Patent Publication Number: US-2021177188-A1

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

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European Application No. 19216429.1, filed Dec. 16, 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 challenge with these steam generators is how to find a way to start the generation of steam with the steam generator as fast as possible, in order to be able to start operation of the household device quickly. 
     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 speed up operating a steam generator and a household device provided with it. 
     This object is achieved with a method having the features of claim  1 , a steam generator having the features of claim  22  and a cooking device having the features of claim  24 . Advantageous and preferred 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. 
     The steam generator comprises a water container, two heating devices on the water container, in particular applied to an outside thereof. The two heating devices are located separated from each other in different height regions viewed in vertical direction. One heating device is an upper heating device and the other heating device is a lower heating device. More separate heating devices could be provided. A flat first temperature detection device is provided and covers a temperature detection area. This temperature detection area includes at least the area covered by the heating devices, preferably also a surface of the water container between the heating devices. It can even be overlapping over the heating devices onto the outer surface of the water container or its lateral wall, respectively. The first temperature detection device can comprise a temperature sensitive layer, preferably a dielectrical isolation layer. At least one second temperature detection device in the form of a spot-like temperature sensor is provided, being located on the water container in a region of an upper border of the upper heating device or slightly above the upper heating device, respectively. A control device for evaluating the first temperature detection device and the second temperature detection device and for activating and deactivating the two heating devices is provided, preferably comprising a microcontroller. The control device can be for the steam generator alone or be provided for the whole cooking device. 
     The method comprises the steps of at first filling the water container with water, which is being made for a predefined first filling duration D 1 , for example 10 sec to 30 sec. After this first filling duration D 1  has elapsed, the lower heating device is activated, and the upper heating device stays deactivated. The water container is still continuously filled further with water. In case the first temperature detection device detects a temperature exceeding a first predefined temperature threshold Tthr, the lower heating device is deactivated again, wherein the water container is still further filled with water. So the water container is continuously filled with water. When the first temperature detection device does not detect any temperature above the first temperature threshold Tthr, which should take place rather quickly, for example after 5 sec to 10 sec of cooling down the water container in the region of the lower heating device, but only a temperature below the first temperature threshold Tthr, the lower heating device is activated again. This may be tried for two or three times until the first temperature detection device does not detect any temperature above the first temperature threshold Tthr for a duration of at least 5 sec, in particular at least 10 sec. Then the control device defines the quantity of water in the water container to have a level l 2 , which is as high as 60% to 90% of a height of an upper rim of the lower heating device so that it is covered sufficiently by water to be activated continuously. Filling of the water container is continued, but now the steam generator does already generate steam, the steam generating having started preferably about 10 sec or 20 sec after the first temperature threshold Tthr has been reached for the first time. 
     As soon as the second temperature detection device detects a temperature of at least 95° C., preferably at least 95° C. for a time of more than 3 sec, which may mean that the temperature is from now on constantly above 95° C., the upper heating device and the lower heating device are both activated for heating operation. This temperature of at least 95° C. can be taken as a sign that now a sufficient amount of water is inside the water container not to cover the upper heating device, but to have sufficient steam or splashing water on the inside of the water container to absorb sufficient heat of the upper heating device that this heat can help for a fast steam generation, whereas the first threshold temperature Tthr is not reached too fast for the upper heating device to be deactivated again after only 2 sec to 5 sec. 
     This serves to provide a very fast generation of steam by activating the lower heating device at an early point of time when most probably it must be deactivated at least once, possibly also twice. But some heat can already be given into the water. Also an activation of the upper heating device when not water but mainly steam is there to absorb the heat can help to speed up the steam generation. Even if the first threshold temperature Tthr may be reached after activation of the upper heating device, only this upper heating device is deactivated for some seconds while the lower heating device continuously is still active. 
     The filling of water into the water container can be continued even further in an embodiment of the invention with the upper heating device being activated, wherein this filling may only be stopped when the first temperature detection device does not detect a temperature above the first temperature threshold Tthr for a second time for a second duration D 2  of between 5 sec to 10 sec after the time before. This is then taken as a sign that now the water level is high enough to cover most part of the upper heating device so that the first temperature threshold Tthr is not reached by the heating operation of the upper heating device. 
     In a further embodiment it is preferably provided that in case the first temperature detection device detects a temperature above the first temperature threshold Tthr after the upper heating device has been activated, only the upper heating device is deactivated. The reason for this is that the water level at first falls below the level that is needed for a stable operation of the upper heating device, and only later on below the level that is needed for a stable operation of the lower heating device. Preferably the lower heating device can also be deactivated if the first temperature detection device again detects a temperature above the first temperature threshold Tthr after 2 sec to 5 sec after deactivating the upper heating device. Even if this is not very probable in a regular operation of the steam generator, it can be a reasonable measure. 
     In an even further embodiment a filling rate of filling water into the water container is not known. If the duration after having started filling the water container until the second temperature detection device detects a temperature of above at least 95° C. is less than a predefined filling duration threshold D 3 , a filling rate can be reduced by at least 5%, preferably by at least 20%. This is then a sign that the filling with water takes place faster than expected. The reduction may be made by clocking a pumping action of a water pump filling water into the water container with the filling rate being the pump filling rate. Alternatively, a valve for filling water may be partly closed to reduce the flow of water through it. 
     In a further embodiment, the upper heating device is deactivated for at least 3 sec after the filling the water container has been stopped for the first time, if the first temperature detection device detects a temperature above the first device threshold Tthr. This is a sign that the upper heating device has generated too much heat. More water is then filled into the water container, which will cool it down and absorb more heat generated by the upper heating device, and the upper heating device is then activated again. 
     In one embodiment, the lower heating device and the upper heating device are both activated simultaneously and together for a high rate of steam generation. They can be activated with their maximum power, wherein preferably they only can be activated with one single power rate. 
     In another embodiment, for generating a low rate of steam generation, preferably 20% to 90% below the high rate of steam generation mentioned before, only the lower heating device is activated and the upper heating device is deactivated. Then the lower heating device alone can be activated with the single power rate mentioned before. 
     Preferably a signal for stopping or starting to fill water into the water container can be generated by monitoring the first derivation by time of a temperature signal of the first temperature detection device. This allows for an easier detection of surpassing any given or predefined value. 
     It is possible to use for deactivating or activating at least one of the two heating devices, preferably only the upper heating device, an absolute value of the temperature signal of the first temperature detection device or the first derivation of this temperature signal by time. It is further possible, after having filled the water container with water and having stopped the filling process, when the first derivation of a temperature signal of the first temperature detection device by time is above a threshold value Vthr, filling water into the water container is started again. This may be for a predefined second filling duration D 2  or until a temperature signal of the first temperature detection device has reached a temperature signal value of a point of time of 2 sec to 20 sec before filling the water container with water has started. 
     In another embodiment, the temperature detected by the first temperature detection device is monitored and a predefined duration is measured until the lower heating device has not been deactivated by a temperature signal of the first temperature detection device for at least 5 sec. The same can be done for the upper heating device until this is not deactivated by a temperature signal of the first temperature detection device. The time is measured until a lower heating device has not been deactivated for at least 5 sec, and then a measured filling rate FRm is determined by dividing a known water volume being defined by the location of the lower heating device on the water container by the time that has elapsed when the lower heating device has been activated or has not been deactivated. 
     It is further possible to compare the measured filling rate FRm with a predefined filling rate FRd which is known for this steam generator. If the measured filling rate FRm is more than 10% below the predefined filling rate FRd, it is determined that a calcification of the filling pump or of a filling valve is too high and a decalcification must be made because the filling rate is too low. Preferably a signal can be output for a user to execute such a decalcification process of the filling pump or that a repair must be made. Such a measured filling rate FRm can be even 20%, 30% or 40% below the predefined filling rate FRd for this action to take place. 
     In an embodiment, the control device can take an increased calcification as a trigger to start and/or stop filling the water container with water with a delay, preferably a delay of 3 sec to 10 sec. This takes the reduced filling rate into account. 
     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 with two heating elements, 
         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 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. 
     
    
    
     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 3 278 691 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 form of a closed area layout running around the water container  12  or having a circumferential layout, respectively. This 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. Both heating elements can have an electrical power of 750 W. 
     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 activating a filling pump  31 , alternatively opening a valve, water W from the fresh water tank  30  can be pumped into the water container  12 , for example until a water level l is reached as is shown here, which is for example about slightly higher than a middle region of the upper heating element  18 , but below temperature sensor  22   a . This is a water level that can be regarded as very good for generating steam with a high rate with high heating power. 
     Two water levels are shown in dashed lines, wherein a lower water level l 2  is a water level at which, when it is reached and surpassed by water W, the heat generated by the lower heating element  20  in its activation state is absorbed by the water W. The temperature stays beneath a critical temperature Tthr. So the first temperature detection device with the dielectric isolation  16  does not lose its isolating properties, in consequence no leakage current can be measured. If the water level l is beneath lower water level l 2 , the heat of the lower heating element  20  is not sufficiently absorbed, the temperature rises and reaches the critical temperature Tthr. So a small leakage current starts to flow from the lower heating element  20  through the dielectric isolation  16  to the metallic lateral wall  13  where it can be detected. When the critical temperature Tthr is surpassed, the leakage current has reached a critical value and a critical temperature is detected by control  25  and measuring apparatus  26 . In consequence, the lower heating element  20  is deactivated; the upper heating element  18  has not even been active. 
     In similar manner an upper water level l 1  is a water level at which, when it is reached and surpassed by water W, the heat generated by the upper heating element  18  in its activation state is absorbed by the water W. The temperature stays beneath a critical temperature Tthr. In the region of the lower heating element  20  the temperature will most probably not surpass 100° C. for all the heat is absorbed by water W. So the first temperature detection device with the dielectric isolation  16  does not lose its isolating properties, in consequence no leakage current can be measured. If, however, the water level l is beneath upper water level l 1 , the heat of the upper heating element  18  is not absorbed sufficiently, and the same as described before will happen. So if the water level l is above the upper water level l 1 , the steam generator can be operating with both heating elements  18  and  20  activated and without overheat problems. 
     From the base plate  14  a water outlet  34  leads via an outlet pipe  36  to an outlet pump  37 . 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 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 . 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 through a steam channel  49  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. Furthermore, control  25  is provided for the whole steam cooking device  40 . The fresh water tank  30  is provided inside the steam cooking device  40  and is connected via a fresh water pipe  32  and filling pump  31  to the steam generator  11  or its water container  12 , respectively. 
     From the flow diagram of  FIG. 3  an example of the method according to the invention for operating the steam generator  11  in the steam cooking device  40  can be taken. After operation has started, the filling pump  31  is activated in the second step and starts pumping water W from the fresh water tank  30  into the water container  12 . As a filling rate FR of this filling process may not be known, which is due to the fact that the parameters of integrating the steam generator  11  and fresh water tank  30  with fresh water pipe  32  and a valve or the filling pump  31  may not be known to the manufacturer of the steam generator  11 . Furthermore, water level sensors directly reacting to a level of water shall be avoided according to the invention. So this step of filling fresh water into the water container  12 , which is defined to have been totally empty before, takes place for a first filling duration D 1  which may be, for example, for 10 sec up to 20 sec. Then in the third step the upper heating element  18  stays deactivated, whereas the lower heating element  20  is being activated and Heat 2 is on. If the water level until this moment has not yet reached the level l 2 , the temperature at the lower heating element  20  will quickly rise as its heat cannot be sufficiently absorbed by water W inside the water container  12 . So for example after 5 sec to 8 sec after activating the lower heating element  20 , the first temperature detection device detects that a temperature T 1  has reached and surpassed the first temperature threshold Tthr. As the upper heating element  18  is not active, this critical temperature must be somewhere in the region of the lower heating element  20 . So it is deactivated again and Heat 2 is also off, see the fourth step. Then the temperature will sink beneath the first temperature threshold Tthr, and according to step  5  the lower heating element  20  is activated again. If now for a time t of more than 6 sec the temperature T 1  does not reach and surpass the first temperature threshold Tthr, this can be taken as a sign that now, as water has still been continuously filled into the water container  12 , a water level l has reached and surpassed the water level l 2 . If, on the other hand, after less than 6 sec or 5 sec the critical temperature of T 1  is reached and surpassed again, the same cycle will be started all over. As water is continuously filled or pumped into the water container  12 , the water level l will continuously rise until the water level l 2  has been reached. 
     As at the water level l 2  the quantity of water W in water container  12  is sufficiently high to continuously operate the lower heating element  20 , which will heat the water W up to the boiling point to generate steam S for the operation of the steam cooking device  40 , this generation of steam took place rather quick, as obviously only a relatively small quantity of water had to be heated. If the quantity of water would be even less, the lower heating element  20  would have been deactivated more often, which may be taking place for a few times, whereas even during this phase the lower heating element  20  has been active for short periods of time. 
     After step  6 , when heating only takes place with lower heating element  20 , and still water W is continuously filled or pumped into the water container  12 , which is also being heated for generating steam, this hot or boiling water reaches with its heat the upper temperature sensor  22   a . The temperature of the upper temperature sensor  22   a  is monitored by the control  25 , and as soon as it reaches a temperature of about 95° C., the water level l will have reached the level l 1 , which is shown in  FIG. 5 . From  FIG. 5  it can be taken that the level l 1  is somewhat beneath the position of the upper temperature sensor  22   a . However, in practice when the quantity of water W corresponds to this level l 1 , due to the boiling action of the water W and steam rising up, which also contacts the inside of the lateral wall  13 , the temperature rises even without direct contact of very hot or boiling water at the inside of lateral wall  13  opposite the upper temperature sensor  22   a.    
     As the level l 1  of the water W has about the same heat absorbing effect to the upper heating element  18  as the lower water level l 2  for the lower heating element  20 , the upper heating element  18  can be activated. It is possible to wait for a short time, preferably 2 sec to 4 sec, for example 3 sec, after first detecting a temperature T 2  of more than 95° C. at the upper temperature sensor  22   a , until the upper heating element  18  is activated. The lower heating element  20  already is active, so Heat 1 is on and Heat 2 is on according to step  7 . For safety reasons it is checked by monitoring the first temperature detection device whether after a few seconds, for example after a second duration D 2  between 5 sec to 10 sec, and whether after activating the upper heating element  18  the temperature T 1  reaches and surpasses the first temperature threshold Tthr. If this might be the case, the filling of water W into water container  12  is continued, whereas the upper heating element  18  is deactivated. Filling of water W is still continued, whereas only if the temperature again is below the first temperature threshold Tthr, the upper heating element  18  is activated again. 
     For the further operation of the steam generator, as the water level l can be supposedly in the region of the upper heating element  18 , a pumping of fresh water W into the water container  12  can be stopped after the upper heating element  18  has not been turned off due to a temperature surpassing the first temperature threshold Tthr for 10 sec or 20 sec, for example. If, after that, the temperature T 1  again reaches the first temperature threshold Tthr, this means that the water level l has fallen beneath the upper water level l 1  and fresh water W has to be filled in again. This is being done according to the last steps of the flow diagram of  FIG. 3 . 
     This operation can continue as long as steam S is required, and as long as this steam should be generated with a high steam generating rate. 
     If, for whatever reason, steam S is only required with a rather low steam generating rate, which may even be after a period of a high steam generating rate, only the lower heating element  20  is operated or activated, respectively. Then the lower heating element  20  is operated without starting the filling pump  31  again. As soon as a first derivation by time of the temperature T 1  reaches a critical value, which means that irrespective of the absolute value of the temperature T it starts to rise rather quickly, the filling pump  31  is activated again to pump more fresh water W into the water container  12 . This can be made for a certain given time, for example for 10 sec or for 20 sec. Alternatively, the filling pump  31  can be active until the temperature measured by the first temperature detection device has reached an absolute value that has been detected 2 sec to 20 sec, for example 5 sec to 10 sec, before the significant rise of the first derivation by time of the temperature signal T 1  has been detected. 
     In a further embodiment of the invention, the lower temperature sensor  22   b  can also be used, in particular if only the lower heating element  20  shall be operated for a lower rate of steam generation. The temperature measured by the second temperature sensor  22   b  is a good indicator whether the water level l of water W inside the water container  12  has reached its height or is still below. If the water level l has surpassed the second temperature sensor  22   b , its temperature will constantly be about 100° C., which can be used as a sign that now or maybe 10 sec to 20 sec afterwards, the filling pump  31  can stop filling water W into the water container  12 . 
     It is also possible with the invention to detect a filling rate of the filling pump  31 . This can be done for example by measuring the time until the water level l 2  has been reached or even until the upper water level l 1  has been reached. Up to this upper water level l 1 , the filling pump  31  has continuously pumped fresh water W into the water container  12  that has been empty before. Of course, some water has already been evaporated as steam S from the water container  12  by heating with the lower heating element  20 , partly also by heating with the upper heating element  18 . This quantity of water W evaporated as steam S is, however, neglected for a rough determination of the filling rate. The quantity of water corresponding to water level l 1  and water level l 2  is known in control  25 . The filling rate can then simply be calculated by dividing the known quantity al level l 1  or level l 2  by the time needed to reach this water level. For an even more precise calculation of the filling rate, the time of activation of the heating elements can be taken into account, and from experiments their steaming rate can be known. From the two parameters a steaming rate can be calculated, which results in an amount of water evaporated by the heating elements. This amount of water has to be added to the water quantity mentioned before as a correction of the calculation for the filling rate, and this sum is then divided by the time needed to reach the corresponding water level. If the filling rate has become smaller during use of the steam generator or the cooking device, respectively, for example during some weeks or months or after 40 to 50 operation cycles, this is a sign for problems with the filling pump or a filling valve, respectively. A repair or a service action can then be called for.