WO1997043873A1

WO1997043873A1 - Electric heaters

Info

Publication number: WO1997043873A1
Application number: PCT/GB1997/001323
Authority: WO
Inventors: Vernon James Donnelly; John Crawshaw Taylor; Martin J. Shepherd
Original assignee: Strix Limited
Priority date: 1996-05-15
Filing date: 1997-05-15
Publication date: 1997-11-20
Also published as: CN1543265A; CN1221549A; CN1128567C; GB2332845A; CN100471346C; GB9610097D0; GB9824979D0; GB2332845B

Abstract

A die-cast heater (2) for a liquid heating vessel comprises a heating element (4) arranged in substantially a single turn about the periphery of the heater and having overlapping end portions (34). The overlapping end portions (34) allow the heating element (4) to heat substantially the entire periphery of the heater (2). The die-cast element carrier (3) comprises integral bosses (30) which are in direct thermal contact with the heating element (4) and serve as thermal contact locations for bimetallic actuators of a control for the heater (2). The boses (30) smooth out transient fluctuations in the temperature registered by the bimetallic actuators to prevent inappropriate operation.

Description

Electric Heaters

The present invention relates to electric heaters, and in particular to electric heaters adapted to form the base of a liquid heating vessel.

Liquid heating vessels are known in which at least a portion of the bottom of the vessel is constituted by a die cast heater. Such heaters comprise a sheathed electric heating element which is die cast into one side of an aluminium or aluminium alloy plate whose other side forms the inner surface of the vessel base. Typically such heaters are clamped against the bottom of the vessel wall by clamping screws, with a seal being interposed between the heater and the bottom of the vessel wall.

From a first aspect, the invention provides a die- cast heater comprising a sheathed heating element cast into an element carrier, wherein the heating element of the heater is arranged in at least a single turn around the periphery of the heater, and has overlapping end portions such that heat is provided around substantially the whole periphery of the heater.

Preferably the heating element is arranged in substantially a single turn, and the terminal pins of the element are preferably arranged close together. This facilitates connection of the terminals to certain control units, such as those described in WO 95/34187.

Most preferably the ends of the element project from the carrier, generally perpendicularly thereto, and are generally parallel.

The ends of the element overlap to provide heating over substantially 360°. The element may have to extend around more than 360° to provide heating around fully 360°, in view of the fact that the "cold tails" of the element may extend back into the element from its ends for some considerable distance. The cold tails can overlap closely since they effectively generate little or no heat, thereby preventing localised hot spots on the periphery of the heater, which is particularly advantageous where the heater is mounted to a plastic- walled vessel, because such hot spots may develop sufficient heat proximate the plastics wall of the vessel to damage the wall, for example in a dry switch on condition of the heater. In one embodiment, therefore, substantially only the cold tail portions overlap. Of course, depending on how far the cold tails extend back into the element, it is possible in such arrangements that the heated portion of the element may extend less than 360° around the periphery of the heater, for example 330°-360°. Such arrangements are intended to fall within the scope of the invention.

A localised region of higher temperature may be tolerated in the central region of the heater when it is desired to provide a larger heat output without substantially increasing the operating temperature of the periphery of the heater.

Thus, in the preferred embodiment, the overlapping end portions of the heating element may be directed inwardly from, preferably substantially perpendicularly to, the periphery of the heater. The inwardly directed end portions may be heated along at least a portion of their length to provide additional heat output in the central region of the heater. The overlap of the end portions will result in a higher local power density in the central region of the heater. Preferably, the overlapping end portions extend substantially fully across the central region of the heater, so that the terminal pins are located close to the periphery of the heater to allow maximum space for location of a control for the heater above the central region of the heater.

Preferably the carrier has means such as one or more studs, pillars or the like for mounting a thermally sensitive control thereto.

Furthermore, the carrier may be formed with one or more thermal contact locations for one or more thermally sensitive actuators, for example bimetallic actuators, of a control. In a particularly advantageous embodiment, the thermal contact location is provided on a boss in direct thermal contact with the heating element. The boss acts as a heat sink to draw heat away from the element, such that the temperature of the boss will more accurately reflect the temperature of the element. This is advantageous in a dry boil or dry switch-on situation where the thermally sensitive control should operate before any part of the heater overheats. Furthermore, the boss will, by virtue of its thermal inertia, smooth out transient fluctuations in the element temperature which may cause the bimetallic actuator to operate inappropriately. The size and position of the boss needed to give desired response times may be determined empirically.

In the case of the overlapping end portions extending into the central region of the heater, the boss provides a platform raised above the base level of the element carrier, so that thermal contact of a bimetallic actuator with the heater may be maintained when the control of the heater is positioned above the heating element.

Preferably the boss extends laterally, more preferably laterally inwardly, from the heating element.

The provision of a heat conducting boss is believed to be a novel arrangement, and thus from a further aspect the invention provides a die-cast heater comprising a sheathed heating element cast into an element carrier, wherein the element carrier is formed with at least one boss in direct thermal contact with the heating element, the boss providing a thermal contact location for a bimetallic actuator of a control.

The invention also extends to a die cast heater in accordance with the invention having a thermally sensitive control unit mounted thereto preferably to a central region thereof.

Some preferred embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which:-

Fig. 1 shows a plan view of a die cast heater in accordance with a first embodiment of the invention;

Fig. 2 shows a section along the line 2-2 of Fig.

1;

Fig. 3 shows a section along the line 3-3 of Fig.

1;

Fig. 4 shows a plan view of a die cast heater in accordance with a second embodiment of the invention;

Fig. 5 shows an enlarged partial section along line 5-5 of Fig. 4;

Fig. 6 shows an enlarged section along line 6-6 of Fig. 4; and

Fig. 7 shows an enlarged section along line 7-7 of Fig. 4

Like reference numerals have been used to denote like parts in each embodiment shown.

With reference to Fig. 1, a die cast heater 2 comprises an element carrier 3 of a castable aluminium alloy, into which is cast a sheathed heating element 4.

The carrier is generally circular, and the element 4 extends substantially around the periphery, its longitudinal axis being indicated by dotted lines 6 in Fig. 1. As shown in Fig. 2, the sheathed heating element comprises an outer metallic sheath 8, for example of mild steel, copper or stainless steel inside which is arranged a coiled heating wire 10 of nichrome or similar resistive heating material, and which is insulated from the metal sheath 8 by an electrically insulating filling such as magnesium oxide 12. The ends of the heating wire 10 are connected at points indicated by reference numerals 14 and 16 to so-called cold tails 18 of the element which are copper or similar rods which extend out from the end of the sheath to form terminals 20 for connection to an electrical supply.

As can be seen from Figs. 1 to 3, the ends of the element 4 overlap within the casting and project substantially perpendicularly from the carrier by a distance sufficient to allow connection to the electrical supply. It will also be seen from Fig. 1 that the end portions overlap so that the element 4 extends for over 360° around the periphery of the carrier 3. In fact, it is substantially the cold-tails 18 of the element 4 which overlap which means that there is relatively little heating occurring in the region 22 defined therebetween. Thus direct heating does not extend completely around 360° of, but only around approximately 330°. However, an advantage of overlapping the cold tails 18 is that local overheating will not occur at the periphery of the heater as might otherwise happen if heated portions of the element 4 overlap in this region.

Since the element 4 is arranged substantially around the outside of the carrier, the central region 24 thereof is left clear for mounting a control to the carrier plate. In particular, it is suitable for an integrated control of the type described in WO 95/34187 which provides both an overheat protection and backup protection function. To accommodate such a control, the carrier is formed with mounting pillars 26 to receive mounting screws or the like for the control and is also provided with particular areas 28 to receive the bimetallic actuators of the control.

With this particular type of control the adjacent arrangement of the terminals 20 of the element is particularly advantageous since it minimises the amount of material required to connect the control to the terminals.

The element may be mounted to or in the base of a liquid heating vessel, for example by being clamped against a lower edge thereof by suitable means.

Figs. 4 to 7 show a heater 2 according to a second embodiment of the invention. The heater 2 of Figs. 4 to 7 is similar to that of Figs. 1 to 3 and thus only the differing features of this embodiment will be described herein in detail.

As shown in Fig. 4, the overlapping region 22 of the heating element 4 of heater 2 is located in the central region 24 of the heater. The heating element 4 is generally circular around the majority of the periphery of the heater 2 with overlapping heated portions 34 extending in the same direction into the central region 24 of the heater 2. The overlapping heated portions 34 connect to the cold tails 18 of the element at points designated by reference numerals 14 and 16 in Fig. 4. The connection of the cold tails 18 to the overlapping heated portions 34 is shown in more detail in Fig. 5. The location of the terminals 20 and the cold tails 18 close to the periphery of the heater 2 inside the peripheral path of the heating element 4 allows the maximum extent of the heating element 4 without causing a hot spot close to the periphery of the heater 2. By overlapping the heating element 4 in the central region 24 of the heater 2, it is ensured that the maximum length of heating element is provided, both at the periphery of the heater 2 and in the central region 24. A heat output of 3 kW can easily be provided with this element layout in a heater having an outer diameter of 11 cm. Figs. 6 and 7 show the construction of the heater 2 in section.

As can be seen in Figs. 4 and 6 two bosses 30 on the element carrier 3 extend laterally from the inner edge of the peripheral turn of the heating element 4. The bosses 30 are solid and are formed integrally as part of the casting of the element carrier 3. The bosses 30 are formed to be in direct thermal contact with the sheath 8 of the heating element 4 to ensure good heat transfer from the heating element 4 to the bosses 30.

The bosses 30 are dimensioned so that they have sufficient thermal capacity to act as effective heat sinks from the heating element, and are substantially unaffected by transient fluctuations in the temperature of the heating element 4. The upper surface of each boss 30 provides a thermal contact location 28 for a bimetallic actuator of a control (not shown) for the heater, for example an integrated control as described in WO 95/34187, and is therefore correspondingly dimensioned. The height of each boss 30 above the base level of the element carrier 3 is sufficient to allow a control to be fitted to the heater 2 over the overlapping portions 34 of the heating element 4. The mounting pillars 26 for the control are also arranged at a suitable height for mounting the control above the heating element 4.

Of course certain modifications may be made to the various embodiments discussed above without departing from the scope of the invention. For example, the upper surface of the die cast element may be treated, for example anodised, to improve its aesthetic and corrosion resistance properties. Furthermore, the plate may form substantially the whole, or just a portion of the base of the vessel.

Claims

1. A die-cast heater comprising a sheathed heating element cast into an element carrier, wherein the heating element of the heater is arranged in at least a single turn around the periphery of the heater, and has overlapping end portions such that heat is provided around substantially the whole periphery of the heater.

2. A heater as claimed in claim 1, wherein substantially only the cold tail portions of the heating element overlap.

3. A heater as claimed in claim 1, wherein the overlapping end portions are directed inwardly from the periphery of the heater.

4. A heater as claimed in claim 3, wherein the overlapping end portions are substantially perpendicular to periphery of the heater.

5. A heater as claimed in claim 3 or 4, wherein the overlapping end portions are heated in use along at least a portion of their length.

6. A heater as claimed in any of claims 3 to 5, wherein the overlapping end portions extend across a central region of the heater.

7. A heater as claimed in claim 6, wherein the terminal pins of the heating element are located close to the periphery of the heater.

8. A heater as claimed in any preceding claim, wherein the terminal pins of the heating element extend perpendicularly to the plane of the heater. - 9 -

9. A heater as claimed in any preceding claim, wherein the element carrier is formed with at least one boss in direct thermal contact with the heating element, the boss providing a thermal contact location for a bimetallic actuator of a control.

10. A die-cast heater comprising a sheathed heating element cast into an element carrier, wherein the element carrier is formed with at least one boss in direct thermal contact with the heating element, the boss providing a thermal contact location for a bimetallic actuator of a control.

11. A heater as claimed in claim 9 or 10, wherein the boss extends laterally from the heating element.

12. A heater as claimed in claim 11, wherein the boss extends inwardly of the heating element.