GB2052227A

GB2052227A - Improvements in Electric Immersion Heaters

Info

Publication number: GB2052227A
Application number: GB7922637A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-06-29
Filing date: 1979-06-29
Publication date: 1981-01-21
Also published as: GB2052227B

Abstract

An electric immersion heater particularly for an electric kettle comprises a heater element having a stainless steel sheath mounted in an element head also formed of stainless steel or the like. The head is pressed out and formed from a sheet to provide a cup shaped member having a helical rim formed at its outside end and engaged with an internal thread in mounting ring 14. The hot return portion 6 of the element is brazed to the head opposite a snap acting actuator 12 forming a thermal cut out. The head is mounted in the wall 13 of a kettle employing a sealing washer 15, which need not be as heat resistant as hitherto in view of the poor thermal conductivity of the head resulting in a lower overshoot temperature at the periphery. <IMAGE>

Description

SPECIFICATION Improvements in Electric Immersion Heaters This invention relates to electric immersion heaters of the kind which are used in liquid heating vessels such as jugs, urns and particularly electric kettles. Such heaters comprise a heater element mounted in an element head, and a thermal control unit including an electric switch and a thermally responsive actuator arranged to open the switch and cut off the supply of power to the element on overheating of the element. This may occur either if the, for example, kettle is switched on dry or if it boils dry in use. The sensing by the actuator of the overheating of the element is conventionally achieved by welding or brazing a portion of the element which becomes hot in use (called the hot return portion) to the element head. The construction and operation of such heaters is conventional and is well known to those in the art.

It is therefore known that the element heads of such heaters are conventionally and almost invariably constructed of brass, because of its ease of forming, ready availability and high thermal conductivity (R=1.04 joules cm/cm2s deg C at 1000C). The latter has been thought conducive to rapid operation of the actuator by the rapid transfer of heat through the head, which needed to be of the order of 1 .25 mm thick to withstand the stresses on the element which might be expected in normal use, for example when cleaning the element.

A disadvantage of brass heads arises from consideration of the seal required between the head and the wall of the, for example, kettle.

Being such a good conductor of heat, in an overheating situation the temperature at the periphery of the head rises considerably, and keeps on rising afterthe actuator has terminated the power supply. This so-called overshoot temperature in this region may rise to as much as 2500C and if the necessary seal is not to be damaged it must be made of a material which can withstand this order of temperature. Silicon rubber seals are suitable for this purpose, but they are expensive. If such high temperatures could be avoided, seals of less expensive material could be used and significant savings in manufacturing costs obtained.

This problem of overshoot temperature, its effect on the seal as well as its effect on other parts of the control units which have to be designed and constructed to withstand the high temperatures, has means that the operating temperature of the actuator has had to be kept low, usually about 1 300C with a small tolerance, usually +50C. This factor brings its own disadvantages; for example in regions of high water hardness, scale tends to build up around the element and head, and heat generated in the hot return portion is inhibited by the scale from being conducted away by the water. The result is that the head heats up instead and the temperature can rise to the operating temperature of the actuator and cause nuisance tripping.

I have now discovered that it is possible to overcome these difficulties by constructing the element head from a material having the hitherto considered unlikely property of a low thermal conductivity as compared to that of brass. It has now been found during the course of my investigations that it is unnecessary to provide high thermally conductive material for rapid transfer of heat via the element head to the thermally responsive actuator to prevent the overheating of the control unit in a boil dry/switch on dry condition and in fact the provision of an element head having a relatively low thermal conductivity as compared with brass has a distinct advantage in that speaking in general terms the heat of the element arising from its thermal capacity is retained in the element when the supply of power is cut off by operation of the electrical switch whereby the element may rise to a very high temperature but the temperature of the control unit being isolated from the heater element remains relatively low.

Thus viewed from one aspect, the invention provides an electric immersion heater comprising a heater element mounted in an element head and a thermal control unit including a thermally responsive actuator arranged to open a switch on overheating of the element to terminate the power supply thereto in which the element head includes a plate of flat or dished configuration formed from metal sheet having a thickness of between 0.3 mm and 1 mm and low thermal conductivity as compared with brass, the element having a hot return portion thermally connected to the head in close proximity to the actuator.

Now in view of the thinness of the head the metal will need to have a high mechanical strength as compared to that of brass. Stainless steel is a preferred metal, but in view of its susceptibility to stress corrosion other alloys such as Monel, Inconel or Incoloy may be preferred.

The preferred thickness for the head is less than 0.75mm and preferably substantially 0.5mm.

The advantages obtained by the invention will now be apparent. Because of the poor thermal conductivity of the head metal, and its thinness, the heat from the hot return portion is conducted radially away from the hot return portion connection only at a very slow rate, thus confining the region of high temperature to a localised hot spot closely proximate to the actuator. This means that the actuator may be set to trip at a higher temperature, say 1 400 C+1 OOC, than before without damaging the seal or components of the control unit by the overshoot temperature.

Tests have shown that the overshoot temperature can be reduced to as little as 1900C at the periphery of the head, in accordance with the invention. Raising the actuator tripping temperature reduces the risk of nuisance tripping.

Less expensive seals can be used.

In a preferred embodiment the hot return portion is connected to the element head over a confined area spaced inwardly from the periphery thereof. This is in direct contrast to the conventional brass headed heater where the hot return portion is connected right across the head.

The effect of this feature of the invention is to increase still further the confinement of the hot spot to an inner region of the head and thus lower the overshoot temperature at the periphery of the head. To ensure that the thermal contact is made over only such a confined area, the head may be formed with a raised portion to which the hot return portion is connected. Adjacent parts of the hot return portion thus remain spaced from the head and are to a degree insulated therefrom by the air gap therebetween.

The provision of such a raised portion can be made a particularly convenient feature of the invention, since it may be pressed out of the head and may therefore be made of such a shape that the corresponding depression on the other, control unit, side of the head snugly receives the actuator and provides a mounting therefor. The actuator is thus in the optimum position to detect heat from the hot return portion, and does this relatively quickly in view of the thinness of the headwhile the remaining parts of the head remain relatively cool.

Forming the head from a relatively strong metal sheet such as stainless steel as discussed above can give rise to unexpected economies of manufacture. In accordance with a preferred feature of the present invention the head can be provided with an integral cylindrical flange adapted to be passed through the kettle wall mounting aperture, the outer end of said flange being provided with a helical rim forming substantially a single thread turn. This is a distinct improvement over existing heads formed from brass in the form of a plate, since brass is insufficiently strong to be able to form a locking thread as a turned out rim. Instead it has been necessary to provide threaded pillars extending from the head for engagement by locking bolts or nuts from outside the kettle, the operations necessary to form such pillars involving high costs of manufacture.

The forming operation from sheet metal stock to provide the flange and helical rim is not intrinsically difficult and therefore not expensive, and the amount of extra metal required does not increase the manufacturing cost prohibitively. Any such increase can be offset by savings in assembly costs. Thus, the use of low conductivity high strength metal previously thought to be quite unsuitable for this purpose in fact enables one to obtain the assembly benefits of early designs of heater without the difficulties and expense associated therewith.

It will however be appreciated that mounting studs or nuts of a kind conventional in the art may also be used for securing the heater to the water heating vessel. The main advantage of the invention however is the reduction in overshoot temperature. As explained above, this arises by reducing the thermal conductance of the head in a radial direction, which is directly dependent on a factor which is the product of the thermal conductivity (R) of the head metal and its thickness. For a brass head this "overshoot factor" is approximately (R=0.104x (thickness=-1 .2mm=0.1 25. For a steel head where R=0.5 and the thickness is less than 1 mm the overshoot factor is less than 0.05. For thicknesses between 0.3mm and 0.75mm the factor is between 0.015 and 0.0375. For a stainless steel head (R=0.165) and thickness 0.4mm, the factor is 0.0065.The ratio of this last factor to that of a brass head is about 1:1 9 which demonstrates the effectiveness of the invention in reducing overshoot.

Another advantage afforded by the invention is that the quality of the plastics moulding of the control unit may be lessened, a cheaper type of reinforced nylon being employed. A significant saving in cost may be achieved by employing a seal of any acrylonitrile rubber clamped between the element head and the vessel wall as opposed to higher quality silicon rubber seals in current use. Silicon rubber seals are available at a cost of 13p whereas acrylonitrile seals may be purchased for 8p; this is a very significant saving in manufacturing costs.

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, 575mm long tube was employed having a watts density ratio of 45/50 watts cm-l. It is now possible to employ for a 3 kw heater a shorter element, say 62.5mm shorter (which represents a saving in manufacturing costs of 5p) with a higher watts density of at least 55watts cam~1.

The invention may be viewed from a number of further interrelated aspects within the broad concept of the invention. Thus in accordance with a second aspect of the invention there is provided an electrical immersion heater comprising a heater element and a thermal control unit, the heater element having an element head and a hot return portion of the element being thermally connected to the element head, the element head having a relatively low thermal conductivity as compared that of brass, a control unit of the heater including an electrical switch and a thermally responsive switch actuator being arranged to detect heat from said head at a position in proximity to said thermal connection of said hot return portion to said head, and said switch actuator being taken from a selected batch of actuators having operating temperatures of at least 1400C+100C (as compared with known heaters wherein the actuator is taken from a selected batch having operating temperatures of 1300C+50C). Thus the actuator may be selected from a batch having a greater spread in operating temperatures (+1 00C as opposed to +50C), and the median operating temperature of the batch may be substantially higher, 1 400C as opposed to 1 300C. A 50C tolerance in the manufacture of such actuators involves exacting manufacturing techniques and a high level of quality control.

Raising the tolerances to f100C and the operating temperature to 1 400C facilitates manufacture considerably and reduces the level of quality control required, effecting a considerable reduction in manufacturing costs.

In accordance with a third aspect of the invention there is provided an electrical immersion heater comprising a heater element and a thermal control unit, the heater element having an element head and a hot return portion of the element being thermally connected to the element head, the element being in the form of an elongate cylindrical member having a minimum watts density ratio of 55 watts cm-1, the element head having a relatively low thermal conductivity as compared with brass, a control unit of the heater including an electrical switch and a thermally responsive switch actuator being arranged to detect heat from said head at a position in proximity to said thermal connection of said hot return portion to said head.

Elements are usually formed as convoluted cylindrical members comprising a sheath of chromium plated copper tube with a fine coiled heating wire extending therethrough and in 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 relatively low thermal conductivity, e.g. stainless steel, 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 need not be employed, plating tending to flake off at high temperatures such as may be encountered with the present invention.

There is thus provided in accordance with a fourth aspect of the present invention an electrical immersion heater comprising a heater element and a thermal control unit, the heater element having an element head and a hot return portion of the element being thermally connected to the element head, the element being in the form of an elongate cylindrical member having an outer tubular sheath of a material having a low thermal conductivity as compared with copper, the element head having a relatively low thermal conductivity as compared with brass, a control unit of the heater including an electrical switch and a thermally responsive switch actuator being arranged to detect heat from said head at a position in proximity to said thermal connection of said hot return portion to said head.

The sheath of the element may 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 from this aspect of the invention is that in hard water areas where the elements tend to develop a scale of minerals, 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 the element.

The material of the sheath should have a low thermal conductivity as compared with copper (R=0.382 joule mm/mm2s deg C at 100 C) (the values for thermal conductivity quoted in this specification are taken from "Physical and Chemical Constants"-Twelfth Edition-Kaye and Laby-Longmans); thus stainless (18/8) steel (R=0.0165 joules mm/mm2s deg C at 100 C) is suitable as the sheath material although because of possible susceptibility to stress corrosion other metals having a low thermal conductivity and being non-corrosive may be employed, for example Monel metal (R=0.024), Inconel or Incoloy.

The hot return portion of the element may be connected directly to the element head by welding or brazing in known manner. However as an alternative it is possible to provide a metal linking member thermally connecting the head with the hot return portion which may then be arranged some distance away from the head. The advantage of this arrangement is that whilst the necessary thermal connection with the head is made at a location which must because of design constraints be some distance from the kettle base, all the heated parts of the elements can be disposed adjacent the kettle base and this means that a smaller quantity of water than hitherto is necessary to immerse the element for safe operation. This represents a considerable saving in energy when only a small quantity of boiling water is required.

Even greater energy savings may be obtained when, as in a preferred embodiment of the invention, the element is disposed in a well formed in the base of the kettle, the arrangement being such that on substantially filling the well the element is adequately immersed for safe operation. The depth of the wall may be such as to accommodate the conventional circular section element, but if this is compressed into a D-shape or "race track" section the depth required for immersion is less and the well may be made correspondingly shallower, with further concomitant energy savings.

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 of substantially flat or dished configuration, and the switch unit including 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. 14101954 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.

In order that the invention may be readily understood certain preferred embodiments thereof will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 is a perspective view of an element head, Fig. 2 is a sectional view along the line 2-2 of Fig. 1, Fig. 3 is a sectional view along the line 3-3 of Fig. 1, Fig. 4 is a vertical section through a heater mounted in a kettle wall, Fig. 5 is a vertical section through a further embodiment of heater, Fig. 6 is a plan view of a further embodiment of element and head, and Fig. 7 is a sectional view along the line 7-7 of Fig. 6.

The element head shown in Fig. 1 is a member made from 18/8 stainless steel 0.5mm thick, pressed out and formed from a sheet. The main body of the head comprises a plate 1, formed with a contoured raised portion 2 as shown and provided with two element receiving apertures 3 and a steam pipe receiving aperture 4. This head is designed for use with control units with a steam control, i.e. one which cuts off the power supply to the element when the water being heated boils, as well as those without such a steam control, in which latter case the aperture 4 is left blind.

The raised portion 2 is also formed with a depression 5 in the region of the intended connection with the hot return portion 6 of the element. Depression 5 provides a mounting for a snap action bimetallic actuator 12 as will be described.

Extending from plate 1 is an integral substantially cylindrical flange 8 whose outer end is rolled over to form a helical rim 9 which provides a single turn of thread.

Fig. 2 shows the cold leads 10 of the element mounted in the apertures 3. A particularly advantageous manner of effecting this mounting is to place solder rings in the wells formed by the apertures 3, insert the cold leads therethrough and simpiy heat the assembly by torch. The steam pipe 11 if provided may be mounted in the same way and at the same time as well as the attachment of the hot return portion 6 to the head.

From Fig. 3 can be seen the snug fitting of snap action bimetallic actuator 12 in the well formed in the head. As can be seen from Fig. 4 the actuator 12 is in close proximity to the connection of the hot return portion 6 to the head so as to detect accurately the temperature of the hot return portion.

Fig. 4 shows more details of the control unit and the mounting thereof in an aperture in kettle wall 13. An internally threaded ring 1 4 is engaged with the bent out rim 9 and screwed home drawing the head into tight engagement with sealing washer 1 5. As mentioned, previously, this washer may now be of a less expensive and less temperature resistant material than hitherto in view of the very low rate of radial heat conduction through the head. Furthermore, the threaded ring 14 includes a substantial number of turns so that it may be used with kettle walls of varying thickness, for example stainless steel, ceramic or plastics.

The actuator 12 is selected from a batch having a mean operating temperature of at least 1400C+100C as compared with known arrangements in which actuators are taken from selected batches having an operating temperature of 1 300C+50C. The switch actuator snap acts between oppositely dished configurations with changes in temperature and is mounted on a thermally insulating pillar 16 of the control unit.

On overheating of the element the actuator snaps through and the lower end drives control member 1 7 to open a pair of contacts (not shown) and cut off the supply of power to the element.

Referring now to Figure 5, the heater element 214 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 56.4 watts cm-'. 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 part 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 the side wall of the kettle 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 to pins of an electrical plug 66 formed in the control unit 216. 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.

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 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 64 carrying a movable contact 76 of the switch via a push rod 78.

The base wall 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 operation 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 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 arrangements where the element has a more open form.

In accordance with the invention the element head 40 of stainless (18/8) steel is provided having a low thermal conductivity (R=0.0165 joule mm/mm2s deg C at 100 C) as compared with brass (k= 0.104 joule mm/mm2s deg C at 1000C) 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 1300C+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 cm1.

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 be conducted through to 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 other 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 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 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 1800 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 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 100. 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 100, is connected to the head by a copper strip 110, 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 11 2 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.

Claims

1. An electric immersion heater comprising a heater element mounted in an element head and a thermal control unit including a thermally responsive actuator arranged to open a switch on overheating of the element to terminate the power supply thereto in which the element head includes a plate of flat or dished configuration formed from metal sheet having a thickness of between 0.3mm and 1 mm and low thermal conductivity as compared with brass, the element having a hot return portion thermally connected to the head in close proximity to the actuator.

2. An immersion heater according to claim 1 in which the thickness of the head is less than 0.75 mm.

3. An immersion heater according to claim 2 in which the thickness of the head is substantially 0.5mm.

4. An immersion heater according to claim 1,2 or 3 in which the head is made of stainless steel, Monel, Inconel or Incoloy.

5. An immersion heater according to any of the preceding claims in which the hot return portion is connected to the element head over a confined area spaced inwardly from the periphery thereof.

6. An immersion heater according to claim 5 in which the head is formed with a raised portion to which the hot return portion is connected.

7. An immersion heater according to claim 6 in which the raised portion is pressed out of the head so as to form a depression on the other side which snugly receives and provides a mounting for the actuator.

8. An immersion heater according to any of the preceding claims in which the head is provided with an integral cylindrical flange adapted to be passed through the kettle wall mounting aperture, the outer end of said flange being provided with a helical rim forming substantially a single thread turn.

9. An immersion heater according to any of the preceding claims in which the element is substantially 51 Omm long and has a watts density of at least 55 watts cm-1.

10. An immersion heater according to any of the preceding claims in which the cross section of the element is D-shaped or of race track section.

11. An electric immersion heater comprising a heater element and a thermal control unit, the heater element having an element head and a hot return portion of the element being thermally connected to the element head, the element head having a relatively low thermal conductivity as compared that of brass, a control unit of the heater including an electrical switch and a thermally responsive switch actuator being arranged to detect heat from said head at a position in proximity to said thermal connection of said hot return portion to said head, and said switch actuator being taken from a selected batch of actuators having operating temperatures of at least 1400C+100C.

12. An electric immersion heater comprising a heater element and a thermal control unit, the heater element having an element head and a hot return portion of the element being thermally connected to the element head, the element being in the form of an elongate cylindrical member having a minimum watts density ratio of 55 watts cm-', the element head having a relatively low thermal conductivity as compared with brass, a control unit of the heater including an electrical switch and a thermally responsive switch actuator being arranged to detect heat from said head at a position in proximity to said thermal connection of said hot return portion to said head.

13. An electric immersion heater comprising a heater element and a thermal control unit, the heater element having an element head and a hot return portion of the element being thermally connected to the element head, the element being in the form of an elongate cylindrical member having an outer tubular sheath of a material having a low thermal conductivity as compared with copper, the element head having a relatively low thermal conductivity as compared with brass, a control unit of the heater including an electrical switch and a thermally responsive switch actuator being arranged to detect heat from said head at a position in proximity to said thermal connection of said hot return portion to said head.

14. An electric immersion heater according to any of the preceding claims in which the element sheath is formed of stainless steel.

1 5. An electric immersion heater substantially as hereinbefore described with reference to Figs.

1 to 4 of the accompanying drawings.