METHOD OF OPERATING A STEAM GENERATOR, STEAM GENERATOR AND COOKING DEVICE WITH A STEAM GENERATOR

Abstract

A steam generator has a water container and an upper heating device and a lower heating device, a first temperature detection device covering a temperature detection area including the area covered by the two heating devices, a control device for monitoring and evaluating the first temperature detection device and for controlling the activation state of the two heating devices. A flat first temperature detection device covers the outside of the container, and a second spot-like temperature sensor is located on the water container in a region of an upper border of the upper heating device. Both temperature detection devices are used to activate and deactivate the two heating devices such that at first the lower heating device is activated to start generating steam as fast as possible.

Claims

1. A method of operating a steam generator, said steam generator comprising: a water container, two heating devices on said water container, which are located in different height regions separated from each other in vertical direction, one said heating device being an upper heating device and an other said heating device being a lower heating device, a flat first temperature detection device covering a temperature detection area, wherein said temperature detection area includes at least an area covered by said heating devices, a second temperature detection device in form of a spot-like temperature sensor being located on said water container in a region of an upper border of said upper heating device, a control device for evaluating said first temperature detection device and said second temperature detection device and for activating and deactivating said two heating devices, with steps of: filling said water container with water for a predefined first filling duration D.sub.1, after said first filling duration D.sub.1 has elapsed, said lower heating device is activated, wherein said water container is continuously filled further with water and wherein said upper heating device is not activated, in case said first temperature detection device detects a temperature exceeding a first temperature threshold T.sub.thr, said lower heating device is deactivated, wherein said water container is filled further with water, as soon as said first temperature detection device does not detect any temperature lying above said first temperature threshold T.sub.thr, but only lying below said first temperature threshold T.sub.thr, said lower heating device is activated again, as soon as said first temperature detection device does not detect any temperature above said first temperature threshold T.sub.thr for a duration of at least 5 sec, said control device defines a quantity of water in said water container to be as high as 60% to 90% of a height of an upper rim of said lower heating device, as soon as said second temperature detection device detects a temperature of at least 95° C., said upper heating device and said lower heating device are both activated for heating operation.

2. Method according to claim 1, wherein said upper heating device is activated and said water container is continuously filled with water, wherein said filling with water is stopped as soon as said first temperature detection device does not detect a temperature above said first temperature threshold T.sub.thr for a second time for a second duration D.sub.2 of between 5 sec to 10 sec.

3. Method according to claim 1, wherein in case said first temperature detection device detects a temperature above said first temperature threshold T.sub.thr after said upper heating device has been activated, only said upper heating device is deactivated.

4. Method according to claim 3, wherein if said first temperature detection device again detects a temperature above said first temperature threshold T.sub.thr after 2 sec to 5 sec after deactivating said upper heating device, said lower heating device is also deactivated.

5. Method according to claim 1, wherein a filling rate of filling water into said water container is not known, and if said duration after starting filling said water container until said second temperature detection device detects a temperature of above at least 95° C. is less than a predefined filling duration threshold D.sub.3, said filling rate is reduced by at least 5%.

6. Method according to claim 5, wherein said filling rate is reduced by clocking a pumping action of a water pump filling water into said water container with said filling rate being said pump filling rate.

7. Method according to claim 1, wherein after filling said water container with water has been stopped for a first time, if said first temperature detection device detects a temperature above said first temperature threshold T.sub.thr, said upper heating device is deactivated for at least 3 sec, wherein more water is filled into said water container, and said upper heating device is then activated again.

8. Method according to claim 1, wherein for generating a low rate of steam generation, only said lower heating device is activated and said upper heating device is deactivated.

9. Method according to claim 8, wherein for generating a rate of steam generation being 20% to 90% below a high rate of steam generation, only said lower heating device is activated and said upper heating device is deactivated.

10. Method according to claim 1, wherein a signal for stopping or starting to fill water into said water container is generated by monitoring a first derivation by time of a temperature signal of said first temperature detection device.

11. Method according to claim 1, wherein for deactivating or activating at least one of said two heating devices, an absolute value of a temperature signal of said first temperature detection device is used or a first derivation of the temperature signal by time of said first temperature detection device is used.

12. Method according to claim 11, wherein at least said upper heating device is deactivated or activated.

13. Method according to claim 11, wherein after having filled said water container with water and having stopped said filling process, and said first derivation of said temperature signal of said first temperature detection device by time is above a threshold value V.sub.thr, filling water into said water container is started again.

14. Method according to claim 13, wherein said filling water into said water container is started again for a predefined second filling duration D.sub.2.

15. Method according to claim 13, wherein said filling water into said water container is started again until said temperature signal of said first temperature detection device has reached a temperature signal value of a point of time of 2 sec to 20 sec before filling said water container with water has started.

16. Method according to claim 1, wherein said temperature detected by said first temperature detection device is monitored and a predefined duration is measured until said lower heating device has not been deactivated by a temperature signal of said first temperature detection device for at least 5 sec, and the same is being done for said upper heating device until said upper heating device is not deactivated by said temperature signal of said first temperature detection device, wherein a time is measured until a lower heating device has not been deactivated for at least 5 sec, wherein a measured filling rate FR.sub.m is determined by dividing a known water volume being defined by a location of said lower heating device on said water container by a time that has elapsed when said lower heating device has been activated or has not been deactivated.

17. Method according to claim 16, wherein said measured filling rate FR.sub.m is compared with a predefined filling rate FR.sub.d which is known for said steam generator, wherein in case said measured filling rate FR.sub.m is more than 10% below said predefined filling rate FR.sub.d, it is determined that a calcification of said filling pump is too high and a decalcification must be made or it is determined that a repair of said filling pump must be made.

18. Method according to claim 17, wherein a signal is output for a user to execute a decalcification process of said filling pump or that a repair of said filling pump must be made.

19. Method according to claim 17, wherein said measured filling rate FR.sub.m is 20%, 30% or 40% below said predefined filling rate FR.sub.d.

20. Method according to claim 17, wherein said control device does take an increased calcification as a trigger to start and/or stop filling said water container with water with a delay.

21. Method according to claim 20, wherein a delay is 3 sec to 10 sec.

22. A steam generator designed to perform said method according to claim 1, said steam generator comprising: a water container, two heating devices on said water container, which are located in different height regions separated from each other in vertical direction, one said heating device being an upper heating device and an other said heating device being a lower heating device, a flat first temperature detection device covering a temperature detection area, which temperature detection area includes at least an area covered by said heating devices, a second temperature detection device in a form of a spot-like temperature sensor being located on said water container in a region of an upper border of said upper heating device, a control device for evaluating said first temperature detection device and said second temperature detection device and for activating and deactivating said two heating devices.

23. The steam generator according to claim 22, wherein said two heating devices are on an outside of said water container.

24. A cooking device with: a steam generator according to claim 22, a cooking chamber connected with said steam generator and a pump for filling said water container with water.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] 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.

[0021] FIG. 1 a schematic overview of a steam generator according to the invention with two heating elements,

[0022] FIG. 2 a steam cooking device according to the invention with a cooking chamber and a steam generator according to FIG. 1,

[0023] 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,

[0024] 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

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] An upper temperature sensor 22a 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 22b 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 22a and 22b 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 22a and 22b are also connected to the control 25 for evaluation.

[0031] 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 22a. This is a water level that can be regarded as very good for generating steam with a high rate with high heating power.

[0032] Two water levels are shown in dashed lines, wherein a lower water level l2 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 l2, 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.

[0033] In similar manner an upper water level l1 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 l1, 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 l1, the steam generator can be operating with both heating elements 18 and 20 activated and without overheat problems.

[0034] 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.

[0035] 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.

[0036] 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 D1 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 l2, 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 T1 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 T1 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 l2. If, on the other hand, after less than 6 sec or 5 sec the critical temperature of T1 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 l2 has been reached.

[0037] As at the water level l2 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.

[0038] 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 22a. The temperature of the upper temperature sensor 22a 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 l1, which is shown in FIG. 5. From FIG. 5 it can be taken that the level l1 is somewhat beneath the position of the upper temperature sensor 22a. However, in practice when the quantity of water W corresponds to this level l1, 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 22a.

[0039] As the level l1 of the water W has about the same heat absorbing effect to the upper heating element 18 as the lower water level l2 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 T2 of more than 95° C. at the upper temperature sensor 22a, 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 D2 between 5 sec to 10 sec, and whether after activating the upper heating element 18 the temperature T1 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.

[0040] 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 T1 again reaches the first temperature threshold Tthr, this means that the water level l has fallen beneath the upper water level l1 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.

[0041] 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.

[0042] 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 T1 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 T1 has been detected.

[0043] In a further embodiment of the invention, the lower temperature sensor 22b 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 22b 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 22b, 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.

[0044] 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 l2 has been reached or even until the upper water level l1 has been reached. Up to this upper water level l1, 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 l1 and water level l2 is known in control 25. The filling rate can then simply be calculated by dividing the known quantity al level l1 or level l2 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.