GB2052226A - Improvements In or Relating To Electric Kettles - Google Patents

Abstract

In an electric kettle, the major part of the element 14 including the hot return portion 46 is disposed close to the base 4 of the kettle. The thermal control unit 16 includes a temperature responsive actuator 72 mounted in an upper region thereof and adapted to switch off the kettle on overheating. Heat is conducted to the region of the actuator 72 from the hot return portion 46 by means of a conductive linking member 48 of greater thermal conductivity than the element head 40. The element head 40 has a lower thermal conductivity than brass to limit heat conduction from the region adjacent actuator 72. <IMAGE>

Description

SPECIFICATION Improvements in or Relating to Electric Kettles This invention relates to electric kettles i.e.

kettles which incorporate an electric immersion heater including a heater element and a thermal control unit for preventing overheating of the element, and to electric immersion heaters therefor.

Kettle elements are conventionally disposed close to the base of the kettle but since it is necessary to cover the element with water for the protection of the element while boiling water, it is usually necessary to fill the kettle with a minimum of about 1 pint of water. This results in considerable energy wastage where only a small amount of hot wate, for example to fill a single cup or mug, is required; the time taken for the water in the kettle to boil is correspondingly and unnecessarily long.

The large amount of water required to cover the element results from the fact that at least a heated part of the element is spaced at some height from the base of the kettle. This arises from the design of the element in the following way: the thermal control unit includes a thermally responsive actuator for actuating an electrical switch to terminate the power supply on overheating of the element. The actuator is disposed near the top of the element head, and it is conventional to weld or braze a heated portion of the element (usually called the hot return portion) to the element head in the region of the actuator i.e. near the top of the head, and some distance from the kettle base.Thus while the main body of the element may be disposed close to the base, the hot return portion is still at some height from the base, and this height determines the minimum amount of water for the kettle, which is correspondingly greater than would be the case if only the main body of the element were to be covered.

It is an object of the invention to provide an improved kettle and immersion heater which may be used with a lesser minimum quantity of water than that described above.

According to the invention there is provided in or for an electric kettle, an electric immersion heater comprising a heater element having a head formed of a material having a low thermal conductivity as compared with that of brass, a thermal control unit including an electric switch and a thermally responsive actuator adapted to operate said switch to terminate the power supply to the element on overheating of the latter, and a metal linking member interconnecting said head at a position in proximity to said actuator with a heated portion of said element disposed there beneath and adjacent to the base of the kettle in use so as to conduct heat from said heated portion to said actuator.

Thus in accordance with the invention the hot return portion need not be connected directly to the element head but is connected by way of said metal linking member, and the position and configuration of the element are not constrained by the necessity of bonding the hot return portion directly to the head in a particular position determined by design considerations in the location of the thermally responsive actuator in the thermal control unit.

Thus in accordance with the invention the major part of the element including the hot return portion but apart from the cold leads can be positioned beneath, or at the base of, the element head whereby the element can be positioned closely adjacent the base of the kettle. In other words, the heated portions of the element may be disposed in a plane closely adjacent the base of the kettle and there is no necessity to raise the hot return portion of the element to a position in proximity to the thermally responsive actuator of the thermal control.

Thus only a small amount of water may be required to cover the heated portions of the element. If it is required for example to boil a cupful of water it is not necessary as has been the case previously to fill the kettle to a considerable depth in order to cover the element and thus protect the element whilst the water is boiled. The minimum amount of water which may be boiled in the kettle is reduced to about one half pint resulting in energy conservation and quicker boiling times when small amounts of boiling water are required. A convenient way of expressing this feature is in terms of the ratio of the maximum volume of water the kettle can boil to the minimum volume the kettle can safely boil.

This is a convenient way of expressing the energy saving potential of the kettle. In a conventional 3 pint kettle employing the heater in accordance with the invention such ratio is (3:-)=6. For conventional 3 pint kettles employing conventional elements currentiy on the market the ratio is somewhere between 2 and 3 and at best 4.

Since the element head is of a thermal conductivity low as compared with that of brass, heat cannot readily diffuse throughout the head and a localised hot spot is created in the region of the connection of the linking member to the head and in the region of the thermally responsive actuator. Thus the control unit is protected in a boil dry/switch-on-dry condition from overheating by general thermal conduction through the head.

A further advantage obtained by the invention is a reduction in the over shoot temperature at the periphery of the control unit. In conventional heaters where the hot return portion is brazed directly to the head this is normally done across a diameter or chord of the head giving a "hot spot" of large area extending closely to the periphery.

When the power supply is terminated the temperature of the hot spot continues to rise for a while and undesirably high temperatures may be reached at the periphery. For this reason, peripheral seals need to be formed of temperature resistant, and therefore expensive, material.

However, in the present invention the heat from the hot return portion is concentrated in a small localised hot spot in the region of the connection of the linking member to the element head. Thus the heat must diffuse radially of the element head a comparatively long distance to reach the periphery of the element; in consequence of this and the lower thermal conductivity of the element of the head the overshoot temperature at the periphery of the element head is much reduced.

The shape, configuration, dimensions and thermal conductivity of the linking member will be determined by the necessity to conduct a sufficient amount of heat to the element head in the region of the thermally responsive actuator to create the localised hot spot at a sufficiently high temperature in a boil dry/switch on dry condition to heat the actuator to its operating temperature.

The metal part may conveniently be a strip of e.g.

copper extending from the hot return portion, positioned below the element head, to the top of the element head. Alternatively the hot return portion may be positioned in close proximity to the head, and the linking member be formed as a sheet, one face being bonded to the hot return portion and the other face being bonded to the head. In any event, the thermal conductivity of the linking member is preferably greater than that of the head.

The head of the element may be of any suitable material, but stainless steel is preferred in view of its high strength, corrosion resistance, ready availability and particularly low thermal conductivity. For example the thermal conductivity of 1 8/8 stainless steel is only about 0.0165 joules mm/mm2s deg C at 1 000C as compared with a value of about 0.104 joules mm/mm2s Deg C for brass ("Physical and Chemical Constants" (12th edition) by Kaye and Laby-Longmans).

In view of its strength a stainless steel head can have a much smalier thickness (for example 0.3 mm to 0.75 mm preferably about 0.5 mm for 1 8/8 steel) than a conventional brass head (about 1.25 mm). This thinness in combination with the much lower conductivity reduces thermal diffusion from the hot spot to the periphery of the head and is the main factor in reducing temperature overshoot at the periphery of the head as mentioned above. The minimum thickness of the steel head will be that which enables the head to be sufficiently strong to resist bending stresses arising in use of the kettle.

The main body of the head may be in the form of a plate of generally flat configuration but including a dished portion adapted to receive the cold leads and house the actuator. Preferably the head will also have an integral raised cylindrical flange adapted to extend through the usual aperture in the kettle wall and be engaged from outside to draw the head into sealing engagement with the kettle wall. A sealing washer will be disposed between the head and the wall, but in view of the reduced overshoot at the periphery of the head this need not be as temperature resistant, and therefore expensive, as with conventional heads. For securing the head in position the outer end of the cylindrical flange may be bent outwardly to form a single helical thread for engagement by a suitable threaded member.

As stated above the element is preferably shaped so that all the heated portions of the element (i.e. apart from the cold leads) are disposed in a plane closely adjacent the base of the kettle. Within that plane, the element may have a sinuous configuration of any conventional type; alternatively and as preferred the element may be curved in the form of a spiral being connected to one cold lead at the outer periphery of the spiral and being connected to the other cold lead at the innermost part of the spiral.

An additional source of saving arises in that since the control unit remains at a relatively low temperature in an overshoot condition, the overall length of the heating element may be shortened and provided with a higher watts/density ratio so that in a switch on dry/boil dry condition the temperature of the element may rise to cherry red heat (much higher than previously was possible without overheating the control unit. Thus previously for a 3 kw element, an 8 mm diameter, 23 inch long tube was employed having a watts density ratio of 45/50 watts cam~'. It is now possible to employ for a 3kw heater a shorter element, say 62.5 mm shorter (which represents a saving in manufacturing cost of 5p), with a higher watts density of at least 55 watts cam~:.

Elements are usually formed as a convoluted cylindrical member comprising a sheath of chromium plated copper tube with a fine coiled heating wire extending therethrough and a filling of compacted magnesium oxide (MgO). As an aid in reducing temperature overshoot at the element head, the chromium plated copper tube may be replaced by a tube of non-corrodable metal of relatively low thermal conductivity as compared with copper, e.g. stainless steel, monel, inconel or incoloy, so that heat is not conducted to the head by way of the cold leads of the element. This has the added advantage that a plated material is not employed, plating tending to flake off at high temperatures such as may be encountered with the present invention.The sheath of the element may then rise to a high temperature in a boil dry/switch on dry condition without damaging the element or thermal control unit. An important advantage arising with this preferred feature of the invention is that in hard water areas where the elements tend to develop a mineral scale, principally calcium carbonate, the element may be deliberately switched on dry without any water in the vessel and the resulting high temperature of the element automatically descales it.

The use of the linking member described above permits all the heated parts of the element, including the hot return portion to be located at a level considerably below the thermally responsive actuator. This furthermore allows the element to be disposed in a well provided in the base of the kettle body. Such an arrangement provides additional and substantial energy savings where only a small quantity of boiling water is required, since it can be arranged that only the well need be filled with water to cover those portions of the element which become hot, and the well may have a volume of only one third of a pint. To reduce the volume of the well still further, the base of the well may be provided with one or more upstanding ridfges to extend into respective free space(s) formed by the turns of the element.

Thus a heater in accordance with the invention employed in a kettle having a volume of 3 pints and the aforesaid well having a volume of 1/3 pint, the energy saving maximum/minimum ratio of the kettle is 9 which compares as stated above with between 2:1 and 3:1 and at best 4:1 with conventional kettles.

Where the element is of a preferred, compact spiral form, the well for the element may surround the element and have a similarly small area in plan. This has the advantage that where the kettle is placed on an inclined surface such as a draining board the element is less likely to be exposed when the wall is filled than for example when a kettle, having a well with a larger area in plan receiving a larger conventional element, is placed on an inclined surface.

As a means of'still further decreasing the minimum volume of water which can be boiled safely by the kettle and increasing the energy saving ratio, the cross sectional shape of ths sheath of the element may be altered from the conventional circular shape to a rectangular, triangular;, D-shape or "race track" section having a lower height dimension. Thus the amount of water required to cover the element is reduced, and the depth of the well, if provided, may likewise be reduced.

The heater in accordance with the invention may incorporate measures for increasing sensitivity to element overheating and it is particularly preferred to incorporate the invention as disclosed and claimed in Patent No. 1470365, i.e. a thermally-controlled electric immersion heater for mounting on the wall of a liquid heating vessel including a heater unit and a switch unit, the heater unit including a heater element carried by a heater head in the form of a plate substantially flat or dished configuration, and the switch unitincluding at least the movable contact of an electrical switch for interrupting the electrical supply to the heater element and thermally responsive switch actuating means mounted to make direct thermal contact with said head plate and comprising a stressed sheet of bimetal movable with changes in temperature between oppositely dished configurations with a snap-action, mounted on a thermally insulating portion of the switch unit so that there is in use no substantial conductive heat loss from the heat conducted to the bimetal sheet from the head plate.

In addition other measures may be incorporated to increase thermal sensitivity, in particular the invention disclosed and claimed in Patent No. 1401954 wherein there is disclosed an electric immersion heater head, the wall of said head adjacent which a bimetal sheet of a control unit extends in use being provided with a protrusion positioned so as to be situated in a concavity of the bimetal sheet, close to or touching the wall.

Preferred embodiments of the invention will now be described with reference to the accompanying drawings, wherein.

Figure 1 is a plan view partly broken away of an embodiment of a domestic kettle incorporating an electric immersion heater; Figure 2 is a sectional view of the kettle along the line 2-2 of Figure 1; Figure 3 is a fragmentary sectional view of the thermal control unit and heater head of the kettle of Figures 1 and 2; Figure 4 is a plan view of a preferred form of heater element for the heater according to the invention; and Figure 5 is a sectional view of the element along the line 5-5 of Figure 4.

Referring to the kettle shown in Figure 1 and 2, the kettle has a cylindrical side wall of stainless steel 2, a base wall 4 of stainless steel, from which feet 5 depend, a handle 6, a spout 8, a filling aperture 10 (the lid for the aperture not being shown) and an electrical immersion heater 1 2 comprising a heater element 14 and a thermal control unit 1 6.

The heater element 14 is mounted to a head 40 formed as a flat plate of 0.4 mm thick stainless steel. The periphery of the head is formed by a rolling over operation to provide an annular rib 42 having a base wall 44. Cold leads 42 of the element extend through the head from the lower region of the head. The element extends in a plane closely adjacent to the base wall of the kettle in a spiral configuration.

One cold lead of the element extends to the outer periphery of the spiral, the other cold lead extends to the innermost part of the spiral. The element has a cylindrical outer sheath of stainless steel and has a watts density ratio of 56.5 watts cam~1. The spiral extends in a plane level with the base of the element head 40 and the portion 46 of the spiral form nearest to the element head 40 provides a hot return portion and is thermally connected to the head by means of a linking member 48 comprising a strip of copper. The strip of copper 48 is disposed in a vertical plane perpendicular to the element head 40 and has a laterally extending tab portion 50 at the top for connection to the head 40. The strip 48 has at its bottom a flange 52 extending on either side of the strip connecting the strip to the hot return 46.The strip has a thickness of 1.5 mm, a height of 36 mm. The rear edge of the strip is vertical and the front edge tapers at 1 50 towards the head to provide a strip 12 mm at the base narrowing to a neck 5 mm wide immediately below the junction of the strip 48 with the head.

Tab portion 50 has a semicircular configuration and is brazed or welded in a complementary shaped recess formed by pressing the element head 40, such pressing providing on the opposite side of the head a protrusion 54.

The head 40 is mounted adjacent an aperture in side wall 2 and the cold leads and three internally screw threaded pillars 60 which are secured to head 40 extend through the aperture.

An annular acrylonitrile rubber seal or grommet 62 is mounted to the edge of the aperture. The cold leads are connected via an electrical switch 64 to pins of an electrical plug 66 formed in the control unit 1 6. Fixing screws 68 extending through apertures in a plastics body 70 of the control unit engage in the mounting pillars 60 and serve to clamp the heater element and control unit to the sidewall 2. A bimetallic thermally responsive switch actuator 72 which snap acts between oppositely dished configurations with changes in temperature is mounted on a thermally insulating plastics pillar 74 of the plastics body 70 of the control unit 1 6 and is disposed in direct thermal contact with protrusion 54 formed in the head, the actuator 72 by virtue of its dished configuation enveloping the protrusion.A smear of silicone grease heat transfer compound may be added between the protrusion 54 and the actuator 72 to improve the thermal conduction path therebetween.

The actuator 72 is coupled to a leaf spring 74 carrying a movable contact 76 of the switch 64 via a push rod 78.

The base wall 4, formed as a pressing of stainless steel, is shaped to define a well 80 surrounding the heater element of generally circular configuration in plan, that is complementary to the outer shape of the spiral element The well is sufficiently deep in relation to the disposition of the element therein that with only the well filled with water the element is adequately immersed for safe protection of the heater when the kettle is switched on. The area of the element is about 1/4 of the area of the base of the kettle and the area of the well in plan is about 2/5 of the area of the base of the kettle. The volume of well 80 amounts to 1/3 pint which is equivalent to a cup or beaker full of water. This volume of water is sufficient to cover the element and therefore the kettle may be switched on safely with only this small amount of water in the kettle.

An advantage of the spiral from of heater element arises from its compactness, since should the kettle be placed on an inclined surface such as a draining board there is less risk of a heated portion of the element being disposed above the water line when the well is filled, as compared with arrangements where the element has a more open form.

In accordance with the invention a a element head 40 of stainless (18/8) steel is provided having a low thermal conductivity (R=0.01 65 joule mm/mm2s deg C at 100 C) as compared with brass (R=0.104 joule mm/mm2s deg C (at 100 C) and being 0.4 mm thick (this compares with brass heads having thicknesses of 1.25 mm).

The thermally responsive bimetallic snap acting actuator is taken from a selected batch of actuators having an operating temperature of at least 1400C +100C. (which compares with known arrangements wherein actuators are taken from selected batches having an operating temperature of 1 300C+50C) The seal 62 is formed of acrylonitrile rubber (known seals are formed of more expensive silicon rubber). The material, fiber reinforced nylon, of control unit body 70 is of a lower quality than that that has previously been employed. The element has an outer sheath of stainless steel and has a watts density ratio of about 56.5 watts cml.

In use of such an immersion heater in accordance with the invention, if a boil dry/switch on dry condition occurs, the element may rise to a very high temperature. Heat will permeate through an actuator 72 from hot return 46 via copper strip 48 and the protrusion 54 in element head 40. Since the head is formed as a sheet of thin stainless steel heat will not readily diffuse throughout the head and a localised hot spot will be created at the protrusion 54, the outer parts of the element head remaining relatively cool.Thus with a break temperature of the actuator of 1400 C, the overshoot temperature of the actuator as measured at the interface between base wall 44 of rib 42 and seal 62 (which is where the major part of the heat will be trasmitted to the thermal control unit from the element in a boil dry/switch on dry condition) is about 1 200C. This compares with overshoot temperatures in known arrangements of 2200C. It has been found that with actuators in accordance with the invention employing a stainless steel head and a copper strip connecting the head with a plated copper sheathed element, the overshoot temperature rose to a maximum of about 1 700C as measured at the interface of wall 44 and seal 62.

With a stainless steel head and a plated copper sheathed element having a hot return brazed directly to the head, the overshoot temperature rose to about 180"C.

This great reduction in overshoot temperature in the region where heat is transmitted to the thermal control unit brings many advantages, in particular in cost savings in that safety margins and quality control may be substantially reduced.

With a stainless steel element head and an element sheath of stainless steel, it is possible without damage to any part of the immersion heater or kettle to deliberately switch the kettle on dry. The element will rise to a very high temperature and where the kettle is used in hard water areas where a large amount of minerals is deposited on the element as scale during use of the kettle, such scale will be blown off the element by the high temperature of the element and unequal expansion between the scale and the element.

Referring now to Figures 4 and 5 there is shown a heating element which may be incorporated in a kettle similar to that of Figures 1 and 2. The element includes a stainless (18/8) steel head 100 formed as a plate of dished configuration from a sheet 0.4 mm thick. Cold leads 102 of the element are mounted centrally of head 100. The leads are angled downwardly as at 104 so that the remainder of the element is disposed in a plane level with the bottom of the head.

The element has a plated copper sheath and is of double loop configuration and the outer portions of the loops curve inwardly as at 1 06.

The hot return portion 108 is disposed adjacent to but spaced from the bottom of the head 100. The hot return 108 is connected to the head by a copper strip 110. Whilst the copper strip is of constant width, the copper strip is twisted as shown so as to extend between the cold leads 1 02 and the upper portion of the head. The face of head plate 100 remote from the hot return is formed with a protrusion 112 against which a bimetallic snap-acting thermally responsive actuator (not shown) is disposed in use of the element. The protrusion 11 2 is formed in a pressing operation and the copper strip 110 is connected to the head plate 100 by welding the end of strip 110 in the recess 114 formed by the pressing operation. The head plate is provided with three internally screw threaded pillars 11 6 for mounting purposes.

When the element of Figures 4 and 5 is mounted in a vessel similar to that shown in Figure 1, it will be understood the shape of the well in the base of the kettle will be adapted to accommodate the shape of the element.

In use of the element in a boil dryXswitch on dry condition the copper strip 110 conducts the heat of the element to the protrusion 112 to create a localised hot spot at the protrusion whereby a thermally responsive bimetallic snap acting actuator detects the over heating of the element and actuates a switch to cut off power to the element.

Claims (9)

Claims

1. In or for an electric kettle, an electric immersion heater comprising a heater element having a head formed of a material having a low thermal conductivity as compared with that of brass, a thermal control unit including an electric switch and a thermally responsive actuator adapted to operate said switch to terminate the power supply to the element on overheating of the latter, and a metal linking member interconnecting said head at a position in proximity to said actuator with a heated portion of said element disposed therebeneath and adjacent to the base of the kettle in use so as to conduct heat from said heated portion to said actuator.

2. An immersion heater according to claim 1 in which the major part of the element including the hot return portion is positioned beneath or at the base of the element head.

3. An immersion heater according to claim 2 in which the linking member is a strip extending from the hot return portion to the top of the element head.

4. An immersion heater according to claim 1 or 2 in which the hot return portion is positioned in close proximity to the head, and the linking member is formed as a sheet one face being bonded to the hot return portion and the other face being bonded to the head.

5. An immersion heater according to any of the preceding claims in which the thermal conductivity of the linking member is substantially greater than that of the head.

6. An immersion heater according to any of the preceding claims in which the head of the element is of stainless steel.

7. An immersion heater according to any of the preceding claims in which the element is curved in the form of a spiral being connected to one cold lead at the outer periphery of the spiral and being connected to the other cold lead at the innermost part of the spiral.

8. An immersion heater according to any of the preceding claims in which the element tube of non-corrodable metal of relatively low thermal conductivity as compared to copper.

9. An immersion heater according to claim 9 whose cross-sectional shape is rectangular, triangular, D-shape or race track section having a low height dimension.

1 0. Immersion heaters substantially as hereinbefore described with reference to the accompanying drawings.