DE29619759U1 - Electric radiant heater - Google Patents

Description

(4627) St/kr, Ceramaspeed Ltd.

Electric radiant heater

The invention relates to an electric radiant heater for stoves with a glass ceramic cover plate, in particular to such a heater that heats up quickly to the radiant temperature and with a high operating temperature and very even heat distribution over an area of the glass ceramic cover plate intended as a cooking surface.

The present invention provides a radiant heater for stoves with a glass ceramic cover plate with a bowl-shaped carrier made of a base layer and an outer edge, both made of heat-insulating material and with at least one electrical resistance heating element on the bowl-shaped carrier, wherein the at least one resistance heating element has a negative temperature coefficient of electrical resistance over a first temperature range from 0 ⁰ C to at least 700 ⁰ C and a positive temperature coefficient of electrical resistance in a second temperature range above the first temperature range.

Preferably, the first temperature range can, for example, range from 0 to 1000 ⁰ C or from 0 to 900 ⁰ C or most preferably from 0 ⁰ C to 800 ⁰ C.

Preferably, the shell-shaped carrier is arranged in a metal shell. The base layer of the shell-shaped carrier is preferably made of microporous heat-insulating material.

The at least one electrical resistance heating element can be made of refractory semiconducting material, in particular silicon carbide and most preferably alpha-silicon carbide. The material can contain one or more dopants or additives in order to set the required electrical resistance and/or temperature coefficient of the electrical resistance.

(4627) St/kr, Ceramaspeed Ltd.

The at least one electrical resistance heating element can be present in the heater in the form of at least one elongated rod, wire or strip.

Preferably, there is at least one elongated strip with a rectangular cross-section, which is advantageously supported by its edge on the shell-shaped support. The heating element with a rectangular cross-section has a height that is significantly greater than its thickness when supported in this way.

Such edge support may be provided by securing the at least one elongate strip to the opposite end portions of the element in the shell-shaped support. Preferably such that a portion of the strip in the middle portion between the end portions is spaced from the base layer of heat insulating material. The end portions of the strip may be held by mounting elements, typically comprising ceramic components. These are arranged in edge portions of the shell-shaped support.

The at least one elongate strip or strips may be substantially straight or curved or have a curved shape. When the at least one elongate strip is arranged to extend across the tray-shaped support, if desired, a region between the end regions may have a height and/or thickness different from corresponding ones of the end regions. Preferably, in the central region, the height and/or thickness is greater than in regions on either side of the central region so that the required temperature distribution over the length of the strip is achieved during operation of the heater.

The at least one elongated strip may taper in height and/or thickness from the central region to the regions on either side.

The at least one electrical resistance heating element can have end regions that have a lower electrical resistance than the remaining part of the element.

(4627) 5t/kr, Ceraiuaspeed Ltd.

The end areas are designed and constructed for connection to supply lines and should have a lower temperature than the remaining part of the element during operation of the heater.

When the at least one electrical resistance heating element is made of silicon carbide, the end regions may typically be formed of beta-silicon carbide or alpha-silicon carbide enriched with silicon, while the remaining part of the element is made of alpha-silicon carbide.

The enrichment of the alpha-silicon carbide occurs by incorporating silicon into the pores of the alpha-silicon carbide. Each of the end areas can also or alternatively be provided with a sleeve or a layer of a metal or metal alloy with good electrical conductivity.

If desired, at least one additional heating element can be arranged in the bowl-shaped carrier, wherein this at least one additional heating element or several heating elements differ from the type of electrical resistance heating element or differ in material therefrom. Such at least one additional heating element can, for example, be an infrared lamp or at least one spiral-shaped resistance wire heating element or at least one resistance heating element in the form of a corrugated band. Combinations of the additional heating elements of different types can also be used.

Each infrared lamp may typically be a halogen lamp. Each spiral resistance heating element or the resistance heating element in the form of a corrugated ribbon may typically be made of an iron-chromium-aluminium alloy. The at least one or more additional heating elements may be arranged and connected so that they can be independently energized. However, they may also be arranged and connected so that they are temporarily or permanently connected in series or parallel with the at least one electrical resistance heating element.

5t/kr, Ceramaspeed Ltd.

The shell-shaped support may also be a metal shell containing the base layer of heat-insulating material and the outer edge of heat-insulating material. The base layer of heat-insulating material is usually a microporous heat-insulating material.

The heater of the invention has a rapid light-up time to radiation, which can be less than 1.5 seconds. Although the electrical resistance of the at least one electrical resistance heating element is relatively high in the cold state at the beginning of energization, it then drops rapidly, accompanied by a rapid increase in temperature, reaching a minimum value of electrical resistance at the point where the temperature coefficient of resistance changes from negative to positive. The subsequent increase in electrical resistance prevents thermal overheating of the heating element.

High operating temperatures of, for example, 1500 ⁰ C are obtainable with the at least one electrical resistance heating element and provide excellent performance and durability of the heater and good heat distribution is achieved, particularly with an element of rectangular cross-section mounted so as to be supported over a narrow edge.

The invention will now be described in more detail by way of example with reference to the drawings.

Fig. 1 is a perspective view of an embodiment of the radiant heater according to the present invention.

Fig. 2 is a side view of a modified form of heating element for use in the heater of the invention.

Fig. 3 is a plan view of another modified form of heating element for use in the heater of the present invention.

Fig. 4 is a plan view of another modified form of heating element for use in the heater of the present invention.

(4627) St/kr, Ceramaspeed Ltd.

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Fig. 5 is a perspective view of another embodiment of the heater according to the invention.

Fig. 6a is a perspective view of another embodiment of the heater according to the invention.

Fig. 6b is a cross-section of the heater of Fig. 6a and shows its arrangement under a glass-ceramic cover plate.

Fig. 7 is a perspective view of another embodiment of the heater according to the invention.

Fig. 1 shows an electric radiant heater for stoves with a glass ceramic top plate, which is constructed as follows. A round metal bowl 1 contains a base layer 2 of electrically and thermally insulating material, such as microporous thermally and electrically insulating material. An outer rim 3 of well known shape is present and contacts the underside of the glass ceramic top plate (not shown) when the heater is placed in a stove for operation. The outer rim is made of thermally and electrically insulating material, such as bonded ceramic fibers, bonded glass fibers or vermiculite.

An elongated electrical resistance heating element 4 in the form of a strip with a rectangular cross-section is arranged standing on a narrow edge on the base layer and extends diagonally across the shell, being at a distance from the base layer 2.

The element 4 is introduced into the heater through openings in the outer edge of the shell 1 and is held by inserts 5, which have ceramic components arranged diametrically opposite one another in the edge regions of the base layer 2. Such inserts 5 can usually be introduced into the shell 1 during the formation of the base layer 2.

The electrical resistance heating element 4 has a negative temperature coefficient of electrical resistance over a

(4627) St/kr, Cerainaspeed Ltd.

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a first temperature range from 0 ⁰ C to at least 700 ⁰ C and in a second temperature range which is higher than the first temperature range, a positive temperature coefficient of electrical resistance. The first temperature range can, for example, range from 0 ⁰ C to 1000 ⁰ C or from 0 ⁰ C to 900 ⁰ C or from 0 ⁰ C to 800 ⁰ C.

The heating element 4 is made of refractory semiconductive material and typically has a central heat-radiating region of alpha-silicon carbide. This may be used in pure form or may contain a proportion of beta-silicon carbide and/or small amounts of impurities or dopant material. The element may have end regions 6, 7 with lower electrical resistance than that of the central region. One possible strip of material for the element 4 is crystallized alpha-silicon carbide, marketed under the brand name Hot Rod by Kanthai AB. This material has a porosity of 30% so that the opposite end regions can be enriched or filled with silicon to reduce the electrical resistance in the end regions. Alternatively, the strip-shaped heating element 4 may have a central region of dense recrystallized or reacted alpha-silicon carbide and end regions of lower electrical resistance of beta-silicon carbide that are reacted to the central region.

Electrical leads (not shown) are connected to the end portions 6, 7 of the heating element 4, for example by clamping or by using small fastening screws which extend through holes in the ends of the elements and are fastened with suitable nuts.
To assist the connection, the end regions 6, 7 of the heating element 4 can be provided with sleeves or metallic coatings with good electrical conductivity, for example those made of aluminum or an alloy such as Kovar.

(4627) St/kr, Ceramaspeed Ltd,

If desired, several heating elements 4 can be arranged in the heater, for example parallel and spaced apart from one another.

In the case of a heater with a diameter of 165 mm, the strip-shaped heating element 4 standing on one edge can, for example, have a strip height of about 6 mm and a strip thickness of about 1.2 mm and a central radiating part of about 140 mm in length. When energized with 240 volts, the heating element 4 has an output of about 1300 watts.

A temperature limiter 8 of known design is used, with its rod-shaped sensor extending over the heater. The temperature limiter 8 serves to interrupt the supply of voltage to the heating element 4 at a predetermined temperature in order to avoid thermal destruction of the glass ceramic cover plate during operation of the heater in a stove.

When the resulting heater is switched on in the oven, the electrical resistance of the heating element 4 drops very quickly, the element heats up and reaches a minimum value of electrical resistance at about 800-900 ⁰ C. If the temperature of the heating element continues to rise, the electrical resistance increases again and the electrical current decreases, thus avoiding overheating. A final operating temperature of about 1500 to 1550 ⁰ C is obtained.

From a visual point of view, the heater appears to the user as rapidly lighting up after switching on in about one second or possibly less with an attractive bright light and any operating behavior when the heating element is operated at a temperature of 1500 to 1550 ⁰ C.

Because the end regions of the heating element 5 have a lower electrical resistance than the central middle region, the end regions are operated at a lower temperature than the central region. This is desirable for the

St/kr, Ceramaspeed Ltd.

From the point of view of the assembly and service life of the element as well as due to the cooling behavior of the element when the heater is switched off.

The heat distribution of the heater across the glass ceramic cover plate can be optimized to suit requirements by using a heating element 4 with uneven thickness and/or height, for example as shown in Fig. 2 and 3. In Fig. 2, a heating element 4 is shown which has a greater height in the central region which is greater than that in the regions on either side of the central region. The central region tapers in height towards the adjacent parts on the sides. The central region therefore has a lower temperature during operation than the regions immediately adjacent to the central region on either side, so that the temperature in the edge region of a zone of the glass ceramic cover plate through the heater is higher.

An alternative is shown in Fig. 3. Instead of increasing the height of the element 4 in the central region, an equivalent effect can be achieved by increasing the thickness of the element 4 in the central region compared to the thickness of the regions on either side of the central region.

A combination of the heating elements 4 shown in Fig. 2 and 3 is also possible with a central region that has a greater thickness and greater height than the immediately adjacent regions on the sides.

Curved or bent shapes of the electrical resistance heating element may be advantageous in some cases to further optimize heat distribution. Such a curved heating element 40 is shown in Fig. 4 and finds particular application in an oval heater as shown in Fig. 5. The heater of Fig. 5 has a circular heating surface 9 and a part-circular heating surface 10 in an oval metal shell 100. The structure of the heater for heating zone 9 is essentially the same as the heater shown in Fig. 1 and the same reference numerals as in Fig. 1 are used to identify the components.

(4627) St/kr, Ceramaspeed Ltd.

The same design details also apply to the part-circular heating surface 10, with the exception that it is a part-circle or sickle shape, so that a curved heating element 40 is used for optimal heat distribution, as shown in Fig. 4. This is a modification of the heating element in Fig. 2.

The curved heating element 40 can also be designed as described in connection with Fig. 1.

The embodiment of the heater shown in Fig. 5 is a specific example having a circular heating surface 9 with a diameter of 165 mm with a strip-shaped heating element 4 made of silicon carbide as described with reference to Fig. 1. The strip-shaped heating element 4 has a height of about 6 mm and a thickness of about 1 mm and the radiating part has a length of about 140 mm. When supplied with a voltage of 240 volts, the heating element 4 has a power of about 1100 watts. The part-circular heating surface 10 shown in Fig. 5 is heated by a curved strip-shaped heating element 40 as shown in Fig. 4, but with a central region of greater height of about 6 mm than the immediately adjacent regions on each side which have a height of only 5 mm. The radiant length of element 40 is approximately 150 mm and the thickness approximately 1 mm. When a voltage of 240 volts is applied, the element has an output of approximately 40 watts. The shape and contour of element 40 results in very good heat distribution over the semi-circular heating area. Heating areas 9 and 10 can be operated independently of one another or together.

For some applications, it is advantageous to provide the heater according to the invention with one or more other shapes and types of heating elements in addition to the electrical resistance heating element 40. One such embodiment of a heater is shown in Fig. 6a and 6b. The heaters shown in Fig. 6a and 6b are essentially constructed in the same way as the heater shown in Fig. 1 and described in this context, with the exception that a further

(4627) St/kr, Ceramaspeed Ltd.

Heating element in the form of a ring-shaped halogen lamp is provided in the edge heating area of the heater. The lamp 11 is arranged so that it extends over the heating element 4. The lamp 11 can be operated independently of the heating element 4 or in series or parallel with the heating element 4. It is particularly advantageous to connect the lamp 11 in series with the strip-shaped heating element 4. The additional lamp 11 gives increased heating power in the edge area of the heater. The strip-shaped heating element 4 also acts as a ballast resistor to dampen the on-resistance of the infrared lamp when the heater is switched on in a cold state and a high inrush current is a result of the lamp 11 having a heating filament, such as tungsten, which has a substantially positive temperature coefficient of resistance. In the specific example of the heater shown in Fig. 6a, this has a diameter of 200 mm and gives an output of 1800 watts at 240 volts. The annular lamp 11 has an inner diameter of about 136 mm and produces 1350 watts at 160 volts. The lamp is connected in series with the silicon carbide strip heating element 4 as described with reference to Fig. 1. The strip heating element is about 2 mm thick and about 8 mm high with a radiating central region about 100 mm long and end regions of lower electrical resistance each about 60 mm long. The material composition and base layer for the heating element 4 are the same as described with reference to Fig. 1.

When switched on in a cold state, the electrical resistance of the series circuit of the combination of lamp 11 and heating element 4 is about 32 ohms, the current being about 7.4 amperes at a voltage of 240 volts. The inrush current that would otherwise occur in the lamp is therefore attenuated. In this state, element 4 has about 1643 watts and lamp 11 only about 131 watts. Under these conditions, heating element 4 heats up more quickly than lamp 11. As heating element 4 heats up, its resistance drops and the current increases from an initial level of about 7.4 amperes to a maximum of about 14 amperes. In this state and with a continuous increase in the electrical resistance

(4527) St/kr, Ceramaspeed Ltd

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level in the lamp 11, the temperature of the lamp increases rapidly and exceeds the temperature of the heating element 4. After a few seconds, the electrical resistance of the series circuit of the combination of element 4 and lamp 11 is at a value of about 30 ohms with a current of 8 amperes and a total power of about 1920 watts.

As the glass ceramic plate heats up, the temperature of the heating element 4 and the lamp 11 continues to increase. Although the resistance of the tungsten wire in the lamp 11 does not change significantly further, the resistance of the heating element now increases from about 6 ohms to about 8 ohms as a result of the positive temperature coefficient of electrical resistance at such a temperature. Therefore, in the final operating state, the series connection of the combination has a resistance of about 32 ohms and the heater has an output of 1800 watts.

During operation of the heater, the heat emitted by the infrared lamp is essentially more than 50% of the total heat emitted by the heater.

Such a heater satisfies the stringent start-up specifications of the competent authorities. For this reason, the element 4 is connected in series with one or more lamps 11 during operation in order to operate it with a minimum electrical resistance at about 900 ⁰ C.

Fig. 7 shows a further embodiment in which the heater is designed essentially as described with reference to Fig. 1, with the exception that there is a further additional heating element made of a spirally wound wire, which is arranged in the edge region of the heater on the base layer 2. The additional heating coil 12 can be operated independently of another heating element or can be connected in series or parallel with the heating element 4. Such an additional heating element 12 is advantageous in order to create additional power, especially in large-area heaters. Furthermore, it can be advantageous to connect further heating elements 12 in series with the heating element 4 in order to

(4527) St/kr, Ceraroaspeed Ltd.

to dampen the inrush current in element 4 at temperatures at which the element has a coefficient of electrical resistance with a negative sign.

In the specific example, the strip heating element 4 is made of silicon carbide as described in connection with Fig. 1. It has a height of about 10 mm and a thickness of about 1.2 mm. In a heater of 200 mm diameter, the radiating central region of the element 4 has a length of about 150 mm. The spiral wire heating element 12 is made of, for example, an iron-chromium-aluminium alloy and is connected in series with the element so that at a voltage of 240 volts the total power of the heater is 1800 watts. Of this, 1350 watts are provided by the heating element 4 and 450 watts by the wire heating element 12.

Instead of a spiral-shaped wire heating element 12, a band-shaped heating element with a corrugated band can also be used, for example made of an iron-chromium-aluminium alloy, which rests with its narrow edge on the base layer 2.

Instead of being arranged in the edge area of the heater, the element 12 can also be arranged on each side of the strip-shaped heating element 4.

One or more additional heating elements, such as a spiral wire heating element as shown in Fig. 7 or a lamp 11 as shown in Fig. 6a, may also be arranged in a heater as shown in Fig. 5 if required.

(4627) St/kr, Ceramaspeed Ltd·

19

LIST OF REFERENCE SYMBOLS

1 round or oval metal bowl

2 Base layer made of electrical and thermal insulating material

3 Outer edge

4 electrical resistance heating element in elongated form

5 Use

6 and 7 End areas of the resistance heating element

with lower electrical resistance than the middle area

8 Temperature limiter

9 circular heating surface

semicircular, sickle-shaped heating surface ring-shaped halogen lamp additional heating element curved heating element metal bowl

Claims (29)

(4627) St/krj Ceramaspeed Ltd. PRIVACY CLAIMS 1. Electric radiant heater for stoves with a glass ceramic cover plate with a bowl-shaped carrier made of a base layer (2) and an outer edge (3), each made of heat-insulating material, and with at least one electrical resistance heating element (4, 40) on the bowl-shaped carrier, characterized in that the at least one resistance heating element (4, 40) has a negative temperature coefficient of electrical resistance over a first temperature range from 0 ⁰ C to at least 700 ⁰ C and a positive temperature coefficient of electrical resistance in a second temperature range above the first temperature range. 2. Radiant heater according to claim 1,
characterized in that the shell-shaped carrier made of heat-insulating material is arranged in a metal shell (1). 3. Radiant heater according to claim 1,
characterized in that the base layer (2) is of shell-shaped microporous heat-insulating material. carrier from 4. Radiant heater according to claim 1,
characterized in that the first temperature range extends from 0 ⁰ C to 1000 ⁰ C. 5. Radiant heater according to claim 4,
characterized in that the first temperature range extends from 0 ⁰ C to 900 ⁰ C. 6. Radiant heater according to claim 5,
characterized in that the first temperature range extends from 0 ⁰ C to 800 ⁰ C. St/kr, Ceramaspeed Ltd» 7. Radiant heater according to one of the preceding claims, characterized in that the electrical resistance heating element (4, 40) is made of fireproof semiconducting material. 8. Radiant heater according to claim 7,
characterized in that the refractory semiconducting material is silicon carbide. 9. Radiant heater according to claim 8,
characterized in that the silicon carbide is at least partly alpha-silicon carbide. 10. Radiant heater according to one of claims 8 or 9, characterized in that the silicon carbide contains one or more dopants or additives in order to create the required behavior of the electrical resistance and/or the temperature coefficient of the electrical resistance. 11. Radiant heater according to one of the preceding claims, characterized in that the electrical resistance heating element (4, 40) is in the form of at least one elongated rod, wire or strip. 12. Radiant heater according to claim 11, characterized in that the elongated strip has a rectangular cross-section, the narrow edge of which is supported on the base layer (2) and the height of the strip is substantially greater than its thickness. 13. Radiant heater according to claim 12, characterized in that the edge of the resistance heating element (4, 40) is supported on the base layer (2) in that at least its opposite end regions (6, 7) are supported in the shell-shaped carrier. St/kr, Ceramaspeed Ltd. 14. Radiant heater according to claim 13,
characterized, that the central region located between the end regions of the resistance heating element or heating elements (4, 40) is at a distance from the base layer (2) made of heat-insulating material. 15. Radiant heater according to one of claims 13 or 14,
characterized, that the end regions (6, 7) of the resistance heating element (4, 40) are held in inserts (5) in the edge region (3) of the shell-shaped carrier. 16. Radiant heater according to claim 15,
characterized, that the inserts (5) contain ceramic components. 17. Radiant heater according to one of claims 12 to 16,
characterized, that the at least one strip-shaped resistance heating element (4, 40) has a substantially straight, bent or curved shape. 18. Radiant heater according to one of claims 12 to 17,
characterized, that the strip-shaped resistance heating element or elements (4, 40) crosses the shell-shaped carrier and has (have) a central region between the end regions (6, 7) whose height and/or thickness differs from the corresponding height and/or thickness in immediately adjacent regions on each side of the central region, so that the temperature distribution required during operation of the heater is obtained over the strip-shaped resistance heating element. 19. Radiant heater according to claim 18,
characterized, that the height and/or thickness of the resistance heating element (40) in its central region is greater than the corresponding height (4627) St/kr, Ceraraaspeed Ltd. · and/or thickness in the areas immediately on each side of the central region. 20. Radiant heater according to claim 19,
characterized, that the height and/or thickness of the at least one strip-shaped resistance heating element (4, 40) tapers from the central region to the adjacent regions. 21. Radiant heater according to one of the preceding claims,
characterized, that the at least one electrical resistance heating element (4, 40) has end regions (6, 7) whose electrical resistance is lower than the resistance of the remaining part of the heating element (4, 40). 22. Radiant heater according to claim 21,
characterized, that the at least one resistance heating element (4, 40) is made of silicon carbide and the end regions (6, 7) are made of beta-silicon carbide or silicon-enriched alpha-silicon carbide and the rest of the element is made of alpha-silicon carbide. 23. Radiant heater according to claim 21 or 22,
characterized, that each end region (6, 7) of the resistance heating element (4, 40) is provided with a sleeve or layer made of metal or a metal alloy with good electrical conductivity. 24. Radiant heater according to one of the preceding claims,
characterized, that at least one additional heating element (11, 12) with the at least one heating element (4, 40) different type or made of different material in the shell-shaped carrier. 25. Radiant heater according to claim 24,
characterized, that the at least one additional heating element (11, 12) comprises at least an infrared lamp or at least one spiral-shaped (4627) 5t/kr, Ceramaspeed Ltd. · · Resistance wire heating element (12) or at least one resistance heating element in the form of a corrugated band. 26. Radiant heater according to claim 24 or 25,
characterized, that contains a combination of at least one electrical resistance heating element (4, 40) with additional heating elements made of a different material or of a different type from the resistance element (4, 40). 27. Radiant heater according to claim 25,
characterized, that the infrared lamp(s) is (are) a halogen lamp (11). 28. Radiant heater according to claim 25,
characterized, that the spiral resistance wire heating element(s) (12) or the resistance heating element(s) in the form of a corrugated strip are made of iron-chromium-aluminium alloy. 29. Radiant heating element according to one of claims 24 to 28,
characterized, that the at least one additional heating element (11, 12) is connected in such a way that it can be supplied with energy independently of the at least one resistance heating element (4, 40) or is temporarily or permanently connected in series or parallel with the at least one resistance heating element (4, 40).