GB2316283A - Insulating film between heating track and thermal actuator - Google Patents

Abstract

Methods for modifying polypeptide agents to enhance their transdermal electrotransport flux are provided. The polypeptide is modified by reducing the potential of the polypeptide for forming * small Greek alpha *-helical or * small Greek beta *-sheet segments. In particular, amino acid residues known to stabilize * small Greek alpha *-helical and * small Greek beta *-sheet segments can be replaced with destabilizing residues and known helix breakers. Modified molecules and compositions including the molecules are also provided.

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. Typical of such heaters are socalled thick-film printed heaters, in which the track material is printed, must commonly screen printed, onto a glass, ceramic or glass ceramic (hereinafter collectively referred to as glass) layer typically provided on a metallic support surface. To prevent corrosion and oxidation of the track material, a thin glass "overglaze" layer is usually provided over the track.

It has been proposed, for example in EP-A-574310 and WO 96/18331, to use such heaters in liquid heating vessels. In EP-A-574310 an insulating glass substrate and heater track is provided on the underside of the base of a metal vessel, while in WO 96/18331, the substrate and track are provided on a metal plate which can be mounted in the base of, for example, a plastics vessel.

In such applications, the heater should be provided with a thermally-sensitive 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. Such controls are shown in WO 95/34187 and in our co-pending application GB 9525493.4.

In order to meet safety requirements, once the control has been assembled to the heater, the electrical insulation between them must be able to withstand a 1500V flash test, in which a potential of 1500 V is applied between the heater and the control.

It has been found that whilst the glass overglaze layer provided normally provided over the heater track provides some degree of electrical insulation, due to the porosity of the glass, in order to provide adequate insulation to meet the flash test, it must be made relatively thick, for example several hundred microns as opposed to its normal thickness of say 15 microns.

However, such a thick layer will also act to insulate the control thermally from the heater to a significant degree, which means that in an overheat situation as described above, heat may not be conducted sufficiently quickly to the thermally sensitive actuator of the control, which may lead to the heater seriously overheating, and thus possibly damaging itself or the liquid heating vessel, before the control operates.

Furthermore, it has been found that as the actuator of the control operates, it will impact upon the glass layer, which may then crack and wear away, which is clearly undesirable.

The present invention seeks to overcome the above problems, and from a first aspect provides in combination, an electric heater of the type comprising a resistive heating track provided on an electrically insulating layer, and a thermally sensitive control for the heater operable in the event that the heater overheats so as to interrupt or reduce the electrical supply to the heater, said control having a thermally sensitive actuator in good thermal contact with said heater, wherein a thin, non-glass, electrically insulating film is provided between the control and the heater.

Thus in accordance with one aspect of the invention, a thin film of a material which provides the necessary electrical insulation, but which by virtue of its low thickness offers reduced thermal insulation compared to a glass layer of comparable electrical insulation, is used. This will allow the control to operate sufficiently quickly to prevent serious overheating of the heater and at the same time provide the requisite electrical insulation.

The film should preferably be impact resistance, so as to resist damage by repeated operation of the actuator. Preferably, therefore, the film is of a compliant or flexible material, and is most preferably of plastics or contains plastics. This will not only resist impact damage, but will also accommodate relative expansion between the heater and the control.

The concept of providing an impact or wear resistant barrier between the actuators and the heater is in itself novel, and from a second aspect therefore, the invention provides in combination, an electric heater of the type comprising a resistive heating track provided on an electrically insulating layer, and a thermally sensitive control for the heater operable in the event that the heater overheats so as to interrupt or reduce the electrical supply to the heater, said control having a thermally sensitive actuator in good thermal contact with said heater, an impact or wear resistant layer being provided between the actuator and the heater.

Although it would be possible, for example, to precoat the surface of the heater (or even the surface of the control or its actuators) with a film of insulating material, the film is preferably a separate component sandwiched between the heater and the control. This has the advantage of ease of application, and avoids the need for a further step in the manufacture of the heater itself. Furthermore, suitable insulating films are obtainable commercially and can easily be cut to form the desired shape of insulator.

The film may, for example, be supplied together with the control, retained in position by suitable means, so that the control and film can be mounted as a sub-assembly to the heater.

To assist in the mounting of the film, it may be preferred, however to provide one or both sides with an adhesive, for example a contact or thermosetting plastic adhesive.

The particular insulating material used, will depend not only upon the electrical insulation required and speed of control response, but also the intended operating temperature of the heater.

For lower temperature operations, materials such as aramid papers (eg. Nomexs paper) and polyester films (eg. Mylar@) may be suitable. However, in the case of water heating vessels, what is required is a material which will resist sustained exposure to temperatures of the order of 1200C (a typical track temperature when water is boiling in a vessel), but which will also be able to withstand exposure to substantially higher temperatures (over 300 or more) for short periods in an overheat condition. A polyimide film available under the trade name Kaptons has been found particularly suitable for this purpose.

The thickness of the material chosen will depend primarily on the thermal resistance and electrical resistivity of the material. However, ease of handling is also a consideration. For example whilst a Kaptons films of 25 micron thickness will provide adequate electrical insulation, it is too thin to handle effectively. A film thickness of at least about 50 micron is thus preferred. A thickness of about 50 micron gives a good compromise between good thermal conduction and ease of handling.

As stated above, the invention is particularly applicable to liquid heating vessels, and the invention also extends to a liquid heating vessel comprising a combination of the invention. More particularly, the heater is preferably provided on the underside of the base of or for such a liquid heating vessel.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 shows a schematic plan view of an embodiment of the invention; and Fig. 2 is an exploded schematic section along the line II-II of Fig. 1.

Referring to the Figures, an electric heater 2 forming or forming part of the base of a liquid heating vessel comprises a stainless steel base plate 4, approximately 0.5 mm thick, on which is provided, in any convenient manner, a 100 micron thick glass insulating layer 6. An arcuate window 8 is provided in the glass layer 6 to allow the passage of two mounting studs 10 for a thermally sensitive control unit which are welded or otherwise bonded to the base plate 4. These studs will also act to earth the base plate 4 in use.

A serpentine, thick film printed heating track 12 about 15 microns thick is provided on the insulating layer 6 and has a pair of terminals 14 for connection to an electrical supply.

The whole heater is provided with a glass overglaze layer 16 about 15 microns thick to protect the track 12 from corrosion and oxidation. The window 8 also extends through the overglaze layer 16.

A thermally sensitive control, for example of the type described in WO 95/34187 or GB 9525493.4 is arranged in good thermal contact with the heater 2 in order to interrupt or reduce the supply of electrical energy to the heater 2 in the event that the heater overheats.

The control comprises a metallic plate 18, whose outline is shown in Fig. 1, which mounts a pair of snapaction bimetallic actuators 20 of the type described in GB 1542252 to which further reference can be made for details of the actuators. The actuators 20 are mounted on the surface of the plate facing the heater base plate 4 by their tongues 22, and when they reach their operating temperature change curvature with a snapaction, causing a peripheral portion 24 to move and open a set of contacts in the control via a push rod (not shown).

The mounting plate 18 also has a pair of openings 26 to allow for the passage of the control mounting studs 10.

Arranged between the mounting plate 18 and the heater 2 is an electrical insulating layer formed by a generally trapezoidal washer 28. This washer acts to insulate the base plate 4 from the plate 18.

The washer 28 has a pair of elongate slots 30 through which the control mounting studs 10 can extend.

The washer in this embodiment is made from 50 micron thick Kaptons 200 VN film. This is a polyimide film which gives excellent electrical insulating properties and which has been found to provide good resistance to wear and to tearing as a result of impacting by the control actuators.

In use, the washer 28 and mounting plate 18 of the control are slipped over the mounting studs 10, and the plate 18 firmly clamped to the heater 2. This traps the washer 28 between the plate 18 and the heater and brings the bimetallic actuators 20 into good thermal contact with the heater 2. If necessary, a proprietary heat sink compound may be provided between the actuators 20 and the heater 2 to improve the thermal contact therebetween.

In tests using the above Kaptone film, a 2.2 kw heater track, and bimetallic actuators chosen to operate at approximately 140"C, switch-off times of 3 to 4 seconds have been achieved in overheat conditions. At the same time completely adequate electrical insulation between the base plate 4 and the mounting plate 18 was obtained in 1500V flash tests.

It will be appreciated that many modifications may be made to the above described embodiment without departing from the scope of the invention. For example other particular plastics materials may be used for the washer 28 depending on the intended temperature of operation of the heater. Furthermore, other materials such as mica could also be used, as they too will provide a satisfactory balance of electrical and thermal insulation.

Furthermore, the base plate 4 may be provided, for example, with a peripheral channel to engage the wall of a plastics vessel, in the manner described in WO 96/18331.

Also, whilst in the preferred embodiment, an overglaze has been provided over substantially the whole plate, a window may be provided in the overglaze in the region of the track against which the control abuts.

Claims (11)

Claims

1. In combination, an electric heater of the type comprising a resistive heating track provided on an electrically insulating layer, and a thermally sensitive control for the heater operable in the event that the heater overheats so as to interrupt or reduce the electrical supply to the heating track, said control having a thermally sensitive actuator in good thermal contact with said heater, wherein a non-glass, electrically insulating film is provided between the actuator and the heater.

2. A combination as claimed in claim 1 wherein said film is of a flexible material.

3. A combination as claimed in claim 1 or 2 wherein said film is of, or contains, plastics.

4. A combination as claimed in any preceding claim wherein said film is a separate component sandwiched between the heater and the control.

5. A combination as claimed in claim 4 wherein said film contains an aramid paper, polyester or polyimide.

6. A combination as claimed in claim 5 wherein said film is at least about 50 ym thick.

7. A combination as claimed in claims 4 to 6 wherein said film is provided on at least one side with an adhesive.

8. A combination as claimed in any preceding claim wherein said heater is provided on the base of or for a liquid heating vessel.

9. An electric heater of the type comprising a resistive heating track provided on an electrically insulating layer, further comprising a thin, non-glass, electrically insulating film over a part thereof intended to receive a thermally sensitive control for the heater.

10. In combination, an electric heater of the type comprising a resistive heating track provided on an electrically insulating layer, and a thermally sensitive control for the heater operable in the event that the heater overheats so as to interrupt or reduce the electrical supply to the heater, said control having a thermally sensitive actuator in good thermal contact with said heater, an impact or wear resistant layer being provided between the actuator and the heater.

11. A liquid heating vessel comprising a heater or combination as claimed in any preceding claim.