GB2215169A - Electric liquid-heating assembly - Google Patents

Abstract

In order to increase the initial rate of heating of the backplate (12) upon dry operation of the relevant appliance, the element (20) is so configured that, intermediate its ends, at least two separate portions (22, 24) thereof are in direct (or indirect) heat transfer relationship with the backplate (12). Consequently, the time taken for the primary cut-out, e.g. bimetal (18), to trip is reduced compared to a conventional element wherein only a single central portion of the element is connected to the backplate. This, in turn, means that the further temperature rise, after cut-out, is less steep than hitherto with reduced risk of deterioration of components and/or "nuisance" tripping of an irreversible secondary cut-out at an elevated temperature. <IMAGE>

Description

ELECTRIC LIQUID-HEATING ASSEMBLY This invention relates to an electric liquid-heating assembly, for an appliance such as an electric kettle.

Known electric liquid-heating assemblies designed for this purpose generally comprise a body which houses a temperature-sensitive cut-out, provides means whereby the assembly can be connected to a power supply and has a substantially flat metal backplate which can be sealed relative to an aperture in the appliance, and metalsheathed resistance heating element which extends from and has its ends sealingly connected to the backplate with a central portion of the sheath in direct thermal transfer relationship with the metal backplate, so that if the assembly has power supplied to it whilst there is little or no water in the appliance the temperature rise at the central portion of the element activates the cut-out to cut off the power. In this respect, it is usual for the central portion of the element sheath to be brazed to the metal backplate.

Such liquid-heating assemblies generally work well, but they do have certain drawbacks.

Typically, the cut-out (which normally includes a bimetal will trip some 14 to 18 seconds after operation of the element under completely dry conditions, as can occur if the appliance is accidentally switched on empty or has been switched on full and left to boil dry. During this period of time the element generates a considerable quantity of heat which continues to flow into the backplate even after the cut-out has tripped. This may lead to the temperature of the body rising to between 2200C and 250 C which is rather high for some of the materials used therein.For example, a nylon insulating material which is usually used to mount the cut-out on the backplate can withstand temperatures in the region of 230 C for short periods only before beginning to deteriorate. When this insulating material is heated above about 200 C, incipient softening and minor permanent dimensional changes occur which may well loosen its connection with other parts, such as the usual pins for an external plug connection, the backplate and the bimetal of the cut-out, and lead to a general lowering of the safety standards of the applicance.

Of particular importance is the effect of such excess heating on the mounting of the bimetal of the cut-out.

This strip is usually secured to the insulating material by a fastener in the form of a pin or screw. If the fastener is adversely affected by excess heating or due to excess heating of the insulating material, the operation of the bimetallic strip may be sufficiently hindered to increase the trip times. When this happens, the element remains hot for a much longer period with the attendant likelihood of destruction of the element and/or irreparable damage to the appliance.

It will be understood from the foregoing that repeated dry operation of such a liquid heating assembly, necessitating operation of the cut-out, will lead to a deterioration in the efficiency of the cut:-ntlt: dtie to overheating of the elements and the body of the assembly.

Nowadays many heating assemblies of the aforesaid general type have a dual cut-out, namely a primary arrangement which includes a bimetal and will act in the conventional manner to cut off power in circumstances of dry operation, and a secondary "fail-safe" arrangement which includes a melting fuse and which acts, similarly, to cut off power, but at a higher temperature, normally reached only if the bimetal has failed. The power cut-out effected by the bimetal is readily reversible and, excepting for the slight deterioration explained above, the appliance will function normally again once the bimetal has cooled and returned to its normal position. In contrast, the secondary cut-out is irreversible as the melting of the fuse permanently breaks the circuit. After this occurs the appliance will no longer function without replacement of the heating assembly.Nevertheless, the purpose of the secondary cut-out, namely to prevent complete destruction of the appliance, which would be extremely dangerous and would otherswise be the result in the event of failure of the bimetal cut-out, will have been fulfilled.

It will be appreciated that the continued heating of the backplate of the body after the primary cut-out has tripped, which effect is exacerbated whenever the trip time is prolonged, owing to deterioration of components or otherwise, gives rise to a risk, in a heating assembly with a dual cut-out, that the secondary cut-out may he tripped even when the bimetal has already functioned. As this renders the appliance us-eless, it is obviously unsatisfactory.

It is an object of the present invention to provide an electric liquid-heating assembly of improved construction such that the risk of post-cut-out overheating of the body leading to deterioration of the cut-out efficiency and/or to "nuisance" tripping of a fail-safe secondary cut-out are significantly reduced.

Pursuant hereto the invention provides an electric liquid-heating assembly comprising a body which houses a temperature-sensitive cut-out, provides means whereby the assembly can be connected to a power supply and has a substantially flat metal backplate which can be sealed relative to an aperture in the appliance, and a metalsheathed resistance heating element which extends from and has its ends sealingly connected to the backplate, characterised in that, intermediate the ends of the element at least two separate portions of the element are in thermal transfer relationship (direct or indirect) with the metal backplate so as to activate the cut-out rapidly upon dry operation of the appliance.

By substantially doubling the area of contact between intermediate portions of the element and the backplate compared to previously known heating assemblies of this type, the rate of heat transmission to the backplate and cut-out upon dry operation is increased and the trip time of the cut-out is reduced, typically from about 15 seconds to 11 seconds for a 2.2 KW element. Although the temperature of the backplate will continue to rise after cuit-out, it will do so less steeply than hitherto now that the period of power supply (i.e. the period of dry operation prior to cut-out) has been doctered.Thus, although the initial temperature rise is steeper with the proposed construction, the risk of overheating of components or of reaching an elevated temperature where nuisance" tripping of a fail-safe, but irreversible cutout will occur is significantly reduced compared to previously known assemblies.

The respective intermediate portions of the element may advantageously be directly connected to the backplate by brazing, as is conventional. Alternatively, however, they may be welded, soldered or otherwise fastened thereto so long as they are in direct thermal contact therewith.

Indirect connection of the respective intermediate portions to the backplate may be similarly achieved with an intervening piece of some kind merely being disposed there between, the portions being brazed or otherwise fixed to said piece which is itself firmly connected to the backplate.

In a further alternative arrangement a first intermediate portion of the element may not inpinge upon the backplate, or any intervening piece at all. It may, instead, be connected to a second intermediate portion (by brazing or otherwise), which in turn is connected to the backplate (with or without an intervening piece). Thermal transmission then takes place from the first portion to the backplate only via the second portion.

There are two conventional element configurations wherein a single central portion of the element is brazed to the backplate, mainly a double-loop configuration (See Fig. 1) and a spiral configuration. Both of these can be modified in accordance with the invention to have two separate portions of the element, intermediate its ends, connected to the backplate. To accomplish this practically, however, it will probably be necessary for the element to be longer than was previously the case to allow for the additional looping thereof.

As previously implied, the invention can advantageously be applied to liquid-heating assemblies having either a single bimetal cut-out or a dual cut-out.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a plan view of a conventional liquid-heating assembly with an element of double loop configuration; Fig. 2 is a perspective view of a preferred practical embodiment in accordance with the present invention; Fig. 3 is a diagrammatic fragmentary cross-section illustrating the connection of the element to the backplate in the conventional assembly; Fig. 4 is a view corresponding to Fig. 3 illustrating the connection of the element to the backplate in the embodiment of the invention shown in Fig. 2.

Fig. 5 is a view similar to Fig. 4, additionally showing n primary cut-out nrrangemellt; Fig. 6 is a view similar to Figs. 4 and 5, but showing a secondary cut-out arrangement; Fig. 7 is a graph of temperature against time of dry operation for a conventional liquid-heating assembly and for comparable assembly in accordance with the invention; Fig. 8 is a schematic fragmentary cross-section of an alternative embodiment in accordance with tulle invention and Fig. 9 is a similar view of another modified embodiment of the invention.

The preferred electric liquid-heating assembly of the invention, as illustrated in Figs.2 and 4 to 6, consists of a body 10 and an element 20. The body 10 includes a metal cup constituted by a substantially flat metal backplate 12, in the form of a disc of about 5 cm in diameter, and integral cylindrical side walling 14 extending from one side of the disc. The backplate 12 extends outwardly of the walling 14 to provide a sealing flange which can be clamped against the surrounding edge of an aperture in the side of a kettle (not shown). At a spacing from the backplate 12, the walling 14 is reinforced by an external sleeve 11 of metal. In use, a nylon insulator {not shown) mounting three connection pins for engagement by a plug rests inside the body 10. One of these pins will be an earth and will be connected directly to the backplate 12.The other two pins are the means by which power is supplied to the element 20. One of these is connected to one of a pair of contacts 15, 16 (See Fig. 5) of a primary cut-out arrangement, which includes a bimetal 18. The cut-out will be described in somewhat greater detail later.

The above-described structure of the liquid-heating assembly is basically the same as a conventional assembly, and, for ease of comparison, analogous components have been indicated in Figs. 1 and 3 by the same reference numerals surmounted by The difference between the assembly of the invention and the conventional assembly lies in the configuration of the element 20, 20' and its connection to the backplate 12, 12'. In each case the element 20 comprises a spiral heating coil extending between terminal conductors and enclosed in a tubular metal sheath, with some form of temperature resistant insulating material between the coil and the sheath. The detailed construction of the element is not illustrated as it is not pertinent to the invention and is well known in the art.Each end of the element's metal sheath is brazed to the backplate 12, around apertures in the plate through which the conductors penetrate.

In the conventional arrangement, shown in Figs. 1 and 3, the element 20' extends from each end outwardly from the backplate 12' then curves outwardly and backwards at each side to provide a loop, the central portion 13 of which contacts the backplate 12' and is united therewith by brazing. It will be noted from Fig. 3 that the central portion 13 is connected to the backplate 12' at a position closely adjacent the bimetal 18' of the primary cut-out for effect heat transmission from the element to the bimetal 18' via the backplate 12'.

In contrast, in the assembly in accordance with the invention, as shown in Figs. 2 and 4, the element 20 is longer and is of spiral configuration. However, the exact configuration is not critical as it could equally well be a double loop (as per Fig. 1) with an extra loop. The most important feature here is that intermediate the ends of the elements 20, two separate portions 22, 24 (See Fig. 4) contact the backplate 12 and are united therewith by brazing. These two portions 22, 24 lie alongside each other and, as previously, connect to the backplate 12 at a position closely adjacent the bimetal 18 of the cut-out.

In view of the enhanced area of contact between the element 20 and the backplate 12, compared to the conventional assembly, transmission of heat to the bimetal 18 is more rapid, resulting in quicker cut-off upon dry operation of the appliance.

Turning back to consideration of Figs. 5 and 6, the cut-out in this embodiment consists of primary and secondary arrangements, illustrated, respectively, in Figs 5 and 6. The contacts 15, 16 of the primary arrangement are attached to the insulator (not shown). One of these, 15 is provided with a push rod 17, which the bimetal 18, when heated, will press against to force the contacts 15, 16 apart and act as off power supply to the element 20.

The contacts 25, 26 of the secondary arrangement are also attached to the insulator. One of these 25, is biased by a spring 27 to move away from the other 26 and thus break the circuit supplying power to the element 20. However, the spring bias is countered by a melting fuse 28 inserted between the contact 25 and the backplate 12. At a certain temperature, several degrees higher than the temperature at which the bimetal 18 should act to cut-off power, the fuse 28 will melt, thus opening the contacts 25, 26 and permanently disabling the assembly. The secondary arrangement is, thus, a "fail-safe", designed to operate, only when the bimetal cut-out fails to prevent severe irreparable damage to the element 20 and indeed the entire appliance (kettle).

As has already been explained in the introduction to this specification, upon dry operation of the appliance, and following operation of the primary cut-off, the temperature of the backplate still continues to rise for some time. With the assembly in accordance with the invention, where at least two portions of the element are fixed to the backplate, the resulting quicker operation of the primary cut-out (about 11 seconds) also means that the subsequent continued rise is less steep than with a slower trip time (about 14 seconds). This is clearly shown in Fig. 7 where graph A is for the conventional assembly and graph B for the assembly of the invention. T1 is the temperature at which the primary cut-out should trip and T2 is the temperature at which the secondary cut-out should trip.Two advantages emerge from this slower continued temperature rise (prior to actual cooling) . Firstly, overheating of all the components, leading to their deterioration upon repeated dry operation, is minimized.

Secondly, the likelihood of the secondary cut-out being inadvertently activated (so-called "nuisance" tripping) is minimized as it is much less likely that the secondary trip temperature will be reached.

The invention is not restricted to the precise details of the illustrated embodiment and variations are quite possible. For example, as mentioned, the exact configuration of the element does not matter as long as two separate portions, intermediate the ends are in contact with the backplate. These may not always be side-by-side although that is probably most convenient and most effective. Connection of the element portions to the backplate may be by any suitable means allowing heat transfer e.g. brazing, soldering, welding. Also a secondary cut-out is not essential to the invention, although the concept of the invention is clearly particularly useful where such is present.

Two specific alternative arrangements for connection of intermediate portions of the element to the backplate are shown in Figs. 8 and 9, where the same reference numerals have been used for parts equivalent to those in the preceding Figs. 2 and 4 to 6.

In Fig. 8, the second intermediate portion 24 is brazed to the first intermediate portion 22 and only the latter is brazed to the backplate 12.

In Fig. 9, the two intermediate portions 22, 24 are both brazed to an intervening piece 30, which is connected to the backplate 12. Also in this embodiment the two portions 22, 24 are in contact with each other.

Both Figs. 8 and 9 represent arrangements for indirect thermal transfer at least one of the intermediate portions to the backplate. These arrangements should give satisfactorily improved performance compared to conventional heating assemblies, but the arrangenent described in detail above (Figs. 2 and 4 to 6) is thought to be most favourable.

Claims (1)

1. An electric liquid-heating assembly for an appliance, such as an electric kettle, comprising a body which houses a temperature-sensitive cut-out, provides means whereby the assembly can be corrected to a power supply and has a substantially flat metal backplate which can be sealed relative to an aperture in the appliance, and a metalsheathed resistance heating element which extends from and has its end sealingly connected to the backplate, characterised in that, intermediate the ends of the element, at least two separate portions of the element are in thermal transfer relationship (direct or indirect) with the metal backplate so as to activate the cut-out rapidly upon dry operation of the appliance.

2. An assembly as claimed in claim 1 wherein the respective intermediate portions are connected to the backplate by brazing.

3. An assembly as claimed in claim 1 or 2 wherein the respective intermediate portions are connected to the backplate side-by-side.

4. An assembly as claimed in any preceding claim wherein the element is of spiral or helical configuration.

4. An assembly as claimed in any preceding claim wherein the cut-out includes a bimetal which serves to reversibly break the power supply to the circuit at a predetermined elevated temperature.

6. An assembly as claimed in claim 5 wherein the cut-out additionally includes a separate meltable fuse which serves to irreversibly break the power supply circuit to the element at a temperature greater than that which activates the bimetal.

7. An electric liquid-heating assembly substantially as hereinbefore described with reference to an as illustrated in Figs. 2, 4, 5 and 6, or in Fig. 8, or in Fig. 9 of the accompanying drawings.