GB2330290A - Die-cast heating member - Google Patents

Abstract

An electric heater for a liquid heating vessel is formed by an electric heating element 2 cast into a die-cast carrier 12, preferably of aluminium or an alloy thereof. The electric heater is adapted to be completely immersed in the liquid to be heated. A minimum water level indicator 30 may be provided.

Description

ELECTRIC HEATERS The present invention relates to electric heaters and in particular to electric heaters for liquid heating vessels.

Traditionally, liquid heating vessels, for example kettles, have included a sheathed heating element positioned in the vessel so as to be immersed in the liquid to be heated. The heating element usually comprises a stainless steel sheath in which is provided a coiled electrically conductive heating wire and an electrically insulating packing material, such as magnesium oxide.

Electrical connection to each end of the heating wire is made via terminals which are inserted into the sheath a short distance and do not themselves generate heat. Thus the unheated portions of the element provided by the terminals are known as "cold tails".

Normally, the cold tails project through, and are secured to an element "head" which provides the interface, usually in one side of the vessel, between the interior of the vessel ("wet side") and the "dry side" of the vessel which houses the control for the element.

Element controls often comprise dry switch-on overheat protection (DSOP) mechanisms which prevent the element from overheating if power is supplied to the element in the absence of liquid in the vessel. Such mechanisms rely on actuation in response to the temperature of the element and are usually provided on the dry side of the head. Thus, thermal contact between the element and the dry side is established by a "hot return", which is a portion of the element in thermal contact with the head.

Stainless steel sheathed heating elements of the type described above are in common usage. However, stainless steel is a poor conductor of heat and is, despite its name, subject to corrosion. Furthermore, such elements are subject to uneven heating and local hot spots when limescale forms on their surface.

As an alternative to the immersed sheathed element described above, die-cast plate heaters have been proposed. Such heaters form the base of the liquid heating vessel and heat the liquid via the upper surface of the heater. This type of heater has an electric heating element provided underneath, and in direct thermal contact with, a die-cast base.

However, the provision of a heater in the base of the vessel presents many problems for the control of the heater, as known control devices for use with sheathed heating elements must be completely redesigned for this application.

The present invention attempts to overcome at least some of the disadvantages of known heaters.

According to the present invention, there is provided an electric heater for a liquid heating vessel comprising an electric heating element cast into a diecast carrier, wherein the electric heater is adapted to be completely immersed, in use, in a liquid to be heated.

The heater according to the invention may be used with standard controls, as it may be located in the liquid heating vessel in the same manner as known sheathed heating elements. However, the die-cast carrier acts quickly to transfer heat throughout the whole heater, such that an even heat distribution is achieved and the possibility of local hot spots due to limescale deposition is reduced. In addition, the shape of the heater according to the invention is determined by the die-cast carrier and thus greater flexibility in the aesthetic design of the heater is provided.

Moreover, the electric heating element may be of a construction similar to a standard sheathed heating element, but the sheath may be constructed of a material that is not resistant to water corrosion, and is therefore less expensive, as the heating element may be entirely enclosed by the carrier. For example, the sheath may be constructed of a steel other than stainless steel.

The carrier may be of any castable, thermally conductive material, for example aluminium or an alloy thereof. The carrier may be arranged to substantially follow the shape of the heating element set into it. In this case, voids may be defined in the carrier where loops occur in the element. Preferably, however, the carrier forms a substantially solid component containing the element, with regions of the carrier filling gaps in the shape of the element. In this way, the distribution of heat in the carrier can be increased due to the higher number of thermal pathways, resulting in a more even heat distribution across the whole heater.

To heater may comprise a head portion which is integrally formed with the carrier, for example during the casting of the carrier, for mounting the heater in an opening in a wall of the vessel, with the cold tails of the element projecting out from the head. The head portion may be an integral part of the casting, being formed of the casting material, or a separate component cast into the heater.

The heater is preferably provided with an integrally formed mounting location for a thermally sensitive actuator of a control unit, for example a dimple.

Preferably, the element is provided with a "hot return" portion in close thermal contact with the mounting location. Advantageously, a void is defined in the carrier proximate the hot return portion to allow the liquid in the vessel to circulate around the hot return and thereby prevent overheating of the hot return and consequent nuisance actuation of the DSOP mechanism during normal operation of the heater. This feature is particularly advantageous if the hot return comprises two overlapping sections of the heating element formed by a complete turn of the heating element. In this arrangement, heat from two lengths of element may be transferred to the DSOP mechanism to improve the efficiency of DSOP actuation. The void proximate the hot return may be defined within the turn of the heating element forming the hot return.

The invention also extends to a liquid heating vessel comprising a heater according to the invention.

Some embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figures la to 1d show, respectively, a top view, a side view, a rear view and a front view of a heating element for a heater according to the present invention; Figures 2a to 2d show in a partially transparent representation, respectively, a top view, a side view, a rear view and a front view of an electric heater according to the invention, showing the location of the heating element of Figures la to 1d within the carrier; Figures 3a to 3e show, respectively, a top view, a side view, a rear view, a front view and an underneath view of the electric heater of Figures 2a to 2d; Figure 4 shows in a partially transparent view the heater of Figures 2 and 3 mounted to a control; Figure 5 shows in a partially transparent view a heater according to a second embodiment of the invention; and Figure 6 shows a yet further embodiment of the invention.

Figure 1 shows four views (la-ld) of an electric heating element 2 for an electric heater for a liquid heating vessel according to the invention. The element 2 has two electrical terminals 4 for connection to a power supply via a control for the element 2. The terminals are connected to a coiled heating wire inside the element 2 which is packed in an electrically insulating medium. The outer sheathing of the element 2, in Figure 1, is of metal, such as stainless steel, and protects the delicate heating wire during the casting process. The choice of the material for the sheathing is wider than for conventional electric heaters, because the sheathing is enclosed by the die cast carrier.

The terminals 4 project a short distance into the sheathing before they connect to the heating wire.

Thus, these portions of the element are unheated and are known as the "cold tails". The cold tails prevent heating of the electrical components connected to the element.

The element 2 extends between the two terminals 4 in a continuous, tortuous configuration. The diameter of the sheath is 6.5mm and its overall length is 463mm, with an active heated length of 384mm. This gives a watts density of 28.05 Wcm-2 over the active surface of the element at an operating power of 2.2kW and 30.60cm2 at 2.4kW. The configuration comprises two symmetrical elongate loops 6 which loop from the inside of the configuration outwards and return towards the terminals 4 to form a complete turn 8 below the cold tails. The complete turn 8 provides two overlapping portions 10 of the heating element 2 at the furthest extent of the element 2 towards the terminals 4. These overlapping portions 10 form a "hot return" of the element 2 which is in thermal contact with the control 22 (not shown in this figure) of the element 2 such that the temperature of the element 2 can be monitored by the control 22.

The provision of two portions 10 of the element as the hot return ensures that more heat is transmitted to the control 22, thereby facilitating more accurate operation of the control. This is the purpose of the complete turn 8 in the configuration of the element 2.

Figure 2 shows four views (2a-2d) of a heater according to a first embodiment of the invention in a partially transparent view so that the location of element 2 described above can be seen within the diecast carrier 12. The element 2 is cast within the carrier 12, which is made of aluminium or a suitable castable alloy of aluminium, such that only a portion of the cold tails and the terminals 4 are exposed. The space inside the loops 6 of the element 2 is filled by the material of the carrier 12. However, a hole 14 is left through the carrier in the centre of the complete turn 8. This hole 14 allows liquid to circulate close to the hot return 10 such that the temperature of the hot return is maintained close to that of the liquid.

In this way, local overheating of the hot return and consequent nuisance actuation of the dry switch-on protection (DSOP) mechanism of the control is prevented.

The carrier comprises a head portion 16 which when the heater is fitted in a liquid heating vessel forms the barrier between the liquid in the vessel and electrical connections to the heater. Thus the heating element 2 is on the "wet side" of the head and the terminals 4 are on the "dry side of the head 16. The head 16 when fitted to the liquid heating vessel is provided with an annular seal (not shown) to ensure that liquid does not leak from the wet side to the dry side.

The head is also provided with mounting locations such as studs on the dry side to which a control unit 22 for the heater may be attached. In this embodiment the head is integrally cast with the carrier, although a nonintegral head is also possible.

The dry side of the head portion 16 comprises a dimple 20 against which in use abuts a bimetallic actuator 24 of the control unit 22, as shown in Figures 3 and 4. The control unit 22 is of the type described in GB 2181598, the applicant's R7 control. The bimetallic actuator 24 will operate to break the power supply to the heater in the event that the temperature of the element 2, and thus the hot return 10, rises above a predetermined temperature, as would occur if the element 2 was energised in the absence of a liquid or if the vessel boiled dry.

The hot return 10 is in close thermal contact with the dimple 20 so as efficiently to transfer heat to the actuator 24, to ensure a rapid operation of the control in an overheat condition.

The head 16 also provides on its dry side a location 26 for a back-up DSOP member of the control 22.

This location comprises a raised portion 26 of the head which facilitates thermal transfer from the hot return 10 to the DSOP back-up member. The back-up member (not shown) may for example be a plastics pin which is arranged to melt at a temperature associated with overheating of the element if the bimetallic actuator 24 fails to operate.

Figure 3 shows five views (3a to 3e), of the heater of Figure 2. The arrangement of the outer surface of the carrier is more aesthetically pleasing than conventional heaters and the provision of the die cast carrier allows for greater freedom in the design of the appearance of the heater. The outer surface of the carrier 12 may be coated with a cosmetic coating which will tend to prevent the build-up of limescale on the heater. Coatings of hydrophilic materials have been found to be particularly effective in this regard, for example Fluoralon B5517 or E8588. Alternatively, the surface of the aluminium carrier may be anodised.

Figure 5 shows a partially transparent plan view of a second embodiment of the invention. In this embodiment the configuration of the element 2 is substantially the same as that of the previous embodiment, although in this embodiment the overall sheath length is 494.5mm with an active heated length of 374.5mm giving a watts density of 28.76 Wcm-2 at 2.2kw and 31.38cm~2 at 2.4 kw. The carrier 12' of this embodiment is configured to substantially follow the configuration of the element 2' such that voids 24 are defined in the carrier 12' within the loops 6 of the element 2'. A further void 26 is defined between the loops 6.

Figure 6 shows three views 6a to 6c of a further embodiment of the invention, again for use with a control unit as described above. The main difference between this embodiment and those described above is that in this embodiment no void is provided in the region of the hot return 10". This considerably facilitates manufacture of the element 2' ', which as can be seen from the phantom lines does not have any fully looped portions, and has a shape more akin to a conventional sheathed element. In this embodiment, the element is of copper plated mild steel and, is of 6.6mm outer diameter, it has an overall length of 355mm and a heated length of 320mm giving a watts density of 33.15cm~2 at 230v/2.2kW.

This embodiment also includes a minimum liquid level indication 30 provided on the casting.

In summary, it will be seen from the foregoing that the invention provides a die-cast heater which may be fully immersed in a liquid to be heated.

Claims (9)

1. Claims: 1. An electric heater for a liquid heating vessel comprising an electric heating element cast into a diecast carrier, wherein the electric heater is adapted to be completely immersed, in use, in a liquid to be heated.
2. 2. An electric heater as claimed in claim 1, further comprising a head portion for forming a barrier in use between the liquid in the vessel and the electrical connections to the heating element, formed integrally with the carrier.
3. 3. An electric heater as claimed in claim 2 wherein said head portion is a cast portion of the carrier.
4. 4. An electric heater as claimed in any preceding claim wherein the heater has a mounting location for a thermally sensitive actuator, and the electric heating element comprises a portion in close thermal contact with the mounting location.
5. 5. An electric heater as claimed in claim 4 wherein a void is defined in the carrier proximate the said element portion such that in use the liquid to be heated circulates therearound.
6. 6. An electric heater as claimed in claim 4 or 5, wherein the said element portion comprises two overlapping sections of the heating element formed by a complete turn of the heating element.
7. 7. An electric heater as claimed in claims 5 and 6 wherein the void is defined within the complete turn of the heating element.
8. 8. An electric heater as claimed in any preceding claim wherein the carrier is formed of aluminium or an alloy thereof.
9. 9. A liquid heating vessel comprising a heater as claimed in any preceding claim.