GB2316847A

GB2316847A - Support for electric heatering element

Info

Publication number: GB2316847A
Application number: GB9617812A
Authority: GB
Inventors: Keith Barrie Doyle
Original assignee: Strix Ltd
Current assignee: Strix Ltd
Priority date: 1996-08-27
Filing date: 1996-08-27
Publication date: 1998-03-04
Anticipated expiration: 2016-08-27
Also published as: GB9617812D0; GB2316847B

Abstract

There is disclosed a heater 2 comprising a support plate 4 having a generally planar portion 12. An electrically insulating layer 16 is formed on the planar portion 12 of the support plate 4 and a resistive heating track 18 is provided on the insulating layer 16. The support plate 4 is formed with a region 14 which is raised in the direction opposite to the side of the plate 4 supporting the heater track 18. The raised region 14 has, or is adapted to receive, heating means 22 therein.

Description

Electric Heaters The present invention relates to electric heaters and in particular to heaters of the type comprising a resistive heating track laid down on an electrically insulating substrate. In particular, the invention relates to so-called thick-film heaters, in which a thick film heating track is applied, most commonly screen printed, onto a glass, ceramic or glass ceramic (hereinafter collectively referred to as glass) layer typically provided on a metallic support surface.

It has been proposed, for example in WO 96/18331, to use such heaters in liquid heating vessels. In such applications, the heater is provided on the underside of the base of a metal vessel, or on a metal plate which can be mounted in the base of, for example, a plastics vessel. The heater is provided with a thermallysensitive control which operates to interrupt or reduce the electrical supply to the heater in the event that the heater overheats, as would happen, for example, if the vessel was switched on without any liquid in it, or if it boiled dry. Typical controls comprise a thermally sensitive actuator such as bimetallic actuator arranged in good thermal contact with the heater.

It has been proposed to mount a flat heater at a slope in the base of the vessel, so that a peripheral edge of the heater plate opposite to a pouring spout becomes exposed first as liquid in the vessel boils away. Without overlying liquid to conduct heat away from it, the temperature of the exposed portion of the plate will rise above that of the rest of the plate.

Accordingly, the control is arranged in thermal contact with that part of the heater track such that it will operate when that part overheats.

The present invention proposes an alternative to the above which obviates the need to mount a plate at an angle in a vessel base. From a first aspect the invention provides a heater comprising a support plate, an electrically insulating layer formed on the support plate and a resistive heating track provided on the insulating layer, characterised in that the support plate is formed with a portion which is raised in the direction opposite to the side of the plate supporting the heater track, said raised region having, or being adapted to receive, heating means therein.

The raised region will be exposed before the rest of the heater plate as the level of liquid in a liquid heating vessel falls as would happen if the vessel boils dry in the event of the failure of a boiling control or the lid of the vessel being left off. As this region is heated, its temperature will begin to rise above that of the surrounding plate, and a thermally sensitive control can then be arranged in good thermal contact with the raised area to operate before the local temperature rises above a predetermined temperature.

The raised region may conveniently be formed in the plate by pressing.

The raised region may be formed in any desired position on the plate, but preferably is arranged generally centrally of the plate.

The support plate may be generally planar, with the raised region upstanding from the plane of the plate.

However, other arrangements may be envisaged. For example, the plate may be domed, with the raised region being arranged at the top of the dome. Alternatively, the plate may be generally frustoconical, with the raised region being formed at the apex of the plate.

The heating means for the raised region may take a number of forms. In a first embodiment, the insulating layer and the heating track may extend into the raised region. Such an arrangement is facilitated by the method disclosed in our co-pending application PCT/GB96/00182 in which a flexible green ceramic tape with a printed element is applied to the support plate and then fired. Such a process overcomes the problems of screen printing insulating layers and heater tracks onto non-planar surfaces.

Alternatively, a separate heater, such as a metal foil heater may be provided within the raised region, with suitable insulation, for example a plastics film, arranged between the plate and the heater.

Preferably the heater is arranged electrically in series with the heater track, and conveniently it can be arranged to extend between two portions of the heater track. Electrical connection to the track can be made, for example via spring contacts attached to the auxiliary heater which can resiliently engage the heater track.

The heater can be mounted within the raised region by an adhesive or other bonding material capable of withstanding the anticipated operating temperatures.

The adhesive could be provided on the insulating film arranged between the heater and the plate.

As stated above, in use a thermally sensitive control will be mounted in good thermal contact with the heater of the raised region. Most conveniently, this may comprise a bimetallic or equivalent actuator, suitably coupled to a set of switch contacts arranged in the electrical supply to the heater track. If necessary, an electrical insulating film may be provided between the actuator and the heater.

It will be appreciated that the invention extends to the combination of a heater in accordance with the invention and a thermally sensitive control arranged in good thermal contact with the heating means within the raised region, and to a liquid heating vessel comprising a heater or combination in accordance with the invention, the heater being mounted in, or providing a part of the base of the vessel.

It will also be appreciated that the invention may be used for other or additional purposes to those described above. For example, instead of, or in addition to, acting to disconnect the electrical supply to the heater track, it could provide a visual or audible indication that, for example, the liquid level has fallen below a certain level and that some action is required on the part of the user. Furthermore more than one raised region may be provided around the plate, and / or a number of regions of differing depths may be provided to provide different indications or to perform certain functions when different water levels are reached.

Some preferred embodiments of the invention will now be described by way of example only, with reference o the accompanying drawings in which: Figure 1 shows a bottom plan view of a plate of a heater in accordance with the invention; Figure 2 shows a schematic exploded section through a heater according to the invention, along the line II II of Figure 1; and Figure 3 shows a schematic exploded section through a second heater according to the invention, along the line II-II of Figure 1; With reference to Figures 1 and 2, a first heater 2 in accordance with the invention comprises a .5mm thick stainless steel support plate 4. The support plate has a peripheral groove 6 for receiving the lower edge 8 of the wall 10 of a plastics liquid heating vessel in the base of which the heater is mounted. Such a method of mounting is disclosed in Wo96/18331.

The plate 4 has a generally planar section 12 and a central region 14 which is raised with respect to the plane of the planar section 12.

A glass insulating layer 16 and a resistive heating track 18 are provided on the undersurface 20 of the plate 4, and extend over both the planar section 12 and the raised central section 14 to form heating means 22 on the inner surface of the region 14. This can be achieved by the method disclosed in our co-pending application PCT/GB96/00182 in which a flexible green ceramic tape is reprinted with a resistive heating track and then applied to the plate 4. The tape and track are then fired together on the plate. Thus in this embodiment, the heating means 22 in the raised region are actually an integral part of the track 18 itself.

In the embodiment of Figure 3, however, although the plate 4 is the same as in the embodiment of Figure 1, the glass insulating layer 16 and the resistive heating track 18 are provided only on the planar section of the plate 4, and do not extend into the raised section 14. In this embodiment, heating is provided by a metal foil heater 24 which is mounted against the undersurface 26 of the raised region. The foil heater 24 is insulated from the plate 4 by an insulating plastics disc 28 of Kapton, and has spring contact portions 30 which make electrical contact with contact portions 32 of the track 18. Alternatively, the contacts could be welded or soldered to the track.

In this embodiment, the foil heater 24 could be adhesively retained against the undersurface of the raised section 14, for example by an adhesive coating provided on both sides of the insulating disc 28.

Alternatively, it may be retained in position by other fixing means or merely by being resiliently urged against the base of the raised section.

In both embodiments, a thermally sensitive control, for example containing a bimetallic actuator 34 or the like, is urged against the heating means 22,24 of the raised region 14. This is shown schematically in Figures 2 and 3. The actuator 34 is insulated from the heating means 22,24 by a further plastics insulating disc 36. The actuator operates to open a set of switch contacts in the electrical supply to the main heater track 18 upon reaching a predetermined temperature.

In operation, the heater 2 is installed in the base of a liquid heating vessel, and when energised acts to heat liquid within the vessel to boiling. If the vessel is not provided with a boiling control which switches the heater off when the liquid in the vessel boils, or if such a control should malfunction, the liquid level 38 in the vessel will drop as the liquid boils away.

Eventually the liquid level will fall to the point at which the upper surface of the raised region 14 of the plate 4 becomes exposed, as shown in Figures 2 and 3.

When this point is reached, the heat generated by the heating means 22,24 arranged in contact with the undersurface 26 of the raised region 14 will not be conducted away by liquid, and the temperature of that surface will begin to rise above that of the rest of the plate 4. When this temperature reaches the operating temperature of the bimetallic actuator 34, this will operate to disconnect the supply to the heater, thereby avoiding potentially dangerous overheating of the heater and possible damage to the vessel.

Of course, many modifications can be made to the above embodiments within the scope of the invention.

For example, a number of raised area may be provided distributed around the plate to overcome the problem of the vessel not being placed on a level surface.

Further, the plate need not be generally planar, but could be domed or frustoconical in section. A frustoconical shape is shown, schematically, in phantom lines 40 on Figure 2. Also, the plate may be configured to be mounted in the base of a vessel by means other than the arrangement shown, for example by simply being clamped in the base from below.

Furthermore the heater of Figure 2 could be mounted to the plate by welding part of the bimetallic actuator 34 to the plate 4 through corresponding location holes in the heater foil 24 and Kapton film 36, thereby trapping the heater foil 24, the Kapton film 36 and bimetallic actuator firmly in place.

Claims

1. A heater comprising a support plate having a generally planar portion, an electrically insulating layer formed on the said planar portion of the support plate and a resistive heating track provided on the insulating layer, characterised in that the support plate is formed with a region which is raised in the direction opposite to the side of the plate supporting the heater track, said raised region having, or being adapted to receive, heating means therein.

2. A heater as claimed in claim 1 wherein the raised region is formed by pressing.

3. A heater as claimed in claim 1 or 2 wherein the raised region is formed generally centrally of the plate.

4. A heater as claimed in any preceding claim wherein said support plate is generally planar with the raised region upstanding from the plane of the plate.

5. A heater as claimed in any of claims 1 to 3 wherein said support plate is domed or frustoconical.

6. A heater as claimed in any preceding claim further comprising heating means arranged within the raised region.

7. A heater as claimed in claim 6 wherein said heating means comprises a section of the resistive heating track extending into the raised region.

8. A heater as claimed in claim 6 wherein said heating means is separate from said heating track.

9. A heater as claimed in claim 8 wherein said heating means comprises a metal foil heater.

10. A heater as claimed in claim 8 or 9 wherein said heating means is arranged in series with said heater track.

11. A heater as claimed in claim 10 wherein said heating means is provided with spring contacts for contacting respective sections of said track.

12. A heater as claimed in any of claims 8 to 11 wherein said heating means is adhesively secured within said raised section.

13. A heater as claimed in any preceding claim in combination with a thermally sensitive control arranged in good thermal contact with the heating means within the raised section.

14. A liquid heating vessel comprising a heater or combination as claimed in any preceding claim, the heater being arranged in the base of the vessel.