GB2331147A

GB2331147A - Gas burners

Info

Publication number: GB2331147A
Application number: GB9823494A
Authority: GB
Inventors: Dieter Thumfart; Micheal Kahlke
Original assignee: Electrovac AG
Current assignee: Electrovac AG
Priority date: 1997-11-07
Filing date: 1998-10-27
Publication date: 1999-05-12
Anticipated expiration: 2018-10-27
Also published as: GB2331147B; GB9823494D0

Abstract

A device for limiting the temperature of gas cooking hobs, the heating space (4) of which is delimited by an electrically conductive burner housing (1), comprises a temperature sensor (6), formed from a tube (7)(Fig.4) having an inner bar (8), and a switching head (9) which is connected to the said temperature sensor and which carries at least one contact (10) capable of being actuated by the latter, and an electrode (12) which runs in the heating space (4) and is fixed to the temperature sensor (6) and/or the switching head (9) and which can be connected to the first terminal of a current-measuring means (22), the second terminal of which is connected to the burner housing (1).

Description

Temperature limiter with sensor electrode The invention relates to a device for limiting the temperature of gas cooking hobs, the heating space of which is delimited by an electrically conductive burner housing, the said device comprising a temperature sensor, formed from a tube having an inner bar, and a switching head which is connected to the said temperature sensor and which carries at least one contact capable of being actuated by the latter.

Cooking hobs are often formed from a glassceramic plate (ceran plate) serving as standing surface for cooking utensils and having a heating space located under it. Such cooking hobs can be heated in various ways, for example electrical heating resistors, halogen lamps, gas or the like may be indicated. In every heating variant, overheating of the ceran hob may cause it to be destroyed, therefore such overheating must be avoided.

The devices described in the introduction are normally used for this purpose. When their temperature sensor detects a temperature which is too high for the ceran hob, it actuates the contacts arranged in the switching head, and this actuation causes the gas supply to be reduced or stopped.

When the cooking hob is heated by gas, it is necessary to provide, in addition to the temperature limiter mentioned, a means by which it is possible to check whether ignition of the gas has taken place within a specific time after the opening of the gas supply line.

Insofar as it has not been possible for the gas to be ignited within the predetermined time, the gas supply must be interrupted. The user of the cooking hob is advised of unsuccessful ignition by corresponding signal means, and, if he wishes to continue to operate the cooking hob, he must make a new attempt at ignition.

The ignition checking means has hitherto been formed by an electrode which extends into the combustion space, is held at a distance from the burner housing and is electrically insulated from the burner housing in the region where it passes through the latter and which is connected to the burner housing, outside the heating space, via a current-measuring means.

A gas flame ionizes the atmosphere of the heating space, thus generating a potential difference between the electrode and the burner housing, the said potential difference driving a slight compensating current (approximately in the range of 5 to 10 4A) across the currentmeasuring means. If such a current can be measured, ignition is evaluated as being successful and the gas supply can be maintained. If, on the other hand, no current is detected, the attempt at ignition has remained unsuccessful, and the gas supply must be interrupted and a corresponding signal transmitted.

The electrode of this checking means has hitherto taken the form of a component designed separately from the temperature limiter. There was therefore the need to pierce the wall of the heating space, that is to say the burner housing, for the temperature limiter, on the one hand, and for the electrode, on the other hand; furthermore, when the cooking hob is being assembled, the two separate subassemblies have to be installed separately.

In conclusion, this type of design is relatively complicated and time-consuming.

The object of the invention, therefore, is to develop a device of the type mentioned in the introduction, in such a way as to avoid the disadvantages mentioned and to ensure that the temperature limiter and the electrode can be mounted in a particularly simple manner.

This is achieved, according to the invention, in that there is provided an electrode which runs, at least in sections, in the heating space and is fixed to the temperature sensor and/or switching head and which can be connected to the first terminal of a current-measuring means, the second terminal of which is connected to the burner housing.

Since the electrode is connected to the temperature limiter, it can be mounted, together with the latter, in a common perforation of the burner housing, and a separate perforation for the electrode of the ignition checking means, the said perforation being provided in the prior art, can therefore be dispensed with.

According to a particularly preferred embodiment of the invention, there can be provision for fixing the electrode to a hollow-cylindrical insulator which, inside it, partially receives the expansion bar, is supported at its first end on the switching head and, at its second end, supports the temperature sensor tube.

The geometric dimensions of the limiting device are therefore scarcely changed at all, but particularly good electrical insulation of the electrode from the temperature sensor bar, usually manufactured from electrically conductive material, is nevertheless achieved.

It proved advantageous to design the electrode as a hollow cylinder, because it can thereby be arranged so as to surround the bar or upper insulator and, in this case, needs to be secured merely against displacements along the longitudinal axes of the bar or insulator.

Furthermore, in this embodiment, the electrode, despite having a relatively short longitudinal extent, has a surface necessary for generating a sufficiently high ionization current.

Another feature of the invention can be that the electrode is enclosed, in particular regions, by a preferably hollow-cylindrical insulator.

This ensures sufficient insulation relative to the burner housing.

According to a particularly preferred embodiment of the invent ion, there can be provision for the electrode to be formed by the bar of the temperature sensor.

Consequently, there is no need for any additional component at all to be fixed to the temperature sensor or to the switching head. It is merely necessary, within the switching head, where one end of the bar is accessible, to provide the latter with an electrical lead for the current-measuring means.

According to a particularly preferred development of the invent ion, there can be provision for the electrode to be capable of being connected to the first terminal of a high-voltage source, the second terminal of which is connected to the burner housing.

Consequently, the electrode, provided per se as a sensor electrode of a means for monitoring correct ignition, can perform the function of gas ignition in cooperation with the burner housing, so that the functions of all three means necessary for operating a gas cooking hob, namely temperature limitation, ignition and ignition monitoring, are carried out by a single subassembly. It is particularly preferred, in this case, to use the bar of the temperature sensor as an electrode.

Furthermore, in this respect, there can be provision for fixing to the burner housing a metallic tongue, of which the distance from the electrode is less than the shortest distance of the burner housing itself or than the shortest distances of all the metal parts connected to the burner housing from the electrode.

By means of such a tongue, the gas path to be covered by the high voltage can be defined in terms of position and direction and consequently adapted to achieve a good ignition result.

The invention is described in more detail below with reference to the preferred embodiments illustrated in the accompanying drawings in which: Figure 1 shows a plan view of a cooking hob equipped with a device according to the invention; Figures 2a,b show sections along the line I-I through a cooking hob according to Figure 1, in each case with different possibilities for gas distribution; Figure 3 shows a plan view of a first embodiment of the invention; Figure 4 shows a second embodiment of the device according to the invention in section; Figure 5 shows a third embodiment of the invention in the illustration according to Figures 2a,b, and Figure 6 shows the embodiment according to Figure 5 in the same illustration, supplemented by electrode circuitry which makes it possible for the electrode to be used as both an ignition and a sensor electrode.

As is evident in Figures 1 and 2, a cooking hob heated by gas has an electrically conductive burner housing 1 and a plate 2 made of metal, glass-ceramic (ceran) or the like, the surface 3 of which forms the cooking surface. Arranged in the heating space 4 formed between the plate 2 and the burner housing 1 are nozzles 5 for supplying fuel gas and for distributing the latter to the entire heating space 4. As illustrated in Figures 2b, 5 and 6, the fuel gas may also be supplied via a single feed line 13 and distributed via a distribution means which is fixed to the inner wall of the burner housing 1.

The distribution means may be formed, for example, from a plate 14 (Figure 2b) provided with small bores 14' or from a nonwoven 18 which is held by a supporting plate 19 provided with large perforations 20 (Figure 5).

Furthermore, a device for limiting the temperature is provided within the heating space 4. As illustrated in detail in Figure 4, this device comprises a temperature sensor 6 which is formed from a tube 7 having an inner bar 8. At the end facing away from the switching head 9, the bar 8 is designed to project beyond the tube 7 and is provided with a stop 17, such as a screwed-on nut, welded-on sleeve or the like.

At the end on the switching-head side, the bar 8 likewise projects beyond the tube 7 and is prestressed in the direction of the switching head 9 by means of a helical spring 48. The bar 8 thereby comes to bear with the stop 17 on the tube 7.

The bar 8 is made from a material, the coefficient of thermal expansion of which is higher than that of the tube material, so that, as a result of the longitudinal expansion of the bar 8 in the event of an increase in temperature, a contact 10 is actuated via a switching button 49 and an electrically insulating actuating part. As a result of the actuation of this contact 10, when a predeterminable maximum temperature is reached their heating capacity is reduced, in the case of the cooking plate heated by gas the throttling of the gas supply is induced.

In addition to a device of this type for temperature limitation, it is necessary, for operating a cooking hob heated by gas, to have a means by which it is possible to check whether ignition of the gas has taken place within a specific time after the opening of the gas supply line. As already mentioned in the introduction, a means of this type comprises an electrode 12 which is arranged, at least in sections, in the heating space 4 and which can be connected to the burner housing 1, outside the latter, via a current-measuring means 22.

According to the invent ion, this electrode 12 is fixed to the device for temperature limitation, more precisely to its temperature sensor 6 and/or to its switching head 9. The electrode 12 can be connected to the first terminal of a current-measuring means 22, this being symbolized in the drawings by the switch 16, and the burner housing 1 is connected to the second terminal of this current-measuring means 22 (earth).

In the simplest case illustrated in Figures 2a,b, the electrode 12 is of strip-like design and is fixed to the tube 7 of the temperature sensor 6. In order to ensure that the potential difference between the electrode 12 and burner housing 1, the said potential difference being caused by the ionization of the heating-space atmosphere when the gas is burning, is not short-circuited in the region where the electrode 12 passes through the burner housing 1, the electrode 12 is surrounded, at least in this passage region, by an insulator 35 (see Figure 2a).

In addition to being fixed to the temperature sensor 6, the electrode 12 may also be fixed to the switching head 9, as illustrated in Figure 3. The disadvantage of this solution, however, is that either two separate perforations for the temperature sensor 6 and the electrode 12 or one common, but correspondingly large perforation has to be provided in the burner housing 1.

Figure 4 shows a particularly preferred design/fixing of the electrode 12. Here, a hollow-cylindrical insulator 36 is provided, which, inside it, partially receives the bar 8, is supported at its first end on the switching head 9 via the supporting pedestal 52 and, at its second end, supports the temperature sensor tube 7. The electrode 12 is fixed to the outer cylindrical surface of this insulator 36.

The electrode 12 can be strip-like, as above, but is preferably designed as a hollow cylinder, and this hollow-cylindrical electrode design may also be provided when the electrode 12 is fixed to the bar 8.

The electrode thereby has a relatively large surface, whilst at the same time having a short longitudinal extent, thus leading to the generation of a sufficiently high potential difference between it and the burner housing 1. Furthermore, in the case of a hollowcylindrical design, particularly simple fixing to the insulator 36 or to the tube 7 is possible: the inside diameter of the electrode 12 is greater than the outside diameter of the tube 7 or insulator 36 to such a small extent that a firm fit is obtained between the electrode 12 and the tube 7 or insulator 36.

According to the embodiment shown in Figure 5, the bar 8 of the temperature sensor 6 is used as the electrode 12. For this purpose, the temperature sensor 6 should not extend over the entire diameter of the heating space 4 and be located with its end facing away from the switching head 9 outside the burner housing 1, as was the case in the embodiments described hitherto, but, instead, must terminate within the heating space 4. As already described with regard to Figure 4, that end of the bar 8 which faces away from the switching head 9 is designed to project beyond the tube 7 and is therefore located, uninsulated, in the heating space 4. In order to obtain a sufficiently high potential difference between the bar 8 and burner housing 1, the said potential difference resulting in a compensating current value capable of being detected by the current-measuring means 22, the bar 8 is designed to be greatly lengthened beyond the stop 17.

If the bar length necessary for achieving a sufficiently high potential difference between the bar 8 and burner housing 1 were to cause the bar tip to come to bear on the burner housing 1, in order to prevent this the bar section located, uninsulated, in the heating space 4 may be deformed, for example, bent along a meander-like path or, as illustrated in Figure 6, simply folded through 180 .

In addition to a device for temperature limitation and a means for checking the ignition of the gas, cooking hobs heated by gas must have a means for igniting the fuel gas. According to the known prior art, such a means is formed, for example, by a resistor capable of being connected to a voltage source and therefore of being heated to a surface temperature sufficient for gas ignition or by a spark plug comprising two electrodes which form a narrow point between them and can be connected to a high-voltage source and which consequently generate arcs between them.

Both embodiments have hitherto taken the form of subassemblies separate from the temperature limiter, they therefore had to have their own perforations in the burner housing and, during the assembly of the cooking hob, they had to be inserted into this perforation separately from the other subassemblies, namely the temperature limiter and the electrode 12.

According to the invention, there is provision for the electrode 12 to be capable of being connected to the first terminal of a high-voltage source 15, the second terminal of which is connected to the burner housing 1. The electrode 12 therefore not only performs the hitherto discussed function of the electrode of a means for checking gas ignition, but also the gas ignition function. Specifically, the electrode 12 and burner housing 1 act as a spark gap, in which arcs suitable for igniting the gas located in the heating space 4 are formed when the electrode 12 is connected to the highvoltage source 15.

The arcs are always formed between the electrode 12 and the metal part which is at the shortest distance from it and which is connected to the burner housing 1.

This metal part will be the gas distribution means, that is to say the plate 14 or nonwoven 18, insofar as these parts are manufactured from electrically conductive material. If there is no gas distribution means (Figure 2a) or if the gas distribution means present is manufactured from electrically insulating material, the arcs are formed directly between the electrode 12 and the burner housing 1 by running through a perforation of the gas distribution means or by puncturing the latter.

However, as illustrated explicitly in Figure 5, in all the embodiments, a metallic tongue 21 fixed to the burner housing 1 may be provided as the metal part located nearest to the electrode 12, the distance of the said tongue from the electrode 12 being less than the shortest distance of the burner housing 1 itself or than the shortest distances of all the metal parts connected to the burner housing 1 from the electrode 12.

If the electrode 12 is used both as a sensor electrode and as an ignition electrode, it can be present in any of the embodiments described, but it is necessary to ensure that the electrode 12 is sufficiently insulated, in the passage region, from the remaining metallic components of the device for temperature limitation and from the metallic burner housing 1, in order to prevent the high voltage from passing through to these metallic components.

The design of the electrode 12, as illustrated in Figures 2a,b, is therefore possible only when, on the one hand, the tube 7 provides sufficient electrical insulation for the bar 8 and when, on the other hand, the insulator 35 surrounding the electrode 12 in the region where the latter passes through the burner housing 1 provides sufficient insulation from the burner housing 1.

As illustrated in Figure 2b, in this case the insulator 35 may also be lengthened beyond this passage region, with the result that spark formation is correspondingly displaced in the direction of the centre of the heating space.

The requirements of sufficient insulation of the electrode 12 from the bar 8 and burner housing 1 in the passage region are satisfied, in particular, by the type of design/arrangement of the electrode 12, as illustrated in Figure 4: The hollow-cylindrical insulator 36, which, inside it, partially receives the bar 8, is supported at its first end on the switching head 9 via the supporting pedestal 52 and, at its second end, supports the temperature sensor tube 7, provides sufficient insulation for the bar 8. The electrode 12 is fixed to the outer cylindrical surface of this insulator 36, and there is provided, surrounding the electrode 12, an insulator 35 which, when the electrode 12 is designed as a hollow cylinder, is likewise hollow-cylindrical and ensures insulation from the burner housing 1. The lead 41 to the electrode 12, the said lead being led through the insulator 35 and being capable of being connected to the highvoltage source 15, is likewise appropriately insulated via insulating bodies 46, 47, in order to avoid arcovers.

It is particularly preferred, if the electrode 12 has the double use just explained, for the latter to be formed by the bar 8. The high voltage introduced via that end of the bar 8 which is located on the switching-head side consequently results in arcs formed between the bar end and the nearest metal part connected to the burner housing 1. As already explained further above, in Figure 5 this metal part is a metal tongue 21 integrally formed on the burner housing 1. In this embodiment, it is necessary to ensure, in particular, that sufficient insulation preventing arc-overs from the bar 8 to the contacts 10 is provided between that end of the bar 8 located on the switching-head side and the switching contacts 10.

Finally, Figure 6 illustrates the control of an electrode 12 which performs both an ignition and a sensor function and which is formed by the bar 8.

When the cooking hob is put into operation, a control circuit 23 must first connect the bar 8 to the high-voltage source 15 and, immediately after an arc or a predeterminable number of arcs have been generated, must connect the bar 8 to the current-measuring means 22.

The current-measuring means 22 can act on a valve arranged in the gas supply line, in order to stop the gas supply if an attempt at ignition has been unsuccessful.

As already mentioned, the double use of the electrode 12 is also possible in all other types of electrode design, and, for this purpose, the control circuit 23 illustrated in Figure 6 and just described would also have to be provided there.

It will, of course, be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

Claims

PATENT CLAIMS 1. Device for limiting the temperature of gas cooking hobs, the heating space of which is delimited by an electrically conductive burner housing the said device comprising a temperature sensor formed from a tube having an inner bar and a switching head which is connected to the said temperature sensor and which carries at least one contact capable of being actuated by the latter, characterized by an electrode which runs, at least in sections, in the heating space and is fixed to the temperature sensor and/or the switching head and which can be connected to the first terminal of a current-measuring means the second terminal of which is connected to the burner housing.
2. Device according to Claim characterized in that the electrode is fixed to a hollow-cylindrical insulator which, inside it, partially receives the expansion bar is supported at its first end on the switching head and, at its second end, supports the temperature sensor tube.
3. Device according to Claim 1 or 2, characterized in that the electrode is designed as a hollow cylinder.
4. Device according to Claim l, 2 or 3, characterized in that the electrode is enclosed, in particular regions, by a preferably hollow-cylindrical insulator (35).
5. Device according to Claim 1, characterized in that the electrode is formed by the bar of the temperature sensor.
6. Device according to one of Claims 1 to 5, characterized in that the electrode can be connected to the first terminal of a high-voltage source (15), the second terminal of which is connected to the burner housing.
7. Device according to Claim 6, characterized in that a metallic tongue is fixed to the burner housing the distance of the said tongue from the electrode being less than the shortest distance of the burner housing itself or than the shortest distances of all the metal parts connected to the burner housing (1) from the electrode.
8. Device for limiting the temperature of gas cooking hobs, substantially as hereinbefore described with reference to the accompanying drawings.