WO2001095670A1 - Improvements relating to electric heating elements - Google Patents

Abstract

In order to accommodate different heating tracks with different power densities on a thick film heating element the thick film heating element is formed with a multi-layer structure in which different heating tracks are in different layers separated by electrical insulation.

Description

IMPROVEMENTS RELATING TO ELECTRIC HEATING ELEMENTS

Field of the Invention:

This invention concerns improvements relating to electric heating

elements and more particularly concerns thick film heating elements which

comprise a resistive heating track or layer formed on a substrate which

commonly comprises stainless steel with an electrically insulating layer

between the stainless steel and the resistive heating track or layer, though

ceramics material substrates are also known. Such thick film heating

elements are nowadays used in liquid heating appliances, such as kettles and hot water jugs for example, and also find wider application.

Background of the Invention:

It has previously been proposed to provide more than one heating

track on a thick film heating element, see for example GB-A-2 343 352,

GB-A-2 335 588 and our British Patent Application No. 0013545.9. In

such previous applications for thick film elements with multiple tracks,

the power density of the additional heater track(s) has always been

sufficiently low to allow it to be located on the heating element surface and

not occupy too much space. However, where the power required from the

additional tracks(s) becomes significant, there may be insufficient space on an

element disk to accommodate the required tracks. An example of an

appliance where this is the case is one which is intended to heat

a range of different liquids, where a high power (2-3kW) is needed for rapid boiling of water, and a lower power (6-700 W), and lower power density,

is needed for heating milk, without the milk becoming burnt on the base of the

vessel. The low power density of the lower power track means that it cannot

be tightly laid into a small area, but may occupy as much space as the high

powered track. A large element diameter to give sufficient area is not

desirable, since this leads to appliance styling difficulties and high costs.

Objects and Summary of the Invention:

It is thus the principal object of the present invention to overcome or at

least substantially reduce the abovementioned problem.

According to the present invention this object can be achieved by a

thick film heating element having a multi-layered construction with different

heating tracks on different layers which are separated by electrically insulating

material.

More particularly, in order to overcome the abovementioned problem,

it is proposed in accordance with the present invention to print two or more

tracks one on top of the other, separated by a thin dielectric layer. With this

proposal, which is based on multi-layer thick film electronics technology, the

withstand voltage for the intermediate layer does not have to be as great as the

basic insulation required between the mean heating element and the steel

substrate which has to withstand a high voltage test of 1250N. The

intermediate layer would only have to withstand 250N, perhaps with a high

voltage test of only 500N. thus the intermediate insulating layer can be

thinner than the main insulation, say 60-80 microns compared to 150 microns for example. The increased total insulating thickness between the additional

track(s) and the substrate will raise the running temperature of that track, but

since we are aiming for a low power density, the resulting temperature rise

will be acceptance. The reduced thickness of the intermediate layer will also

assist in keeping temperature rises down.

The manufacturing process for such a thick film heating element might

for example comprise the following steps:

1. Onto a prepared steel substrate print the main dielectric layer and fire;

2. Print the main heater track and fire;

3. Print the intermediate insulating layer and fire;

4. Print the additional heater track and fire;

5. Print silver connection pads and fire; and

6. Print a protective cover coat and fire.

The various firing operations may be combined by firing multiple layers at

once, as is usual in the manufacture of thick film elements, and operations 5

and 6 may be reversed or combined.

The additional track(s) may include other features, such as an

adjustable resistance control track as described in our aforementioned British

Patent Application No. for example, and tappings from either track

may be provided to give access for controls or indicators, similar to that

described in our GB-A-2 329 759 or GB-A-2 325 396. Connections to the

underlying track could be made through holes in the intermediate insulating layer, and access to the tracks for bimetal or other sensors can also be similarly provided.

The above and further features of the present invention are set forth in the appended claims and will be clearly understood from consideration of the

following description which is given with reference to the accompanying

drawings. Description of the Drawings:

Figures 1A to IE illustrate various steps in the manufacture of an exemplary thick film heating element embodying the present invention; and Figure 2 is a perspective cross-sectional view of a thick film heating

element manufactured by the steps of Figures 1 A to IE. Detailed Description of the Embodiment:

In the following, since the manufacture of thick film heating elements is well known, only brief details of the various manufacturing steps will be

described.

Referring to the drawings, Figure 1A illustrates the provision of a

dielectric layer 1 on one surface of a stainless steel substrate 2. The dielectric

material can for example consist of a glass, ceramics or glass ceramics

material which is applied onto the surface of the substrate in powder form and

is then fired, as is well known. Figure IB shows the provision of a main

heater track 3 on top of the dielectric layer 1, for example by means of a

printing step employing electrically conductive ink followed by a firing step.

Note that the form of the track 3 that is shown in Figure IB is exemplary only. Figure 1C illustrates the provision of an intermediate electrically

insulating layer 4 over the track 3, but with connector terminal parts 5 and 6 of

the track 3 left exposed in a window opening 7 provided in the layer 4. A

second window opening 8 exposes a portion of the main heater track 3 for

access by a thermally-responsive control (not shown). Figure ID shows the

provision of a further heating element track 9 on top of the intermediate

electrically insulating layer 4 and Figure IE shows the provision of a further,

protective electrically insulating layer 10 overlying the further heating element

track 9, the layer 10 being formed with window openings 11, 12 and 13

which, respectively, register with the window opening 7 in the intermediate

insulating layer 4, provide access to the terminal parts 14 and 15 of the

heating element track 9, and expose a portion of the additional heating

element track 9 for access by a further thermally-responsive control (not

shown).

At appropriate stages in the manufacturing process as abovedescribed

the terminal parts 5, 6, 14 and 15 are provided with low resistance silver

coatings which may also be provided, as is well known, at the locations where

the heating element tracks 3, 9 turn back upon themselves so as to avoid the

phenomenon of current crowding.

Figure 2 is a perspective cross-sectional view of a thick film heating

element manufactured as described above. It is to be noted that the

thicknesses of the various layers are greatly exaggerated for the sake of clarity. Having described the invention in the foregoing in relation to a

particular embodiment, it is to be well appreciated that the embodiment is in

all respects exemplary and that modifications and variations thereto are

possible without departure from the spirit and scope of the invention as set

forth in the appended claims.

Claims

Claims:

1. A thick film heating element having a multi-layered construction in

which different heating tracks are provided at different layers which are

separated by electrically insulating material.

2. A thick film heating element as claimed in claim 1 comprising a metal

substrate provided with an electrically insulating layer upon which at lest one

heating track is provided, and wherein the thickness of said electrically

insulating layer is greater than that of an electrically insulating layer overlying

said at least one heating track.

3. A thick film heating element as claimed in claim 1 or 2, wherein

window openings are provided in the one or more electrically insulating layers

which cover the heating tracks so as to provide access to terminal portions of

the tracks.

4. A thick film heating element as claimed in claim 1 or 2 or 3 wherein at

least one window opening is provided in one or more of the electrically

insulating layers of the element for enabling access to at least one of the

heating element tracks by a thermally-responsive control.

5. A thick film heating element as claimed in any preceding claim

wherein a main heating track is provided at one layer and an additional heating track is provided at another layer together with a means for adjusting

the power output of such additional track.

6. A thick film heating element as claimed in claim 5 wherein said power

adjustment means comprises a resistive track coupled to said additional

heating track, said resistive track being configured to enable its resistance to

be adjusted by operation of a control associated therewith.

7. A thick film heating element substantially as herein described with

reference to the accompanying drawings.

8. A liquid heating appliance incorporating a thick film heating element

as claimed in any of the preceding claims.

9. A liquid heating appliance as claimed in claim 8 having a high power

(eg. 2 to 3kW) heating track for rapid boiling of water and a low power (eg. 6

to 700W) heating element for the heating of milk without the milk becoming

burnt on the heating element.

10. A method of manufacturing a thick film heating element comprising:

(i) preparing a metal (eg. stainless steel) substrate;

(ii) providing an electrically insulating layer on a surface of said substrate;

(iii) providing a first heating track on said electrically insulating layer; (iv) providing a second electrically insulating layer overlying said first

heating track;

(v) providing a second heating track on said second electrically insulating

layer; and

(vi) providing a third, protective electrically insulating layer overlying said

second heating track; wherein access openings are provided in said second and third electrically insulating layers which enable terminal portions of said first and second heating tracks to be accessed for supplying electricity to the tracks.