NL2001690C2 - Device and method for generating steam, and heating element for use in such a device. - Google Patents

Description

Device and method for generating steam, and heating element for use in such a device

The invention relates to a device for generating steam. The invention also relates to a 5 heating element for use in a device according to the invention. The invention further relates to a method for generating steam by means of a device according to the invention.

In conventional steamer ovens, a steam generator is positioned in the bottom of the 10 steamer oven cavity. This steam generator comprises a steam cup for holding water to be evaporated by heat generated within the oven. Under the steam cup a sheathed heating element is positioned. During operation heat is transferred to the steam cup and subsequently from the steam to the water to be evaporated. The water supply is controlled by the application of a so called bird drink dispenser principle, which 15 commonly comprises an inverted water bottle having a dispensing opening extending from the lowermost portion. Gravitational pull on the water creates a partial vacuum at the upper region of the bottle that prevents unhindered outflow through the opening. Typical problems of this known steam generator are limescale forming within the steam cup, and cleaning of the steam generator as such. Moreover, a further problem of the 20 known steam generator is that it has been found difficult to secure a dosed and continuous supply of water to the steam cup. In case the water level in the steam cup is too low, then the heating element will run dry easily, which could damage the steam generator, and moreover could lead to relatively dangerous situations. On the other hand, in case the water level in the steam cup is too high, the time-to-steam will be 25 unfavourably large, which could substantially delay a cooking process within the steamer oven.

It is an object of the present invention to provide an improved device for generating steam, which can be controlled relatively accurately.

30

Accordingly, the present invention provides a device according to the preamble, comprising: at least one steam vessel for collecting water to be evaporated, at least one heating element for evaporating water contained by said steam vessel, at least one water supply valve for providing water to said steam vessel, detecting means for detecting the 2 water level within the steam vessel, and a control unit connected to said water supply valve and said detecting means, said control unit being arranged to open said water supply valve upon detection by said detecting means of the actual level dropping below a predetermined critical minimum water level in order to (re)fill the steam vessel with 5 water. By detecting the actual water level within the steam vessel, either directly or indirectly by the detection means, and by refilling the steam vessel in case the detected actual water level has dropped below a predetermined critical water level, the water supply to the steam vessel in the device according to the invention can be controlled in a relatively accurate manner, as a result of which running dry of the heating element can 10 be prevented. The water supply valve can be considered either as a conventional valve or as a pump. In this context opening of the water supply valve means activating of the water supply valve to allow a pass through of water. Preferably, the control unit is also arranged to close (deactivate) the water supply valve after a certain length of time dependent on the flow rate of the water, and the volume and design of the steam vessel.

15 Alternatively, the control unit can be arranged to close the water supply valve upon detecting the actual water level within the steam vessel reaching a predetermined critical maximum water level. In this manner, the water level within the steam vessel can be kept relatively accurately between a critical minimum water level and a critical maximum water level to prevent running dry of the heating element on one side and to 20 secure a satisfying time-to-steam on the other hand.

The detection of the actual water level within the steam vessel can be realised in various manners, wherein it is conceivable to directly measure the water level within the steam vessel, for example by using a floating element. However, in this boiling environment 25 such a direct, preferably accurate, detection will commonly be hard to realise. Hence, it is often preferred to measure other process parameters which are directly or indirectly related to the actual water level within the steam vessel as will be elucidated hereinafter.

In a preferred embodiment the detecting means are arranged to detect running dry of at 30 least a part of said heating element, and wherein the control unit is arranged to open the water supply valve upon detection by said detecting means of running dry of at least a part of said heating element. The detection of running dry of the heating element will be directly related to the water level within the steam vessel. A situation of (close to) running dry can for example be detected by detecting the actual temperature and/or 3 resistance of the heating element. In case the detection means detects the temperature and/or resistance of the heating element exceeding a predetermined critical threshold value, a situation of running dry of the heating element is considered to be detected, as a result of which the control unit will open the water supply valve to refill the steam 5 vessel at least partially.

The steam vessel may be formed by a conventional steam cup under which a separate heating element could be positioned, wherein heat generated by the heating element can be transferred to the steam vessel, and wherein the heat absorbed by the steam vessel 10 can be transferred to water contained by the steam vessel thereby generating steam. However, in order to increase the thermal efficiency of the device according to the invention, the steam vessel is preferably at least partially defined by the heating element. By integrating the heating element at least partially with the steam vessel, water to be evaporated can be heated directly, rather than indirectly, by the heating 15 element which is favourable from an energetic point of view. Commonly, the heating element forms a bottom side of the steam vessel, wherein one or multiple side walls, preferably a truncated cone-shaped side wall, of the steam vessel could be coupled to the heating element in a substantially mediumtight manner. In this context it will be clear that various configurations and designs of the steam vessel will be imaginable for 20 a person skilled in the art. In a particular preferred embodiment a substantially flat top surface of the heating element defines a bottom part of the steam vessel at least partially. Applying a substantially flat top surface of the heating element will facilitate to perform a controlled and accurate detection of the water level within the steam vessel. More preferably, the top surface of the heating element is oriented substantially 25 horizontally. This implies that the water level above each part of the top surface of the heating element is equal. However, a drawback of this horizontal orientation of the top surface of the heating element defining the steam vessel is that the complete heating element will commonly run dry several times during operation in order to trigger the control unit and hence the water supply valve. In this embodiment running dry of the 30 heating element will be more common that exceptional, which may likely affect the life expectancy of the heating element. Hence, under circumstances it could be preferable in case the top surface of the heating element is oriented tilted with respect to a horizontal line. The angle of tilt is preferably situated between 2° and 30°. By placing the substantially flat top surface of the heating element under an angle it is always defined 4 which part of the top surface of the heating element is running dry firstly. Consequently, the design of the heating element can be optimised for this latter tilted application in order to limit stress within the heating element and to extend the life expectancy of the heating element, and hence of the steam generating device as such.

5

The heating element of the device according to the invention can be of various nature and can e.g. be formed by a sheathed (tubular) heating element. Preferably, the heating element of the device according to the invention is, however, formed by a thick-film heating element, comprising a heat-conducting substrate for heating, at least one first 10 dielectric layer arranged on said substrate, and at least one electrically conductive heating track arranged on said first dielectric layer. Commonly, the substrate is formed by a stainless steel or ceramic plate, onto which successively the first dielectric layer and resistive paste are applied. The individual layers are commonly applied by screen printing, wherein each layer is dried and fired afterwards. A thick-film element is 15 preferably over other conventional (tubular) heating element, since a thick-film has a relatively low (thermal) mass, a small heat storage capacity, a quick response, and is able to spread and to emit heat equally to the water to be evaporated. Different preferred embodiment of the thick-film heater are presented below.

20 In a preferred embodiment the detecting means is at least partially integrated with the heating element, wherein the detecting means comprises: at least one electrically conductive sensor track arranged on the first dielectric layer at a distance from the heating track, and at least one second dielectric layer arranged on the first dielectric layer, which second dielectric layer connects to at least a part of the heating track and to 25 at least a part of the sensor track. Preferably, the sensor track is enclosed between both dielectric layers. More preferably, at almost the same temperature the electrical resistance of the first dielectric layer is higher than the electrical resistance of the second dielectric layer. In case the temperature of the heating element will exceed a predetermined critical value, a leakage current will flow from the heating track via the 30 second dielectric layer to the sensor track. By sensing this leakage current, for example by connecting the sensor track to an ammeter or voltmeter, overheating of the heating element and therefore a situation of running dry can be detected in a relatively sensitive and reliable manner, as a result of which the water supply valve can be opened by the control unit. The heating track may be positioned between the first dielectric layer and 5 the second dielectric layer, resulting in a more or less parallel layout of the heating track with respect to the sensor track. Alternatively, the at least one heating track is positioned on a side of the second dielectric layer remote from the first dielectric layer, which could be favourable in case the sensor track would for example formed by a 5 sensor grid. Preferred embodiments are disclosed in more in detail in the international application WO 2006/083162 and in the non-published international application PCT/NL2008/050360 which are both incorporated herein by reference in its entirety and for all purposes.

10 In another preferred embodiment the heating element further comprises thermal protective means covering at least one heating track section, said thermal protective means being adapted to generate a short circuit of said at least one covered heating track section at a predetermined increased temperature of the heating track thereby increasing the temperature of said at least one covered heating track section such that said at least 15 one covered heating track section will be destroyed at least partially rendering said heating track irreversibly interrupted. Due to this short circuit the thermal protective means and hence the covered heating track section will reach a high temperature being such that the covered heating track section will melt and/or evaporate, and will hence be destroyed. Preferably, the control unit is also connected to the heating element to ensure 20 that the element does not reach excessive temperatures by preventatively switching off the heating element at a predetermined critical temperature.

In a particular preferred embodiment the heating element comprises: a first heating track having a relatively low power density, said first heating track being arranged on a 25 higher part of the steam vessel, and a second heating track having a relatively high power density, said second heating track begin arranged on a lower part of the steam vessel. The first heating track and the second heating track may be connected to each other. However, it is also conceivable that both tracks are separated electrical circuits.

As already mentioned above, such a particular layout of the heating tracks can be 30 favourable in case the heating element is for example oriented tilted. To detect running dry of the heating element, and in particular of a part of the substrate heated by the first heating track, the detecting means is preferably at least coupled to a part of the heating element comprising the first heating track. Alternatively the change in resistance of the low power density track can be used to detect running dry. Alternatively a separate 6 temperature sensing device (for instance a bimetal, an NTC, a PTC, a thermo element) can be used to sense running dry of the low power density track of the heating element. This specific layout is however not merely applicable to a tilted heating element. It is also thinkable that the second heating track (high power density) is arranged on a 5 bottom side of the steam vessel, and that the first heating track (low power density) is arranged on another side, such as a side wall, of the steam vessel.

As already mentioned it is not preferably to directly detect the water level within the steam vessel by a mechanical means such as floater. This boiling environment will 10 commonly be too turbulent to come to a reliable detection of the actual water level. In addition to the preferred embodiments presented above to achieve a relatively accurate detection of an actual water level within the steam vessel, the device alternatively comprises a cold water vessel for collecting water to be fed to the steam vessel. By applying the cold water vessel and the steam vessel as communicating vessels, and by 15 detecting the water level within the cold water vessel, the actual water level within the steam vessel can be determined. Measurement of the water level in the cold water vessel can be established by all know detection means, for instance a floater, resistive or capacitive measurements, or thermal measurements as described above. The cold water vessel may also be integrated with the steam vessel, wherein the water reservoirs of the 20 cold water vessel and the steam vessel are mutually separated, e.g. by a separation wall. In case of detection of the actual water level dropping below a predetermined critical minimum water level, the control unit will open the water supply valve to refill the cold water vessel, resulting in a desired refill of the steam vessel.

25 An alternative manner to detect the amount of water contained by the steam vessel is by weighting the amount of water contained by the steam vessel, based on which the actual amount and hence the water level in the steam vessel can be calculated.

The water supply valve will commonly be arranged to be connected to a water supply, 30 in particular a water mains. The water supply may alternatively be formed by a supply container making part of the device according to the invention, wherein the supply container can be refilled manually or automatically. The supply of water to the steam vessel can alternatively - and eventually additionally - be based on the recirculation of (condensed) steam.

7

In case the water supply valve is formed by a pump, the pump flow rate of the pump, i.e. the number of units of water volume per unit of time, is preferably regulable. It can be advantageous to regulate the pump flow rate so as to be able to meet the water supply 5 requirement in relatively simple manner. More preferably, the control unit is adapted to regulate the pump flow rate subject to the water level related information gathered by the detecting means. By regulating the flow rate of the pump the water supply to the steam vessel could easily be optimised.

10 In another preferred embodiment the device further comprises an after heater arranged to superheat steam received from the steam vessel. Superheated steam is particularly suitable for cooking, as the superheated steam rapidly conducts heat onto the food to be cooked. As the steam is superheated, there is little condensation on the food, so that the embodiment is applicable to ‘dry frying’, where food is cooked rapidly at high 15 temperature without fat or oil, thus providing a healthy alternative to fried food.

Superheated steam is also particularly applicable to sterilizing, degreasing and cleaning applications. The after heater to generate superheated steam is preferably formed by a flow-through heater to efficiently convert the steam into superheated steam. The flowthrough heater preferably comprises an elongate heated flow path, more preferably in a 20 spiral. The flow path optionally includes one or more baffles. The flow-through heater preferably comprises an heating element in thermal contact with the heated flow path, wherein said heating element is more preferably a thick-film heating element. Preferred embodiments are disclosed in more detail in the international application WO 2007/037694 which is incorporated herein by reference in its entirety and for all 25 purposes.

The invention also relates to a heating element for use in a device according to the invention, characterized in that the heating element comprises: a first heating track having a relatively low power density, and a second heating track having a relatively 30 high power density. Advantages and further embodiments of this particular heating element have been elucidated above in a comprehensive manner.

The invention further relates to an electrical appliance provided with a heating element according to the invention. Appliances according to embodiments of the invention may 8 provide steam either as a primary function, such as a steam cleaner or as a secondary function, such as a steam milk frother for making cappuccino coffee. Appliances may have the option to dispense hot water or steam, either through the same outlet or through different outlets. Other appliances that may incorporate the embodiments that 5 dispense steam or other gas include wallpaper strippers, steam irons, water purifiers, food steamers, dishwashers, floor cleaner, carpet, curtain or furniture cleaners and sterilization equipment for medical, dentistry or food sterilisation applications. The appliance may be portable, or form part of a domestic, industrial, commercial or laboratory processing unit.

10

The invention moreover relates to a method for generating steam by means of a device according to the invention, comprising the steps of: A) filling the steam vessel with water to be evaporated, B) activating the heating element thereby evaporating at least a fraction of water initially contained by the steam vessel, C) detecting the water level 15 within the steam vessel by means of the detecting means, and D) opening the water supply valve to refill the steam vessel upon detection of the actual water level dropping below a predetermined critical minimum water level. More preferably, during step C) a situation of running dry or a situation close to running dry, of at least a part of said heating element, and that during step D) the water supply valve is activated upon 20 detection of a situation of running dry of at least a part of the heating element.

Alternatively, during step C) the water level is detected by measuring a leakage current originating from at least one heating track of the heating element. Alternatively during step C) the water level is detected by measuring a change in temperature and/or resistance of at least one heating track of the heating element. It is also conceivable that 25 during step C) the water level is detected by measuring the water level in a cold water vessel communicating with the steam vessel. In a preferred embodiment the method further comprises step E) comprising leading the steam generated in the steam vessel through an after heater for generating superheated steam. The method preferably also comprises step F) comprising closing the water supply valve to terminate refilling of the 30 steam vessel upon detection of the actual water level exceeding a predetermined critical maximum water level.

The invention will be elucidated on the basis of non-limitative exemplary embodiments shown in the following figures. Herein: 9 figure 1 shows a cross-section of a first embodiment of a device for generating steam according to the invention; figure 2 shows a cross-section of a second embodiment of a device for generating steam according to the invention; 5 figure 3 shows a cross-section of a third embodiment of a device for generating steam according to the invention; figure 4 shows a cross-section of a fourth embodiment of a device for generating steam according to the invention; figure 5 shows a cross-section of a fifth embodiment of a device for generating steam 10 according to the invention; figure 6 shows a cross-section of a sixth embodiment of a device for generating steam according to the invention; figure 7 shows a perspective view of a steam oven according to the invention; and figure 8 shows a schematic diagram of a superheated steam generating apparatus 15 according to the invention.

Figure 1 shows a cross-section of a first embodiment of a device 1 for generating steam according to the invention. The device 1 comprises a steam vessel 2 partially filled with water 3 to be evaporated. A bottom side of the steam vessel 2 is defined by a thick-film 20 heating element 4, comprising a metal or ceramic substrate 5, a first dielectric layer 6 arranged on said substrate 5, a sensor track 7 arranged on said dielectric layer 6, a second dielectric layer 8 arranged on said first dielectric layer 6 and said sensor track 7, and a heating track 9 arranged on said second dielectric layer 8. The device 1 further comprises a control unit 10 to which the heating track 9 and the sensor track 7 are 25 connected. An ammeter 11 is applied to measure the leakage current through the sensor track 7. The control unit 10 is connected to a pump 12 being arranged to refill the steam vessel 2. In case of running dry of the heating element 4, the temperature of the heating element will rise rapidly resulting in a decrease of the resistance of in particular the second dielectric layer 8. Consequently, a leakage current will flow from the heating 30 track 9 via the second dielectric layer 8 to the sensor track 7. In case a leakage flow is detected by the ammeter 11 the control unit 10 will switch on and preferably regulates the flow rate of the pump 12 to refill the steam vessel to counterbalance the situation of running dry of the heating element 4. In an alternative embodiment of the device 1, the heating track 9 can be positioned, at a distance from the sensor track 7, in between the 10 first dielectric layer 6 and the second dielectric layer 8, wherein the heating track 9 and the sensor track 7 preferably have a substantially parallel layout.

Figure 2 shows a cross-section of a second embodiment of a device 13 for generating 5 steam according to the invention. The device 13 comprises a steam vessel 14 partially filled with water 15 to be evaporated. A tilted bottom of the steam vessel 14 is defined by a (tilted) thick-film heating element 16, comprising a heat-conductive substrate 17, a dielectric layer 18 arranged on said substrate 17, a first heating track 19 with a relatively low power density arranged on a high part of said dielectric layer 18, and a second 10 heating track 20 with a relatively high power density arranged on a low part of said dielectric layer 18. The first heating track 19 is connected to a detection element 21 for detecting a running dry state of this higher part of the heating element 16. The detection element 21 can be of various nature, wherein the detection element 21 can for example be adapted to detect a change in temperature or resistance of the first heating track 19.

15 In this embodiment also a sensor layer (not shown) could be applied as shown in figure 1 in order to detect a situation of running dry. The first heating track 19 is purposively provided a low power density, since this elevated part of the heating element 16 will alternate between a running dry state and a wetted state. The low power density layout of the first heating track 19 prevents the generation of substantial material stress within 20 this elevated part of the heating element 16 thereby preserving a satisfying life expectancy. Moreover, a low power density heating track 19 is commonly substantially less dangerous than a high power density heating track 20. The first heating track 19 is coupled to a detection element 21 for detecting a situation of running dry of the elevated part of the heating element 16. A control unit 22 connected to said detection element 21 25 is adapted to activate a pump 23 in case of running dry of at least a part of the heating element 16. A water supply container 24 is connected to the pump 23 to enable refilling of the steam vessel 14. The second heating track 20 may optionally also be connected to a second detection element (not shown) providing an additional safety provision in case the detection element 21 would not function properly. In the shown state of the device 30 13, the control unit 22 has just activated the pump 23 to refill the steam vessel 14.

Figure 3 shows a cross-section of a third embodiment of a device 25 for generating steam according to the invention. The device 25 comprises a steam vessel 26 partially filled with water 27 to be evaporated. A bottom of the steam vessel 26 is partially 11 formed by a first thick-film heating element 28. A centre bottom part 29 of the steam vessel 26 is elevated with respect to the adjacent first thick-film heating element 28, wherein the elevated bottom part 29 of the steam vessel 26 is provided with a second thick-film heating element 30 having a low power density. The first heating element 28 5 comprises a substrate 31, a dielectric enamel layer 32 arranged on said substrate 31, and at least one heating track 33 arranged on said enamel layer 32, said heating track 33 having a relatively high power density. A state of running dry of the second heating element 30 can be detected by means of a detection element 34 connected to said second heating element 30. Based on the information gathered by the detection element 10 34 a control unit 35 can actuate a valve 36 of a water conduit 37 connected to the water mains to be able to timely refill the steam vessel 26 to prevent a state of running dry of the first heating element 28.

Figure 4 shows a cross-section of a fourth embodiment of a device 38 for generating 15 steam according to the invention. The device 38 shown in figure 4 is constructively more or less similar to the device 25 shown in figure 3. The device 38 according to figure 4 comprises a steam vessel 39, also referred to as steam cup or steam chamber, which is partially filled with water 40 to be evaporated. A bottom of the steam vessel 39 is formed by a first thick-film heating element 41. A side wall 42 of the steam vessel 39 20 is provided with an elevated second heating element 43 having a low power density.

The first heating element 41 comprises a substrate 44, a dielectric enamel layer 45 arranged on said substrate 44, and at least one heating track 46 arranged on said enamel layer 45, said heating track 46 having a relatively high power density. A state of running dry of the second heating element 43, and hence of the steam generating device 38 as 25 such, can be detected by means of a detection element 47 connected to said second heating element 43. Based on the information gathered by the detection element 47 and provided to a control unit 48 can actuate a valve 49 of a water conduit 50 connected to a water supply container 51 to be able to timely refill the steam vessel 39 to prevent a state of running dry of the first heating element 41. Optionally the water supply 30 container 51 is refilled by recirculation of condensed steam (not shown in this figure)

Figure 5 shows a cross-section of a fifth embodiment of a device 52 for generating steam according to the invention. The device 52 comprises a steam vessel 53 partially filled with water 54, said steam vessel 53 comprising a substantially truncated cone- 12 shaped side wall 55 and a bottom defined by a thick-film heating element 56. The heating element 56 is constructively substantially similar to the first heating element 41 shown in figure 4. The device 52 further comprises a cold water vessel 57 which is in open communication with the steam vessel 53 by means of a connecting conduit 58 5 passing through the heating element 56. The cold water vessel 57 is provided with an optical sensor 59 to detect the actual water level within the cold water vessel 57. The optical sensor 59 is connected to a control unit 60 to process the information gathered by the optical sensor 59 and to activate a pump 61 for refilling the cold water vessel 57 and consequently the steam vessel 53. Since the actual water level within the cold water 10 vessel 57 is directly related to the actual water level within the steam vessel 53, the actual water level within the steam vessel 53 can be monitored by means of the optical sensor 59, and hence a situation of running dry of the heating element 56 can be prevented.

15 Figure 6 shows a cross-section of a sixth embodiment of a device 62 for generating steam according to the invention. The device 62 comprises a vessel 63 which is in fact divided into a cold water vessel 63a and a steam vessel 63b by means of a dividing wall 64. In the embodiment shown the vessel 63 is partially filled with water 65 to be evaporated. The cold water vessel 63a and the steam vessel 63b are in open 20 communication with each other. Consequently the water level in both vessels 63a, 63b is equal. A bottom of the vessel 63 is defined by a thick-film heating element 66 comprising a metal substrate 67, a dielectric layer 68 arranged on said substrate 67, and a heating track 69 applied to said dielectric layer 68. The heating track 69 is applied such that the heating element 66 is adapted to heat in particular the water present in the 25 steam vessel 63b. No heating track is applied underneath the cold water vessel 63a to counteract undesired heating of water contained by the cold water vessel 63a. A floating element 70 is applied in the cold water vessel 63a which substantially freely floats on the water level within the cold water vessel 63a. The floating element 70 is connected electrically with a control unit 71. Said control unit 71 is further connected to a pump 30 72, wherein the control unit 71 is arranged to activate the pump 72 dependent on the detected water level within the vessel 63. In this manner the water level within the vessel 63 can be dosed to prevent both a situation of running dry of the heating element 66 and a situation wherein an excessive amount of water would be present in the vessel 13 63. In this manner, the amount of steam generated in an amount of time can be optimised.

Figure 7 shows a perspective view of a steam oven 73 according to the invention. The 5 steam oven 73 comprises a cooking compartment 74 which is presently closed by a door 75. Within the cooking compartment 74 a device 76 for generating steam according to the invention is applied. Embodiments of this steam generating device 76 have been described above.

10 Figure 8 shows a schematic diagram of a superheated steam generating apparatus 77 according to the invention, wherein pressurized steam is provided to an inlet 78 of a flow-through heater 79, which superheats the steam to produce superheated steam at an outlet 80. The pressurized steam is generated by a steam generator 81, which is supplied with liquid. A temperature sensor 82 at the outlet 80 of the heater 79 provides an input 15 to a controller 83 arranged to control the steam generator and/or the heating power of the heater 79 so as to maintain a predetermined temperature at the outlet 80. Preferably, the steam generator 81 generates steam substantially free from liquid. The steam is then superheated by the flow-through heater 79, which may be a spiral flow-through heater substantially as disclosed in WO 2005/080885, although preferably baffles are provided 20 within the spiral channel so as to slow the flow of steam; the baffles may be provided by a spring arranged within the channel. In one experimental example, the flow-through heater draws 320 W and outputs superheated steam at up to 260°C at only 0.25 bar overpressure. Superheated steam is produced very quickly, within 60 seconds of starting the apparatus.

25

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the 30 claim. Use of the verb “comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims (31)

Device for generating steam, comprising: - at least one steam vessel for holding water to be evaporated, - at least one heating element for evaporating water contained in the steam vessel, - at least one water supply valve for supplying the steam vessel supplying water, - detection means for detecting the water level in the steam vessel, and - a control unit connected to the water supply valve and the detection means, wherein the control unit is adapted to, after detection by the detection means, that the current water level has fallen below a predetermined critical water level, open the water supply valve to refill the steam vessel with water. Device as claimed in claim 1, characterized in that the detection means are adapted to detect dry cooking of at least a part of the heating element, and in that the control unit is adapted to open the water supply valve after detection by the detection means of drying at least dry a part of the heating element. 20 Device as claimed in claim 1 or 2, characterized in that the steam vessel is formed at least partially by the heating element. 4. Device as claimed in claim 3, characterized in that a substantially flat top surface of the heating element forms at least one bottom part of the heating element. Device as claimed in claim 4, characterized in that the upper surface of the heating element is substantially horizontally oriented. 30 Device according to claim 4, characterized in that the upper surface of the heating element is oriented obliquely. 2001690 7. Device as claimed in any of the foregoing claims, characterized in that the heating element comprises a heat-conducting heating substrate, at least one first dielectric layer applied to the substrate, and at least one electrically conductive heating track arranged on the first dielectric layer. 5 Device as claimed in claim 7, characterized in that the detection means comprise: - at least one electrically conductive sensor track arranged on the first dielectric layer remote from the heating track, and - at least one second dielectric layer arranged on the first dielectric layer, which second dielectric layer connects to at least a part of the heating track and to at least a part of the sensor track. 9. Device as claimed in claim 8, characterized in that at least one heating track is placed between the first dielectric layer and the second dielectric layer. 10. Device as claimed in claim 8 or 9, characterized in that at least one heating track is placed on a side 20 of the second dielectric layer remote from the first dielectric layer. 11. Device as claimed in any of the claims 7-10, characterized in that the heating element further comprises thermal protection means which cover at least one heating track segment, wherein the thermal protection means are adapted to create a short circuit of the at least one covered heating track segment at a predetermined increased temperature of the heating track, wherein the temperature of the at least one covered heating track segment will rise such that the at least one covered heating track segment will be destroyed at least in part, irreversibly interrupting the heating track. Device as claimed in any of the foregoing claims, characterized in that the heating element comprises: - a first heating track with a relatively low power density, wherein the first heating track is arranged on a higher part of the steam vessel, and - a second heating track with a relatively high power density, wherein the second heating track is arranged on a lower part of the steam vessel. 13. Device as claimed in claim 12, characterized in that the second heating track is arranged on a bottom part of the steam vessel, and in that the first heating track is arranged on another side of the steam vessel. 14. Device as claimed in any of the claims 7-11, and claim 12 or 13, characterized in that the first heating track is arranged on a higher part of the first dielectric layer, and in that the second heating track on a lower part of the first first dielectric layer is provided. Device as claimed in any of the claims 11-14, characterized in that the detection means are adapted for detecting a part of the steam vessel being boiled dry by the first heating track. 20 Device as claimed in any of the claims 7-11,13, or 14, characterized in that the detection means are adapted to detect a change in temperature and / or a change in resistance of at least one heating track of the heating element. 25 Device as claimed in any of the foregoing claims, characterized in that the device further comprises a cold water vessel for collecting water to be supplied to the steam vessel. Device as claimed in claim 17, characterized in that the cold water vessel and the steam vessel are communicating vessels, and that the detection means are adapted to detect the water level in the cold water vessel. Device as claimed in any of the foregoing claims, characterized in that the water supply valve is adapted to be connected to a water supply, in particular a water pipe. Device as claimed in any of the foregoing claims, characterized in that the water supply valve is a pump, and in that the control unit is adapted to regulate the pump flow rate in dependence on the water level related data and the data collected by the detection means. Device as claimed in any of the foregoing claims, characterized in that the device further comprises a post-heater for overheating steam received from the steam vessel. 22. Device as claimed in claim 21, characterized in that the post-heater is formed by a flow-through heater. Heating element for use in a device according to any one of claims 1-13, characterized in that the heating element comprises: - a first heating track with a relatively low power density, and - a second heating track with a relatively high power density. An electrical device provided with a heating element according to any of claims 1-22. 25. Method for generating steam by means of a device according to any one of claims 1-22, comprising the steps of: A) filling a steam vessel with water to be evaporated, B) activating the heating element whereby at least a fraction of the water initially absorbed by the steam vessel is evaporated, A method according to claim 25, characterized in that during step C) a dry boiling state or an almost dry boiling state of at least a part of the heating element is detected, and that in step D) the water supply valve 5 is opened after detection of a dry boiling state of at least a part of the heating element. 27. Method as claimed in claim 25 or 26, characterized in that during step C) the water level is detected by measuring the leakage current coming from at least one heating track of the heating track. 28. Method as claimed in any of the claims 25-27, characterized in that during step C) the water level is detected by measuring a change in temperature and / or resistance of at least one heating track of the heating element. A method according to any one of claims 25-28, characterized in that during step C) the water level is detected by measuring the water level in a cold water vessel communicating with the steam vessel. 20 A method according to any one of claims 25-29, characterized in that the method further comprises step E) comprising passing through a post-heater the steam generated in the steam vessel for generating superheated steam. C) detecting the water level in the steam vessel by means of the detection means, and D) opening the water supply valve for filling the steam vessel after it has been detected that the current water level falls below a predetermined critical water level. A method according to any one of claims 25-30, characterized in that the method further comprises a step F) comprising closing the water supply valve to, after detection of the current water level exceeding a predetermined critical maximum water level, filling of the steam vessel. 2001690