GB2052225A - Improvements In or Relating To Electric Kettles - Google Patents

Abstract

An electric kettle incorporating an electrical immersion heater (12) including a heater element (14) and a thermal control unit (16) for preventing over heating of the heater element (14) is provided with a base (4) having a well (30) therein and the heater element (14) is disposed within said well (30) so that with only the well filled with water the element is adequately immersed for safe operation of the heater. <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. Such 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 a considerable energy wastage where only a small amount of hot water, 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.

It is an object of this invention to reduce the minimum quantity of liquid which has heretofore been required in the use of electric kettles.

Accordingly the present invention provides an electric kettle whose kettle body is provided with a base having therein a well within which the heater element is disposed the arrangement being such that the element may be sufficiently immersed for safe operation thereof by substantially only filling the well.

Thus in use of the kettle according to the invention it can be arranged that only the well need be filled with water to cover those portions of the element which become hot. This determines the minimum volume of the water which may be boiled within the kettle safely without danger to the element and such volume is reduced considerably as compared with known arrangements. Thus in accordance with the invention a kettle may be designed to have a minimum volume of only say one third pint (a cupful) as opposed to one pint or more which has been the case with known kettles.

The kettle in accordance with the invention may employ an immersion heater element of any suitable shape and size, the well of the kettle being generally complementary in shape to the element. For example, a conventional element has two cold leads extending parallei to each other from the element head, the element then being doubled back in a double open loop configuration so that a hot return portion of the element is secured to the element head. The outer form of such an element when viewed in plan is generally oblong and the well will have a similar form in plan closely surrounding the element; the element and well will extend from the element head to or near to the opposite side wall of the kettle.The well may be provided with raised elongate ribs which upstand within the open loops of the element to reduce still further the volume of water required to fill the well.

With such an element it has been found necessary to secure the hot return portion to or near to the top of the element head, the hot return portion in consequence being spaced some considerable distance from the base of the kettle.

This necessity has arisen from design constraints imposed on the thermal control unit. In particular it has been found necessary to position a thermally responsive actuator sensitive to a boil dry/switch-on-dry condition controlling a switch of the control unit close to the top of the head (see Specifications Nos. 1,470,365, 1,470,366, 1,470,367), and for proper control of the heater in a boil dry/switch-on-dry condition it is necessary to position the hot return portion in close proximity to the actuator at the top of the element head. With the hot return portion of the element positioned some distance away from the base of the kettle, it is necessary to raise the base wall to surround the element head and the hot return portion in order to provide a well of sufficiently small volume to boil only a small quantity of water.This results in a reduction in the overall volume of the kettle.

In a preferred embodiment of the invention however, this problem is overcome by connecting the hot return portion of the element to the heat by a metal member providing a heat conductive path therebetween. Since the hot return is not connected directly to the element head but by way of said metal member, the shape, configuration and disposition of the element head may be independent of design constraints imposed on the thermal control unit. Thus it is unnecessary in contrast to the arrangement described above to raise the base wall of the kettle to surround the element to provide a well of sufficiently small volume since the heated parts of the element can be positioned below and spaced from the element head closely adjacent to the base of the element; in consequence the total volume of the kettle will not be significantly reduced.

By employing said metal member providing said thermally conductive path, it is possible to design elements with configurations which were not thought previously to be practicable. One preferred configuration is to provide the heated parts of the element in the form of a spiral disposed in a plane adjacent to or lower than the bottom of the element head with one cold lead extending to the outer periphery of the spiral and the other cold lead extending to the innermost part of the spiral. This results in a very compact element, the area of the element in plan being relatively small, being about + the area of the base of a conventional 3 pint kettle. The area in plan of the wall surrounding the element is in consequence small.One important advantage of a compact element and well is that if the kettle is placed on an inclined surface such as a draining board and the well is filled with water there is far less risk of part of the element being exposed and not immersed in water as compared with the risk for conventional elements having a more open configuration and a large area in plan.

As described above, a conventional 3 pint kettle may be adapted to incorporate the invention without any significant reduction in volume. Thus the maximum volume of water the kettle can boil is 3 pints which may be compared with the minimum safe volume of water that can be boiled, i.e. the volume of the well, which may be 1/3 pint. The ratio of such maximum volumes is 9 and is a convenient way of expressing the energy saving potential with a kettle according to the invention. With conventional 3 pint kettles in current use, the energy saving ratio is generally 2 or 3 and at best 4.

The depth of the said well is determined by the clearance of the element from the bottom of the kettle, the thickness of the element and the depth of water above the element. To retain the maximum capacity of the kettle and the largest variation in maximum and minimum capacity it is advantageous that the depth of the well in the kettle is kept to a minimum.

Thus it is advantageous that the thickness of the element is kept to a minimum as the clearance below the element is controlled by manufacturing tolerances and the depth of water above the element is also fixed by the amount necessary to cover the element under all normal usage conditions such as when the kettle is placed on a sloping surface such as a draining board.

With a copper element it is usual practice to use 8 mm diameter circular section tube with a wall thickness of 0.75 mm. Clearly a flatter element would require less water to cover it, so the cross sectional shape of the element sheath may be altered from its conventional circular shape to a rectangular or triangular shape having a flattened profile i.e. a smaller height dimension).

Alternatively it is possible to flatten the element from a circular section to a "race track" section having a maximum thickness of 5 mm and a width of 11 mm. The flattening improves the compaction of the magnesium oxide powder insulation and reduces the thermal distance of most of the wire from the sheath.

This reduction in the thermal distance allows the wire to run at the same temperature as a conventional element but to emit more power per unit length, so that a shorter element is possible with all its cost savings.

In flattening the element experiments have shown that the circumference of the sheath is stretched from 25.2 mm to 28 mm thus increasing the surface area, as of course a round tube has the minimum surface area for a given length and volume. This increased surface area also improves the emission of heat from the element and helps reduction in the length, and hence the cost reduction.

It is advantageous to use as small a diameter tube as possible to start with, though manufacturing difficulties limit this to a practical minimum of about 6 mm with copper elements, and then to flatten the whole of the active length of the element.

The wall thickness of the element tube is determined by the requirement to withstand the bending up of the element and the stresses occurring in normal use such as attempts to clean the element. Here the use of rust resistant ferrous materials may offer certain advantages over copper. The latter work hardens on forming and is not a strong material so the element walls need to be correspondingly thick. This makes the element stiff and inflexible but the use of a stronger ferrous material, enables the wails to be made thinner.

For example, it is possible to manufacture an element using Monel metal with a diameter of 6 mm and a wall thickness of only 0.5 mm. With its much stronger but thinner sheath a Monel element is flexible, and this allows the element to move due to unequal heating of the sheath as bubbles form and dissipate from the water. The movement of the sheath naturally helps prevent the build up of scale on the element, and if any does form it is easily dislodged either by springing the element by hand or switching it on dry, so that the excess temperature and unequal expansion cracks off the scale which is usually CaCO3.

In experiments with boiling small quantities of water such as a teacup full very quickly it was also found that stratification of the water is a problem. If the element is more than a few millimetres from the bottom of the well of the kettle then there is insufficient time during the heating of the top portion of the water to boiling point to stir the whole volume as is usual. This can result in the steam produced operating a steam controlled off switch, but on pouring out the water and hence mixing it in the process, the temperature of the water will be less than 1 000C.

Thus it is advantageous to mount the element as low as possible in the well to minimise this stratification. With a stiff copper element it is difficult to get a good seal on the head if the element actually touches the bottom of the kettle, but with a springy Monel element it is possible to allow the element to touch the bottom of the well.

To prevent the element touching the bottom over too large an area and producing hot spots it has been found that it is advantageous to raise two ridges on the base of the kettle at 60C to each other and allowing a slight interference angle between the element and the kettle base.

The stirring of the water in the well to break down the stratification can also be improved by using an element having a triangular or D-shaped cross-section, the biggest side of the element or the flattened portion of the D being at the bottom.

The greatest wattage is emitted from these flat portions, and being at the bottom of the element they are as low in the well as possible. Also, the flat areas create turbulence to help stir up the cooler water under the element as the maximum wattage per square millimetre is produced in the middle of the flat area. In contrast with the smooth action of a round element these flattened forms help ensure that the whole of the well volume reaches 1000C. For instance a Monel element of circular cross section of 6 mm diameter can be pressed into a triangular or Dshaped section with a height of only 4 mm; With 1 mm clearance under the element and 5 mm water over the element the well need only be 10 mm deep in comparison with the 19 mm with a conventional copper element of circular crosssection.

The base of the well is kept in the same position and the raised portion of the well reduced as compared with that shown in the accompanying drawings, thus increasing the maximum water capacity and reducing the minimum water capacity. The reduction in the depth of the well facilitates fabrication of the kettle base. This is particularly so in the case of a spun kettle where the well has to be pressed as a separate operation.

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 a first embodiment of 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 plan view partly broken away of a second embodiment of a domestic kettle incorporating an electric immersion heater; Figure 4 is a sectional view of the kettle along the line 4-4 of Figure 3; Figure 5 is a fragmentary sectional view of the thermal control unit and heater head of the kettle of Figures 3 and 4; Figure 6 is a plan view of a further embodiment of heater element for a kettle according to the invention; Figure 7 is a sectional view of the element along the line 7-7 of Figure 6;; Figure 8 is a cross-section through an element having a circular element on an enlarged scale; and Figure 9 is a cross-section through the element of Figure 8 after it has been compressed in the vertical direction.

Referring to the kettle shown in Figures 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 1 0 (the lid therefore not being shown) and an electrical immersion heater 12 comprising a heater element 14 and a thermal control unit 1 6.

The heater element is mounted to a chromium plated brass head 1 8 formed as a plate of dished configuration. Cold leads 20 of the element extend from the head in the lower region of the head. The element extends in a plane closely adjacent base wall 4 in a double loop configuration to the other side of the kettle; a hot return portion 21 of the element extends above the cold leads and is welded or brazed directly to the head 1 8. The head 1 8 is mounted adjacent an aperture in side wall 2 and the cold leads, and internally screw threaded mounting pillars (not shown) which are secured to head 18, extending through the aperture. The cold leads are connected to an electrical switch in the control unit.Fixing screws extending through apertures in a body of the control unit to engage in said mounting pillars and serve to clamp the heater element and control unit to the sidewall 2. A bimetallic thermally responsive switch actuator controlling the electrical switch is disposed in direct thermal contact with a protrusion 22 formed in the head, the actuator by virtue of its dished configuration enveloping the protrusion. A smear of silicone grease heat transfer compound may be added between the protrusion 22 and the actuator to improve the thermal conduction path therebetween. The hot return portion 21 is brazed to the head 1 8 at a position directly opposite the protrusion 22.

The base wall 4, formed as a pressing of stainless steel, is shaped to define a well 30 surrounding the heater element 14. The well has a deepened scoop portion 32 to accommodate the head plate 1 8. The base wall 4 provides a circumferential platform 34 which is at a higher level than any heated portion of the heating element 14. The base wall 4 also provides two upstanding elongate ribs 36 which are disposed within the loops of the element.

The volume of well 30 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 with only the well filled with water the element is adequately immersed for safe operation of the heater when the kettle is switched on.

Should the kettle boil dry or be switched on dry the temperature of the element will quickly rise, this rise in temperature being detected by the switch actuator in the control unit by heat from the hot return 21. When the temperature of the actuator rises by a predetermined amount the switch in the control unit is actuated to switch off the element 14.

Referring now to the second embodiment of a domestic kettle shown in Figures 3, 4 and 5 parts similar to those shown in Figures 1 and 2 are indicated by the same reference numerals. In this embodiment however, the heater element 14 is mounted to a head 40 formed as a flat plate of 0.4 mm thick 18/8 stainless steel. The stainless steel has a low thermal conductivity (R=0.01 65 joule mm/mm2 sdeg C at 100 C) as compared with brass (R=0.104 joule mm/mm2 s deg C at 100 C) and is 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 batch of actuators having an operating temperature of at least 140+1 00C (which compares with known arrangements wherein actuators are taken from batches having an operating temperature of 1 30+50C).

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 20 of the element extend through the head from the lower region of the head, to join with the main body of the element which extends in a plane closely adjacent to the base wall of the kettle in a spiral configuration. One cold lead of the element 14 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 cm-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 metal 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 portion 46. The strip has a thickness of 1.5 mm and a height of 3.6 cm. The rear edge of the strip is vertical and the front edge tapers at 15 towards the head to provide a strip 1.2 cm at the base narrowing to a neck 0.5 cm 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 seal or grommet 62 of acrylonitrile rubber (as opposed to the more expensive conventional silicon rubber) is mounted to the edge of the aperture. The cold leads are connected via an electrical switch 64 to pins 66 of an electrical plug formed in the control unit 1 6.

Fixing screws 68 extending through apertures in a fibre reinforced nylon 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 73 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 configuration 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 14. 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 operation of the heater when the kettle is switched on. The area of the element is about i of the area of the base of the kettle and the area of the well in plan is about 2/5 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 form 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 the arrangement shown in Figures 1 and 2 where the element has a more open form.

If a boil dry/switch-on-dry condition occurs, the temperature of the element rises to a very high level. Heat will permeate through to actuator 72 from hot return portion 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 1 400C, the overshoot temperature of the actuator as measured at the interface between base wall 44 of the rib 42 and seal 62 (which is where the major part of the heat will be transmitted 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 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 1 800C.

This 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 temperature of the element will rise to a very high level and cause any deposited mineral scale on the element to be blown off by the high temperature of the element and unequal expansion between the scale and the element.

Referring now to Figures 6 and 7 there is shown a further heating element which may be incorporated in a kettle similar to that of Figures 1 and 2 with suitable adaptation of the well. The element is mounted in a stainless steel head 100 formed as a plate of dished configuration. Cold leads 102 of the element are mounted centrally of head 1 00. The leads are angled downwardly as at 104 so that the main body of the element is disposed in a plane level with the bottom of the head.

The element is of double loop configuration and the outer portions of the loops curve inwardly as at 1 06. The hot return portion 108, which is disposed adjacent to but spaced from the bottom of the head 1 00, is connected to the head by a copper strip 11 0, which provides a heat conductive path therebetween as in the previously described embodiment. Whilst the copper strip is of constant width, it is twisted as shown so as to extend between the cold leads 102 and the upper portion of the head. The face of head plate 100 remote from the hot return portion 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 112 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.

It will be noted that in the embodiments of Figures 3 to 7 the use of heat conducting copper strips permits the main part of the heating elements to be mounted substantially level with the bottom of the heater heads, despite the necessity for the thermally responsive actuators to be mounted at the top of the heads. This means that the bottom of the well may be at the same level as the base of the kettle (as well illustrated in Figure 4) in contrast to the embodiment of Figure 2. The available capacity of the kettle is, therefore, increased given the same overall dimensions.

It is possible to so design the element that a shallower well than that shown in Figure 2, 4 or 5 may be used. This can be achieved by flattening the element from its conventional circular section to a D section or a "race track" section. Figure 9 shows such a "race track" section as obtained by compressing the circular section element shown in Figure 8. The depth of water required to cover such a flattened element is less, so the well may be made shallower. Thus the capacity of the kettle is greater for the same overall dimensions, and can approach the volume of a conventional 3 pint kettle having a flat base wall.

It will thus be seen that in accordance with the invention it can be arranged that only the well need be filled with water for the element to be adequately immersed for safe operation of the heater. Whilst this is a preferred arragement it will be realised that whilst the main body of the element will always be disposed within the well it is possible that part of it could project above the well. In this case, to immerse the element would require slightly more water than that necessary to fill the well only, but substantial savings in energy, as mentioned previously, would still be obtained.

Claims (10)

Claims

1. An electric kettle whose kettle body is provided with a base having therein a well within which the heater element is disposed the arrangement being such that the element may be sufficiently immersed for safe operation thereof by substantially only filling the well.

2. An electric kettle according to claim 1 in which the well has a similar plan form to and closely surrounds the element.

3. An electric kettle according to claim 1 or 2 in which the well is provided with one or more raised ribs which upstand within one or more open loops of the element.

4. An electric kettle according to claim 1,2 or 3 in which a hot return portion of the element is connected to the element head by a metal member providing a heat conductive path therebetween.

5. An electric kettle according to claim 4 in which the heated parts of the element are provided in the form of a spiral disposed in a plane adjacent to or lower than the bottom of the element head with one cold lead extending to the outer periphery of the spiral and the other cold lead extending to the innermost part of the spiral.

6. An electric kettle according to any of the preceding claims in which the cross section of the element sheath is of rectangular D-shape or triangular shape whose height is less than the width thereof.

7. An electric kettle according to any of claims 1 to 6 in which the element is flattened from a circular section to a race track section.

8. An electric kettle according to claim 7 in which the element tube wall is of ferrous material.

9. An electric kettle according to any of the preceding claims wherein the well has a capacity of substantially 0.19 1.

10. Electric kettles substantially as hereinbefore described with reference to the accompanying drawings