IMPROVEMENTS TO LIQUID BOILING APPARATUS

Abstract: In a liquid heating apparatus (10) such as an electric kettle, a method is provided of controlling operation of an electrical element (11), for example to switch off the power supply to the element on boiling of the liquid. In the method, the temperature of the element (11) is measured at regular predetermined intervals by a sensor (14). A microprocessor (15) is provided linked to the sensor (14) and to a timer (16) and is capable of determining the rate of change of the temperature of the element (11) at any given time after the element (11) has been switched on. The microprocessor (15) can thereby detect when the rate of change in the temperature of the element (11) exceeds a predetermined threshold, which has been calculated according to the power rating of the electrical element, subsequent to a predetermined initial heating period for the element (11). Thus, the microprocessor (15) can control the means (12) for cutting-off or varying the electrical power supply to the element (11) dependent on the difference between said actual rate of change and said predetermined value. In this way, the apparatus can be used to detect an initial boiling of the liquid when the bubbles of vapour which initially form within the liquid congregate over the surface of the element and insulate it from the liquid for a short time causing the temperature of the element at this period of time to increase more rapidly.

Description: IMPROVEMENTS TO LIQUID BOILING APPARATUS The present invention relates to a liquid boiling apparatus incorporating an electrical element and in particular to but not exclusively to water boiling apparatus such as an electric kettle. In a conventional apparatus used for boiling liquid, such as an electric kettle, boiling of the liquid is detected by detecting the presence of the vapour produced as the liquid boils. Typically in an electric kettle, the pressure of the steam produced is used to operate a switch to cut off the electricity supply to the electrical element. However, one disadvantage with such an arrangement is that a considerable quantity of steam has to be produced before the pressure-responsive switch operates, particularly if only a small quantity of water is boiled in a large volume kettle. This increases the risk of the kettle boiling dry. In addition, to detect a dry boil condition, a thermal fuse is usually employed. It has been found, however, that when a liquid being heated by an electrical element in direct contact with the liquid begins to boil, the bubbles of vapour which initially form within the liquid congregate over the surface of the element before rising and discharging into the atmosphere. These bubbles insulate the element from the liquid for a short timr but once a condition of rapid boiling occurs and the liquid is agitated, bubbles of vapour are formed throughout the liquid and any formed over the surface of the element rapidly rise to be replaced by liquid. It will be appreciated that boiling of the liquid could, therefore, be deduced by detection of the period at which the element is insulated by the first bubbles to be formed within liquid for, if the power supply to the element is kept constant, the rate of rise in the temperature of the element at this period of time will increase more rapidly as it is unable to discharge its heat into the liquid. Once rapid boiling occurs, however, the temperature of the element will level off as a steady state condition is reached. It is an object of the present invention to make use of this phenomenon to enable the supply of power to a liquid boiling apparatus to be controlled. Such an apparatus overcomes the aforementioned disadvantage of conventional apparatus and enables boiling of a liquid to be detected without it being necessary to produce significant quantities of vapour and irrespective of the mass of liquid being heated. According to a first aspect of the present invention there is provided a method of controlling the operation of an electrical element for heating liquid comprising the steps of measuring the temperature of the element at regular predetermined intervals; calculating the rate of change of the temperature of the element at each of said predetermined intervals; detecting when the rate of change in the temperature of the element exceeds a predetermined threshold calculated according to the power rating of the electrical element and subsequent to a predetermined initial heating period for the element; and at a predetermined time thereafter cutting-off or varying the electrical power supply to the element thereby to control the subsequent quantity of heat imparted to the liquid by the element. Preferably, in a water heating apparatus with a 2 kW electrical element said predetermined threshold comprises a rate of rise of temperature of 40 * C per second. Preferably also, the time taken for the rate of change of the temperature of the element to reach said predetermined threshold is measured subsequent to said initial heating period for the element and if this time is less than one second the electrical power supply to the element is cut off to prevent a dry boil condition occurring. Preferably also, if the electrical power supply to the element has been switched off after the rate of change in the temperature of the element has exceeded the predetermined threshold and thereafter if the rate of decrease in the temperature of the element is slower than a predetermined rate of change for any given liquid, a warning signal is given whereby a user can be alerted to clean the element. Preferably also, in a water heating apparatus with a 2 kW electrical element said predetermined rate of change comprises a rate of decrease in the temperature of the element of substantially 30 * C per second. Alternatively, if the initial rate of change of the temperature of the element immediately subsequent to said initial heating period for the element is greater than a predetermined rate of change for any given liquid but less than said predetermined threshold, a warning signal is given to prompt a user to clean the element. Preferably also, in a water heating apparatus with a 2 kW electrical element, said warning signal is given if said initial rate of change of the temperature of the element immediately subsequent to said initial heating period is greater than 20* C per second but less than 40 * C per second. According to a second aspect of the present invention there is provided a liquid heating apparatus comprising an electrical element, a sensor for detecting the temperature of the element, and means for cutting-off or varying the electrical power supply to the element, and characterised in that a timer and a microprocessor are provided, the microprocessor being linked to the sensor and to the timer and being capable of determining the rate of change of the temperature of the element at any given time after the element is switched on, comparing said rate of change to a stored predetermined value and controlling said means for cutting-off or varying the electrical power supply to the element dependent on the difference between said actual rate of change and said predetermined value. Preferably, the temperature sensor comprises a thermistor located peripherally of the electrical element. Advantageously, the electrical element comprises a conductive track of a thick film printed circuit formed on a substrate and the temperature sensor comprises a thermistor formed by a conductive track of measurable resistance on the same substrate. Preferably also, the conductive track comprising said thermistor extends parallel to and in close proximity with at least a part of the conductive track comprising the electrical element. Preferably also, the substrate comprises a ferromagnetic steel which will vibrate in use to mitigate the effect of limescale deposition thereon. The present invention will now be described by way of example with reference to the accompanying drawings in which;- Figure 1 is a graph showing the rise in temperature against time of water contained within a liquid boiling apparatus and of an electrical element of such an apparatus ; Figure 2 is a schematic block diagram of circuitry for use in controlling the operation of an electrical element in an apparatus according to the present invention; Figure 3 is a schematic view of a substrate incorporating an electrical element and a temperature sensor for use in an apparatus according to the present invention; and Figure 4 is a graph similar to Figure 1 showing the change in temperature against time of an electrical element in a water boiling apparatus in both a scaled and unsealed condition when the water is heated to boiling point and then subsequently permitted to cool. The scientific phenomenon underlying the present invention will firstly be described in detail with reference to Figure 1. Whilst Figure 1 shows the conditions in a water boiling apparatus such as an electric kettle it should be appreciated that a similar phenomenon would occur in a boiling apparatus for any liquid. Figure 1 is a graph illustrating ideal conditions when a litre of water is heated to boiling point by a 2 kW electrical element in an apparatus such as a kettle. The element itself can be of any conventional type. The rise in the temperature of the water against time is shown by the dashed line 1; the rise in the temperature of the electrical element heating the water is shown by the unbroken line 2; and the rise in the temperature of the electrical element in a dry kettle, ie. in a dry boil condition is shown by the unbroken line 3. As can be seen, when the electrical element is powered the water temperature rises linearly from ambient to boiling point at 100" C before levelling off and remaining at boiling point for as long as heat is supplied to it by the element. The length of time taken for the water to reach boiling point is dependent on the mass of water being heated and the power rating of the element but the relative gradients of the lines 1, 2 and 3 are always the same. In contrast to line 1, the temperature of the element rises rapidly initially at approximately 40* C per second, dependent on its thermal mass, until it reaches a point A when the temperature begins to rise more slowly and its rate of rise becomes the same as that of the water. However, just before boiling of the water occurs, the gradient of the line 2 again begins to rise rapidly at a point B before levelling off at a temperature of approximately 130" C at a point C. The point B in the line " 2 occurs when the bubbles of vapour which initially form within the water are lying over the surface of the element and are thereby insulating it from the water. This causes the temperature of the element to rise sharply. However, as the water begins to boil more rapidly, the insulating layer of bubbles is dispersed as the water becomes agitated and the temperature of the element levels off at point C and remains thereafter in a steady state condition. Line 3 of the graph shows the rise in temperature of the element in a dry boil condition when no water is present in the kettle. Here at point A, the rate of rise in the temperature of the element does not fall but continues to rise sharply. It can be seen that the gradient of the line 3 from the point A onwards is similar to that of the line 2 between points B and C when the element is being insulated by the bubbles of vapour. For a 2 kW element, this gradient comprises a rate of approximately 40" C per second. The present invention uses the fact that the gradient of the line 2 increases rapidly just prior to and at the initial boiling of a liquid because of the aforementioned phenomenon in order to provide a means of controlling the power supply to an electrical element. In addition, dry boil conditions can be monitored. For example, if a 2kW element sustains a rate of temperature rise of 40" C per second for more than approximately a second either after the initial heating period of the element, that is after point A has been reached, or after point B has been reached, then a dry boil condition can be inferred. In these circumstances the power supply to the element can be cut off until the apparatus is manually re-set. With reference to Figure 2, a liquid heating apparatus 10, such as an electric kettle, comprises at electrical element 11 which can linked by a power switch 12 to a source of electricity 13 such as a mains supply. Within the apparatus 10 and in close proximity to the element is a temperature sensor 14 for detecting the running temperature of the element 11. The output from the temperature sensor 14 is connected to a microprocessor 15 which can control the power supply to the element 11 via the switch 12. The microprocessor 15 is also linked to or incorporates a timer 16 which measures the elapsed time from the point at which the element 11 is first activated. In normal operation of the apparatus, the apparatus 10 is at least partially filled with water and the element 11 is activated by operation of the switch 12 to heat the water. As the microprocessor 15 is linked to both the temperature sensor 14 and the timer 15 it can calculate at predetermined intervals the instantaneous rate of change of the temperature of the element 11. In other words it can calculate the gradient of the line 2 in Figure 1 at any given elapsed time. The microprocessor 15 is programmed to make this calculation at predetermined intervals of a second or less throughout the period during which the element 11 is activated and by this means it is then possible to detect the condition of the element 11 when the rate of change of its temperature is between points B and C as shown in Figure 1. It is known, therefore that at point C and beyond the water must be at boiling point. Thus, it is possible to program the microprocessor 15 either to cut¬ off the electrical supply to the element 11 altogether after a further predetermined time when rapid boiling will have been achieved, or if the switch 12 is of a sophisticated type such as a TRIAC, relay or other voltage controlling means, to reduce the power supplied to the element 11 to permit the water in the apparatus to be kept, for example, in a simmering mode. Also, as previously described the microprocessor - can be programmed to take action if a dry boil condition is suspected at any time. In order for apparatus according to the present invention to operate satisfactorily it will be appreciated that the temperature of the element 11 must be detected with some accuracy. It is important, therefore for the sensor 14 to be in close proximity to the element 11. With a conventional wire wound electrical element the temperature sensor could comprise a thermistor wound around the periphery of the element. However, the invention is particularly suited for use with an electrical element which comprises a conductive track of a thick film printed circuit formed on a substrate. The temperature sensor 14 can then comprise a thermistor which is formed by a second conductive track of measurable resistance on the same substrate whereby the temperature of the element can be accurately detected. Such an element will now be described in more detail with reference to Figure 3. As shown in Figure 3, a substrate 17 is provided to which a thick film circuit layout has been applied, in known manner, by printing. The circuit layout comprises a first conductive track 18 constituting the electrical element 11 and a second conductive track 19 constituting the temperature sensor 14 which takes the form of a thermistor. An earth tag 20 is also provided, in conventional fashion, for the element. Preferably, the substrate 17 comprises a ferromagnetic steel. Such a substrate will vibrate slightly in use when an electrical current is passing through the track 18. This is advantageous for use with water heating apparatus as the vibration prevents significant layers of limescale being deposited on the substrate 17 because such layers are eventually broken up by the energy of the vibrations. The track 18 follows a tortuous path over the majority of the area of the substrate 17 to maximize the heated area of the element 11. At its ends, the track 18 terminates in respective contact portions 21 and 22 which are adapted to make electrical connection with an electrical control device which comprises the switch 12 as shown in Figure 2. A third contact portion 23 is also provided connected to the track 18 which is used to take power from the element to power the control circuitry comprising the microprocessor 15 and the timer 16. The track 19 constituting the temperature sensor extends in close proximity with the track 18 around its periphery between first and second terminal pads 24 and 25. These pads 24, 25 are used to connect the track to the microprocessor 15. Up to this point, a water heating apparatus has been described wherein it is expected that the rates of rise of both the water and the element itself will closely follow an ideal, such as described with reference to Figure 1. However, in reality the operation of the element and therefore the efficient heating of the water within the apparatus will be affected by the presence of limescale. As is known, in any water heating apparatus which heats water above 65" C and which is used in hard water areas, limescale tends to build up as a layer within the apparatus. In particular, limescale builds up as a coating over the element itself. Thick film printed circuits formed on substrates are not immune from such limescale build-up but as previously described, those formed on a substrate of ferromagnetic steel tend to be self-cleaning once a significant layer of limescale is present. However, the presence of a limescale layer on an electrical element affects the rate of rise of its temperature when powered. In Figure 4 is shown the change in temperature against time of an electrical element in a water heating apparatus in both a scaled and unsealed condition when the water is heated to boiling point and then subsequently permitted to cool. Unbroken line 4 is equivalent to line 3 in Figure 1 and shows the change in temperature against time of the element in an unsealed condition. Here, however, a short time after a steady state position is reached once the water is boiling, the power to the element is switched off. This occurs at point D and it can be seen that the effect of turning the power off results in a rapid cooling of the element until it reaches the temperature of the water, at point E. Thereafter, the water and the element together cool much more slowly at a rate which will be determined by factors that are of no relevance to the present invention and that are dependent on the environment of the kettle itself and the quantity of water heated initially. Should the element have a layer of limescale, its performance is significantly affected and the change of its temperature against time is shown by line 5. The limescale acts as an insulator placed between the element and the water. When the element is switched on, the temperature of the element again rises rapidly but at point A where the initial rapid rate of rise in the temperature of the unsealed element falls to track that of the water, the rate of rise in the temperature of the scaled element continues to rise for a further short time before reducing. When it does reduce, at point F, the actual running temperature of the element is greater by approximately 20" C over that of the unsealed element. Between point A and point F if the rate of change of the temperature of a 2 kW element is greater than 20" C per second but less than 40" C per second then it can be deduced that it requires de-scaling. Points B and C occur at the same time for the scaled element as the unsealed element so that detection of the boiling of the water is not affected. However, after the power to the element is switched off at point D, the scaled element takes a significantly longer time to cool down to the same temperature, which occurs at point G. In particular it is estimated that in a water heating apparatus with a 2 kW electrical element a rate of decrease in the temperature of the element of less than 30" C per second after point D indicates that the element requires de-scaling. Hence, it is possible by detecting a higher running temperature of the element after the initial period up to point A than would normally be expected, or by detecting either a higher rate of increase in the temperature of the element immediately after point A or a slower rate of decrease in the temperature of the element after the power has been switched of at point D than would otherwise be expected to deduce that the element needs de-scaling or cleaning. It is possible, therefore, to include in the control circuitry shown in Figure 3 a warning signal such as a light which can be illuminated to prompt a user to clean the element. Thus, the invention provides a means whereby the boiling of a liquid in a vessel can be detected by monitoring the temperature of the element without reference to the liquid itself. This means that the use of conventional vapour pressure switches and the thermal fuses used to detect dry boil conditions can be avoided and, combined with the use of thick film elements, that all the element control circuitry can be made solid state. This significantly reduces the quantity of wiring required for such an apparatus and thereby simplifies its assembly. The detection of boiling also enables the apparatus to be simply adapted to incorporate built-in simmer controls and other variable temperature controls. In addition, for a water heating apparatus, such as a kettle, or for other apparatus used for boiling liquids resulting in the deposition of a sediment or scale further analysis of the rates of change in the element temperature can be carried out to enable users to be prompted into cleaning or de-scaling the element to ensure the apparatus operates at maximum efficiency.

Claims: CLAIMS 1. A method of controlling the operation of an electrical element for heating liquid comprising the steps of measuring the temperature of the element at regular predetermined intervals; calculating the rate of change of the temperature of the element at each of said predetermined intervals; detecting when the rate of change in the temperature of the element exceeds a predetermined threshold calculated according to the power rating of the electrical element and subsequent to a predetermined initial heating period for the element; and at a predetermined time thereafter cutting-off or varying the electrical power supply to the element thereby to control the subsequent quantity of heat imparted to the liquid by the element. 2. A method as claimed in Claim 1, characterised in that in a water heating apparatus with a 2 kW electrical element said predetermined threshold comprises a rate of rise of temperature of 40" C per second. 3. A method as claimed in Claim 1 or Claim 2, characterised in that the time taken for the rate of change of the temperature of the element to reach said predetermined threshold is measured subsequent to said initial heating period for the element and if this time is less than one second the electrical power supply to the element is cut off to prevent a dry boil condition occurring. 4. A method as claimed in any one of Claims 1 to 3 , characterised in that if the electrical power supply to the element has been switched off after the rate of change in the temperature of the element has exceeded the predetermined threshold and thereafter if the rate of decrease in the temperature of the element is slower than a predetermined rate of change for any given liquid, a warning signal is given whereby a user can be alerted to clean the element. 5. A method as claimed in Claim 4, characterised in that in a water heating apparatus with a 2 kW electrical element, said predetermined rate of change comprises a rate of decrease in the temperature of the element of substantially 30" C per second. 6. A method as claimed in any one of Claims 1 to 3, characterised in that if the initial rate of change of the temperature of the element immediately subsequent to said initial heating period for the element is greater than a predetermined rate of change for any given liquid but less than said predetermined threshold, a warning signal is given to prompt a user to clean the element. 7. A method as claimed in Claim 6, characterised in that in a water heating apparatus with a 2 kW electrical element, said warning signal is given if said initial rate of change of the temperature of the element immediately subsequent to said initial heating period is greater than 20* C per second but less than 40" C per second. 8. A liquid heating apparatus (10) comprising an electrical element (11), a sensor (14) for detecting the temperature of the element, and means (12) for cutting-off or varying the electrical power supply to the element, and characterised in that a timer (16) and a microprocessor (15) are provided, the microprocessor (15) being linked to the sensor (14) and to the timer (16) and being capable of determining the rate of change of the temperature of the element (11) at any given time after the element (11) is switched on, comparing said rate of change to a stored predetermined value and controlling said means (12) for cutting-off or varying the electrical power supply to the element (11) dependent on the difference between said actual rate of change and said predetermined value. 9. An apparatus as claimed in Claim 8, characterised in that the temperature sensor (14) comprises a thermistor located peripherally of the electrical element (11) . 10. An apparatus as claimed in Claim 8, characterised in that said electrical element (11) comprises a conductive track of a thick film printed circuit formed on a substrate (17) and the temperature sensor (14) comprises a thermistor formed by a conductive track (19) of measurable resistance on the same substrate (17). 11. An apparatus as claimed in Claim 10, characterised in that the conductive track (19) comprising said thermistor extends parallel to and in close proximity with at least a part of the conductive track (18) comprising the electrical element . 12. An apparatus as claimed in Claim 11 or Claim 12, characterised in that the substrate (17) comprises a ferromagnetic steel which will vibrate in use to mitigate the effect of limescale deposition thereon.

Inventors: SHARPLES JAMES HENRY, SHARPLES, JAMES, HENRY
Applicants: PIFCO LTD, SHARPLES JAMES HENRY, PIFCO LIMITED, SHARPLES, JAMES, HENRY
Application year: 1996
Main classification: A47J27
IPC codes: A47J27