Patent Publication Number: US-2021177187-A1

Title: Method of operating a steam generator, steam generator and cooking device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European Application No. 19216086.9, filed Dec. 13, 2019, the contents of which are hereby incorporated herein in its entirety by reference. 
     FIELD OF APPLICATION AND PRIOR ART 
     The invention is directed to a method of operating a steam generator, in particular in a cooking device or a household device operating with steam. Furthermore, the invention is directed to such a steam generator as well as to a cooking device with such a steam generator. Steam generators for cooking devices for cooking purposes or the like are known for example from EP 2 366 315 B1 or EP 2 397 755 B1. A problem with these steam generators is how to find a way to empty a water container of the steam generator after operating of a steam household device, in order to avoid problems with hygiene in the water container due to water staying too long in the water container. 
     Problem and Solution 
     The object of the invention is to provide a method of operating a steam generator as well as such a steam generator and a cooking device with such a steam generator with which problems of the prior art can be avoided and, in particular, it is possible to optimize operating a steam generator. 
     This object is achieved with a method having the features of claim  1 , a steam generator having the features of claim  11  and a cooking device having the features of claim  19 . Advantageous and preferred design embodiments of the invention are the subject matter of the further claims and will be explained in more detail hereunder. Some of the features are only described for the method, only described for the steam generator or only described for the cooking device. Independently of this, they can be applied to such a method, such a steam generator or such a cooking device independently and on their own. The wording of the claims by way of explicit reference is incorporated in the content of the description. 
     In the method according to the invention, the steam generator comprises a water container with a circumferential wall, preferably in the form of a vertically oriented cylinder, for example with round-cylindrical cross-section. Two separate heating devices are provided or located on the wall of the water container, in particular on a lateral outside of the wall of the water container. The two heating devices are located in different height regions separated from each other in vertical direction. One heating device is an upper heating device and the other heating device is a lower heating device such that the upper heating device is above the lower heating device. A first temperature detection device is provided covering a temperature detection area in which a temperature is measured, in particular the reaching of a predefined first temperature threshold is measured. The temperature detection area includes at least the area covered by the two heating devices, wherein preferably also an area of the water container or its lateral wall, respectively, between the two heating devices is covered. A control device for monitoring and evaluating the first temperature detection device is provided, which also controls the activation state of the two heating devices, which can mean that the control device activates or deactivates the two heating devices. The method provides the steps of filling the water container with water and operating the steam generator afterwards by heating with at least one of the two heating devices that are being activated, preferably by the control device. Steam is then generated for operation of a steam cooking device with this steam, preferably for cooking purposes. After the operation of the cooking device is finished, for example because a respective steaming program is finished, steam generating is finished because no more steam is required. After that, at least one heating device is activated, wherein preferably at least the lower heating device is activated or both heating devices are activated, until the first temperature detection device detects that the predefined first temperature threshold named above has been reached. Also the upper heating device can be activated for the first time. This predefined first temperature threshold may be chosen such that it guarantees operating the steam generator in a safe and secure mode, wherein if this first temperature threshold would be exceeded for more than 20° C. or 40° C., a serious damage to the steam generator could occur. 
     As soon as the first temperature detection device detects the reaching of the predefined first temperature threshold, one heating device is deactivated, preferably the upper heating device if it is active or the lower heating device if the upper heating device is not active. If more heating devices than only the lower one had been activated, only they are being deactivated. Then remaining water in the water container is pumped off for a predefined first pumping duration or the lower heating device is activated again until the first temperature detection device detects that a pre-defined first temperature threshold has been reached again. This pumping off may occur with a constant pump rate, in particular by means of an outlet pump provided in the steam generator or in the cooking device, respectively. It is possible that this outlet pump is provided only for the purpose of pumping off remaining water from the water container. Activation of the lower heating device also serves to remove water from the water container. 
     The aim of the invention is to reduce not only a pumping duration for emptying the water container, but mainly to remove or evaporate water from the water container until a certain defined level of water is reached before pumping off is started. 
     When both heating devices are activated and heat the water remaining in the water container after finishing steam generating, they remove the water by evaporation until the upper heating device does not have sufficient thermal contact with water in the water container to dissipate its heat, which results in the first temperature detecting device to detect the first temperature threshold. Then the water level is most probably in the lower region of the upper heating device or between the two heating devices. In any case, this water level can be determined from the construction of the steam generator and a few experiments. 
     When only the lower heating device is activated and heats the water remaining in the water container after finishing steam generating, this water is preferably evaporated until the water level is too low to absorb a large or substantial part of the heat generated by the activated lower heating device. This may mean for example that the water level is at less than half the height of the lower heating device in vertical direction, or preferably at less than 20% of this height. In any case, it can be measured from experiments with such a steam generator at which water level the activated lower heating device effects to reach the predefined first temperature threshold. 
     If the water level is known, the quantity of water remaining in the water container can easily be determined and, if the pump rate of the outlet pump is known, the predefined first pumping duration can be determined rather exactly. This allows for the outlet pump not to have to pump substantially more time than needed, because this should be avoided in order to avoid unnecessary noise at the end of operation of the cooking device e.g. by the outlet pump running dry. Usually it is preferred to be on the safe side when pumping off remaining water so that really no water is left in the water container. However, the outlet pump should not work too long if not necessary, especially not when running dry. 
     If the pump rate of the outlet pump is not known, one of the heating devices, preferably only the lower heating device, is activated again to evaporate water after the predefined first pumping duration, wherein this activation takes place until the predefined first temperature threshold is reached again, which is being detected by the first temperature detection device. This means that at least now there probably is not much water left in the water container. 
     In an embodiment of the invention, the lower heating device is activated again after pumping off or after the predefined first pumping duration has elapsed. Preferably, only the lower heating device is activated and not the upper heating device. It can be provided that the lower heating device is activated until the first temperature detection device detects that the first temperature threshold has been reached again. Then the lower heating device is deactivated again. Such a renewed activation of the lower heating device serves for evaporating the last remains of water in the water container, preferably to really dry the inside of the water container by heat. Then no pumping action may occur anymore. 
     In a further embodiment of the invention, the sequence described before with activating only the lower heating device for drying the inside of the water container is carried out at least two times, preferably five to ten times. This provides for complete evaporation and thus removal of remaining water in the water container by heating. A duration of such an activation of the lower heating device may be between 1 sec and 10 sec, preferably between 2 sec and 5 sec. Alternatively, a duration may be until the first temperature detection device detects that the first temperature threshold has been reached again. 
     It may be the case that the pump rate of the outlet pump for pumping off water from the water container is not known as described before. Then it is advisable to be able to adapt the pumping duration. For this purpose, it can be provided that a pumping duration of pumping off the remaining water from the water container for future pumping off is determined in the control device from the predefined first pumping duration used at the beginning of pumping off. This predefined first pumping duration is being decreased or lowered by the control device in case, during the step of heating again with the lower heating device after pumping off remaining water, the first temperature threshold is reached after less than 4 sec, preferably less than 3 sec. This means that no or almost no water has been left in the water container. Alternatively, the sequence is being carried out once more and the duration until the first temperature threshold is reached for the second time is 5% to 20% shorter than the duration until the first temperature threshold is reached for the first time. In a preferred embodiment of the invention, the predefined first pumping duration is decreased or lowered by 10%, alternatively decreased by 20%. 
     On the other hand, the pumping duration of pumping off the remaining water from the water container for a future pumping off can be increased from the predefined first pumping duration by the control device when, in the step of heating again with the lower heating device after pumping off remaining water, the first temperature threshold is reached only after more than 3 sec, in particular after more than 5 sec or 6 sec. This means that a substantial amount of water is left in the water container after pumping off for the predefined first pumping duration, which means that this pumping duration should be increased. Alternatively, the predefined first pumping duration may be increased if the duration until the first temperature threshold is reached for the second time is more than 20% longer than the duration until the first temperature threshold is reached for the first time. In particular, it may be that the second time is more than 50% longer. It is advisable to increase the pumping duration from the predefined first pumping duration by 10% or even 20%. Such an increased pumping duration can make sure that the remaining water in the water container after operating the cooking device is really removed. 
     In a preferred embodiment of the invention, such an adaptation or decreasing or increasing of the pumping duration is being made each time that remaining water from the water container is pumped off. This can allow for the control device to find a pumping duration after for example five to fifteen or twenty sequences of operating the cooking device or the steam generator, respectively, to find such an optimum pumping duration. In case such an optimum pumping duration has been found by the control device, the step of carrying out a sequence of activating the lower heating device for a rather short time to evaporate remaining water from the water container can be done away with or, alternatively, be at least reduced to being carried out only for two times. 
     In a further embodiment of the invention, after the initial evaporation of the water from the water container and before pumping off, a first pause period should be made before the water is pumped off out of the water container. Such a first pause period may be between 20 sec and 5 min, preferably less than 1 min. This helps for the water in the water container to cool down somewhat in order to protect the outlet pump from excess temperature. 
     Alternatively, directly after the initial evaporation of the water from the water container the water is being pumped off out of the water container. This helps to be quicker in removing the water. Furthermore, it may be used for sterilizing the inside of the pump, for example an outlet pump. It may also serve to easily evaporate the water in a faster way if after the pumping off the lower heating device must be activated again, because the residual heat in the water container is still higher. 
     In addition to the first temperature detection device which covers a temperature detection area and also detects a temperature threshold only at any location in this temperature detection area, without exactly localizing it, a temperature sensor being designed for point-like detection of a temperature may be provided. Preferably, this temperature sensor is arranged on a lateral outside wall of the water container. In particular, this point-like temperature sensor can be arranged in a region between the upper heating device and the lower heating device. It may be provided that no heating device is in the region of this temperature sensor or at a distance of less than 5 mm from this temperature sensor. So a certain minimum distance between the heating devices and this temperature sensor is given. This serves for the temperature sensor being rather independent in measuring and not influenced by the heating devices. This temperature sensor may serve for detecting whether boiling water is at a height level inside the water container at a height corresponding to the location of the temperature sensor. 
     In a further embodiment of the invention, the lower heating device can be placed or arranged less than 20 mm from a baseplate of the water container. Preferably it is placed less than 8 mm in vertical direction over the baseplate. This provides for generating heat and heating water in the water container already at a very low level of water, which means that when water is being filled into the water container, a steam generating process can be started rather quickly. Furthermore, this serves for evaporating and removing remaining water from the water container by being able to provide the heat necessary for that at a lower height level. 
     These and further features will emerge not only from the claims but also from the description and from the drawings, wherein the individual features may be realized in each case individually or severally in the form of sub-combinations in an embodiment of the invention and in other fields, and may constitute advantageous and independently protectable embodiments for which protection is claimed here. The division of the application into individual sub-sections and sub-headings does not restrict the statements made here in terms of their general applicability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, embodiments of the invention will be described in detail with reference to the drawings. Throughout the drawings, the same elements will be denoted by the same reference numerals. 
         FIG. 1  a schematic overview of a steam generator according to the invention, 
         FIG. 2  a steam cooking device according to the invention with a cooking chamber and a steam generator according to  FIG. 1 , 
         FIG. 3  a flow diagram according to a first embodiment of a method of operating a steam generator according to the invention, wherein both heating elements are operated in the beginning, 
         FIGS. 4 and 5  a simplified view on the steam generator according to  FIG. 1  with two different water levels which are reached with both heating elements or with only one heating element activated, 
         FIG. 6  a modification of the flow diagram of  FIG. 3 , wherein either both heating elements are activated or both heating elements are deactivated before pumping is started, 
         FIGS. 7 and 8  two water levels l 1  and l 3  similar to  FIGS. 4 and 5 , which can be reached with a modified embodiment of the invention, and 
         FIG. 9  a further modification of the flow diagram of  FIG. 3 , wherein only the lower heating element is used for evaporating water. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In  FIG. 1  a steam generator  11  according to the invention is shown, the steam generator  11  having a water container  12  which advantageously is in the form of a vertical or upright round-cylindrical container. Water container  12  has a circumferential lateral wall  13  and a base plate  14 , which are connected in watertight manner. Lateral wall  13  is made of metal, preferably of stainless steel. Base plate  14  can also be made of the same material, alternatively from synthetic material being connected in watertight manner to the lateral wall  13 . On top of water container  12  a kind of cover or lid for collecting or concentrating steam S might be provided, for example as known from EP 3278691 A1. This is, however, not important for the invention. In any case, the steam S can rise from the water container  12  and be guided in a steam channel or the like to the location of its application. This is described later in  FIG. 2 . 
     The outside of lateral wall  13  is preferably mostly covered by a dielectric isolation  16  in the form of a thin layer. Dielectric isolation  16  preferably contains glass or glass ceramic and can be fabricated preferably according to DE 10 2013 200 277 A1 or WO 2007/136268 A1. It is only important that the material of this dielectric isolation  16  is adapted so as to change its resistance behavior strongly at the temperature threshold mentioned before, which is preferably a temperature between 150° C. and 300° C. 
     An upper heating element  18  is applied onto the dielectric isolation  16  as shown here, which is also denominated by heat  1 . The upper heating element  18  may be applied in meandering form or in the form of several parallel stripes or in the form of a closed area layout being circumferential around the water container  12 , which is but known from the prior art as mentioned before, for example according to US 2017/0086257 A1. It is preferably a thick film heating element. 
     A lower heating element  20  is applied on the outside of lateral wall  13  directly onto the dielectric isolation  16 . The lower heating element  20  is also denominated by heat  2 , and is basically in a shape or form similar to the upper heating element  18 , preferably also according to US 2017/0086257 A1. It is important that the upper heating element  18  is located above the lower heating element  20  in vertical direction, which direction in this case is extended from base plate  14  at right angle upwards, which is also the direction that the steam S is taking which rises from water W in the water container  12 . There is a distancing ring region  21  between the two heating elements  18  and  20  which is ring-like or runs circumferentially around water container  12 . The width of the ring region  21  may be between 5 mm and 20 mm. The heating elements  18  and  20  correspond to the heating devices described before. 
     The upper heating element  18  is activated by a switch  27  with which it is connected, wherein switch  27  preferably is connected to an energy source, for example a mains connection of the steam generator  11 . This is not shown here, but can easily be conceived by a person skilled in the art. In similar manner, the lower heating element  20  is connected to a switch  28  which is also connected to the same energy source. The switches  27  and  28  are controlled by a control  25  which is the control for the whole steam generator  11 , preferably also for a corresponding steam cooking device according to  FIG. 2 . Control  25  is also connected with a connection  17  to the lateral wall  13  of the water container  12  to measure a leakage current as explained before from one of the heating elements  18  and  20  through the dielectric isolation  16 . By connection of the control  25  to switches  27  and  28 , a measuring apparatus  26  connected to the control  25  can measure the leakage current through the dielectric isolation  16 . For details of such a measuring of leakage current it is pointed to WO 2007/136268 A1 and DE 10 2013 200 277 A1 named before, which is easy for the person skilled in the art to conceive and to put into practice. 
     An upper temperature sensor  22   a  is provided on the outside of lateral wall  13 , in this case also on the dielectric isolation  16  and slightly above the upper heating element  18  in the vertical direction. A lower temperature sensor  22   b  is placed between the upper heating element  18  and the lower heating element  20  in the ring region  21 , preferably also placed onto the dielectric isolation  16 . This is mainly for the reason so that there is sufficient electrical isolation to the metallic lateral wall  13  of the water container  12 . The temperature sensors  22   a  and  22   b  can be made for point-like temperature detection, for example as NTC temperature sensors in SMD construction manner. They should be attached with a good thermal contact to the lateral wall  13  so as to detect its temperature or the temperature of water W inside the water container  12  and potentially being right on the other side of the lateral wall  13 . Temperature sensors  22   a  and  22   b  are also connected to the control  25  for evaluation. 
     For filling water W into the water container  12 , a fresh water tank  30  is provided which can also be a connection to a fresh water pipe. By operating a valve  31 , alternatively a pump, water W from the fresh water tank  30  can be filled into the water container  12 , for example until a water level l is reached as is shown here, which is about as high as a middle region of the upper heating element  18 . Two water levels are shown in dashed lines, wherein an upper water level l 1  is the water level at which, when it is reached and fallen below, the upper heating element  18  in its activation state generates so much heat that the temperature detection device, which is mainly made up of the dielectric isolation  16  together with the control  25  and the measuring apparatus  26 , detects a sudden rise in a leakage current from the upper heating element  18  to the lateral wall  13 . This is a clear sign of a temperature being too high somewhere in this region, or reaching a first temperature threshold, respectively, so that at least the upper heating element  18  is deactivated as explained before. If the water level l has been above water level l 1  before and the water has been steamed off, the water level l 1  is usually constantly as is shown here, which means that it is a known value which allows for a rather exact calculation of the quantity of water being inside the water container  12 . 
     In similar manner the lower water level l 2  is the water level which, when it is fallen below from a higher water level, effects a temperature rise in the region of the lower heating element  20 . This temperature rise corresponding to passing the first temperature threshold can again be detected by the temperature detection device or at the dielectric isolation  16 , respectively, so that the lower heating element  20  is deactivated before a critical temperature is reached. In the same manner as water level l 1 , water level l 2  is usually rather constantly reached at this point and also allows for a rather exact calculation of the quantity of water W being inside the water container  12  when the temperature at the dielectric isolation  16  reaches this first temperature threshold. 
     From base plate  14  a water outlet  34  leads via an outlet pipe  36  to an outlet pump  37  which may correspond to the outlet pump described before. Alternatively, a valve could be provided in the outlet pipe  36  instead of the outlet pump  37 , or in addition to it. The outlet pump  37  leads to an outlet  38  which may be a waste water outlet into a sewage. The outlet pump  37  is also connected to control  25  and is preferably controlled by control  25 . 
     In  FIG. 2  a steam cooking device  40  is shown schematically which has a housing  41  and a cooking chamber  43  inside the housing  41 . The cooking chamber  43  can be closed with a chamber door  44  above which a steam outlet  45  is provided. Inside the cooking chamber  43  food  46  can be placed for being cooking with the help of hot steam. 
     A ventilator  48  is provided for transporting steam from the steam generator  11  with water W in the water container, wherein the steam S is blown into the cooking chamber  43  in known manner through a steam channel  49 . Furthermore, control  25  is provided for the whole steam cooking device  40 . A fresh water tank  30  is provided inside the steam cooking device  40  and is connected via a fresh water pipe  32  to the steam generator  11 . 
     Also in  FIG. 2  it is shown that an outlet pipe  36  out of the steam generator  11  leads to an outlet pump  37  which can pump water out of the steam cooking device  40  via the outlet  38 . 
     In  FIG. 3  the flow diagram is shown which illustrates the method according to one embodiment of the invention which starts here after operation of the steam cooking device  40  for a certain duration. The start of the operation according to the flow diagram in the uppermost box means that now the water container will have to be emptied, but no more steam is required for operation of the steam cooking device  40 . So the process of emptying the water container  12  is started. According to the second box, in this embodiment of the invention the lower heating device  20  and the upper heating device  18  are activated or, respectively, heat  2  and heat  1  are in the on-state. They heat an unknown quantity of water inside the water container, and this quantity of water is unknown because the water level l is not known after finishing the steam generating for the steam cooking device  40 . Both heating devices  18  and  20  are active until a temperature measured by the first temperature detection device or at the dielectric isolation  16 , respectively, reaches the pre-defined first temperature threshold Tthr. This first temperature threshold Tthr has been pre-defined in the control  25 , it may be between 150° C. and 300° C. From the dimensions of the steam generator  11  as well as from experiments it is known that, after generating steam S for the operation of the steam cooking device  40 , and both heating element are activated again for emptying the water container  12 , a certain water level  11  has been reached when for the first time the temperature reaches the first temperature threshold Tthr. This first water level l 1  is shown in  FIG. 4  in the simplified steam generator  11 . The first water level l 1  is in this case at a height of about the lower third of the upper heating element  18 . It is obvious that this water level l 1  could, due to some influencing factors, also be somewhat higher or somewhat lower. 
     In this case, because there is much water W left in the water container  12  according to the water level l 1 , and the noise of the outlet pump  37  shall be reduced as far as possible, some more water should be removed by evaporating. As such, according to the third box from above in  FIG. 3 , only the lower heating element  20  is activated as heat  2 . From  FIG. 4  it can be taken that it would not make any sense to activate the upper heating element  18  again, because a temperature would very fast reach the first temperature threshold Tthr again. So a second heating sequence is started which is after the first heating sequence, and at least one of these sequences is a prerequisite according to the definition of the invention as above. 
     If now with only the lower heating element  20  activated a temperature reaches the first temperature threshold Tthr again, this means that the water level l 2  according to  FIG. 5  has been reached. Also the water level l 2  is in the lower third of the height of the lower heating element  20 . It is also possible for this water level l 2  to vary somewhat, although it can be predicted rather exactly. So after also the lower heating element  20  has been deactivated, evaporating of water W from the water container  12  in the form of steam S is stopped and, at the water level  12 , the outlet pump  37  is activated and starts the pumping sequence. As a pumping rate of the outlet pump  37  may not be known exactly or even roughly, a certain predefined first pumping duration tO can be used. After this predefined first pumping duration tO has elapsed, the outlet pump  37  is stopped and also the pumping sequence ends. This predefined first pumping duration tO can be calculated according to an average of typical pumping rates of outlet pumps used for this purpose. 
     After the pumping sequence has ended, it is not known whether there is some water W left in the water container  12  which should also be removed. It should be avoided to start the pump again or have it still active for the risk of loud pumping noise. For this reason a drying sequence is started where only the lower heating element  20  is activated and starts heating action. If some water should be still inside the water container  12 , it is evaporated again. As the water level now is definitely lower than the water level l 2 , which has already effected that the temperature has quickly reached the first temperature threshold Tthr, and now definitely less water is inside the water container  12 , it can be expected that the lower heating element  20  is only activated for a rather short time which may be some seconds, for example 3 sec to 6 sec. Then of course after reaching the first temperature threshold Tthr, the lower heating element  20  is deactivated again such that the temperature again falls below the first temperature threshold Tthr. 
     According to the flow diagram, the lower heating element  20  is then activated again for evaporating potentially remaining water in the water container  12 . Then again the temperature may reach the first temperature threshold Tthr rather quickly, for example after 3 sec to 6 sec, which leads again to a deactivation of the lower heating element  20 . If now the second duration of activation of the lower heating element  20  is the same as the first duration, the conclusion is that there is no more water left inside the water container  12  and the drying cycle needs not be repeated for a third time. In consequence, the control  25  regards the water container  12  as being completely empty or dry and the operation is finished. 
     It can be provided that the control  25  is able to learn, which means that if the last cycle of drying by repeatedly activating and deactivating the lower heating element  20  occurs for a number of five to ten times, this means that the outlet pump  37  has not removed the largest part of the water remaining in the water container  12 . This leads to the control  25  increasing the first pumping duration somewhat, for example by 10% or 20%. The next time that the water container  12  has to be emptied from water after operating the steam cooking device  40 , the control  25  can check whether at the end of the sequence of the flow diagram according to  FIG. 3  one, two or three times repeating the last cycle of drying by activating the lower heating element  20  for a rather short time is sufficient. If this is the case, this slightly increased first pumping duration is now being used in the future. If the control  25  can see that the last cycle of drying by activating the lower heating element  20  still needs to be repeated for more than three times, the first pumping duration is increased again, preferably again by 10% or 20%. In the next case that the water has to be removed completely from the water container  12 , the same is repeated again until a new first pumping duration has been found that needs only one, two or three cycles of activating the lower heating element  20  to completely dry the water container  12 . 
     If, on the other hand, in the last sequence of drying the water container  12  by activating the lower heating element  20  for a rather short duration, the first temperature threshold Tthr is reached after less than 3 sec for example, this means that already now no more water seems to be left in the water container  12 . Although this may basically seem to be welcome, it bears the risk that the outlet pump  37  has been pumping for too long. This means that the generation of noise of the outlet pump has been too long and could potentially be shortened. In this case the control  25  decreases or lowers the predefined first pumping duration somewhat, preferably by 10% or 20%. This new reduced first pumping duration is, similar to what has been described before, now being used for the next time the water has to be completely removed from the water container  12 . If again in the last sequence of drying the water container  12  according to  FIG. 3 , activating the lower heating element  20  leads to reaching the first temperature threshold Tthr in less than 3 sec for the first time or also for the second time, but at least for the first time, that the pumping duration may still be too long. In consequence, the control  25  can decrease or lower the first pumping duration once more by 10% or 20%. The control  25  can then apply this new decreased pumping duration the next time. 
     Similar to what has been described before, this adaptation or decrease of the first pumping duration tO is made until it takes the lower heating element  20  in the drying sequence according to  FIG. 3  for example three times about 3 sec until the first temperature threshold Tthr has been reached after activation of the lower heating element  20  to evaporate any water in the water container  12 . 
     While the  FIGS. 4 and 5  with the water levels l 1  and l 2  show the method described before according to  FIG. 3 , the  FIGS. 7 and 8  show according to a flow diagram according to  FIG. 6  that water is pumped from water level l 1  down to a low water level l 3 . The first two steps of the flow diagram according to  FIG. 6  correspond to the ones of  FIG. 3  together with the temperature reaching or passing the predefined first temperature threshold Tthr. This means that the water W is about at a water level l 1 . In the case of  FIG. 6 , heating is stopped and no additional heating step with reduced heating power is performed. Both heating elements  18  and  20  are deactivated, heat is off in each case. In the next step, the outlet pump  37  is turned on and starts pumping out water W of the water container  12 . This is done for a predefined first pumping duration tO′, which is obviously much longer than the pumping duration tO described in relation to  FIG. 3  because the quantity of water that needs to be pumped off is larger, see  FIGS. 7 and 8 . If the pumping rate is known as in the case at hand, the water is pumped off such that after the first pumping duration tO′ the water level l 3  according to  FIG. 8 . This water level l 3  as a quantity difference toll is chosen such that even with some variation of water level l 1 , the outlet pump  37  does not run dry in any case. 
     In the next step, the lower heating element  20  is activated in a drying cycle which has been described before. Only this time the lower heating element  20  may need more than three cycles to remove or evaporate all the water left in the water container  12 . If now the control  25  recognizes that each time after activating the lower heating element  20  the temperature reaches the predefined first temperature threshold Tthr after less than 3 sec, the water container  12  is defined to be completely dry and drying of the water container  12  is finished. 
     In the flow diagram according to  FIG. 9 , as a variation of the one according to  FIG. 8 , only the lower heating element  20  is activated according to the second step, whereas the upper heating element  18  is deactivated. The water level at the beginning is unknown. In consequence, the lower heating element  20  is heating the steam generator  11  alone and is evaporating any water being present until the temperature at the dielectric isolation  16  reaches the predefined first temperature threshold Tthr. According to what has been described with regard to  FIGS. 3 to 5 , this means that the water level l 2  has most probably been reached. Then the step of evaporating is stopped and the further steps starting with pumping by turning on the outlet pump  37  correspond to what has been described with regard to  FIGS. 3 to 5 . So this need not be repeated again. 
     In an embodiment of the invention it may be provided that the absolute power of the lower heating element  20  could be higher or lower than the absolute power of the upper heating element  18 . Preferably, both heating elements  18  and  20  have the same power, for example 750 W as a maximum continuous power. 
     A typical pumping rate could be in the range of 50 to 200 ml/min. A typical quantity of water in the water container  12  could be 50 ml at water level l 2  and 120 ml at water level l 1 . Preferably, the first temperature detection device with the dielectric isolation  16  has a predefined first temperature threshold of about 300° C.