Electrically powered immersion heating elements and controls therefor

An electric immersion heating element has an elongate, metal-sheathed, electrically insulated, resistance heating portion (1) engaged at its ends in bores formed in an element head (2) which may be formed of a synthetic plastics material, a heat transfer element (8) formed of a high thermal conductivity material extends from a heated part of the resistance heating portion adjacent one of its ends through the element head to enable the element temperature to be sensed from the opposite side of the head, and a tubular enclosure (7) extends from the element head (2) and surrounds the part of the resistance heating portion of the element. The element construction enables dry-boil protection and automatic switch-off on boiling to be achieved by use of a single thermal sensor coupled to the heat transfer element (8) since the steam generated in the enclosure when water boils causes water to be expelled from the enclosure thereby mimicing a dry boil situation.

This Application is a 371 of PCT/GB93/00501 filed on Mar. 10, 1993. 
FIELD OF THE INVENTION 
This invention concerns improvements relating to electrically powered 
immersion heating elements and controls therefor and more particularly 
concerns immersion heating elements and controls for water boiling vessels 
such as kettles, hot water jugs, pots, pans, urns and laboratory equipment 
for example. 
BACKGROUND OF THE INVENTION 
A conventional immersion heating element for an electric kettle or hot 
water jug comprises an elongate, metal-sheathed, electrically insulated, 
resistance heating element coupled to an element head plate serving for 
mounting of the element in a wall of the kettle or hot water jug, the 
resistance heating element being curved back towards the element head 
plate to define a so-called hot return portion which is secured to the 
element head plate, generally at a level above the general plane of the 
element. 
A control for switching off the supply of power to such a heating element 
in the event of the associated vessel being switched on dry (a so-called 
dry boil situation) or being allowed to boil dry commonly comprises a 
bimetallic element or other thermally responsive switch actuator which is 
held in close thermal contact with the rear of the element head in the 
region thereof where the hot return portion of the element attaches to the 
front of the element head plate. An example of such an element protection 
control is the X1 control manufactured by Otter Controls Limited which is 
substantially as described in GB-A-2194099 with reference to FIGS. 3A, 3B 
and 3C of the drawings thereof. 
It is further well known to provide a control for switching off the heating 
element when water boils in the vessel, and this is conventionally 
accomplished by provision of a second bimetallic element or other 
thermally responsive switch actuator located so as to be subject to 
impingement of steam thereon when water boils in the vessel. An example of 
such a steam sensing control is the J1 control manufactured by Otter 
Controls Limited which is substantially as described in GB-A-2248519 with 
reference to FIGS. 5A, 5B and 6 of the drawings thereof. 
A single sensor electronic control for a water boiling vessel is described 
in GB-A-2228634. This control utilizes a thermistor and senses element 
temperature as a function of the electrical resistance of the thermistor 
and the onset of boiling as a function of the rate of change of the 
thermistor resistance. However, even with the current state of modern 
electronics a circuit capable of switching a mains load cannot readily be 
manufactured at a price competitive with bimetallic controls. 
A single sensor control is proposed in GB-A-1 143 834 (Matsushita) but has 
never been manufactured, so far as we are aware. According to this 
proposal, a heating element as above described has an apertured enclosure 
provided on the element head plate in the region where the element hot 
return portion attaches thereto. By enclosing the hot return part of the 
element within an enclosure provided with small openings sufficient to 
admit water to the interior of the enclosure when the kettle is cold, the 
proposal of GB-A-1 143 834 was that the steam generated within the 
enclosure on boiling would drive the water from the enclosure whereupon 
the hot return portion of the element would overheat and cause the 
bimetallic or other control to operate. In accordance with the arrangement 
proposed in GB-A-1 143 834, a single bimetallic or other control will 
operate both when a kettle boils and when a kettle is switched on dry. As 
mentioned above, the proposal of GB-A-1 143 834 has not to our knowledge 
ever been manufactured despite its apparent simplicity and the promised 
advantage of a single sensor bimetallic control providing both element 
overtemperature protection and boil sensing. An automatic kettle or hot 
water jug thus will conventionally have separate element protection and 
steam sensing controls, and furthermore will commonly have a secondary or 
back-up element protection function operable in the event of failure of 
the primary element protection control. 
SUMMARY OF THE INVENTION 
The present invention provides a new and improved heating element 
construction which enables a single control to perform both element 
protection and boil detection functions. According to the present 
invention in one of its aspects, the ends (cold tails) of an elongate, 
metal-sheathed, electrically insulated, resistance heating element are 
engaged with an element head advantageously (though not essentially) 
formed of an appropriate heat-resistant synthetic plastics material, a 
heat transfer element formed of a high thermal conductivity material 
extends from a heated part of the element adjacent to one of its ends and 
through the element head to provide for sensing of the element temperature 
by means of an appropriate control, and an enclosure surrounds the said 
heated part of the element portion, the arrangement being such that in use 
of the heating element with a water boiling vessel the enclosure will fill 
with water when the vessel is filled and the onset of boiling and the 
consequent generation of steam in said enclosure will cause the water 
substantially to be expelled therefrom so as to mimic a dry boil situation 
sensible by an appropriate thermally-responsive control thermally coupled 
to said heat transfer element. 
The above and further aspects of the invention are set forth in the 
appended claims and, together with the advantages thereof, will be clearly 
understood from consideration of the following description of exemplary 
embodiments which are illustrated in the accompanying drawings.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
Referring to FIG. 1, the electrically powered immersion heating element 
shown therein comprises an elongate, metal-sheathed, electrically 
insulated, resistance heating portion 1 which, as is well known, is 
constructed so that the internal resistance heating wire of the element 
does not extend fully to the ends of the element and in consequence the 
ends of the element, the element cold tails, remain relatively cool in 
operation. The element cold tails are received within respective bores 
provided in a heat resistant moulded plastics element head portion 2 so 
that electrical terminal portions 3 and 4 of the element extend through to 
the opposite side of the head, grommet seals 5 (one only of which can 
partly be seen) formed of a temperature resistant material such as 
silicone rubber for example sealing the passage of the element cold tails 
through the head and serving furthermore to support the element in the 
head. Note that the resistance heating portion 1 of the heating element 
does not have a hot return portion such as is commonly provided in 
conventional heating elements. 
The detailed construction of the element head portion 2 is a matter of 
design, but notable in this respect is the tubular forward projection 6 
which extends from and is formed integrally with the head and surrounds a 
part of the heating element 1 adjacent to one end thereof. This tubular 
projection 6 defines an annular enclosure 7 surrounding a part of the 
resistance heating portion of the heating element, and a thermal linkage 8 
formed of a high thermal conductivity material extends from this part back 
through the element head portion 2 to the opposite side of the head. The 
thermal linkage can for example comprise a copper or brass strap assembled 
with the element in any convenient manner, for example as a tubular 
extension of or an internal or external addition to the conventional 
element sheath. 
In use of the thus described heating element with a thermally responsive 
switch controlling the supply of electric power to the heating element and 
with a thermally responsive switch actuator, for example an appropriate 
snap-acting bimetal, in thermal contact with the end of the abovementioned 
thermal linkage, the element will operate as follows. Firstly, in a dry 
boil situation, namely when the element is switched on without there being 
any water in the associated vessel, the element will rapidly overheat and 
the resultant element over-temperature will be transferred via the thermal 
linkage to the thermally responsive switch actuator so as to cause the 
switch to go open circuit. Secondly, in a normal boil situation, water 
will fill the enclosure 7 defined by the forwardly projecting tubular head 
portion 6 until the water reaches boiling temperature, whereupon the 
generation of steam within the enclosure will expel the water therefrom; 
the portion of the heating element within the enclosure 7 will then see a 
quasi dry boil situation, will rapidly overheat and will cause the 
thermally responsive switch to operate. The illustrated element thus 
enables dry boil protection and boil sensing to be achieved by use of but 
a single sensor element. 
The invention is advantageous, furthermore, in that it enables a stainless 
steel heating element to be used rather than the more conventional copper 
or brass. Stainless steel heating elements, that is to say heating 
elements wherein the element sheath is formed of stainless steel, are 
advantageous in that they are much less susceptible to furring and 
corrosion in use, but they are not popular with element manufacturers on 
account of difficulties experienced in brazing the stainless steel element 
sheath to the element head, particularly at the conventionally provided 
hot return portion of the resistance heating portion of the element. Not 
only do such problems not arise with a heating element in accordance with 
the present invention, but also it is actually preferred to use a heating 
element having its sheath formed of stainless steel or of a material 
having a similarly low thermal conductivity as compared to copper or brass 
since the stainless steel does not readily conduct heat from the element 
part that is within the enclosure into the water surrounding the main part 
of the element, thereby enabling a more responsive control of element 
temperature to be achieved. 
FIG. 2 shows an arrangement substantially identical to that of FIG. 1 
except that the enclosure 7 is defined by a forward tubular extension of 
the grommet seal 5 rather than by an integrally formed part of the element 
head 2. FIG. 2 also shows the two grommet seals 5 interconnected for 
convenience by means of an integral strap. 
In use of an immersion heating element as herein described with an X1 
control manufactured by Otter Controls Limited as aforementioned the X1 
control would not perform a permanent switch-off function when the bimetal 
in the control responded to water boiling in an associated vessel because 
the bimetal would reclose its switch contacts after a period of time 
sufficient to enable it to cool and return to its `cold` condition. The 
bimetal would thus cycle between its `cold` and `hot` conditions and the 
heating element, correspondingly, would be intermittently powered. This 
could be avoided by use of a modified J1 control with the X1, the bimetal 
in the J1 control being replaced by a simple spring and a mechanical 
interconnection being provided between the push-rod in the X1 control and 
the trip lever of the J1. Operation of the X1 control would thus cause the 
J1 control to trip and open the contacts which would remain open, 
regardless of the condition of the bimetal in the X1, until the J1 was 
reset. 
Having thus described the present invention by reference to exemplary 
embodiments, it will readily be appreciated that many changes could be 
made without departure from the general teachings of the invention. Thus, 
for example, whereas it is preferred to form the element head of a 
synthetic plastics material it could alternatively be formed as a deep 
drawn metal article. Also the shape of the resistance heating portion of 
the heating element, that is to say the form into which it is bent for 
purposes of compactness, could be varied, and the detailed shape and form 
of the enclosure could be varied. In particular, the end of the enclosure 
furthest from the head could be formed with an inwardly directed lip so as 
to reduce the gap between the element sheath and the end of the enclosure, 
in which case a small aperture might also be provided in the wall of the 
enclosure to ensure that an air bubble was not trapped therein when the 
associated vessel was filled.