Patent Publication Number: US-7223947-B2

Title: Domestic appliance and heating structure for a domestic appliance

Description:
TECHNICAL FIELD 
     This invention relates to an electric heating structure for a domestic appliance such as an iron, a (deep fat or other) frying pan, a water kettle or a grill and to a domestic appliance including such a heating structure. 
     BACKGROUND ART 
     Many electric heating structures of domestic appliances include a heating element with a positive temperature coefficient (PTC), such as a thick-film resistive heating element. In a PTC heating element, the electric resistance increases with the temperature. 
     A problem of such heaters is that, in operation, the heating power of the heating element reduces as its temperature rises. The increase of the electric resistance as the temperature rises causes a reduction of the current through the heating element, and accordingly of the rate at which electric energy is converted into heat by the heating element. 
     For instance, in silver based heating elements the electric resistance typically increases by at least 0.2% of the room temperature resistance per ° C. This results in a power drop of more than 50% when heating up from room temperature to an operating temperature of the heating element of 250° C. and more than 20% when heating to 100° C. In a heating structure that has for instance been designed for a maximum power consumption of 2000 W, in order to avoid exceeding the maximum power usually available for household use without causing safety fuses or circuit breakers to trip, the maximum power available at 250° C. is therefore less than 1000 W. The high temperature power reduction generally associated to heaters with PTC heating elements causes an increase of the time required for heating to the maximum temperature associated to the selected or pre-set temperature setting, which manifests itself in particular during re-heating such as occurs for instance in thermostatically controlled heaters. Alternatively or in addition, the reduction of the available amount of power at higher temperatures results in a reduction of the conversion rate of a process, such as for instance steam generation, driven by the heating element. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to reduce the power reduction at higher temperatures that occurs in heating structures equipped with a PTC heating element. 
     According to the present invention, this object is achieved by providing a heater according to claim  1 . The invention may also be embodied in a domestic appliance according to claim  11 . 
     By switching-on an additional heating track in parallel and in addition to the first heating track when at least the first heating track has been heated to at least a predetermined extent, additional heating power is provided when the electric power consumption by the first heating track has reduced sufficiently to allow power consumption by a further heating track without exceeding the allowable maximum power consumption. 
     Further aspects, effects and details of particular embodiments of the invention are set forth in the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is schematic representation in cross-sectional view along a vertical, longitudinal midplane of a domestic appliance according to the invention in the form of a steam iron equipped with a heater according to the invention; 
         FIG. 2  is a schematic representation of a heating structure of the appliance according to  FIG. 1 ; 
         FIGS. 3-5  illustrate successive stages of operation of a heating structure according to  FIG. 2 ; and 
         FIG. 6  is a graph showing electric current consumption of some examples of heating structures according to the invention. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
       FIG. 1  shows schematically a cross section of a steam iron according to the invention. The iron comprises a housing  10  to which a soleplate  20  is attached. The housing includes a handle portion  11 . A steam generator  40 , which at the same time serves as a water tank, a compartment  12  accommodating a control circuit, and a control panel  60 , are arranged in the housing  10 . 
     A first heating element  21  including first and second heating track-patterns, for instance of conducting film, and a temperature sensor  22 , for example an NTC resistor, are located on the top side of the soleplate  20 . A second heating element  41  also including first and second heating track-pattern, which may also be made of conducting film, is located on the bottom side of the steam generator  40 . A temperature sensor  42  is disposed on the electrical insulation of the second heating element  41 . The bottom of the steam generator  40  can be provided with a layer  44  of capillary material, which makes the entire surface of the bottom remain moist, even if the steam generator is almost empty and the bottom is standing at an angle or even vertically, as a measure against local overheating of the steam generator  40 . A filling cap  45  is mounted to the housing  10  for closing off a filling passage communicating with a water reservoir of the steam generator  40 . Also communicating with the water reservoir of the steam generator  40  are a steam valve  46 , and a sprinkler  47 . The filling cap  45  is shown as a collapsible filling cap, but a different design is, of course, also possible. The steam valve  46  is used to open and close the steam pipe between the steam generator  40  and the steam passages (not shown) in the soleplate, which open out into steam outlets at the bottom side thereof. The sprinkler  47 , finally, serves for additional moistening of the articles being ironed. 
     The iron housing  10  also has a control circuit for controlling the temperature of the soleplate and the steam production in the steam generator  40 . The control circuit is accommodated in the compartment  12  in the handle  11 . 
     An isolation transformer  51 , for example, is provided for the power supply to the control circuit, and arranged such that the control circuit has no direct contact with the mains voltage, and the control circuit can be driven by means of a low electrical voltage, which ensures greater safety. If there is adequate electrical insulation, the isolation transformer may be omitted. The iron is connected to the electricity mains by means of the flex  100 . A hand presence detector  54 , is included in the control circuit. The control panel  60  is also included in the control circuit and designed for the display of information which is useful to the user, such as an indication of the set temperature of the soleplate  20 , and/or indications whether the soleplate has reached the set temperature, regarding the quantity of water in the steam generator etc. The control panel  60  also has switches for setting the temperature, for setting the degree of steam delivery, for operation of the sprinkler, and for causing the release of an additional steam surge. 
     Two relays  52  and  53  are disposed near the transformer  51 , in order to switch on and off the two heating elements  21  and  41  in response to control signals from the control circuit for thermostatically controlling the two heating elements  21  and  41  in accordance with temperature signals received from the temperature sensors  22 ,  42 . 
     In  FIG. 2 , a heating structure including the temperature sensor  22  is shown schematically. This heating structure for heating the soleplate  20  of the iron according to  FIG. 1  includes a thick-film heating element  21 . 
     The heating element  21  is provided with a first electric heating track  23  which has a positive temperature coefficient, as is typical for thick film heating tracks, especially silver based heating tracks, but also common for other heating tracks for electric heaters. The first electric heating track  23  is included in a circuit  26  connected to a contact plug  24  for connection to the mains and including a switch  25  that is operatively connected to the temperature sensor  22  (for instance an adjustable bimetal or a thermistor) for closing the switch  25  if the temperature of the soleplate  20  is below a switch-on temperature and opening the switch  25  if the temperature of the soleplate  20  reaches a switch-off temperature above the switch-on temperature. 
     The heating element  21  further includes a second electric heating track  27  and a third electric heating track  28 , both included in the same circuit  26  in parallel to each other and to the first heating track  23 . 
     In addition to the switch  25  controlled by a temperature signal from the temperature sensor  22 , the control structure for controlling electric power supply to the heating tracks  23 ,  27 ,  28 , further includes a control element  29 , sensitive to the heating of the first heating track  23  and for switching-on the second and third electric heating tracks  27 ,  28  in parallel and in addition to said first heating track when the first heating track has been heated to a predetermined extent. 
     According to the present example, the control elements  29  is sensitive to temperature of the first heating track  23  for carrying out the switching-on of the second and third heating tracks  27 ,  28  in response to a sensed temperature above a predetermined temperature. 
     The operation of the heating structure according to  FIG. 2 , while the thermostatic control switch  25  is closed is illustrated by  FIGS. 3-5 . In  FIG. 3 , the situation at the time of cold start-up in an ambient room temperature of 25° C. is represented. The first heating track  23  has a resistance of 23 Ohm and at a voltage of 230 V; this results in a current of 10 A and, accordingly, a heating power of 2300 W. A current of 10 A is generally the maximum power that can reliably be drawn from normal domestic wall outlets without causing safety fuses or switches of the domestic power network to trip. Accordingly, the second and third heating tracks  27 ,  28 , which have a combined resistance of 35 Ohm at the ambient temperature, are switched off during start-up from cold. 
     The situation in  FIG. 3  is accordingly indicated in the graph in  FIG. 6 . As can be seen in  FIG. 6 , line  30 , which represents the current through the first heating track  23 , decreases at a rate of about 0.02 A/° C. (i.e. about 0.2% of the current at 25° C. per ° C.). 
       FIG. 4  represents the situation once the first heating track  23  has reached a temperature of 200° C. and is also indicated in  FIG. 6 . The resistance of the first track  23  has increased to 40 Ohm so that the current has decreased to 5.8 A and the heating power has decreased to 1322 W. It is observed that the present invention may also be advantageous if the heating tracks have lower PTC values, for instance as low as 0.05% of the current at 25° C. per ° C. 
     Meanwhile, because the second and third heating tracks  27 ,  28  are mounted to the same thermal conductor as the first heating track  23  and thereby thermally coupled to the first heating track  23 , the temperature of the switched-off second and third tracks  27 ,  28  has increased with the temperature of the first heating track  23 , so that these tracks have also reached a temperature of 200° C. The combined electric resistance of the second and third tracks  27 ,  28  has thereby risen to 52 Ohm. 
     Then as represented by  FIG. 5 , the control element  29  switches-on the second and third heating tracks  27 ,  28 . This causes an additional current of 2.1 A through each of the second and third heating tracks  27 ,  28 , adding a current of 4.2 A to the current of 5.8 A through the first heating track  23 . Accordingly, the total current through the heating structure is brought back to 10 A as also appears from the graph in  FIG. 6 . 
     If the temperature of the heating tracks  23 ,  27 ,  28  continues to rise to 250° C., the resistance of the tracks will continue to rise causing the total current to decrease again to 7.7 A at 250° C. (line  31 ). In another embodiment, partial compensation for such a renewed decrease of the heating power is achieved by providing control elements that cause the third heating track  28  to be switched on in response to a predetermined sensed temperature of at least one of the switched-on heating tracks  23 ,  27  that is higher than the sensed temperature in response to which the second heating track  27  is caused to be switched-on. The second heating track  27  may for instance have a resistance of 52 Ohm at 200° C. so that, at 200° C. , the heating power is again 2300 W. The third heating track may be added at 225° C. to add 1.15 A to again bring the total heating power to 2300 W by adding further heating power when the electric resistance of the first and second heating tracks  23 ,  27  has decreased after the second heating track was switched on in addition to the first heating track  23 . 
     According to the present example, the heating tracks  23 ,  27 ,  28  are thermally connected to each other such that, in operation, the second and third heating tracks  27 ,  28  are heated by the first heating track  23 . Furthermore, the sum of the electric resistance of the first heating track  23  when in condition for switch-on of the second and/or third heating track or tracks  27 ,  28  and of the electric resistance at room temperature of the track or tracks  27 ,  28  to be switched-on, is smaller than the electric resistance at room temperature of the first heating track  23 . Because the second and third heating tracks  27 ,  28  are not switched-on before having been heated by the first heating track  23 , it is nevertheless ensured that, in operation, the combined resistance of the heating tracks  23 ,  27 ,  28  does not drop below the initial resistance of the first heating track  23  when the second and third heating tracks  27 ,  28  are switched-on. Preferably, the heating track or tracks to be switched-on in addition to the first heating track are dimensioned such that the combined power consumption of all active heating tracks just after switch-on of the or each additional heating track is about equal to the initial heating power of the first heating track at room temperature. However, a margin (for instance up to about 25-50% of the power decrease in the active heating tracks to be compensated) may be applied, for instance for safety reasons or in view of available heating tracks or modular design to avoid an increase in the variety of parts used. 
     The effect of dimensioning the heating tracks such that the tracks to be switched-on bring the power back to the original level while in a condition pre-heated by the already active heating tracks is best illustrated by  FIG. 6 . If the second and third heating tracks  27 ,  28  would have been designed to have a combined resistance of 52 Ohm at room temperature, the resistance at 200° C. would have been 77 Ohm, so that the current through the second and third heating tracks at 200° C. would only be 2.4 A and drop to 1.9 A at 250° C. (see dashed line  32 ) instead of to 3.3 A at 250° C. as in the present example. 
     The switching-on of additional heating tracks in response to the active heating track being heated can be applied with particular advantage in appliances in which the first heating track is arranged for heating a medium and wherein said second heating track is arranged for heating the same medium. In particular in the temperature range of thermostatic temperature control, this provides a particularly fast reheating of the heated medium in response to heat withdrawal. Examples of such situations are the positioning of an iron on humid cloth or the feeding of cold or even frozen food to a deep fat fryer. Furthermore, if the heating tracks heat the same medium, it can be ensured relatively easily that the heating tracks to be switched-on at higher temperature are heated by the active heating tracks, so that these heating tracks may be dimensioned for compensating the entire power decrease of the active heating track or tracks at an elevated temperature without causing an undue risk of a too high current through the heating structure. 
     However, to avoid a current through the heating structure higher than the current at cold-start-up, it is preferred that the combined electric resistance of the heating tracks when the heating structure is in condition for switch-on of an additional heating track is equal to or larger than the electric resistance of the active heating track or tracks at room temperature. 
     To avoid that at any temperature, the current through the heating structure is higher than the current at cold-start-up, it is preferred that the sum of the electric resistance of, firstly, the active heating track or tracks when the heating structure is in condition for switch-on of one or more further heating tracks and, secondly, the electric resistance at room temperature of the heating track or tracks to be switched-on is equal to or larger than the electric resistance of the active heating track at room temperature. This is of particular interest if the further heating track or tracks that are to be switched-on in addition to one or more active heating tracks are not reliably heated by the active heating tracks. 
     For instance, in an iron with a steam generator as shown in  FIG. 1 , the first heating track that is switched-on first may be formed by the heater  41  of the steam generator and the additional heating track that is switched-on only if the first heating track is above a predetermined temperature may be formed by the heater  21  for heating the soleplate  20 . This allows to have a steam iron of which the heating structure has a combined power at room temperature that is higher than would be allowable if all heating tracks could be active simultaneously while at room temperature, but which nevertheless allows to heat the soleplate while the heating track for generating steam is active without exceeding the maximum allowable electric power consumption rate, because the heater  21  for heating the soleplate  20  is switched-on only if the temperature of the heating track or tracks  41  of the steam generator is above a suitably set switch-on temperature (for instance 130 to 200° C.). For switching-on one or more further heating tracks  27 ,  28  in response to the temperature of the active (first) heating track or tracks, the control element  29  may for instance be provided in the form of a bimetallic temperature switch sensitive to temperature of the first heating track  23 . 
     Another possibility is to provide the control element  29  in the form of a negative temperature coefficient (NTC) resistance, sensitive to temperature of the first heating track  23 . In such a control element, a small current may also be allowed to pass through the further heating tracks before the switch-on temperature is reached and even at room temperature. The current at room temperature may for instance be a few tenth of a percent or up to a few percent of the current at 200° C. In most NTC-resistances, the resistance decreases exponentially with temperature. A smooth switch-on of the additional heating tracks provides the advantage that the further increase of the resistance of the active heating tracks as temperature rises may be taken into account when dimensioning the heating tracks without allowing the maximum power consumption rate to be exceeded. 
     Within the framework of the present invention many other embodiments than those, which have been described above by way of example, are conceivable. For instance, in the previous examples, the further heating track or tracks have been switched on in response to the sensed temperature of at least the active heating track or tracks. However, it is also possible to provide that the control structure switches-on additional heating tracks in response to other phenomena than the sensed temperature that are normally associated to the temperature of the active heating tracks. 
     For instance, the control element  29  may be sensitive to electric current through the first heating track  23  for carrying out the switching-on of the second and third heating track  27 ,  28  in response to at least current through the first heating track  23  being below a predetermined current. 
     The control element  29  may also include a timer and be adapted for carrying out the switching-on of the second and third heating track  27 ,  28  in response to at least expiration of a predetermined duration of time after switching-on the first heating track  23 , for instance if the purpose of the switching-on of additional heating tracks, while ensuring that maximum allowable power consumption is not exceeded at any time, is mainly to improve the responsiveness to heat withdrawal in use while the time to heat up from cold is relatively unimportant.