Patent Description:
Steam generators for cooking devices for cooking purposes or the like are known for example from <CIT> or <CIT>. 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.

A further steam generator for a cooking device or a steamer, respectively, is disclosed in <CIT>, which shows various steps of how such a steam generator is operated. If after the steam generating operation water remains in a water container, this can be left out via an outlet pipe with an outlet valve.

Another such steam generator for a cooking device is known from <CIT>. When operating this steam generator is accomplished and heating of water in a water container is stopped, it can be drained. Before draining, the temperature of the water is measured such that draining is only effected when the water has cooled down. Draining takes place by opening a drain valve underneath the water container such that the water can simply drain into a removable water drain tank, which again can be removed and emptied separately.

A further steam generator for a cooking device is known from <CIT>. The generation of steam by heating water in a water container is controlled by an electronic control circuit. If water in the water container is left after generating steam, a drain opening can be used.

Another steam generator for a cooking device is known from <CIT>, which steam generator has a housing with an outer housing wall. Inside the housing, an inner partition wall is provided at short distance from said outer wall. The inner partition wall serves to calm down boiling water inside the housing. Heating elements are provided on the outside of the outer wall in the form of a lower heating element and an upper heating element. Both heating elements can be operated separately.

<CIT> discloses another cooking device with a steam generator. It is provided to heat the steam generator and blow air through it after an operation for generating steam. This serves to dry the inside of the steam generator for hygienic reasons.

<CIT> discloses a boiler for heating water, which also serves to generate steam. A water inlet is provided at the upper end of a tank body. A steam outlet is provided at the upper end of the tank, and a water outlet is provided in the lower region of the tank.

<CIT> discloses another boiler, which may also generate steam by heating water. A fan is provided to blow air through the inside of a water tank of the boiler.

The object of the invention is to provide a method of operating a steam generator as well as such a steam household 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 <NUM> and a steam household device having the features of claim. 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 or only described for the steam household device. Independently of this, they can be applied to such a method or such a steam household 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 household device or 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 <NUM> or <NUM>, a serious damage to the steam generator could occur. This predefined first temperature threshold is a temperature between <NUM> and <NUM>.

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 and 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 <NUM>% 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.

According to 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 <NUM> sec and <NUM> sec, preferably between <NUM> sec and <NUM> 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 <NUM> sec, preferably less than <NUM> 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 <NUM>% to <NUM>% 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 <NUM>%, alternatively decreased by <NUM>%.

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 <NUM> sec, in particular after more than <NUM> sec or <NUM> 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 <NUM>% 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 <NUM>% longer. It is advisable to increase the pumping duration from the predefined first pumping duration by <NUM>% or even <NUM>%. 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 <NUM> sec and <NUM>, preferably less than <NUM>. 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, preferably being designed for point-like detection of a temperature, is provided according to the invention. 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 is provided that no heating device is in the region of this temperature sensor or at a distance of less than <NUM> from this temperature sensor according to the invention. 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 <NUM> from a baseplate of the water container. Preferably it is placed less than <NUM> 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.

In <FIG> a steam generator <NUM> according to the invention is shown, the steam generator <NUM> having a water container <NUM> which advantageously is in the form of a vertical or upright round-cylindrical container. Water container <NUM> has a circumferential lateral wall <NUM> and a base plate <NUM>, which are connected in watertight manner. Lateral wall <NUM> is made of metal, preferably of stainless steel. Base plate <NUM> can also be made of the same material, alternatively from synthetic material being connected in watertight manner to the lateral wall <NUM>. On top of water container <NUM> a kind of cover or lid for collecting or concentrating steam S might be provided, for example as known from <CIT>. This is, however, not important for the invention. In any case, the steam S can rise from the water container <NUM> and be guided in a steam channel or the like to the location of its application. This is described later in <FIG>.

The outside of lateral wall <NUM> is preferably mostly covered by a dielectric isolation <NUM> in the form of a thin layer. Dielectric isolation <NUM> preferably contains glass or glass ceramic and can be fabricated preferably according to <CIT> or <CIT>. It is only important that the material of this dielectric isolation <NUM> is adapted so as to change its resistance behavior strongly at the temperature threshold mentioned before, which is a temperature between <NUM> and <NUM>.

An upper heating element <NUM> is applied onto the dielectric isolation <NUM> as shown here, which is also denominated by heat <NUM>. The upper heating element <NUM> 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 <NUM>, which is but known from the prior art as mentioned before, for example according to <CIT>. It is preferably a thick film heating element.

A lower heating element <NUM> is applied on the outside of lateral wall <NUM> directly onto the dielectric isolation <NUM>. The lower heating element <NUM> is also denominated by heat <NUM>, and is basically in a shape or form similar to the upper heating element <NUM>, preferably also according to <CIT>. It is important that the upper heating element <NUM> is located above the lower heating element <NUM> in vertical direction, which direction in this case is extended from base plate <NUM> at right angle upwards, which is also the direction that the steam S is taking which rises from water W in the water container <NUM>. There is a distancing ring region <NUM> between the two heating elements <NUM> and <NUM> which is ring-like or runs circumferentially around water container <NUM>. The width of the ring region <NUM> may be between <NUM> and <NUM>. The heating elements <NUM> and <NUM> correspond to the heating devices described before.

The upper heating element <NUM> is activated by a switch <NUM> with which it is connected, wherein switch <NUM> preferably is connected to an energy source, for example a mains connection of the steam generator <NUM>. This is not shown here, but can easily be conceived by a person skilled in the art. In similar manner, the lower heating element <NUM> is connected to a switch <NUM> which is also connected to the same energy source. The switches <NUM> and <NUM> are controlled by a control <NUM> which is the control for the whole steam generator <NUM>, preferably also for a corresponding steam cooking device according to <FIG>. Control <NUM> is also connected with a connection <NUM> to the lateral wall <NUM> of the water container <NUM> to measure a leakage current as explained before from one of the heating elements <NUM> and <NUM> through the dielectric isolation <NUM>. By connection of the control <NUM> to switches <NUM> and <NUM>, a measuring apparatus <NUM> connected to the control <NUM> can measure the leakage current through the dielectric isolation16. For details of such a measuring of leakage current it is pointed to <CIT> and <CIT> named before, which is easy for the person skilled in the art to conceive and to put into practice.

An upper temperature sensor 22a is provided on the outside of lateral wall <NUM>, in this case also on the dielectric isolation <NUM> and slightly above the upper heating element <NUM> in the vertical direction. A lower temperature sensor 22b is placed between the upper heating element <NUM> and the lower heating element <NUM> in the ring region <NUM>, preferably also placed onto the dielectric isolation <NUM>. This is mainly for the reason so that there is sufficient electrical isolation to the metallic lateral wall <NUM> of the water container <NUM>. 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 <NUM> so as to detect its temperature or the temperature of water W inside the water container <NUM> and potentially being right on the other side of the lateral wall <NUM>. Temperature sensors 22a and 22b are also connected to the control <NUM> for evaluation.

For filling water W into the water container <NUM>, a fresh water tank <NUM> is provided which can also be a connection to a fresh water pipe. By operating a valve <NUM>, alternatively a pump, water W from the fresh water tank <NUM> can be filled into the water container <NUM>, for example until a water level I is reached as is shown here, which is about as high as a middle region of the upper heating element <NUM>. Two water levels are shown in dashed lines, wherein an upper water level l<NUM> is the water level at which, when it is reached and fallen below, the upper heating element <NUM> in its activation state generates so much heat that the temperature detection device, which is mainly made up of the dielectric isolation <NUM> together with the control <NUM> and the measuring apparatus <NUM>, detects a sudden rise in a leakage current from the upper heating element <NUM> to the lateral wall <NUM>. 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 <NUM> is deactivated as explained before. If the water level I has been above water level l<NUM> before and the water has been steamed off, the water level l<NUM> 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 <NUM>.

In similar manner the lower water level l<NUM> 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 <NUM>. This temperature rise corresponding to passing the first temperature threshold can again be detected by the temperature detection device or at the dielectric isolation <NUM>, respectively, so that the lower heating element <NUM> is deactivated before a critical temperature is reached. In the same manner as water level l<NUM>, water level l<NUM> 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 <NUM> when the temperature at the dielectric isolation <NUM> reaches this first temperature threshold.

From base plate <NUM> a water outlet <NUM> leads via an outlet pipe <NUM> to an outlet pump <NUM> which may correspond to the outlet pump described before. Alternatively, a valve could be provided in the outlet pipe <NUM> instead of the outlet pump <NUM>, or in addition to it. The outlet pump <NUM> leads to an outlet <NUM> which may be a waste water outlet into a sewage. The outlet pump <NUM> is also connected to control <NUM> and is preferably controlled by control <NUM>.

In <FIG> a steam cooking device <NUM> is shown schematically which has a housing <NUM> and a cooking chamber <NUM> inside the housing <NUM>. The cooking chamber <NUM> can be closed with a chamber door <NUM> above which a steam outlet <NUM> is provided. Inside the cooking chamber <NUM> food <NUM> can be placed for being cooking with the help of hot steam.

A ventilator <NUM> is provided for transporting steam from the steam generator <NUM> with water W in the water container, wherein the steam S is blown into the cooking chamber <NUM> in known manner through a steam channel <NUM>. Furthermore, control <NUM> is provided for the whole steam cooking device <NUM>. A fresh water tank <NUM> is provided inside the steam cooking device <NUM> and is connected via a fresh water pipe <NUM> to the steam generator <NUM>.

Also in <FIG> it is shown that an outlet pipe <NUM> out of the steam generator <NUM> leads to an outlet pump <NUM> which can pump water out of the steam cooking device <NUM> via the outlet <NUM>.

In <FIG> 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 <NUM> 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 <NUM>. So the process of emptying the water container <NUM> is started. According to the second box, in this embodiment of the invention the lower heating device <NUM> and the upper heating device <NUM> are activated or, respectively, heat <NUM> and heat <NUM> 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 I is not known after finishing the steam generating for the steam cooking device <NUM>. Both heating devices <NUM> and <NUM> are active until a temperature measured by the first temperature detection device or at the dielectric isolation <NUM>, respectively, reaches the pre-defined first temperature threshold Tthr. This first temperature threshold Tthr has been pre-defined in the control <NUM>, it is between <NUM> and <NUM>. From the dimensions of the steam generator <NUM> as well as from experiments it is known that, after generating steam S for the operation of the steam cooking device <NUM>, and both heating element are activated again for emptying the water container <NUM>, a certain water level l<NUM> has been reached when for the first time the temperature reaches the first temperature threshold Tthr. This first water level l<NUM> is shown in <FIG> in the simplified steam generator <NUM>. The first water level l<NUM> is in this case at a height of about the lower third of the upper heating element <NUM>. It is obvious that this water level l<NUM> 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 <NUM> according to the water level l<NUM>, and the noise of the outlet pump <NUM> 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>, only the lower heating element <NUM> is activated as heat <NUM>. From <FIG> it can be taken that it would not make any sense to activate the upper heating element <NUM> 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 <NUM> activated a temperature reaches the first temperature threshold Tthr again, this means that the water level l<NUM> according to <FIG> has been reached. Also the water level l<NUM> is in the lower third of the height of the lower heating element <NUM>. It is also possible for this water level l<NUM> to vary somewhat, although it can be predicted rather exactly. So after also the lower heating element <NUM> has been deactivated, evaporating of water W from the water container <NUM> in the form of steam S is stopped and, at the water level l<NUM>, the outlet pump <NUM> is activated and starts the pumping sequence. As a pumping rate of the outlet pump <NUM> may not be known exactly or even roughly, a certain predefined first pumping duration t<NUM> can be used. After this predefined first pumping duration t<NUM> has elapsed, the outlet pump <NUM> is stopped and also the pumping sequence ends. This predefined first pumping duration t<NUM> 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 <NUM> 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 <NUM> is activated and starts heating action. If some water should be still inside the water container <NUM>, it is evaporated again. As the water level now is definitely lower than the water level l<NUM>, 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 <NUM>, it can be expected that the lower heating element <NUM> is only activated for a rather short time which may be some seconds, for example <NUM> sec to <NUM> sec. Then of course after reaching the first temperature threshold Tthr, the lower heating element <NUM> is deactivated again such that the temperature again falls below the first temperature threshold Tthr.

According to the flow diagram, the lower heating element <NUM> is then activated again for evaporating potentially remaining water in the water container <NUM>. Then again the temperature may reach the first temperature threshold Tthr rather quickly, for example after <NUM> sec to <NUM> sec, which leads again to a deactivation of the lower heating element <NUM>. If now the second duration of activation of the lower heating element <NUM> is the same as the first duration, the conclusion is that there is no more water left inside the water container <NUM> and the drying cycle needs not be repeated for a third time. In consequence, the control <NUM> regards the water container <NUM> as being completely empty or dry and the operation is finished.

It can be provided that the control <NUM> is able to learn, which means that if the last cycle of drying by repeatedly activating and deactivating the lower heating element <NUM> occurs for a number of five to ten times, this means that the outlet pump <NUM> has not removed the largest part of the water remaining in the water container <NUM>. This leads to the control <NUM> increasing the first pumping duration somewhat, for example by <NUM>% or <NUM>%. The next time that the water container <NUM> has to be emptied from water after operating the steam cooking device <NUM>, the control <NUM> can check whether at the end of the sequence of the flow diagram according to <FIG> one, two or three times repeating the last cycle of drying by activating the lower heating element <NUM> 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 <NUM> can see that the last cycle of drying by activating the lower heating element <NUM> still needs to be repeated for more than three times, the first pumping duration is increased again, preferably again by <NUM>% or <NUM>%. In the next case that the water has to be removed completely from the water container <NUM>, 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 <NUM> to completely dry the water container <NUM>.

If, on the other hand, in the last sequence of drying the water container <NUM> by activating the lower heating element <NUM> for a rather short duration, the first temperature threshold Tthr is reached after less than <NUM> sec for example, this means that already now no more water seems to be left in the water container <NUM>. Although this may basically seem to be welcome, it bears the risk that the outlet pump <NUM> 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 <NUM> decreases or lowers the predefined first pumping duration somewhat, preferably by <NUM>% or <NUM>%. 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 <NUM>. If again in the last sequence of drying the water container <NUM> according to <FIG>, activating the lower heating element <NUM> leads to reaching the first temperature threshold Tthr in less than <NUM> 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 <NUM> can decrease or lower the first pumping duration once more by <NUM>% or <NUM>%. The control <NUM> 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 t<NUM> is made until it takes the lower heating element <NUM> in the drying sequence according to <FIG> for example three times about <NUM> sec until the first temperature threshold Tthr has been reached after activation of the lower heating element <NUM> to evaporate any water in the water container <NUM>.

While the <FIG> with the water levels l<NUM> and l<NUM> show the method described before according to <FIG>, the <FIG> show according to a flow diagram according to <FIG> that water is pumped from water level l<NUM> down to a low water level l<NUM>. The first two steps of the flow diagram according to <FIG> correspond to the ones of <FIG> 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<NUM>. In the case of <FIG>, heating is stopped and no additional heating step with reduced heating power is performed. Both heating elements <NUM> and <NUM> are deactivated, heat is off in each case. In the next step, the outlet pump <NUM> is turned on and starts pumping out water W of the water container <NUM>. This is done for a predefined first pumping duration t<NUM>', which is obviously much longer than the pumping duration t<NUM> described in relation to <FIG> because the quantity of water that needs to be pumped off is larger, see <FIG>. If the pumping rate is known as in the case at hand, the water is pumped off such that after the first pumping duration t<NUM>' the water level l<NUM> according to <FIG>. This water level l<NUM> as a quantity difference to l<NUM> is chosen such that even with some variation of water level l<NUM>, the outlet pump <NUM> does not run dry in any case.

In the next step, the lower heating element <NUM> is activated in a drying cycle which has been described before. Only this time the lower heating element <NUM> may need more than three cycles to remove or evaporate all the water left in the water container <NUM>. If now the control <NUM> recognizes that each time after activating the lower heating element <NUM> the temperature reaches the predefined first temperature threshold Tthr after less than <NUM> sec, the water container <NUM> is defined to be completely dry and drying of the water container <NUM> is finished.

In the flow diagram according to <FIG>, as a variation of the one according to <FIG>, only the lower heating element <NUM> is activated according to the second step, whereas the upper heating element <NUM> is deactivated. The water level at the beginning is unknown. In consequence, the lower heating element <NUM> is heating the steam generator <NUM> alone and is evaporating any water being present until the temperature at the dielectric isolation <NUM> reaches the predefined first temperature threshold Tthr. According to what has been described with regard to <FIG>, this means that the water level l<NUM> 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 <NUM> correspond to what has been described with regard to <FIG>. 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 <NUM> could be higher or lower than the absolute power of the upper heating element <NUM>. Preferably, both heating elements <NUM> and <NUM> have the same power, for example <NUM> W as a maximum continuous power.

Claim 1:
A method of operating a steam generator (<NUM>), the steam generator (<NUM>) comprising:
- a water container (<NUM>) with a circumferential wall (<NUM>),
- two separate heating devices (<NUM>, <NUM>) on the wall (<NUM>) of the water container (<NUM>), in particular on a lateral outside thereof, which are provided in different height regions separated from each other in vertical direction, one heating device being an upper heating device (<NUM>) and the other heating device being a lower heating device (<NUM>),
- a first temperature detection device covering a temperature detection area, which temperature detection area includes at least the area covered by the two heating devices (<NUM>, <NUM>), preferably also an area of the water container (<NUM>) between the two heating devices (<NUM>, <NUM>),
- a control device (<NUM>) for monitoring and evaluating the first temperature detection device and for controlling the activation state of the two heating devices (<NUM>, <NUM>),
with the steps of:
- filling the water container (<NUM>) with water (W),
- operating the steam generator (<NUM>) by heating with at least one of the two heating devices (<NUM>, <NUM>) being activated,
- generating steam (S) for operation of a steam household device (<NUM>) and finishing steam generating after operation of the steam household device (<NUM>),
- activating at least the lower heating device (<NUM>), in particular the lower heating device (<NUM>) and the upper heating device (<NUM>), until the first temperature detection device detects that a pre-defined first temperature threshold (Tthr) has been reached, wherein the pre-defined first temperature threshold (Tthr) is between <NUM> and <NUM>,
- deactivating one heating device (<NUM>, <NUM>), in particular deactivating the upper heating device (<NUM>),
- pumping off remaining water (W) in the water container (<NUM>) for a pre-defined first pumping duration (t<NUM>) and activating the lower heating device (<NUM>) until the first temperature detection device detects that the pre-defined first temperature threshold (Tthr) has been reached again.