which consists of a plurality of individual wires (19) which are threaded into one another as a fabric or mesh, preferably as a hose mesh, and are connected in parallel. The heating conductor has a low ratio between mass and rated power, of less than 7 x 10<-3> g/W. <IMAGE>

Description

The invention relates to a radiant heating conductor of an electric radiant heater. Radiant heaters of this type are used for heating plates, in particular glass ceramic plates. They are arranged on an insulating support at a distance from the plate.

From US-A-3 991 298 a radiant heater according to the preamble of claim 1 has become known which has a spirally arranged heating conductor in the form of an upright strip of expanded metal lying in grooves and having a lattice structure. Due to its enlarged surface it should reach its working temperature in about 3 seconds. No further instructions on how this should be achieved can be found in the writing.

A radiant heater has become known from US Pat. No. 3,345,498, which guides elongated heating conductor strips in the slots of insulating buttons which rise from the bottom of a carrier shell. The heating conductors should consist of a nickel-chromium or iron-chromium-aluminum alloy.

From DE-A-39 11 761 a radiation heating device has become known in which the heating conductor is divided into several individual wires which are twisted together. The aim is to increase the lifespan of the radiator because the heat radiation is spread over a larger area. The specific surface load is reduced by increasing the surface while the overall cross-section of the heating conductor remains the same. If several individual wires are twisted or stranded, however, the radiation is also impeded in their radiation, at least on the surfaces facing one another. This document also proposes a different cross-sectional design, for example a star shape of the wires. These are difficult to manufacture and process.

Such heating conductors are also known from DE-A-35 09 985.

From DE-B-1 094 383 tubular radiators manufactured with a non-circular surface are known.

From DE-A-35 45 454 flat heating conductors are known, which are designed in thick-film paste technology. Similar heating conductors made of metal foils are known from EP-A-0 202 969. However, they are enclosed in insulation and are therefore not freely radiating.

For radiant heaters in which the heating conductor is sealed off from the atmosphere by being enclosed in a quartz tube, relatively thin heating conductors can be used which assume very high temperatures and therefore radiate in the visible light range, especially if, for example, a halogenated gas atmosphere in the Tube the heating coil is also protected against evaporation. Such halogen lamps are used for cooking purposes, but are expensive and also pose control problems.

GB-A-2 074 828 attempts to improve the radiation behavior by using a non-circular helix shape.

It has also become known (DE-A-36 23 130) to shorten the glow time of a heating conductor by temporarily increasing the current, for example by connecting a PTC resistor in parallel with part of the heating conductor.

The object of the invention is to provide a radiant heater which, although it is exposed to the atmosphere, has a shortened glow-up time without sacrificing service life.

This object is solved by claims 1, 7 and 15.

The low mass / performance ratio makes it possible to shorten the glow time considerably. The result is a rapidly glowing heating conductor, which has a smaller overall cross-sectional area at a substantially the same surface end temperature as a conventional single-wire heating conductor with a circular cross-section. It has been found that the delay in the glowing is largely caused by the thermal inertia of the heating conductor itself, provided that heat dissipation by the insulating support is kept to an acceptable level. The advantages of the invention are achieved to a particular extent if the radiator is practically only on an insulating support with good thermal insulation properties. Despite the smaller overall cross-section, a sufficient lifespan of the heating conductors can be achieved, even if the heating conductors designed according to the invention consist of several individual conductors connected in parallel. It was even found that the heating conductor lasts longer, if the conditions are comparable, when it is thin. Although this has already been recognized in DE-A-39 11 761, the specific surface load has been reduced there and the effect on the twisting of the individual wires has also been reduced. It was also found that heating conductors made of an iron-chromium-aluminum alloy show better values if their aluminum content is relatively high, for example above 4%, preferably approximately 5%. Your specific heat capacity value of approx. 0.53 [kJ / kg. K] in the range of 300 - 1100 K forms the basis for the value of the mass-power ratio.

The heating conductors can also be designed as flat conductors. When using waveguides, eg thin tubes made of resistance material, it may be possible to dispense with a division into several individual conductors, because this increases the surface / cross-sectional ratio. Furthermore, it is possible to produce from a film which is brought into a predetermined pattern by etching, laser cutting, etc. Round wires, flat wires, rolls from the roll and powder are possible as the starting material, whereby the shaping ranges from straight wire to single or double coils, meanders etc. and can be carried out in the film production by punching, laser cutting, eroding, etching or electroforming . For powder starting material, sintered or foil casting could also be used in addition to plasma spraying. The attachment to the insulating body could be carried out by partial pouring, clamping, nailing, clamping, snapping or other form-fitting holding methods, possibly also by means of a holding structure, such as a rod, a lattice structure or preferably a helix become, while when applying as a layer anchoring is appropriate by appropriate surface design of the insulating support.

The geometry of the application of the heating conductors to the insulating support (macro-geometry) can comprise several, for example ten parallel individual conductors which are arranged in linear, meandering or spiral paths in conventional round wires.

In addition to the helix, the arrangement could also comprise a stranded, stranded or coaxial helix geometry. Flat wires are corrugated in a standing arrangement and can be arranged in a meandering, spiral or other configuration.

In any case, it is advantageous to arrange so that the heating conductor is exposed to as little thermal movement or expansion as possible and the curvature of the heating conductor wall is relatively large in order to keep the aluminum oxide layer that forms on the heating conductor. For this reason, any contact with the heating conductor by elements that scratch it during thermal expansion etc. should be avoided in order not to damage the oxide layer. This protects the heating conductor and enables a long service life.

In a preferred embodiment, the heating conductor can consist of a plurality of individual conductors which are connected in parallel to one another and interact in a network-like manner. This arrangement allows for. a specific surface load of the heating conductor and thus essentially the same surface temperature, the total heating conductor cross-sectional area compared to a single wire significantly lower. However, it is also possible to lower the surface temperature with the same power throughput due to the relatively enlarged radiating surface. The net-like arrangement creates the advantage that on the one hand the individual heating conductors form a coherent structure, but on the other hand hardly shield each other, since they only touch one another at a time and hardly any mutual shadowing occurs. With a largely regular network structure, which is preferred, it is also ensured that the individual conductors at the respective crossing points have essentially the same potential, so that there are no significant voltage differences between them. A formation of an oxide skin, for example made of aluminum oxide, which occurs in most heating conductor materials and is also important for the service life, ensures that the wires are practically electrically insulated from one another at their crossing points.

The heating conductor can consist of a woven, braided or knitted fabric of any binding structure. However, a hose braid is particularly preferred, the individual conductors of which are interwoven in the form of spirals which are rotated in opposite directions. Such a hose mesh is quite stable in itself, although it is flexible enough to be spiral, meandering or the like on an insulating support. to be arranged without fear of the hose kinking or being compressed. The tubular braid can also be formed from groups of individual conductors running parallel to one another. Another advantage is that the relatively small wire diameter Individual conductor which holds the oxide skin protecting the heating conductor very well and is not caused to rub off or flake off by mechanical or thermal movements.

Fig. 1

zeigt einen schematischen, vertikalen Schnitt durch einen Strahlungsheizkörper mit zwei Heizleiter-Varianten in den TeilFiguren 1a und 1b,

Fig. 2 und 3

zeigen vergrößerte Detailansichten der in den Teil- Fig. 1a und 1b gezeigten Heizleiter.

These and further features emerge from the description and the drawing in addition to the claims, the individual features being realized individually or in groups in the form of sub-combinations in one embodiment of the invention and can represent advantageous and protectable designs, An embodiment of the invention is shown in the drawing and is explained in more detail below.

Fig. 1

shows a schematic, vertical section through a radiant heater with two heating conductor variants in the partial figures 1a and 1b,

2 and 3

show enlarged detail views of the heating conductor shown in the partial Fig. 1a and 1b.

The radiant heater 11 is intended for an electric cooking appliance and is pressed onto the underside of a glass ceramic plate 12 on which cooking vessels can stand. The radiant heater 11 contains, in a sheet metal shell 13, an insulating body 14 made of thermally well-insulating and high-temperature-resistant insulating material, for example a microporous silica airgel that can be reinforced with ceramic fibers. Heating conductors 18a, 18b are arranged on the bottom 17 of the radiation space 15 which forms in the insulating support 14 and is surrounded by its edge 16.

One or more heating conductors can be arranged, which are formed in FIG. 1a in the form of a tubular braid, the net-like surface of which consists of individual wires interwoven with one another or of strands of individual wires. In the enlarged detailed representation according to FIG. 2 it can be seen that several strands of two individual wires each run helically in parallel to one another and are interwoven with corresponding strands running in the opposite direction of the helix. The tubular braid is relatively loose, so that stitches are formed with a free mesh area 21 which is at least half the size of the corresponding area occupied by the individual wires 20, but preferably larger than this. This ensures that radiation originating or reflected from the rear can exit through these mesh spaces 21.

Any type of braiding, weaving or knitting is suitable that can be used for a tubular braid. The number of individual wires is determined by the number of wires in the strands and by the number of strands running parallel to each other in each spiral direction. It can be, for example, twelve, as is indicated schematically in FIG. 1a. The heating conductor 18a made of hose mesh lies on the bottom 17 of the insulating body 14 and can be clamped or the like there. be attached, paying attention to the lowest possible thermal contact with the surface. However, it is also possible, as shown for example, the heating conductor 18a in a Include helical coil 22, which is also made of an electrical resistance material. This spiral should have enough gaps in order not to hinder free radiation. Its inner diameter is somewhat larger than the outer diameter of the heating conductor 18a and its lower part 23 is embedded in the insulating body 14 and thus holds the heating conductor largely in a contact-free manner, but well secured in position.

In the event that the hose braid must consist of relatively thin wires and accordingly, especially when heated, becomes unstable, a holding or support spiral 24 can also be drawn into the interior of the hose braid, which in turn is then clamped o . Like. is attached to the insulating support 14. The arrangement of the heating conductors on the insulating support can or the like in spiral, meandering. respectively.

1b shows a heating conductor 18b which is designed in the form of a network-like flat braid made of individual conductors 20. It can be a flat strip, of which only one side is shown in FIGS. 1b and 3, but which on the other side also has an outer edge 26 which is closed by individual conductor bends 25, so that the individual conductors 20 pass from terminal to terminal. This tape can be fastened to the insulating support by clips 27.

The connection is made together at one end, so that all individual conductors are connected in parallel to each other. Since they are all at the same potential, contact between the individual conductors does not lead to a short circuit, so that their attachment is not a major problem and they cannot have any significant differential voltage at each other even at the crossing points.

The individual conductors consist of an iron-chromium-aluminum alloy with an aluminum content of approx. 5% and have such a diameter and length that the total mass / nominal power ratio of the heating conductor consisting of individual wires is smaller than the connection from connection to connection 7. 10⁻³ g / W. If ten individual conductors connected in parallel are arranged in a radiant heater with a diameter of 180 mm and a nominal output of 1700 watts at 230 V and a surface load of 6 W / cm², the heating conductor has a total mass of 9.1 g and accordingly a length of each ten single conductors of 5.12 m, ie a sum of the individual wire lengths of 51.2 m with a wire diameter of 0.176 mm.

In contrast, in the same example, the use of only one wire of 0.817 mm diameter would give the mass of the wire at 41.7 g in order to obtain the same surface load, while three wires have a total mass of 20.16 g with a diameter of approx. Would result in 0.4 mm. It can thus be seen that the mass of the heating conductor can be substantially reduced by the invention while maintaining the surface temperature, although the wire diameter is reduced and the total length of the individual conductors increases. However, the length of the heating conductor itself does not increase if you consider the length of the overall conductor. While the ten individual conductors each have a length of 5.12 m, in the same example the individual wire would be 11.03 m long. It is therefore possible to use a hose network in which the individual coils are not as close together as is usually the case with a heating coil, so that it can be relatively loose and form enough space between them to be irradiated.

A major advantage of the network-like construction of the heating conductor from individual conductors is that the individual conductors can be relatively thin, but gain stability and manageability due to the braided weave. Furthermore, they are thereby determined in their position relative to one another without having to rely on fasteners for this function. A very significant advantage is the fact that the individual conductors essentially only touch one another at the crossing points, i.e. can radiate freely over the vast majority of their total length and scope. In contrast to a stranded wire, in which the conductors are directed towards each other over a very large part of their circumference and therefore cannot radiate freely, this is a very important advantage. The embodiments shown in FIGS. 2 and 3 are only mentioned as examples. All braids, fabrics or knitted fabrics of different types of weave can be used. A particular advantage is the fact that the aluminum content in the heating conductor alloy forms an oxide skin on the individual conductors, which not only protects the heating conductor from further oxidation, but also has an insulating effect, at least in the voltage range that can occur between the individual conductors that there is normally no electrical connection at the crossing points.

The support and holding coils 22, 24 can also be connected in parallel to the heating conductor in order to bring them to the same potential as the actual heating conductor and can absorb, for example, a tenth of the total power. This performance share of the holding or support coils can be controlled via a corresponding extension factor of the coils, ie more or less tight winding or via lower conductivity values and the diameter of these coils.

In the case of the heating conductor networks, fastening means could also be molded in the same way, for example by providing loops or tips which are bent downwards in the edge region and which are pressed or embedded in the insulating carrier material.

For all values given in the examples, in particular the mass / nominal power ratio in g / W, the iron-chromium-aluminum alloy described for the heating conductor, which has a specific heat capacity of approx. 0.53 kJ / kg, was assumed. K has a mean value over the range between 300 and 1100 K (approx. 20 - 800 ° C). Since the mass / power ratio is dependent on the specific heat capacity value c, it changes, in reverse proportion to the change in the heat capacity value (with 1 / c).

Such a heat conductor takes on an annealing temperature within approx. 3 seconds, which is the case at approx. 1100 K (approx. 800 ° C), while the maximum temperature exceeds 1300 K (approx. 1000 ° C), e.g. should be around 1350 K (1050 ° C).

It should be noted that the mass / performance ratio is always based on the nominal output of the heating element concerned, i.e. not an output that is reduced by control or regulation or that is temporarily increased by special measures. It is an essential advantage of the invention that the heat conductor configuration is electronic or other circuitry measures for temporary performance changes are not necessary. The surface of the thin, parallel-connected individual conductors, which is large in relation to their mass, and which can be wires, but also flat strips, ensures favorable radiation conditions, so that the maximum permissible temperatures are not exceeded and the service life is therefore sufficient.

When using flat wires or strips, a ratio of width to thickness of 10 should not be undercut if possible. He or the like in the manner already described by spraying, from a film. be made. In this embodiment, only about half of the surface is freely radiating, while this is a much larger proportion in the arrangement of individual wires, as shown in the left half of the drawing. In the case of training as a helix, possibly as a multiple helix or as a standing flat wire, this percentage can be increased even further, which has advantages for the radiation and thus also for the specific surface load on the heating conductor surface.

It should also be noted that in many applications it is sufficient to produce only a part of the total output of a radiant heater quickly glowing. For example, a heating conductor according to the invention could only be arranged in an annular area around another heating conductor, which can be of conventional design. In this case, the nominal power specification relates only to the partial power of the radiant heater which is designed to be rapidly glowing according to the invention.

Claims (16)

1. Radiant heating conductor, particularly an electrical radiant heater (11) exposed to the atmosphere and which for heating a plate (12), particularly a glass ceramic plate, is to be positioned on an insulating support (14) spaced with respect to the said plate (12) and the substantially freely radiating heating conductor has an increased surface to mass ratio compared with a single round wire configuration, characterized in that the ratio of the mass of the heating conductor (18a, 18b) to its rated output is lower than 7·10⁻³ gramme per Watt [g/w] and the heating conductor (18a, 18b) comprises a plurality of individual conductors (20) arranged in netting-like manner.
2. Heating conductor according to claim 1, characterized in that it comprises a braiding of individual conductors (20) or several parallel individual conductors (20), preferably a hose or tubular braiding.
3. Heating conductor according to one of the claims 1 or 2, characterized in that the individual conductors (20) are wires insulated against one another by surface oxidation.
4. Heating conductor according to one of the preceding claims, characterized in that the heating conductor (18a, 18b) is held by an inner or outer holding structure, such as a coil (22, 24), which is optionally anchored in an insulating support (14).
5. Heating conductor according to one of the preceding claims, characterized in that the netting has netting meshes (21) with a free surface, which is at least half as large and preferably larger than the partial surface of the netting taken up by the individual conductor (20).
6. Heating conductor according to one of the preceding claims, characterized in that it is constructed as a hollow conductor.
7. Radiant heating conductor according to the preamble of claim 1, characterized in that the ratio of the mass of the heating conductor to its rated output is lower than 7·10⁻³ gramme per Watt [g/W] and the heating conductor in the form of a corrugated flat wire is placed vertically on the insulating support (14).
8. Radiant heating conductor according to claim 7, characterized in that the corrugated heating conductor is arranged in meander-like or spiral manner on the insulating support (14).
9. Heating conductor according to one of the preceding claims, characterized in that only part of the entire heating of the radiant heating body (11) is in the form of a heating conductor (18a, 18b) with a low mass/rated output ratio.
10. Heating conductor according to one of the preceding claims, characterized in that the specific surface loading measured in Watts per square centimetre [w/cm²] of the heating conductor (18a, 18b) substantially corresponds to that of a single solid heating conductor with a circular cross-section designed for the same final temperature.
11. Heating conductor according to one of the preceding claims, characterized in that most of the surface of the heating conductor (18a, 18b) is arranged in free emitting manner.
12. Heating conductor according to one of the preceding claims, characterized in that a heating conductor (18a, 18b) which, compared with merely placing on the insulating support (14), is exposed to increased heat dissipation to the insulator, e.g. through the partial embedding of each coil or by the other heat conducting bridges, which are at a short distance from one another, has a mass/rated output ratio of less than 5·10⁻³ gramme per Watt [g/W].
13. Heating conductor according to one of the preceding claims, characterized in that its glow temperature in the stationary state is above 1300 K and preferably below 1600 K.
14. Radiant heating conductor according to one of the claims 7 to 13, characterized in that the width to thickness ratio of the flat wire is not smaller than 10.
15. Radiant heating conductor according to the preamble of claim 1, characterized in that the ratio of the mass of the heating conductor (18a, 18b) to its rated output is lower than 7·10⁻³ gramme per Watt [g/W] and the heating conductor (18a, 18b) is made from an iron-chromium-aluminium alloy with an aluminium percentage above 4 and preferably approximately 5%.
16. Radiant heater with a heating conductor (18a, 18b) according to one of the preceding claims.

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