WO2005055666A1

WO2005055666A1 - Coaxial heating element for a heating device and associated heating device

Info

Publication number: WO2005055666A1
Application number: PCT/EP2004/013622
Authority: WO
Inventors: Christian Seidler; Michael Riffel; Lutz Ose; Jürgen Weber
Original assignee: E.G.O. Elektro-Gerätebau GmbH
Priority date: 2003-12-01
Filing date: 2004-12-01
Publication date: 2005-06-16
Also published as: DE10356914A1

Abstract

The aim of the invention is to provide a coaxial heating element (13, 113), whose response is significantly more rapid than conventional tubular heating elements. To achieve this, the diameter of the inventive element is reduced to 3mm or less. The ratio of the mass of the coaxial heating element to its power rating can be less than 45mg/Watt and the ratio of its volume to its power rating can be less than 9.5mm3/W. The ratio of its power rating to its surface area can be greater than 7 W/cm2, in particular between 8 and 12 W/cm2.

Description

Description Coaxial radiator for a heating device and heating device

Field of application and state of the art

The invention relates to a coaxial heating element for a heating device, for example as radiant heating or oven heating, as well as a heating device.

It is known to use so-called tubular heaters, for example, as oven heaters. Such tubular heaters consist of a tubular jacket, in which a heating conductor envelops the insulating material. The heating conductor is operated as an electrical heating resistor, being insulated from the outer jacket by the insulating material. The advantage of such tubular heaters is that they can be manufactured relatively easily and inexpensively, as well as being technologically manageable. Furthermore, they have a relatively large scope for shaping, so that they can be given a shape of any shape with wide coverage over a wide range. Such a tubular heater is known for example from DE-OS 24 21 842.

A disadvantage of the known tubular heaters is that they take a relatively long time to develop heat or to effectively start heating operation.

Task and solution

The invention has for its object to provide a coaxial radiator mentioned, with which the disadvantages of the prior art

Technology can be avoided and in particular the time until the effective start of heat generation can be reduced. Of Furthermore, the effectiveness of the heating and its even heat distribution should be as good as possible even in full heating mode.

This object is achieved by a coaxial heating element with the features of claims 1, 3, 5 and 7 and in a corresponding application by a heating device with such a coaxial heating element. Advantageous and preferred embodiments of the invention are the subject of the further claims and are explained in more detail below. The wording of the claims is made the content of the description by express reference.

According to the invention, it is provided according to a variant that the coaxial heating element has a diameter that is significantly smaller than conventional tubular heating elements, in particular 3 mm and less. The diameter can even be reduced to 1 mm to 1.5 mm. Conventional tubular heaters have a diameter of just over 6mm. Furthermore, according to one variant, the ratio of the mass of the coaxial heating element to its nominal output, with which it is applied in nominal operation or in the standard operating mode, is less than 50 mg / W or 45 mg / W. A relatively low inertia of the coaxial heating element can be achieved because, due to the small mass, which is considerably less than that of a conventional tubular heating element with a diameter of approximately 6.5 mm, less mass has to be heated and the final temperature can thus be reached more quickly. In an advantageous embodiment, the ratio of the mass to the nominal power is even smaller, advantageously less than 33 mg / W.

The relatively small ratio of mass to nominal output makes it possible to achieve a more dynamic control behavior of the low-mass coaxial radiator. Above all, a significant overshoot after switching off the heating should be avoided. This can temperatures, for example in an oven, can be better regulated.

In a further variant of the invention, with a similarly small diameter of less than 3 mm, the ratio of the volume of the coaxial heating element to its nominal output is less than 9.5 mm ³ / W. This very low power-related volume can also ensure that the coaxial heating element reaches its nominal operating temperature very quickly and that it is advantageously even higher than with conventional tubular heating elements. In a further embodiment of the invention, the ratio of volume to nominal power is even smaller, for example less than 4.7 mm ³ / W, which has led to very positive results in tests.

In yet another variant of the invention, in the case of a coaxial heating element with a small diameter of less than 3 mm, the ratio of its nominal power to its surface or lateral surface is greater than 7 W / cm ² . This means that the surface load can be selected to be very large and thus a relatively high radiation rate is also achieved, since the heating power generated is radiated over the surface. The surface load can be particularly advantageously even greater, for example greater than 8 W / cm ² and going up to a range from 12 to 20 W / cm ² . With active cooling, for example in moving air or in water, the surface load can be even higher. These high surface loads enable this operation with unchanged total or nominal output. With conventional tubular radiators, the total output would have to be increased with such an increased surface load and a constant length. This would lead to problems with the mains connection and would significantly reduce the service life of the radiators. In a further variant of the invention, in the case of a thin and low-mass coaxial heating element described above, the ratio of power or nominal power to the thermal mass of the coaxial heating element is between 95K / sec and 25K / sec. It is advantageously between 87K / sec and 31 K / sec.

Conventional tubular heaters are operated, for example, in an oven with a surface load of 5W / cm ² . If they are used as a grill heater in an oven, the surface load may exceed 6W / cm ² , but not significantly exceed it. These limits are due on the one hand to material-specific parameters of the tubular heater or its individual components. Furthermore, it must also be taken into account that there is a maximum available power at the mains connection, which is due to the nominal voltage of 230 volts and the usual fuse protection with 16 amperes. Furthermore, a higher power density significantly reduces the service life.

The aforementioned features or configurations according to the invention also make it possible to make the coaxial heating element longer and thus, for example, to form larger areas or finer elaborated area distributions with so-called heating focal points. The length can be more than 2 m, advantageously 3 to 8 m. This enables a very even heat distribution to be achieved in an oven. Better heat generation can also be achieved in other types of boiler rooms, in particular also immersed in, for example, a liquid medium such as water.

Another advantage of the small diameter of a coaxial heating element according to the invention is that, for example, smaller bending radii are possible. A geometry of the coaxial heating body can be made more flexible and heat distribution can be better adapted to the desired conditions.

A rated output of more than 2000 watts is considered advantageous. It is particularly advantageously 2700 watts at a normal voltage of 230 volts. With this high output, the value of normal oven heating can be achieved with a conventional tubular heater.

A straight wire can preferably be used as the heating conductor, which in contrast to conventional tubular heating elements is not coiled or is completely straight in the initial state of the heating element. In this way, on the one hand, the corresponding length of the heating conductor can be determined, on which in turn the nominal power depends. Furthermore, due to the reduced space consumption of the heating conductor, the entire coaxial heating element can be made thinner. The heating conductor itself can have a diameter which is between 0.2 mm and 0.9 mm, advantageously between 0.4 mm and 0.75 mm.

A conventional heat conductor alloy can be used as the heat conductor material. The heating conductor wire preferably consists of a Ni-Cr alloy or an Fe-Cr-Al alloy. An alloy ratio of Ni: Cr can be selected here with approximately 80:20. The specific resistance of such a heating conductor material can be about 1, 14 Ωmm ² / m at about 800 ° C. An alloy ratio of Fe: Cr: Al can be chosen approximately with 75: 20: 5. The specific resistance of such a heating conductor material can be approximately 1,40 Ωmm ² / m at approximately 80 ° C.

A jacket wall can be between 0.2mm and 0.3mm thick. With such a relatively thin wall, the total mass and thus the thermal mass and thus the inertia during heating can also be reduced. The thickness of the insulating material can be chosen to be relatively small, particularly in the case of a straight heating conductor. It can be between 0.1 mm and 1.2 mm. This also results in a relatively low thermal resistance between the heating conductor and the jacket, since the temperature difference due to the small masses and short distances is significantly lower than with conventional tubular heaters. For example, it is possible to achieve a surface temperature of the coaxial heating element of more than 800 ° C., advantageously 900 ° C. to 1,000 ° C. While with conventional tubular heating elements the internal heating conductor would have to reach a temperature of well over 1,000 ° C, a considerably lower temperature difference can be achieved with a thin and low-mass coaxial heating element according to the invention. Due to the much thinner insulation layer and the much lower thermal resistance, this temperature difference can be many times smaller than with conventional tubular heaters. It can advantageously be a maximum of 90K, particularly advantageously less than 60K, even less than 30K.

If the coaxial heating element is designed appropriately, it can be achieved that the heating-up time during operation with nominal power is not more than 20 to 30 seconds. These are advantageously less than 20 seconds, particularly advantageously even less than 10 seconds. The heating time is the time within which the radiator begins to glow in the visible area.

The coaxial heating element can be used for oven heating. For this purpose, it is advantageously arranged with a course covering a surface in the upper region of an oven muffle. The ratio of the length of the coaxial heating element to a base area of the oven can be between 0.1 cm / cm ² and 0.6 cm / cm ² . This value, which is considerably higher than that of conventional tubular heaters, also enables better heat distribution. For use in a heating device, a coaxial heating element with the features described above can have a surface-mounted fastening. This is particularly advantageous in that the thin coaxial heating element has a reduced stability, which is significantly below that of conventional tubular heating elements. Such a flat attachment should provide that the coaxial radiator is held at least every 10 cm. This means that only 10cm of the coaxial heating element should be between two fastening points.

For example, such a fastening can have struts that run lengthwise, in particular parallel to one another and / or at equal distances. For example, you can cover an entire cross-sectional area in a boiler room or the area over which the coaxial heating element or the heating device is to run. The attachment of the coaxial radiator can be electrically insulating depending on the desired type of application, but it does not have to be.

Such a coaxial heating element can also be provided, for example, in an oven for the grill function. The faster heating process is particularly energy-saving, especially when it is necessary to reach very high temperatures. Furthermore, shorter grilling times can be achieved, which means a clear gain in comfort for a user.

In addition to the claims, these and further features also emerge from the description and the drawings, the individual features being implemented individually and in groups in the form of sub-combinations in one embodiment of the invention and in other fields, and being advantageous and advantageous Protectable versions can be represented for which protection is claimed here becomes. The division of the application into individual sections and sub-headings do not limit the general validity of the statements made under these.

Brief description of the drawings

Embodiments of the invention are shown schematically in the drawings and are explained in more detail below. The drawings show:

1 is a plan view of a heating device for an oven with a meandering thin coaxial heating element,

2 shows a cross section through a coaxial heating element with a somewhat larger diameter and thin heating conductor, and FIG. 3 shows a cross section through a coaxial heating element with a small diameter and relatively thick heating conductor.

Detailed description of the exemplary embodiments

In Fig. 1 an oven heater 11 is shown, which has a coaxial heating element 13 extending in one plane. The coaxial heating element 13 runs in a meandering manner and has two electrical connections 15, which can be plug connections.

The course of the oven heater 11 shown here, which is fitted in the upper area of an oven, with the electrical connections 15 pointing to the rear, makes it clear that the course of the coaxial heater 13 is very well adapted to the thermal conditions in an oven , It can be clearly seen how there is an increased laying density of the coaxial heating element 13 in the outer region. In the area opposite the electrical connections 15, which is on an oven door, a particularly high heating performance provided. On the other hand, in some cases, no courses of the coaxial heating element 13 are provided in a central region.

A bracket 18 of the oven heater 11 consists of a circumferential frame 19 with struts 20 running between them. These parts are advantageously made of metal or sheet metal, so that they ensure sufficient stability. Fastening the coaxial heating element 13 to the struts should above all be thermally stable, in particular also offer a certain mechanical flexibility. Thus, the coaxial heating element 13 and the oven heating 11 as a whole are protected from damage due to breakage due to excessive thermal expansion. It can also be electrically insulating.

2 shows a section through a coaxial heating element 13 on a scale of 10: 1. This coaxial heater 13 has a relatively large diameter of 3 mm for the variants according to the invention. The thickness of the jacket 22 is 0.25 mm. The thickness of the heating conductor is 0.52mm. There is insulating material 24, for example ceramic powder or granules, between the two. MgO is a suitable material here, for example. The layer thickness of the insulating material 24 is approximately 1 mm.

Such a coaxial heating element 13 can have a length of approximately 3.6 m for a nominal output of 2,700 watts. The specific power per surface is 8W / cm ² . The mass-related power is 22W / g, so that the power-related mass is 0.044g / W. The ratio of power to the thermal mass of the coaxial heating element is approximately 31 K / sec, so that the power-related thermal mass is approximately 0.032sec / K. The power-related volume is 9.4mm ³ / W. FIG. 3 shows a particularly thin coaxial heating element 113. The diameter here is 1.5 mm. The wall thickness of the jacket 122 is again 0.25 mm. While the internal heating conductor 126 is relatively thick at 0.73 mm, the layer thickness of the insulating material 124 is very thin at 0.14 mm. 3 is also shown on a scale of 10: 1.

The coaxial heating element 113 shown here has a length of 7.2 m with a nominal output of 2,700 watts. The specific power is also 8W / cm ² . It could be increased to 12W / cm ² , which would reduce the overall length to 4.8m and the heating conductor diameter to 0.6mm.

The mass-related power is 30.4W / g. At the higher specific power mentioned, a similarly thin coaxial heating element would achieve a mass-related power of 48.4W / g. The power-related mass of the coaxial heating element 113 according to FIG. 3 is approximately 0.033 g / W. The ratio of power to the thermal mass of the coaxial heating element is about 87K / sec, so that the power-related thermal mass is about 0.011sec / K. The power-related volume is around 4.7mm ³ / W.

Claims

claims

1. Coaxial heating element for a heating device, preferably for radiant heating and / or oven heating, wherein the coaxial heating element (13, 113) has a heating conductor (26, 126) running in a tubular jacket (22, 122) and an insulating material between jacket and heating conductor ( 24, 124), characterized in that the coaxial heating element has a smaller diameter than conventional tubular heating elements of less than 3 mm, the ratio of the mass of the coaxial heating element to its nominal output being less than 45 mg / W.

2. Coaxial radiator according to claim 1, characterized in that the ratio of the mass to its nominal power is less than 33mg / W.

3. Coaxial heating element according to the preamble of claim 1, characterized in that the coaxial heating element (13, 113) has a smaller diameter than conventional tubular heating elements of less than 3 mm, the ratio of the volume of the coaxial heating element to its nominal power being less than 9.5 mm ³ / W.

4. Coaxial radiator according to claim 3, characterized in that the ratio of the volume to its nominal power is less than 4.7mm ³ / W.

5. Coaxial heater according to the preamble of claim 1, characterized in that the coaxial heater (13, 113) has a smaller diameter than conventional tubular heaters of less than 3mm, the ratio of its nominal power to the surface of the coaxial radiator is greater than 7W / cm ² .

6. Coaxial heater according to claim 5, characterized in that the ratio of its nominal power to the surface is greater than 8 W / cm ² , preferably 12 to 20W / cm ² .

7. coaxial heater according to the preamble of claim 1, characterized in that the coaxial heater (13, 113) has a smaller diameter than conventional tubular heaters of less than 3mm, the ratio of power to the thermal mass of the coaxial heater between 95K / sec and 25K / sec lies.

8. Coaxial heater according to claim 7, characterized in that the ratio of power to the thermal mass of the coaxial heater is between 87K / sec and 31K / sec.

9. Coaxial heater according to one of the preceding claims, characterized in that its length is greater than 2m, preferably greater than 4.7m.

10. Coaxial radiator according to one of the preceding claims, characterized in that the heating conductor (26, 126) is a straight wire.

1 1. Coaxial radiator according to one of the preceding claims, characterized in that the material of the heating conductor (26, 126) has nickel and chromium, in particular with an alloy ratio Ni: Cr with 80:20.

12. Coaxial radiator according to one of claims 1 to 10, characterized in that the material of the heating conductor (26, 126) comprises iron, chromium and aluminum, in particular with an Fe: Cr: Al alloy ratio of 75: 20: 5.

13. Coaxial radiator according to one of the preceding claims, characterized in that the thickness of the insulating material (24, 124) between the jacket (22, 122) and heating conductor (26, 126) is small, preferably between 0.1mm and 1, 2mm, for a low thermal resistance between the heating conductor and the jacket.

14. Coaxial heater according to one of the preceding claims, characterized in that the surface temperature of the coaxial heater (13, 113) or the jacket (22, 122) is greater than 800 ° C during operation, preferably 900 ° C to 110O ° C.

15. Coaxial radiator according to one of the preceding claims, characterized in that the temperature difference between the outside of the jacket (22, 122) and heating conductor (26, 126) is less than 90K, preferably less than 60K.

16. Coaxial heater according to one of the preceding claims, characterized in that the coaxial heater (13, 113) is an oven heating (11), preferably the ratio of the length of the coaxial heater to a base of an oven muffle between 0.1 cm / cm ² and Is 0.6 cm / cm ² .

17. Heating device, in particular oven heating, characterized in that it has a coaxial heating element (13, 113) according to one of the preceding claims, the coaxial heating element having a surface-mounted fastening (18, 19, 20) can be seen, which preferably holds the coaxial radiator at least every 10 cm.

18. Heating device according to claim 17, characterized in that the fastening has longitudinal struts (20), preferably over the entire cross-sectional area of a heating room, to which the coaxial heating element (13, 113) is attached.