IMPROVEMENTS RELATING TO ELECTRIC HEATING ELEMENTS

Abstract: Thick film heating elements can cause problems if associated water heating vessels are used on a sloping surface so that if the vessel boils dry an elevated part of the heating element is first exposed to overheating and self-destructs. To protect against this the invention proposes to divide the heating element track into parallel-connected main heater and sensor portions, the sensor portion surrounding the main heater portion and comprising a plurality of parallel-connected track portions having NTCR (negative temperature coefficient of resistance) characteristics and connected in series therewith a track portion having a PTCR (positive temperature coefficient resistance) characteristic. By virtue of this arrangement, if one of the NTCR track portions begins to boil dry, so its temperature will increase and its resistance reduce so that more current flow through the PTCR track portion and its temperature rises. With a bimetallic thermally-responsive control arranged to sense the temperature of the PTCR track portion and to switch off at least the main heater portion if it overheats, effective control of the heater operation can be achieved by use of a bimetallic control.

Description: IMPROVEMENTS RELATING TO E ECTRTC HEATTNG ELEMENTS Field of the Invention; This invention concerns improvements relating to electric heating elements and more particularly concerns electric heating elements of the so-called thick film type which comprise a resistance heating track or layer formed on a substrate, for example a printed ink track formed on a stainless steel substrate with an insulating layer between the substrate and the track and, preferably, a further insulating layer overlying the track. The invention is particularly concerned with thick film heating elements for water boiling vessels, such as domestic kettles and hot water jugs for example, but is capable of wider application. Back∑round of the Invention: Controls are well known which are adapted to switch off the power supply to electric heating elements if the element overheats. It is known furthermore that a special problem can arise if thick film heating elements in kettles and hot water jugs are operated on a slope, for example with the appliance standing on a sloping draining board, in that local overheating of the element can occur, leading to failure of the heating element, if any part of the heating element track is exposed above the water level in the appliance. Because thick film heating elements have low thermal mass and limited lateral heat transfer capability, any part of the heating element track exposed above the water level suffers a rapid temperature rise which can result in failure of the element. This can happen if, for example, the lid of a kettle is left off so that the water boils away with the kettle standing on a modest slope. Proposals have been put forward to overcome this problem. In DE-A-1954770.5 (Stiebel Eltron) for example, the proposal is made to counterslope the heating element, but this simple solution only protects for a limited slope and in only one axis. In GB-A-2 316 847 the proposal is made to raise some part of the heating element substrate so that the raised part is subject first to any tendency of the element to boil dry, and in EP-A-0 715 483 the proposal is made to array a plurality of PTCR (positive temperature coefficient of resistance) or other sensors around the heating element periphery so that one sensor at least is responsive to an element overtemperature condition irrespective of the particular slope of the element. All of these proposals give rise to manufacturing difficulties and/or increased costs, particularly in the heating element control system. A further difficulty that arises in regard to providing over-temperature protection for thick film heating elements results from their low thermal mass, which gives rapid rates of temperature rise during dry boil conditions, namely when the heating element is powered in the absence of cooling water. To protect against this, a rapid response thermal cut-out is required. To achieve such a rapid response, a high power density is needed in the track(s) providing heat to the thermal actuator of the heating element overtemperature protection control, and this gives rise to a high running temperature of this heating element track because of the thermal insulation of the dielectric layer(s) and the heating element substrate. This means that a thermal actuator set to operate at a high temperature is required which causes difficulties, at least as regards cost implications, in regard to the manufacture and setting of a stable, snap-acting, bimetallic device operable at the requisite temperatures. Typical temperature settings for overtemperature protection of an immersion heating element of the mineral insulated, metal sheathed, resistance heating type, or an underfloor heating element having such a metal sheathed heating element clamped or clenched to the undersurface thereof, would be 135± 15°C, whereas for a thick film heating element the equivalent temperature setting is around 180±5°C, the tighter tolerance resulting from the problem of slower response from a bimetal of too high a setting. From the foregoing it can be appreciated that special problems are required to be solved if effective heating element overtemperature protection is to be provided to a thick film type heating element which is susceptible to be operated on a sloping surface so that, in use, an indeterminate part of the heating element can be subject to overheating before any other part. Objects and Summary of the Invention: It is the principal object of the present invention to overcome or at least substantially reduce the abovementioned problems. According to the present invention, a thick film heating element comprises a main heater portion and, electrically connected in parallel therewith, a plurality of parallel-connected track portions having an NTCR (negative temperature coefficient of resistance) characteristic in series with a track portion having a PTCR characteristic, the plurality of NTCR track portions being distributed around the main heater portion. In operation of such a heating element with an element overtemperature protection device arranged to be responsive to the temperature of the PTCR track portion and, in response to a sensed overtemperature condition, to switch off the supply of electricity to all parts of the heating element, operation of the heating element at an inclination such that one of the NTCR track portions will be exposed first if the heating element boils dry will cause the temperature of the respective NTCR track portion to rise. This in turn reduces the resistance of the respective NTCR track portion whereby an increased current flow through the series-connected PTCR track portion will occur. Depending on the overall circuit resistance, the increased current flowing through the NTCR track portion will further heat it and further reduce its resistance, thereby increasing the effect. The temperature of the PTCR track portion will rise, thereby increasing its resistance so that an increased proportion of the supply voltage appears across it. This leads to a disproportionate increase in the power dissipated in the PTCR track portion, thereby raising its temperature even if it is still below the water level. This increase in temperature of the PTCR track portion is sensed by the heating element overtemperature protection device which will switch off the heating element before any of the main heater portion is exposed above the water level, thus preventing any damage. The arrangement can be such that the PTCR track section is distributed with the NTCR track sections about the main heater portion of the heating element. With such an arrangement, the PTCR track portion can be the portion first exposed as the heating element boils dry. This will result in the temperature of the PTCR track section increasing, enhanced by the PTCR effect, so that the heating element overtemperature protection device will again be operated. The above and further features of the present invention are set forth in the appended claims and will be best appreciated from consideration of the following detailed description of an exemplary embodiment which is illustrated in the accompanying drawings. Description of the Drawin2s: Figure 1 is a schematic showing of the track layout of an exemplary embodiment of the present invention; and Figure 2 is a schematic circuit diagram representative of the track layout shown in Figure 1. Detailed Description of the Embodiment: Referring first to Figure 1, shown therein is a plan view of the track layout of a thick film heating element embodying the present invention. It is to be noted that the dimensions and proportions of the tracks are schematic only and do not represent a practical form, it being well within the skills of an average skilled thick film heating element designer to transform the schematically illustrated layout of Figure 1 into a practical form. The thick film heating element 1 of Figure 1 might for example comprise a stainless steel disc substrate having an electrically insulating layer, of glass for example, formed on one or both of its major surfaces. The track pattern is then formed onto the insulating layer, for example by a screen printing process employing electrically conductive ink paste which is fired following deposition. An overlying electrically insulating layer, for example of glass, may then be provided on top of the track pattern to protect the same. As shown, the track pattern comprises a main heater portion 2 occupying the major central part of the heating element 1 and, connected in parallel therewith, four NTCR track sections 3 which are connected in parallel with each other by means of interconnecting tracks 4 formed of a highly electrically conductive material such as silver and a PTCR track section 5 which is connected in series with the four NTCR track sections 4, again by means of highly electrically conductive (eg. silver) tracks. First and second terminal portions 6 and 7, formed of a highly electrically conductive material such as silver, are provided for connection to the live and neutral lines respectively of the mains electrical supply, the neutral terminal 7 being connected to one end of the main heating element track 2 and the live terminal 6 being connected to a further terminal portion 8 which serves as a connection point for one terminal of a thermal cut-out device (not shown), the other terminal of the cut-out device being connected in use to yet another terminal portion 9 of the heating element track, the terminal portion 9 being connected to the other end of the main heating element track 2. It can be seen that the terminal portions 7 and 9 also connect to the array of NTCR track portions 3 and the series-connected PTCR track section 5. Figure 2 shows the schematic circuit diagram of the arrangement of the heating element shown in Figure 1. From Figure 1 it can be seen that the main track portion 2 is surrounded by the four NTCR track portions 4 and the PTCR track portion 5. If the heating element is operated on a slope and boils dry, for example because the lid is left off an associated appliance, then one of the sensor tracks, namely the NTCR tracks 4 and the PTCR track 5, will be exposed first and its temperature will rise. If it is an NTCR track portion 4 then its resistance will fall, which allows an increased current to flow through the PTCR track portion 5. Depending on the overall circuit resistance, the increased current flowing through the NTCR track portion will further heat it and reduce its resistance, amplifying the effect. The temperature of the PTCR track portion 5 will rise, increasing its resistance, further increasing the proportion of the supply voltage which appears across it. This will lead to a disproportionate increase in the power dissipated in this track, raising its temperature, even though it is still below the water level. This increase in temperature can be sensed by a thermal cut-out, which will switch off the element before any of the main power track 2 is exposed above the water level, thus preventing any damage. If the track portion which is first exposed is the PTCR track portion 5, then its temperature will rise, enhanced by the PTCR effect, and the thermal cut-out will be tripped. Thus a single thermal cut-out can sense if any of the sensor tracks, either singly or two partially, are exposed by a slope of the heating element in any axis. The embodiment is an example of the operation of the invention and uses four parallel NTCR track portions in series with a single PTCR track portion. Assuming that the power generated in all five sensor tracks is the same and that the power density is around 10 watts per square centimeter, this will give, in a typical element, a running temperature of 120°C. To achieve this means that the resistance of the PTCR track is 1/16 of the resistance of each of the NTCR tracks, assuming an NTCR and PTCR of 0.006/°C, which is typical of suitable materials. With a layout as shown, at the running temperature of 120°C, the sensor tracks would have a power output of 28 watts each. If exposing one of the NTCR tracks 4 resulted in its temperature rising by 100°C, then its resistance would fall to 40%, and the total resistance of the parallel NTCR tracks would fall to 73% of their 120°C value. This would result in an increase in power in the PTCR track portion of over 100% as a result of the increased current and the temperature rise of the PTCR track. The temperature rise of the PTCR track will be roughly proportional to the power increase so its temperature would be expected to rise to 220°C approximately, even though that part of the element was still submerged. It can be seen from this simplified analysis that the invention not only enables heating elements to be operated safely at an inclination, but also enables bimetallic sensors to be used since the low running temperature of the PTCR track portion 5 will allow bimetals of a much lower operating setting to be used, whilst the large temperature rise under fault conditions means that a wide blade tolerance may be used. The track layout shown is only one possible configuration. A different number of NTCR track portions could be used in parallel, or only one, in series with the PTCR part. The fewer the number of NTCR track portions in parallel, the greater will be the effect on the resistance of the combination and the greater will be the effect on the PTCR track portion. However, a smaller number of NTCR track portions means that they must each cover a greater arc angle, which may make them less sensitive to slope angles since only a part of the arc would be exposed. It might be that five NTCR track portions (giving a reduction in resistance to 77% in the example above) would be the maximum to have an effective change in resistance, whilst three NTCR track portions (a reduction to 67%) would be a good compromise to give adequate slope sensitivy. Of course, a number of NTCR and PTCR circuits could be used, each with its own thermal cut-out to provide protection for very large diameter heating elements, and the invention could be adapted to heating elements of any profile. In the embodiment, the PTCR track portion is placed on the element periphery, aligned with the NTCR track portions but the PTCR track portion could be located anywhere on the heating element, for example to align with our X4 protection system which is described in our British Patent Application No. 9808484.1. The slope protection system of the present invention may be combined with other element control systems, for example the steamless control system of our British Patent Application No. 9816645.7, or any simple element protection control such as the X2 control which is described in GB-A-2 283 156. The PTCR track could be arranged to operate one of the X4's bimetals, whilst the other bimetal could be operated conventionally by the main heater track. This would reduce the number of high, close tolerance, bimetals required. Alternatively two NTCR/PTCR circuits could be used, each PTCR track operating one of the X4's bimetals. The limit to such arrangements is the element topology, which limits the shapes and circuits that are practical because of the circuit complexity they cause, together with the need to achieve a uniform high power density over as much of the element as possible to minimise the element area and cost. A feature of this invention is that it can give rise to an unstable circuit, in which the NTCR tracks cause the total sensing circuit resistance to fall as the current rises, leading to thermal runaway which, if not controlled, will destroy the sensor tracks. This instability can be used positively to quicken the response of the system to fault conditions and increase the temperature of the PTCR track more rapidly that either the low power density or low initial temperature would suggest. The conditions for this are fulfilled when the resistance of the PTCR track and/ or its temperature coefficient of resistance is much lower than that of the NTCR network. This, albeit much simpler example, is simpler to the principles of designing fighter aircraft that are aerodynamically unstable so they can react to the controls much more quickly. This contrasts with the proposal made in our British Patent Application No. 9816645.7 to improve the stability of the steamless control system, in which the effects of the NTCR track are controlled by having a much larger ballast PTC resistor. In this latter case any instability would remove the benefits of the voltage compensation provided. A further variation of the present invention would be to switch only the main part of the heating element off when the thermal cut-out operates, leaving the sensing circuit energised. The heat from this would prevent the cut-out from resetting, giving effectively a manual reset by means of a voltage maintained thermal cut-out. To do this would require that the sensor circuit did not thermally run away, as described above, but had a sufficiently low temperature stable state. The invention having been described herein by reference to a specific example and possible modifications, it is to be well appreciated that the described embodiment is in all respects exemplary and that modifications and variations thereto, over and above those described, are possible without departure from the scope of the invention as set forth in the appended claims.

Claims: CLAIMS: 1. A thick film heating element comprising a main heater track portion and, electrically connected in parallel therewith, a plurality of parallel-connected track portions having an NTCR (negative temperature coefficient of resistance) characteristic connected in series with a track portion having a PTCR (positive temperature coefficient of resistance) characteristic, the plurality of NTCR track portions being distributed around the main heater portion. 2. A thick film heating element as claimed in claim 1 wherein there are at least three said NTCR track portions. 3. A thick film heating element as claimed in claim 2 wherein there are four or five said NTCR track portions. 4. A thick film heating element as claimed in any preceding claim wherein the PTCR track portion is distributed with the NTCR track portions around the main heater portion. 5. A thick film heating element as claimed in any preceding claim including terminal portions enabling a temperature sensitive control device to be operatively coupled to the heating element so as to be responsive to the temperature of said PTCR track section for switching off the power supply at least to said main heater track portion. 6. A thick film heating element as claimed in any preceding claim comprising a substrate, formed of stainless steel for example, having an electrically-insulating layer, formed of glass for example, upon one of its major surfaces and having its track pattern formed on said electrically-insulating layer and a further electrically-insulating layer, formed of glass for example, overlying the track pattern. 7. A thick film heating element as claimed in any of the preceding claims wherein there are a plurality of heater track portions each having associated therewith a sensory circuit comprising a plurality of NTCR track portions as aforesaid and a series connected PTCR track portion as aforesaid. 8. A thick film heating element as claimed in any of the preceding claims and including, associated therewith, a thermally-sensitive control device arranged to be responsive to the temperature of the or each said PTCR track portion for controlling the supply of electrical power to at least the associated heater track portion. 9. A thick film heating element as claimed in claim 8 wherein said thermally-sensitive control device has a first sensor responsive to the temperature of the PTCR track portion and a second sensor responsive to the temperature of the respective heater track portion. 10. A thick film heating element as claimed in claims 7 and 8 wherein said thermally-sensitive control device has a first sensor responsive to the temperature of a first PTCR track portion associated with a first main heater track portion and a second sensor responsive to the temperature of a second PTCR track portion associated with a second heater track portion. 11. An electrically powered water heating vessel incorporating a thick film heating element as claimed in any of the preceding claims. 12. A thick film heating element as claimed in any of claims 1 to 10 or an electrically powered water heating vessel incorporating the same, except that rather than a plurality of parallel-connected NTCR track portions there is just one NTCR track portion in series with the PTCR track portion.

Inventors: O'NEILL ROBERT ANDREW, O'NEILL, ROBERT, ANDREW
Applicants: ONEILL ROBERT ANDREW, OTTER CONTROLS LTD, O'NEILL, ROBERT, ANDREW, OTTER CONTROLS LIMITED
Application year: 1999
Main classification: H05B3
IPC codes: H05B1