GB2318713A

GB2318713A - Reduced power density region in an electric heater

Info

Publication number: GB2318713A
Application number: GB9621977A
Authority: GB
Inventors: John Crawshaw Taylor; Ath Lawrence De; Keith Barrie Doyle
Original assignee: Strix Ltd
Current assignee: Strix Ltd
Priority date: 1996-10-22
Filing date: 1996-10-22
Publication date: 1998-04-29
Anticipated expiration: 2016-10-22
Also published as: GB9621977D0; GB2318713B

Abstract

There is disclosed an electric heater 2 of the type comprising an electrically resistive heating track 8 applied to an electrically insulating substrate 6. In a region 10 of the heater intended to receive a thermally sensitive actuator 12 of a thermally sensitive control, a portion of the track 8 is made wider than an adjacent portion of the track so as to reduce the power density per unit area of the track in that region.

Description

Electric Heaters The present invention relates to electric heaters of the type comprising an electrically resistive heating track applied to an electrically insulating substrate.

Typical of such heaters are so-called thick film printed heaters, such as those described in WO 96/18331 and WO 96/17497.

Such heaters are now being proposed for use in liquid heating vessels. In such vessels, it is normal to provide a thermally sensitive control which operates in the event that the heater overheats to disconnect or reduce the power supply to the heater. This prevents potentially dangerous overheating of the heater and vessel in the event that the vessel is switched on without any liquid in it, or boils dry in use. The control typically comprises a bimetallic actuator coupled to a set of switch contacts.

In order to achieve operation of the thermally sensitive control, it has been proposed in WO 96/17497 to increase the power density of the heating track in the region of the control actuator, so as to ensure that the part of the heater runs hotter than the remainder of the heater. This is done by splitting a portion of the track into a number of narrower tracks.

The applicant has discovered, however that such an arrangement presents a number of problems. Firstly, it may lead to a scale build-up on the liquid-facing surface of the heater in the region of the actuator, which, with time, may lead to premature operation of the actuator due to the heat insulating effect of the scale.

Secondly, it means that higher temperature bimetallic actuators than are presently used in liquid heating vessels must be used, which leads to additional expense.

The applicant has found that to mitigate the above problems, it is desirable actually to decrease the power density of the track in the region of the actuator.

Thus according to a first aspect of the invention there is provided an electric heater of the type comprising an electrically resistive heating track applied to an electrically insulating substrate, wherein in a region of the heater intended or adapted to receive a thermally sensitive actuator of a thermally sensitive control, a portion or portions of the track is made wider than an adjacent portion of the track so as to reduce the power density per unit area of the track in that region.

Thus in accordance with the invention, the power density of the heater track is reduced locally by making the track wider in that region. This reduces the local temperature in this region, which reduces the chances of scale building up on the heater thereby improving the reliability of operating the control. It also means that lower temperature, standard actuators may be used.

The invention is extremely easy and cost-effective to implement, since the heating track with the appropriate widths can be applied, for example printed, to the substrate in a single operation in a conventional manner.

From a further broad aspect, therefore the invention provides a method of manufacturing an electric heater of the type comprising an electrically resistive heating track applied to an electrically insulating substrate, wherein the resistive track material is applied to the insulating substrate with a portion or portions of the track intended or adapted to receive a thermally sensitive actuator of a thermally sensitive control being wider than an adjacent region of the track material.

Preferably the track width is increased by over 25% and most preferably over 50%, although the precise increase in width can be chosen to give the desired control response.

It will be appreciated that the invention also extends to a heater as described in combination with a thermally sensitive control having an actuator, most preferably a bimetallic actuator, arranged in good thermal contact with the wider regions of the track.

Most desirably the invention is applied to liquid heating vessels having a heater as described above mounted in, to, or forming the base of the vessel, with the control arranged thereunder.

A preferred embodiment of the invention will now be described by way of example only with reference to the accompanying drawing, Fig. 1.

With reference to Fig. 1, a thick film printed plate heater 2 comprises a stainless steel plate 4 on which a glass ceramic insulating layer 6 is laid down and fired in a conventional manner. A thick film resistive heating track 8 of a resistive material such as palladium silver is then applied in a single operation in a conventional manner such as silk screen printing and fired. A protective overglaze or other protective layer (not shown) may be applied over the track 8.

The heater 2 has two regions 10 intended to receive a pair of bimetallic actuators 12 of a thermally sensitive control, for example of the general type disclosed in WO 96/34187.

The actuators 12 may, for example, be snap-acting actuators of the type disclosed in GB 1542252, and may be suitably urged against the regions 10.

It will be seen that the width of the heating track in the regions 10 is greater than over the remainder of the track. In this embodiment, the nominal track width WN is about 3.9 mm, while in the regions 10, the increased with W1 is about 6.0 mm. This reduces the power density per unit area of the track 8 from about 51.6 Wcm2 over most of the track to about 22.4 Wcm2 in the regions 10.

This has been found to give satisfactory operation of a bimetallic actuator operable at a nominal 125 135"C, without over-heating the heater as a whole, and to slow down the build up of scale in that region of the heater.

The heater 4 may be mounted in any suitable manner in the base of a liquid heating vessel, for example as shown in WO 96/18331.

From the above, it will be seen that the invention permits a selected area of track with the desired heating effect to be produced very easily and cost effectively, to produce a heater which is less likely to build up scale in the region of a control, thereby improving operation of the control.

Of course, the invention is not limited to use in liquid heating vessels and may find application in other heating appliances.

Claims

1. An electric heater of the type comprising an electrically resistive heating track applied to an electrically insulating substrate, wherein in a region of the heater intended or adapted to receive a thermally sensitive actuator of a thermally sensitive control, a portion of portions of the track is made wider than an adjacent portion of the track so as to reduce the power density per unit area of the track in that region.

2. A heater as claimed in claim 1 wherein the track width is increased by over 25%.

3. A heater as claimed in claim 2 wherein the track width is increased by over 50%.

4. A heater as claimed in any preceding claim wherein in combination with a thermally sensitive control having an actuator arranged in good thermal contact with the wider region(s) of the track.

5. A combination as claimed in claim 4 wherein said actuator is a bimetallic actuator.

6. A combination as claimed in claim 5 wherein the nominal operating temperature of the actuator is between about 125"C and 135"C.

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

8. A method of manufacturing an electric heater of the type comprising an electrically resistive heating track applied to an electrically insulating substrate, wherein the resistive track material is applied to the insulating substrate with a portion or portions of the track intended or adapted to receive a thermally sensitive actuator of a thermally sensitive control being wider than an adjacent region of the track material.

9. A method as claimed in claim 8 wherein said track is screen printed.