WO2000067527A1

WO2000067527A1 - Improvements relating to heating elements, particularly in the field of thick film heating elements

Info

Publication number: WO2000067527A1
Application number: PCT/GB2000/001695
Authority: WO
Inventors: Robert Andrew O'neill; Robin Keith Moore
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 1999-05-04
Filing date: 2000-05-04
Publication date: 2000-11-09
Anticipated expiration: 2001-11-04
Also published as: DE60024253T3; DE60024253T2; GB2351894B; EP1177708A1; GB2351894A; EP1177708B1; CN1358402A; EP1177708B2; DE60024253D1; GB9910286D0; CN1162044C; ATE311083T1

Abstract

A thick film heating element for a water boiling vessel is secured to the bottom of the vessel with a heat dispersion layer of copper or aluminium between the vessel bottom and the thick film heating element and extending laterally beyond the edges of the thick film heating element. The thick film heating element is about half the size of a conventional thick film heating element (about 60mm as compared to 110mm) with a corresponding cost reduction, and has at least twice as high a power density (60W/cm2) as compared to 30W/cm2) and yet does not generate excessive noise in operation.

Description

IMPROVEMENTS RELATING TO HEATING ELEMENTS, PARTICULARLY IN THE FIELD OF THICK FILM HEATING

ELEMENTS

Field of the Invention:

This invention concerns improvements relating to electric heating

elements and, more particularly, concerns heating elements of the so-called

thick film type comprising a substrate, commonly formed of stainless steel,

carrying a resistance heating track or layer which, as appropriate having regard

to the nature of the substrate, may be formed on an electrically-insulating

layer, commonly of glass, provided on the substrate. An additional

electrically-insulating layer may be provided over the resistance heating track

or layer as a protective measure. Thick film heating elements are employed in

a variety of applications and are currently becoming popular in the field of

electrically-heated water boiling vessels, domestic kettles and hot water jugs

for example, where their clean appearance as compared to the conventional

immersion heating element of metal sheathed construction has aesthetic

advantages. In addition it is possible with a thick film heating element to

accommodate a greater power density than is readily accommodated with

conventional sheathed heating elements, leading to more rapid boiling times.

Background of the Invention:

As mentioned above, thick film heating elements are commonly

formed on a stainless steel substrate, for example by first providing an

electrically-insulating layer of glass on one or both surfaces of a stainless steel plate or disc and then screen-printing a resistance heating track onto the glass

surface using electrically conductive inks which are then fired. As

abovementioned, a further layer of glass may then be provided over the

resistive track. It is known that the manufacture of thick film heating

elements by this process can give rise to problems of distortion of the heating

element out of its normal planar configuration and that distortions can arise

furthermore in use of the heating element on account of differential thermal

expansion effects. In order to at least alleviate these problems, it has been

proposed to select the materials deposited onto the stainless steel substrate to

have compatible thermal expansion coefficients insofar as is possible and it

has been proposed furthermore to provide layers on both sides of the stainless

steel substrate so as to subject it to similar thermal expansion and contraction

effects from both sides. All of these solutions give rise to cost implications

which, when added to the basic cost of appropriate quality stainless steel

substrates as are required for water boiling vessels, tend to render the product

unattractive on considerations of cost irrespective of its other clear

advantages.

To overcome or at least substantially reduce the distortion problem

abovementioned, the invention of our British Patent Application No.

9908918.7 proposed to form the substrate of a thick film heating element with

a slightly domed curvature, form the heating element track or layer on the

convex surface of the domed substrate, and bond the thus formed thick film heating element to a planar surface to be heated by a process which flattens

the domed thick film heating element onto the planar surface.

Whilst the invention of our British Patent Application No. 9908918.7

promises to overcome the distortion problem, there are a number of other

problems in the manufacture and use of thick film heating elements which

tend to limit their wider adoption, namely:

(i) The materials used are relatively expensive. Anything that can

be done to reduce material content is desirable. This includes a

reduction in thickness of the substrate, and a reduction in size

of the substrate and printed area.

(ii) The usable power density is limited by the noise generated by

such heaters during heating water. The noise principally arises

from local formation of steam bubbles, which rapidly collapse,

because the power density is too high to allow convection

currents to become established close to the element surface,

and especially directly opposite the location of the heater track.

(iii) The limiting power density leads to heaters that are of relatively

large area, leading not only to increased costs but also to an

inability to operate on more than a very small angle of slope. To overcome this problem it has been conventional to employ

costly solutions of multiple protectors or electronics.

(iv) At present the only steel substrate which is successfully used is

400 series stainless steels. These materials have poor

resistance to corrosion and give a low quality cosmetic surface.

To overcome this various coatings have been proposed, which

increases the cost and raises the running temperature of the

heater tracks.

(v) In order to withstand thermal and mechanical shock arising

from abuse of the appliance, it has normally been necessary to

employ relatively thick steel substrates, which increases costs.

(vi) The screen printing techniques presently employed can only be

applied to flat surfaces, with no rims or other projections above

the printing surface. This limits the applications which can

make use of such elements.

(vii) To overcome the limitations of the flat printing process it might

be possible to fabricate the plate into a separate vessel.

However the materials from which such vessels are commonly made (typically 300 series stainless steel) are not sufficiently

compatible to allow straightforward and inexpensive direct

assembly and joining.

Summary of the Invention

To overcome or at least substantially reduce these problems we

propose to use a relatively small heater mounted to a larger vessel, with a

layer of material of relatively high thermal conductivity between the steel

substrate of the heater and the material of the vessel to act to spread the heat

from the heater over a wider area of the liquid heating surface.

Any material may be used for the intermediate heat dispersion layer,

provided it has a thermal conductivity significantly greater than the stainless

steels used, i.e. about 20W/sqM/°C. Preferred materials are copper and

aluminium, chosen for their thermal conductivity, relatively low cost and

compatibility with the proposed assembly processes.

The joining process may be the same as or similar to any of the

existing known methods of attaching aluminium mounting plates as for

Blitzkocher type elements, i.e. impact pressure bonding, welding or brazing,

with any of the known sources of heat. However the preferred method,

because of its controllability, is induction brazing. This technique claims to

give a good quality, repeatable, joint with few voids. The join of the heat

dispersion layer to the vessel may be done first, for example by impact

bonding, and the heating element may be induction brazed separately, giving the opportunity of inspecting the quality of the vessel/dispersion layer joint.

Vents could be left in the part of the dispersion layer which extends beyond

the heating element to allow the escape of flux and fumes generated during

brazing. During the brazing process, the assembly is desirably pressed

together with a clamping force, possibly as much as 4 tons, to ensure that the

plates are flattened together without any gaps in the joint area. This clamping

force enables the invention of our British Patent Application No. 9908918.7 to

be utilized in the practice of the present invention.

The above and further features of the present invention are set forth in

the appended claims and will be explained in the following by reference to an

exemplary embodiment which is illustrated in the accompanying drawings.

Description of the Drawings:

Figure 1 is a schematic side elevation view of a thick film heating

element embodying the present invention;

Figures 2 A and 2B show alternative constructions of the peripheral

edge of the thick film heating element of Figure 1 ; and

Figure 3 is a plan elevation view of the thick film heating element of

Figure 1.

Detailed Description of the Embodiment:

The illustrated embodiment comprises a thick film heating element 1

of relatively small diameter which is bonded to a heat dispersion plate 2,

formed of aluminium or copper for example and having a diameter greater than that of the thick film heating element 1, which in turn is bonded to a

heating surface 3 shown in the example as a thin metal plate adapted to be

fitted into the bottom of a water heating appliance, the alternative edge details

shown in Figures 2A and 2B are, respectively, intended to provide a

water-retaining well around the heating element periphery to facilitate sealing

of the element into a vessel body by providing a cooler sealing environment

and to facilitate interfacing with the vessel body; other edge configurations are

of course possible.

It is conventionally considered that a thick film heating element having

a power density of more than 30W/cm" will give rise to unacceptable noise.

This equates to an overall power output of about 3kW on an element formed

on a 120mm diameter disc with a plain sealing area left all round. It is

proposed according to the teachings of the present invention to use a

significantly higher power density heating element formed on a disc of around

60mm, for example. It is proposed that the dispersion layer will spread the

heat over an area extending about 10mm beyond the disc, thus giving a

heating element of an effective diameter of 80mm. Bearing in mind that the

material costs of a thick film heating element are proportion to its area

(thickness being constant), a conventional 80mm diameter element would use

78% more material than one of 60mm diameter constructed according to the

present invention, which represents a significant saving. There are, of course, approximations in the above, since the power

density of the embodiment will fall rapidly outside the disc region 1, and will

tend to be concentrated towards the centre. However, examination of a

conventional thick film heater on a stainless steel substrate shows that the heat

is not spread evenly over the surface, but is closely concentrated directly

above the tracks with little heating occurring between the tracks. The true

power density is much greater than simply dividing the power by the disc area

would suggest. It is believed that this is the cause of the excessive noise

generation. This effect is caused by the low thermal conductivity of the

stainless steel substrate, which forces the heat to flow perpendicularly to the

plane of the heater, through the thickness, and significantly limits lateral heat

flow. By the addition of the dispersion layer taught by the present invention,

heat can flow laterally as well as transversely, so that a more uniform heat

distribution is obtained on the liquid heating surface. Thus the power density

on the liquid heating surface is close to the calculated value of the power

divided by the surface area, rather than to the value of the power divided by

the (much smaller) heater track area. The result is a lower effective power

density and a significant reduction in noise generated.

As an example of the improvements that can be made, the following

values are taken from present production elements. For a conventional

element of around 110mm diameter, the heater track power density is

68W/cm². Thus, if the dispersion layer is fully effective, the area of the heater could be reduced to less than half, whilst maintaining the same power density

on the liquid heating surface. By adding the further gain in surface area

around the periphery of the heater disc, the heater can be reduced still further

in size. The net result of this is a major reduction in the cost of the materials

of the element. A disc of 77mm diameter has half the area of one of 110mm,

and taking into account the dispersion layer, this gives an element diameter of

approximately 60mm, the value used in the example above. Thus

theoretically it should be possible, with an element of only 60mm diameter, to

achieve approximately the same power density at the liquid heating surface as

is presently achieved with a conventional 110mm diameter element. This is a

reduction to just over one third of the area. The material cost of the element

makes up over 80% of the total element cost, when fully automated. We

believe that the track power density can be still further increased to in excess

of lOOW/cm , leading to still further cost reduction. In the end the limit is

likely to be caused by the loss of area caused by the need to make electrical

connections, with their associated creepage and clearances, and by the area

needed to accommodate any necessary element protector controls as

schematically shown in Figure 3.

This significant reduction in area may be accompanied by a similar

reduction in substrate thickness. At present, substrates of between 1.2 and

1.5mm are used to achieve satisfactory mechanical rigidity and resistance to

thermal and mechanical shock. We propose to reduce the substrate thickness, for example to 0.3mm, to allow the use of high power density without a

penalty in increased track running temperatures. We anticipate that the

improved heat transfer efficiency afforded by the dispersion layer will reduce

the track running temperature to acceptable values, but this will only be

possible by reducing the thermal resistance of the whole sandwich to values

similar to those at present, taking into account the narrow thermal path of the

present designs. The thin substrate becomes possible because of the support

and cushioning of the dispersion layer, further supported by the water treating

plate and/or vessel wall. Thus the complete assembly will be able to

withstand mechanical impact and thermal shock better than an element with a

unitary substrate of the present thickness. To further improve the heat transfer

we propose that the vessel wall, which is preferably formed of a 300 series

stainless steel commonly used in stainless steel cooking vessels like

saucepans, is also reduced to around 0.3mm. This compares with 0.5mm

normally found in stainless steel kettles. The arrangement of the sandwich is

preferably such that the thermal resistance between the printed heater tracks

and the heating surface is not more than that of a conventionally made thick

film heating element on a 1.2mm substrate of 400 series (S430D or S444)

stainless steel, with a dielectric thickness of no more than lOOμ.

The complete heating element sandwich can be made as an "Easifix"

(GB 2330064A) type element or as a Strix "Sure Seal" (WO 96/18331) for

fitting to a moulded vessel, or it may be made directly onto the base of a stainless steel vessel. This latter is a very cost effective method of fitting a

thick film heater to a stainless steel appliance, something that, so far as we are

aware, has only been done by Pifco - Russell Hobbs by using the same (costly)

plastic securing ring as was developed for the Millennium kettle. Examples of

such mountings are in Pifco's GB 2 291 324 and GB 2 319 154 which show

the complexity of the method. It is possible to attach the small thick film

element 1 into a depression, which would otherwise prevent screen printing,

and this gives further advantages in that it allows a well to be formed around

the periphery of the element to retain some water in the event of boiling dry,

which will give protection to any adjacent seals or cosmetic mouldings. Such

protection is given at present by providing an area free of heating element

track around the element periphery, which increases the diameter and hence

the cost of the heating element. An example of such a well is shown in

Sunbeam, CA 1 202 659, applied to a mechanical embedded element. The

ability to attach the element within a depression will allow a thick film printed

element to be used where previously it was not possible. An example is a

hotplate element which may have a raised heating surface surrounded by a

mounting flange, so that the heating surface is above the general level of the

top of the appliance.

A further advantage of the small element proposed by the present

invention, which gives rise to savings in the manufacturing cost of the

elements, is that they may be processed several at a time. The number of elements which can be printed simultaneously is limited by the area of the

screen printer and by the width of the processing ovens. Clearly, by halving

the diameter of the element, four times as many may be printed and processed

together on the same equipment. Depending on the type of equipment used,

this may be achieved by carrying the elements in a strip and separating them

on completion, or by automated handling, placing individual discs into

location jigs for printing and onto the oven belts for drying and firing. It is

believed that retaining the discs in a strip for processing will lead to reduced

distortion, as a result of the support of the strip. In any case, measures such as

this may be desirable to reduce the effects of distortion.

The small diameter element has an additional operational advantage,

in that it is less sensitive to being operated on a slope whilst heating liquids.

The dispersion layer will ensure that, as liquid boils away, the exposed area of

the heater is cooled to some extent by the remaining liquid until a protector

can operate. In addition, any thermal protector will tend to cover a larger

proportion of the heating element and give protection over a wider area.

The proposal of the invention may be considered to be similar in

principle to the Blitzkocher type of heating element construction, where a

sheathed heating element is secured to a heat transfer plate which in turn is

secured to a steel plate which is part of a liquid heating vessel. However the

power density available from a sheathed heating element is limited by the

insulation of the mineral filling of the sheath and by the robustness of the joint between the sheath and the heat dispersion mateπal. If the power density of a

Blitzkocher heating element is too great, the thermal expansion of the sheath

causes it to peel away from its support, leading to further overheating and

subsequent premature failure. We are also aware of thick film ceramic heaters

secured by conductive cement to the base of water heating vessels, such as the

Hywel egg boiler. Such arrangements are limited by the relatively poor

thermal conductivity of the ceramic substrate and are only suitable for low

power density applications.

While the invention has been described in the foregoing with

particular reference to water boiling vessels such as kettles and hot water jugs,

the invention is not limited to such applications and, in particular, could be

used in electric cooker hobs and hotplates for example.

Claims

Claims:

1. A heating element assembly comprising a heating element proper of

thick film construction secured to a surface to be heated with a heat dispersion

layer between the heating element proper and the surface to be heated, the heat

dispersion layer extending laterally beyond the periphery of the heating

element proper.

2. A heating element assembly as claimed in claim 1 wherein the heating

element proper comprises a substrate of stainless steel.

3. A heating element assembly as claimed in claim 2 wherein said

substrate has a thickness of less than 0.5mm.

4. A heating element assembly as claimed in any preceding claim

wherein said heat dispersion layer comprises copper or aluminium.

5. A heating element assembly as claimed in any of the preceding claims

wherein the power density of the thick film heating element, measured over its

total area, is substantially greater than 30W/cm .

6. A heating element assembly as claimed in claim 5 wherein the power

density if the thick film heating element, measured over its total area, is at

least 60W/cm².

7. A heating element assembly as claimed in any of the preceding claims

wherein the area of the thick film heating element is of the order of one third

to one half of the area of the surface to be heated.

8. A heating element assembly as claimed in claim 7 wherein the surface

to be heated is a disc of diameter of around 100 or 110mm and the thick film

heating element is a disc of diameter around 60 to 80mm.

9. A heating element assembly as claimed in any of the preceding claims

wherein the heat dispersion layer extends at least 10mm beyond the periphery

of the thick film heating element.

10. A heating element assembly as claimed in any of the preceding claims

wherein the surface to be heated comprises a heater plate adapted to be

assembled into a plastic bodied liquid heating vessel.

11. A heating element assembly as claimed in claim 10 wherein the heater

plate is formed around its periphery with a well.

12 A heating element assembly as claimed in any of claims 1 to 9 wherein

the surface to be heated compπses the bottom of a liquid heating vessel

13 A heating element assembly as claimed in claim 12 wherein said liquid

heating vessel bottom is formed of stainless steel, preferably 300 seπes

stainless steel.

14 A heating element assembly as claimed in claim 13 wherein said

stainless steel has a thickness of less than 0 5mm, preferably 0 3mm.

15. A heating element assembly as claimed m any of claims 10 to 14

wherein the surface to be heated, viewed from the side to which the thick film

heating element is affixed, has a depression into which the thick film heating

element is affixed

16 A liquid heating vessel having a thick film heating element of such a

high power density that excessive noise generation would occur in operation

were it not for the provision of a heat dispersion layer between the thick film

heating element and the vessel to provide more uniform heat distπbution and

so avoid excessive noise generation

17. A liquid heating vessel having a bottom surface to which there is

attached a thick film heating element, the thick film heating element being

significantly smaller than said bottom surface (e.g. less than half the diameter

thereof) and a heat dispersion layer of high thermal conductivity material (e.g.

copper or aluminium) being interposed between the thick heating element and

said bottom surface, said heat dispersion layer being significantly larger than

the thick film heating element (e.g. approaching twice the diameter thereof).

18. A heater assembly comprising a thick film heating element constructed

so as in itself to provide an output having a predetermined power density (e.g.

greater than 60W/cm²) and, affixed thereto, a heat dispersion member of

greater surface area than the thick film heating element, the power density of

the overall assembly thereby being substantially less than that of the thick film

heating element itself (e.g. only about 30W/cm²).

19. A heating element assembly as claimed in any of claims 1 to 15 and 18

in or for a liquid heating vessel and wherein the thick film heating element is

located in a depression in the bottom of the vessel (as viewed from the outside

thereof).

20. A heating element assembly substantially as herein described with

reference to the accompanying drawings.

21. A liquid heating vessel including a heating element assembly as

claimed in any of claims 1 to 15, 18, 19 and 20.