GB2249244A

GB2249244A - A heating wire in matrix tunnel allows thermal expansion and contraction

Info

Publication number: GB2249244A
Application number: GB9121227A
Authority: GB
Inventors: Josef Hecht
Original assignee: AKO Werke GmbH and Co KG
Current assignee: AKO Werke GmbH and Co KG
Priority date: 1990-10-04
Filing date: 1991-10-04
Publication date: 1992-04-29
Anticipated expiration: 2011-10-04
Also published as: GB9121227D0; DE4031343A1; DE4031343C2; GB2249244B

Abstract

In a process for affixing a heating wire 2 to an insulator support 1, particularly for a glass-ceramic hot plate, the heating wire 2 is cemented to the insulator support 1. The thermal expansion of the heating element must not be impeded, and therefore the heating wire 2 is provided at least at separate points 6 with a temporary coating 12 e.g. of WAX or STARCH prior to the cementing process. The coating 12 is then disintegrated e.g. by ultrasonic or thermal treatment, leading to the formation of tunnels 7, so that the heating wire 2 is not constricted by the cement during operation of the heating element. <IMAGE>

Description

1 1 22,49244 - 1 A PROCESS FOR AFFIXING A HEATING WIRE TO AN INSULATOR

SUPPORT AND A HEATING ELEMENT, PARTICULARLY FOR A GLASS-CERAMIC HOT PLATE This invention relates to a process for affixing a heating wire to an insulator support, and to a heating element containing such a wire, particularly for a glass-ceramic hot plate, wherein the heating wire is cemented to the insulator support. The invention also relates to a heating element in which a heating wire laid in coils is cemented at separate points to an insulator support.

Systems are known in which the heating wire of heating elements for the radiation heating of stoves is cemented onto a shellshaped insulator support at separate points. For this purpose temperature-resi sting ceramic cement is used. It is true that cement of this type is a means of firmly affixing the heating wire to the insulator support. It nevertheless almost completely prevents the wire from moving to adapt itself to the inevitable thermal expansions. It has been f ound that in practice the heating wire always burns through first at the points where it is cemented.

The known cementing process has proved particularly disadvantageous to the durability of the heating wire when the latter consists of molybdenum disilicide and is designed as a "bright radiator".

D.E. 36 40 999 Al proposes holding devices for a heating coil which are to be fitted before the latter is mounted on the 1 - 2 insulator support. The heating wire hooks onto the holding devices. This method does not include a cementing operation.

In the D.E. 27 49 941 Cl the heating wire is secured to the insulator support by means of bars extending over the latter. These bars must be capable of resisting the operating temperature of the heating wire. In the case of "bright radiators" made of molybdenum disilicide, for example, this temperature is about 1300 C. The bars are accordingly expensive.

D.E. 38 12 490 Al describes a radiant heating element with a "bright radiator" and a "dark radiator". The bright radiator is affixed to a separate insulating part by means of clamps. The dark radiator is secured to the insulator carrier by being partly embedded therein. No cementing operation is involved. This system also suffers from the drawback that the heating wires of the bright radiator and dark radiator have very little freedom of movement to enable them to take up thermal expansions.

The object of the invention is to affix the heating wire to the insulator, in the case of a process or a heating element of the type mentioned above,, in such a manner that its thermal expansion will only be impeded by the cement to the minimum extent, the cement nevertheless firmly securing --the heating wire to the insulator support.

According to one aspect of the invention there is provided a process for affixing a heating wire to an insulator support by 0 cementing the wire to the support, wherein prior to the cementing operation the heating wire is provided with a temporary coating, at least at certain separate points thereof at which cement is to be applied.

The coating of the heating wire ensures that the cement, when applied, does not lie directly on the heating wire. The coating thus forms a spacing between the cross section of the resistance heating wire and the cement and then the temporary coating is removed. This result can be obtained by thermal treatment, i.e. by melting, evaporating or burning. The disintegration of the coating can be brought about by means of a separate temperature source or simply by connecting up the heating wire itself to a power source. The coating is preferably subjected to the effect of temperature after the cement has set. The dissolution of the coating may also be affected by chemical, ultrasonic or other means.

The embedding of the heating wire in the cement without direct contact, as a result of the coating, has the additional advantage that any contamination of the cement will not lead to undesirable thermo-chemical reactions in the wire. It also ensures that the formation of a layer of oxide on the surface of the wire, an essential requisite for durability, will not be impeded.

In a preferred embodiment of the invention the coating is applied to the heating wire in a viscous state and allowed to set before the latter is connected to the insulator support. A wax or 4 - starch paste is. for example, suitable for this purpose.

The coating can be applied directly to certain individual points. It can equally well be applied by immersing the heating wire in a bath of the material used for the coating.

The coating can be applied before the heating wire is placed on the insulator support. Its application can nevertheless be deferred until after this has been done.

According to another aspect of the invention there is provided a heating element comprising a heating wire laid in windings on an insulator carrier and cemented to the latter, wherein the cement is spaced from the surface of the heating wire to form tunnels, through which the heating wire extends.

In the tunnels the heating wire is f reely movable in the longitudinal direction. The diameter of the heating wire can also increase with temperature in the tunnels without hindrance. This ensures that the cement will present practically no obstacle to the thermal expansions of the heating wire. In the event of thermal expansions the heating wire will hardly scrape against the cement at all, so that damage to its surface is avoided. The service life of the wire is thereby considerably lengthened. The wire is firmly held by the tunnels transversely to its length. Drop tests have shown that even if the wire consists of molybdenum disilicide, which in itself is brittle, fractures are avoided. In addition, the tunnels maintain the necessary safety spacings of the heating wire.

Further advantageous embodiments of the invention will become apparent from the subsidiary claims and from the description given below.

The drawings are as follows:

Fig.1: A partial view of a heating element for a glassceramic hot plate.

Fig.2: A section along the line II-II of Fig.l.

Fig.3: A section along the line III-III of Fig.l.

Fig.4: A partial view, in perspective, in the zone IV of Fig.1, and Fig.5: A view similar to Fig.l.

A heating wire 2 is placed on an electrically and thermally nonconductive insulator support 1. This latter is laid in meandering coils 3. Its ends 4 are secured to a ceramic body 5 affixed to the insulator support 1. The first heating wire 2 consists of molybdenum disilicide and operates as a bright radiator. It is held at individual points 6 (cementing points) on the insulator support 1 by means of cement.

Each cementing point 6 forms a tunnel 7 (see Fig.3). The cementing points 6 are provided in the zone of the longitudinal sections 8 of the coils 3. In the tunnels 7 the longitudinal sections 8 are f reely movable in the directions a, b (see Fig. 3) 1 so that the cementing points 6 do not impede thermal expansions - 6 of the heating wire 2. In the radial direction, r, the tunnels 7 limit the movement of the heating wire 2, so that the latter cannot lift away from the insulator support 1. The tunnels 7 enable thermal expansion'of the diameter of the heating wire 2 to take place in the radial direction, r.

In the example shown in Fig. 2 the insulator support 1 has a recess 9 to accommodate the longitudinal section 8 of the heating wire 2. Additional adjacent recesses 10, into which the cement 11 can enter, are provided at the cementing points 6.

Fig. 2 shows the coating 12 which is further described below, which surrounds the cross section of the heating wire 2 and which, after its dissolution, forms the tunnel 7. In Fig.3 the coating 12 has been dissolved and the tunnel 7 formed. Furthermore, the longitudinal section 8 in not positioned in a cavity of the insulator support 1 but is positioned at a certain distance, c, above this latter, one zone 13 of the cement 11 being situated between the longitudinal section 8 and the insulator support 1.

The operation of affixing the first heating wire 2 to the insulator support is effected approximately as follows:

After the heating wire 2 has been pre-shaped to form the coil 3 and affixed to the ceramic body 5, it is provided with a coating 12. The coating 12 is applied in a thickness corresponding to the required tunnel 7. The coating 12 may consist, for example, of a wax or starch paste. It is applied in the viscous state and J.

then allowed to set. This is effected by applying the coating material only in the region of the individual places. It can equally well be carried out, however, by immersing the complete pre-shaped heating wire 2 into a bath of the coating material.

After the coating has set the pre-shaped heating wire 2 is placed on the insulator support 1. It is either laid in the cavities or secured provisionally at a distance, c, above the surface of the said support 1.

Cement 1 is then applied to the individual places 6, the cement 11 bonding with the insulator support 1 and hardening. The cement 11 does not come in direct contact with the surface of the heating wire 2. The coating 12 provides a separation between the wire 2 and the cement 11.

The cement 11 need not be applied to the individual points 6 separately. It can equally well be applied in the form of strips of cement 14 to the heating wire 2 placed on the insulator support 1, as illustrated in Fig.5.

As soon as the cement 11 is sufficiently stable in shape the coating 12 is subjected to a temperature at which it melts, evaporates or burns. The coating 12 thus disintegrates, leading to the formation of the tunnels 7. The temperature can be caused to take effect by connecting up the heating wire 2, causing the coating 12 to disintegrate. The system may be so arranged that the coating will not disintegrate until the first time the heating element is put into operation.

Alternatively, the pre-shaped heating wire 2 can be placed on the insulator carrier 1 before the coating 12 is applied. The coating 12, is then at least applied at the separate points 6, where it covers the heating wire 2 before the cement 11 is applied to the insulator support 1 to extend over the coating 12.

A channel 15 is formed on the insulator support 1 in the vicinity of the f irst heating wire 2. A second heating wire 16, preshaped in the f orm of a coil (see Fig. 1 and Fig. 4) is laid in the channel. This heating wire 16 is designed as a "dark radiator".

The heating wire 16 likewise is affixed in the channel 15 in the manner described above. For this purpose the coiled heating wire 16 is wholly or partly provided with the coating 12 bef ore it is introduced into the channel 15.

Cement 11 is applied at individual points 6 in the channel 15 before the insertion of the second heating wire 16. The said heating wire 16, provided with the coating 12, is then pressed into the cement 11 which has not yet set. The cement 11 extends around the cross section of the heating wire 16 and hardens, so that the heating wire 16 is indirectly cemented to the insulator carrier 1.

The coating 12 in this example is then subject to a temperature at which it disintegrates. This leads to the formation of the tunnels 7. The latter enable thermal expansions to occur in the heating wire 16 and prevent the heating wire 16 from detaching itself from the insulator support 1.

In other versions of the invention the coating 12 can be designed to disintegrate from its solidified state by chemical reaction or by ultrasonic means.

The invention can also be applied if the heating wire, taking the form of a heating spiral or heating coil of particularly thin wire, is pressed into a cement bead extending over its entire length.

1 - 10

Claims

1 A process f or af f ixing a heating wire to an insulator support, by cementing the wire to the support, wherein prior to the cementing operation the heating wire is provided with a temporary coating, at least at certain separate points thereof, at which cement is to be applied.

2. A process in accordance with Claim 1, wherein the coating extends around the cross section of the heating wire.

3. A process in accordance with Claim 1 or 2, wherein the coating consists of a material which melts, evaporates or burns, at a temperature not exceeding the operating temperature of the heating wire.

4. A process in accordance with claim 1 or 2, wherein the coating consists of a chemically decomposable material.

5. A process in accordance with Claim 1 or 2, wherein the coating consists of a material that is decomposable by ultrasonic means.

6. A process in accordance with any one of the foregoing claims, wherein the coating is applied to the heating wire in the viscous state and is then allowed to harden before the heating wire is cemented to the insulator supports.

n - 11

7. A process in accordance with Claim 6, wherein the coating is applied at individual points to the heating wire which is preformed in windings.

8. A process in accordance with Claim 6 wherein the heating wire is immersed in a bath of the coating material and is then placed on the insulator support.

9. A process in accordance with any one of the foregoing claims, wherein the heating wire provided with the coating is placed on the insulator support, after which the cement is applied.

10. A process in accordance with any one of Claims 1-8, wherein the heating wire provided with the coating is pressed into cement applied to the insulator support.

11. A process in accordance with any one of Claims 1-7 wherein the heating wire is placed on the insulator carrier, after which the coating is applied.

12. A process in accordance with any one of the foregoing claims wherein the coating consists of wax or a starch paste.

13. A process in accordance with any one of the foregoing claims, wherein the temperature causing the disintegration of the coating is applied when the cement has hardened or dried or has set in the humid state.

14. A process in accordance with any one of the foregoing claims, wherein the heating wire is pre-shaped in the f orm of windings prior to the application of the coating.

15. A heating element, comprising a heating wire laid in windings on an insulator carrier and cemented to the latter, wherein the cement is spaced from the surface of the heating wire to form tunnels, through which the heating wire extends.

16. A heating element in accordance with Claim 15, wherein the cement is applied at individual places on longitudinal sections of the windings of the heating wire.

17. A heating element in accordance with any one of the foregoing claims, wherein the cement is in the f orm of strips of cement laid over a number of sections of individual windings of the heating wire.

18. A heating element in accordance with any one of the foregoing claims, wherein recesses are provided for the heating wire on the insulating support.

19. A heating element in accordance with any one of the foregoing claims, wherein recesses are provided for the cement at individual points on the insulating support.

20. A heating element in accordance with any one of the foregoing claims, wherein the cement serves to space the heating 1 1 1 wire from the insulator support.

21. A process as claimed in Claim 1, substantially as described herein.

22. A heating element substantially as described herein with reference to the accompanying drawings.

23. A glass-ceramic hot plate incorporating a heating element made by the process of any one of Claims 1-14 or 21, or according to any one of Claims 15-20 or 22.