EP3145273B1 - Heating device for heating water and a method for operating such a heating device - Google Patents

Description

Field of application and state of the art

The invention relates to a heater for heating water and a method for operating such a heater for heating water.

It is from the DE 102012213385 A1 It is known that in heating devices for heating water, in particular with metallic carrier and heating element arranged thereon in thick-film technology, rapid regulation is necessary because of the high surface power and the very dynamic processes, since a low thermal inertia is given. Especially with local or large calcifications on a medium side of the carrier, the heat loss decreases sharply by the water to be heated, which very quickly especially a local or a large-scale heating can occur, which would be very harmful. Under certain circumstances, this can even lead to the destruction of the heater.

From the DE 102013200277 A1 a heating device is known in which, so to speak, by means of a dielectric layer between two electrically conductive pads a large-scale monitoring of the heater or a heating element and detection of localized or small-scale overheating or calcification is possible.

From the WO 2010/008279 A1 For example, a heating device is known in which a first dielectric layer made of enamel is applied to a hotplate made of steel. On top of that is applied a conductive thick film which is covered by a second dielectric layer. Heating conductors are first applied to this second dielectric layer. The heating conductors are exposed on their side facing away from the second dielectric layer side.

From the WO 2007/136268 A1 For example, a heater having a similar structure as described above is known. On the underside of a carrier, which may be made of steel, an electrically insulating layer is applied, are applied to the two web-shaped elongated electrodes. In turn, a dielectric layer is applied, which is electrically insulating and advantageously has good thermal conductivity. On this dielectric layer in turn heating conductors are applied, which are also exposed down.

Task and solution

The invention has for its object to provide an aforementioned heating device and a method for their operation, with which problems of the prior art can be solved and it is in particular possible to recognize calcifications on a medium side of the carrier and thus a potential source of danger or to prevent damage to the heater.

This object is achieved by a heating device with the features of claim 1 and a method having the features of claim 5. Advantageous and preferred embodiments of the invention are the subject of further claims and are explained in more detail below. Some of the features are mentioned only for the heater or just for the method. However, they should apply independently and independently of each other for both the heating device and the process can. The wording of the claims is incorporated herein by express reference.

It is provided that the heating device for heating water, which is intended to flow through a carrier in particular or should flow past it, especially on a so-called medium side of the carrier, such a carrier. On this support at least one heating element is applied, which has a single heat conductor or has a plurality of heating conductors connected in series. This is advantageously a DickschichtHeizelement with two electrical connections, between which extend the single or its individual successively connected or adjoining heating element. For example, the heating conductors may each be straight sections of the heating element, which may in particular have a meandering course in particular. It can also be provided a plurality of such trained heating elements, advantageously at least two.

The heating device has at least one planar dielectric layer, which essentially covers the heating conductors or the heating element. The dielectric layer does not necessarily have to rest directly on the or a heating element. It actually has electrically insulating properties, but at temperatures from 200 ° C or only from 300 ° C but decreases their electrical resistance. Such dielectric layers consist for example of glass or glass ceramic and are in the aforementioned DE 102013200277 A1 described in more detail.

A dielectric layer is provided on both sides each with an electrically conductive pad. Thus, these pads are applied directly to the dielectric layer and can in particular detect a current flowing through the dielectric layer or so-called leakage current. In this case, the electrically conductive connection surfaces can have the same overlap, at least as far as an outer contour or maximum clamping of the surface is concerned, depending on how many dielectric layers are provided next to one another on or above the heating elements. One of the electrically conductive connection surfaces can advantageously also be formed over the entire surface or closed.

At least one of the pads is connected to a controller or measuring device of a controller for evaluating a leakage current as a current flow through the dielectric layer. So just the leakage current can be monitored in terms of its time course or a possible rapid increase. Furthermore, the heating element with measuring means for monitoring a Heizleiterstroms by this heating element and thus connected by all its heating conductors. Thus, just the Heizleiterstrom can be monitored or evaluated, in particular with regard to a decrease due to increasing resistance of the heating element with increasing or too high temperature, if the heating element has a positive temperature coefficient of its resistance in one embodiment of the invention. If the heating element in another embodiment of the invention has a negative temperature coefficient of its resistance, an increase in the Heizleiterstroms be recognized due to decreasing resistance of the heating element with increasing or too high temperature. Since the control or measuring device monitors the heating conductor current and, in addition, the voltage can be measured, the heat transfer can also be evaluated in the form of a thermal resistance.

Thus, it is possible to monitor the temperature conditions on the heating element or on the heating device both by means of the leakage current at the dielectric layer and by monitoring the heating conductor current. While a change in the Heizleiterstroms is rather relatively slow and also not change very much in locally limited or small scale calcification, as this yes only a very small area of the heating element is affected, but so to speak, averaged over the surface temperature and Thus, an average overheating of the heater can be detected. The detection of a localized or small-area overheating on evaluation of the leakage current to the dielectric layer is on the one hand considerably faster and on the other hand, so to speak ranges only a few millimeters large area with very high temperature, here the leakage current, which are detected over the entire area can quickly increase sharply. Such overheating is usually caused by calcifications, as will be explained in more detail below, or by emptying a water-filled heater or dry running when used in a pump. There are also so-called hot spots, which are caused by a punctiform or small-scale, not complete detachment of a calcification, which is additionally hindered by a gap between the medium side of the carrier and the limestone layer, the heat transfer. Here, dangerous overheating may occur, although small or locally very limited, but also could cause damage or destruction of the heater.

The carrier is advantageously made of metal. On it, an insulating layer is applied for a layer structure, on which in turn the heating element or the heating elements are applied. As mentioned above, thick-film heating elements are preferred, for example with exactly one or more heat conductors in a total generated meander shape. Be over or on the heating element or the heating elements turn planar dielectric layers applied, advantageously as a closed surface. A planar dielectric layer may substantially cover a rectangle. A dielectric layer also acts as electrical insulation, at least in the range of normal operating temperatures. In turn, an electrically conductive connection area can be applied to the dielectric layer with essentially the same area. Here, any electrically conductive material can be used. The other electrically conductive pad to the dielectric layer is then formed by the heating element or the heating elements or its heating conductors. During operation of the heater these are connected to an operating voltage and flows through the Heizleiterstrom. As the insulation properties of a dielectric layer become smaller, a usually small current can then flow as leakage current through the dielectric layer to the other electrically conductive connection area. This can be detected by the aforementioned connection to a controller.

The controller advantageously has a memory to store reference values for the heating element temperature, dielectric layer signals or a heating conductor current during normal operation. Values can also be stored for abnormal operating states, in particular for locally limited or small scale calcifications with regard to the then expected dielectric layer signals or a heating conductor current as well as values for the aforementioned hot spots and for surface or large-scale calcification. This can be, for example, a limit value for the dielectric current or leakage current, which may not be exceeded or, when it is reached, the heating device has to be switched off.

A power density of the heating element may advantageously be at least 30 W / cm ² or at least 100 W / cm ² . Particularly advantageous, the power density can be a maximum of 150 W / cm ² or even 200 W / cm ² . So a fast responding and very powerful heater is given in a small footprint.

In an embodiment of the invention, the heating elements engage with one another or are arranged in an entangled manner, preferably with heat conductors as sections of the heating elements which run straight and parallel to one another. Thus, the heating elements can advantageously be bifilar, especially in meandering form. Between two parallel heating conductors of a heating element, at least one heating conductor of another heating element can run, in particular parallel thereto.

In an alternative embodiment of the invention, although the heating elements run close to each other, but are separated in terms of area or do not engage in each other. Each heating element takes so to speak, a surface with a closed outer contour, in particular a rectangular surface or a quadrangular surface into which no part of another heating element protrudes.

Advantageously, the heating device has exactly two or three heating elements. Two heating elements can be interlocked or interlocked according to the first embodiment of the invention mentioned above. It may be provided an additional third heating element, which is then separated in terms of area. If exactly three heating elements are provided, they can advantageously according to the second aforementioned embodiment of the invention all three run close to each other and be separated in terms of area.

According to the invention, at least two electrically separate and independently operable heating elements are mounted on the carrier and a single planar dielectric layer is provided on one side of the heating elements for connection to the control or measuring device for detecting a leakage current, wherein the dielectric layer substantially covers all the heating elements , The one pad is then formed in each case by the heating elements. As a result, the area discrimination of the heating elements is achieved. The other pad, for example as an electrode, can either cover the entire surface of the dielectric layer or be divided into a plurality of pads or partial electrodes, the distribution of which in turn corresponds to the heating elements or covers them substantially exactly with their course. This has the advantage that even complicated courses for the heating elements, in particular intermeshing courses, can also be simulated with the connection surfaces.

In the method according to the invention, both the Heizleiterstrom by the heating element or by the heating element and a leakage through the dielectric layer are monitored during operation of the heater for heating water, including its temporal progressions are monitored and may also be stored as Betriebsprotokollierung. There are three cases to be distinguished.

In a first case of a heat conductor with a positive temperature coefficient, ie a PTC heat conductor can be detected at a slow or so to slow drop of the Heizleiterstroms a large scale calcification on a medium side of the carrier or can such a slow drop of Heizleiterstroms as a such large scale calcification can be defined. A large scale calcification grows slowly over the operating time, the conductor temperature rises slowly with the operating time due to the decreasing heat capacity and the Heizleiterstrom falls off. It can various measures follow, such as an indication to a user that a descaling or cleaning of the heater is necessary, or a temporary temporary power reduction with simultaneous display. Such a slow drop in the heating conductor current is when it falls by at least 2% in less than 100 hours. It may also fall by at least 3% to 5% in less than 100 hours to be recognized as such large-scale calcification. The drop in the Heizleiterstroms is evaluated at approximately the same water temperature, since the Heizleiterstrom falls when heating the water because of the thereby rising Heizleitertemperatur also. This will also be later based on the Fig. 5 specifically explained.

A temperature sensor may be provided on the heating device, advantageously at a distance from the heating element or its heating conductors. This can be a small sensor, such as an NTC. The distance should be so large that the temperature sensor detects only the temperature at the carrier and thus the water.

In a second case of a heat conductor with a negative temperature coefficient, ie an NTC heat conductor, a large-scale calcification can be detected on a medium side of the carrier at a slow or so to slow rise of the Heizleiterstroms or can such a slow increase of the Heizleiterstroms as a such large scale calcification can be defined. Otherwise, however, the same values as previously explained for the first case may apply, namely that such a slow increase in the heating conductor current is present when it increases by at least 2% in less than 100 hours. It may also increase by at least 3% to 5% in less than 100 hours.

In a third case, too rapid a rise in the leakage current can be detected as a locally limited or small-scale calcification or an aforementioned hot spot on the medium side of the carrier. This should be noticeable to the controller, but does not necessarily have to lead to a reduction of the heating power or shutdown of the heater. Such a too rapid increase in leakage current occurs when the leakage current increases by at least 30% in less than 20 hours, possibly by at least 30% in less than 5 hours or by at least 50% in less than 10 hours. As a further condition can be provided that an absolute maximum value or limit value for the leakage current is exceeded, not only so that such a localized or small scale calcification or hot spot on the medium side of the carrier is detected, but thus at least reduced or the heating power even the heater is turned off. This absolute In particular, the limit value may be at least 200% of the leakage current which is present at the beginning of the operation of the heating device in the clean state or without calcification of the medium side of the carrier. It is also possible to provide an absolute limit value for the leakage current. Thus it can be provided that the leakage current may amount to a maximum of 20 mA or 30 mA, otherwise a shutdown occurs because of a hot spot.

Here, several heating elements can be operated one after the other individually, wherein in each individual operating case the leakage current is detected at the at least one dielectric layer above the heating element currently being operated and under certain circumstances is also stored as operation logging. There are two ways to differentiate. In a first possibility that the leakage current in the individual operation of the heating elements is the same in each case, in particular by a maximum of 10% to 20% is different, a large-scale calcification on the medium side of the carrier is detected or this is considered as large-scale calcification. In a second possibility that the leakage current in the individual operation of the heating elements is different or at least 10% different, in particular by at least 30% different, a localized or small scale calcification on the medium side of the carrier in the region of the heating element detected with the higher leakage current or it is detected an aforementioned hot spot.

In a further embodiment of the invention can be provided that in the event that a large-scale calcification has been recognized on the medium side of the carrier, a visual and / or acoustic signaling to an operator. This should be reported that a cleaning or descaling of the heater or provided with the heater device should be made. In this case, the operation of the heater can still go on easily, either with full heating power or at least reduced Heating capacity. It can also be provided that when a first limit value for a large-scale calcification has been reached, the heating power has to be reduced by 20% to 50%, but at least the heating device can continue to be operated. When a second limit value is reached, either the operation can then be continued only with low heating power, for example a maximum of 20% to 30%, or else the heating device is completely switched off. A reduction in the power of the heater can be evenly distributed to the individual heating elements if several of them are provided.

In the event that a localized or small-scale calcification or a hot spot has been recognized by rapid increase of the leakage current as explained above, the heating power of the affected heating element can be greatly reduced. In particular, this heating element or, alternatively, the entire heater can also be switched off immediately. This applies in particular if, as explained above, an absolute maximum value or limit value for the leakage current has also been exceeded. Otherwise, there is the risk of sustained damage or even destruction of the heater not only in the area of this heating element, but even the entire heater. Then it is possible that after a waiting time of 2 seconds to 20 seconds, the heater or the affected heating element is turned on again, advantageously with the same heating power as before or immediately with full heating power, so very fast. By a rapid change in temperature, a flaking of a localized or small scale calcification can be effected, possibly even at the hot spot. Under certain circumstances, this process can also be repeated several times, for example twice to five times or even ten times. Only when it is detected in the monitoring of the leakage current and its renewed very rapid increase and possibly a difference between the individual heating elements, that there is still a localized or small scale calcification or the hot spot on the heater or the heating element concerned , which then obviously has not been caused to flake or should be removed, the heating power should be reduced to this heating element as described above and this heating element or possibly even the entire heater are switched off together with a corresponding error message to an operator. The flaking or removal of such a localized or small-scale calcification or a hot spot can namely be recognized by the fact that when the heater is switched on again, in particular with previously set or full heating power, a rapid increase in the leakage current does not take place immediately.

In the case of several heating elements, the performance of this heating element can be reduced or it can be switched off completely in one case after recognizing a locally limited or small scale calcification on the medium side of the carrier in the region of a heating element. At least one further heating element is then operated with unchanged performance, since it is safe in this case. In the other case that a large-scale calcification on the medium side of the carrier is detected, the heating element located there or generally a heating element with at least one further heating element can be connected in series to continue to operate the heater with a reduced overall power. These can generally be two different cases of emergency operation.

To set different power to the heater, the individual heating elements can be interconnected differently, preferably they can be operated serially or in parallel or individually. Advantageously, the heating elements have different power values. They can then be switched in parallel, by power, serial, single, or for maximum power.

In a further embodiment of the invention can be provided that in the event that neither the monitoring of the Heizleiterstroms a slow drop or increase nor the monitoring of the leakage current through the dielectric layer show a rapid sharp increase within seconds or within 1 minute, but at a certain time both a large increase in the leakage current and a relatively rapid decrease or increase in the heating conductor current occur at the same time, the case of emptying a heater provided container or dry running when the heater is used in a pump is detected. Such an empty cooking or the resulting overheating usually affects the entire heating device or the entire carrier. Therefore, no different behavior of several heating elements can be determined here together with the aforementioned rapid and approximately equal increase in the leakage current. In this case, because of the relatively rapid increase in the temperature of the entire heating device, the heating conductor current drops significantly faster in a PTC heating conductor, and it increases significantly faster in an NTC heating conductor than in a large-scale calcification. There is no heat removed. Furthermore, the leakage current increases, but with almost the same speed as the Heizleiterstrom drops or increases, since it is not a local overheating, but a large area or full-surface. In this case, the entire heater should be shut down immediately, because otherwise a further operation makes no sense and above all the risk of damage is too great. In addition, a corresponding signal should be output to an operator.

In a further preferred embodiment of the invention, the method for operating a heating device according to the invention of a dishwasher is used, wherein advantageously the dishwasher has a controller for the operation of a water softening in the dishwasher. It can be provided that at the beginning of the operation of the dishwasher, ie after the first start-up, the water softening in the dishwasher is lowered for a lower water softening. This can be lowered or operated in the lowered state until over a corresponding slow drop or rise of Heizleiterstroms in the manner previously explained a large scale calcification on a medium side of the carrier is detected. In response, signaling to an operator may occur that the carrier or heater is to be descaled manually. The control of the dishwasher can automatically increase the water softening again, in particular increase briefly for a short time and then lower it again, in order to continue working with reduced water softening. For use in a dishwasher, such a heating device according to the invention can for example be installed in a pump for heating and for conveying the water in the dishwasher, as for example the DE 102010043727 A1 shows.

These and other features will become apparent from the claims but also from the description and drawings, wherein the individual features each alone or more in the form of sub-combinations in an embodiment of the invention and in other fields be realized and advantageous and protectable Represent embodiments for which protection is claimed here. The subdivision of the application into individual sections as well as intermediate headings does not restrict the general validity of the statements made thereunder.

Brief description of the drawings

Fig. 1

eine Ausführung einer nicht von der Erfindung umfassten Heizeinrichtung mit einem einzigen Heizelement mit Schichtaufbau in Explosionsdarstellung,

Fig. 2

eine Ausführung einer erfindungsgemäßen Heizeinrichtung mit zwei Heiz-elementen in seitlicher Darstellung,

Fig. 3

eine Draufsicht auf die Heizeinrichtung aus Fig. 2,

Fig. 4 bis 6

verschiedene Diagramme mit Verläufen des Leckstroms und des Heizleiter-stroms und

Fig. 7

ein Diagramm für einen Verlauf des Heizleiterstroms und der Leistung beim langsamen großflächigen Verkalken.

Embodiments of the invention are shown schematically in the drawings and are explained in more detail below. In the drawings show:

Fig. 1

an embodiment of a not included in the invention heater with a single heating element with layer structure in exploded view,

Fig. 2

an embodiment of a heating device according to the invention with two heating elements in a lateral view,

Fig. 3

a plan view of the heater off Fig. 2 .

4 to 6

different diagrams with curves of the leakage current and the heating conductor current and

Fig. 7

a diagram for a course of Heizleiterstroms and the performance of slow large-scale calcification.

Detailed description of the embodiments

In Fig. 1 a first embodiment of a not included in the invention, the heater 11 is shown in an exploded view with an oblique view showing the layer structure. It corresponds to the one from the aforementioned DE 102013200277 A1 , The heating device 11 has a carrier 13, which here consists of metal or stainless steel. It can be flat or flat, alternatively also tubular, as it is from the aforementioned DE 102010043727 A1 is known. On its underside or medium side is to be heated water or flows to be heated Waser over. On the carrier 13, a dielectric insulating layer 15 is provided as the base insulation of the carrier 13, which may consist of glass or glass ceramic. It must be electrically insulated, even at high temperatures. Such a material is known to those skilled in principle for insulation layers.

On the first insulating layer 15, a single heating element 17 is applied with meandering course, consisting of individual successively or serially connected heating conductors 17 '. These are largely straight and connected by curved sections. But it could also be provided a single heating element, which is also considerably wider than the narrow heating element 17 'shown here, see also the Fig. 2 , The heating element 17 is formed as a thick-film heating element made of conventional material and applied by conventional methods. At its two ends are enlarged fields as Heizleiterkontakte 18, which may also be made of other material, such as a usual for thick film heat conductor contact material with significantly better electrical conductivity and especially better contacting properties.

Over the heating element 17, a dielectric layer 20 is applied over a large area, which may be glass-like or a glass layer. The dielectric layer 20, so to speak, closes the heating device 11 or insulates the heating element 17 and seals it and the layer structure, in particular against harmful or aggressive environmental influences. To the electric Contacting the heating element 17 or its Heizleiterkontakte 18, the dielectric layer 20 window 21 just above the Heizleiterkontakten 18 for a known per se Durchkontaktieren.

On the dielectric layer 20, an electrode 24 is applied as an electrically conductive pad, in the form of a large-area layer. This is here just as large as the carrier 13 and the insulating layer 15. The electrode 24 should not overlap directly on the carrier 13 or the heating element 17, since it must be isolated from the carrier 13 and heating element 17. On the electrode 24 may be another cover or insulation layer, but need not. It has two cutouts 25 at the corners, which, together with the underlying windows 21 in the dielectric layer 20, allow a previously described contacting with the heat conductor contacts 18. The heating element 17 or its heating conductor 17 'form the other or first connection surface.

Shown is also a controller 29 with power supply for the heating element 17. The controller 29 has a memory 29 'on. This is known from the prior art and need not be explained in detail. Furthermore, a measuring device 30 is shown, which is connected on the one hand to the electrode 24 via an electrode contact 26 and on the other hand to the heating element 17. As previously explained, the dielectric and resistive properties of the second dielectric layer 20 change with temperature, and the current detected by the measuring device 30 correspondingly increases with increasing temperature. The measuring device then detects this change in the properties of the dielectric layer 20 between heating element 17 and electrode 24.

In Fig. 2 is an embodiment of a heater 111 according to the invention with layer structure to see in a very simplified side view. A carrier 112, which may form a container such as a tube, has at the bottom a medium side 113 as a bottom, along which water 5 flows or is present. This water 5 is to be heated by the heater 111. On top of the carrier 112, a base insulation 115 is provided as an insulating layer. Then, in turn, a heating element 117 is applied, here as a planar heating element or in thick film technology. On the heating element 117, a dielectric layer 119 is applied, in a different planar configuration, as has been explained above and as shown by the Fig. 3 will be shown. On the dielectric layer 119, in turn, an electrode surface 121 is applied as an upper connection surface to the dielectric layer 119 made of electrically conductive material. Their areal Design can also be variable. The heating element 117 also serves as the lower connection surface to the dielectric layer 119, as has been explained above.

On the medium side 113 there is a risk of calcification of the heater 111 with the mentioned risks of an increase in temperature and damage or even destruction of individual heating elements 117 or the heater 111. Therefore, especially at the high power densities mentioned here, make sure that this does not happen ,

To the heater 111 are corresponding to the Fig. 1 or the DE 102013200277 A1 a controller, a memory and a measuring device connected, which is not shown here, but is easy to imagine.

In the Fig. 3 is shown in plan view, the heater 111, which may be either flat or a tube, so that the Fig. 3 in this case shows the unwound carrier. On the carrier 112, two heating elements are applied, namely a first heating element 117a and a second heating element 117b. The heating element 117a forms a partial heating circuit and the heating element 117b forms a partial heating circuit. Both heating elements 117a and 117b are interlaced or run meander-shaped into each other, so that they ultimately heat the same surface of the carrier 112 when they are operated individually, in common operation anyway. Thus, so to speak, a different distribution of the heating power of the heater 111 in itself possible. At maximum desired heating power, both heating elements 117a and 117b are operated in parallel. At minimum desired heating power, the two heating elements 117a and 117b operated in series, possibly also in a kind of emergency operation as previously explained. At an intermediate desired heating power, one of the heating elements 117a and 117b is operated. If they have different power values, the respective power can be generated by the respective individual operation.

Both heating elements 117a and 117b have the same length and four longitudinal sections. Both heating elements 117a and 117b also have interruptions by contact bridges on two adjacent longitudinal sections in a known manner. This can be reduced locally, the heating power. An electrical contacting of the heating elements 117a and 117b takes place via the individual contact fields 118a and 118b and a common contact field 118 '. A plug-in connection 122, which is applied to the contact fields 118 or to the carrier 112, can also be seen schematically, advantageously in accordance with FIG EP 1152639 A2 ,

It is easy to imagine how even a third heating element could for example run separately next to the two heating elements 117a and 117b, or could engage in the central space between the inner heating conductors of the heating element 117a. Under certain circumstances, it could also run along both outer heating conductors of the heating element 117a and would therefore also be virtually interlocked.

On the heating elements 117a and 117b, a single planar dielectric layer 119 is applied from a suitable material, shown here by the cross-hatching. It completely covers both heating elements 117a and 117b and extends as far as the edge of the carrier 112 or shortly before.

In turn, an electrode surface 121 is applied to the dielectric layer 119, specifically as a full-area electrode. Thus, although no separate temperature detection or detection of calcification is possible with distinction into different areas, but ensures a simple structure. The distinction in terms of area is made by the above-described separate individual operation of the heating elements 117a and 117b. The electrode surface 121 is electrically contacted in a manner not shown here, advantageously by means of the plug connection 122.

Based on Fig. 3 It is easily conceivable how a distinction in terms of area is possible, since in a first embodiment, although a large-area dielectric layer 119 is still applied to both or all of the heating elements 117. The electrode surface is then divided into two partial electrode surfaces. In this case, each partial electrode surface runs according to the underlying heating element, under certain circumstances even with exact coverage. Thus, the partial electrode surfaces are separated from each other. At each partial electrode surface then a temperature monitoring can be done exactly and only the underlying heating element. Thus, there is no problem that there is only a single full-surface dielectric layer 119.

In a second embodiment, the dielectric layer could also be divided into two or correspondingly many partial dielectric layers with a profile corresponding to the underlying heating element. Then, a correspondingly formed partial electrode surface is also applied per partial dielectric layer. Here, however, the production cost would be noticeably higher.

In the Fig. 4 is shown schematically how the signal or a leakage current corresponding to the y-axis changes over time. The time course is shown here over many hours, for example over 160 hours as operating time. The solid curve shown A is a normal operation, the slight increase of the course A comes through a slow, flat calcification on the heater 11 or 111 or on the carrier 13 or 113 on the medium side.

The dashed lines shown B represents the occurrence of a localized or small scale calcification or an aforementioned hot spot. The increase in the course of a few hours, for example, 1 hour to 5 hours to the maximum causes more than a doubling of the signal or leakage current at the maximum. Then, during the course B, the small-scale calcification has flaked off or has come off, which is why in this course B the leakage current or the signal just drops again and then again corresponds to the normal course A. Whether the calcification has been completely or incompletely replaced here can not be distinguished on the basis of the waste alone. If the course B then continues parallel to the course A, but with an increased value, it can be assumed that the replacement was not complete. Although this can be recognized, countermeasures are not absolutely necessary.

The dash-dotted curve C means, similar to the course B, forming a renewed localized or small scale calcification. Therefore, it should run in the rise area similar to the course B. However, the calcification does not dissolve here, a hot spot persists, which is why the leakage current or the signal continues to rise. If it reaches a limit value for the leakage current, in this case the limit value G _L , which is, for example, slightly higher than four times the normal leakage current according to the curve A, then this is recognized as dangerous locally limited or small scale calcification with too high a temperature. Then the heat output at the single heating element 17 or at one of the heating elements 117a or 117b is correspondingly greatly reduced or even switched off in order to avoid damage. Via a signaling, not shown, the controller 29 can call an operator for maintenance or for descaling.

In Fig. 5 curves D and E are shown for the Heizleiterstrom I over the time t, again over a time axis of several hours. The continuous curve D corresponds to a normal operation, the slight drop of Heizleiterstroms represents a slow large scale or complete calcification on the medium side of the carrier 13 or 113 dar. Furthermore, a threshold value G _H for the Heizleiterstrom is shown in broken lines, which may be, for example, 90% or 80% of the Heizleiterstroms at the beginning, here it is 90%. If this limit value G _{H is} exceeded, the large-scale calcification on the medium side of the carrier 13 or 113 is too strong, consequently a heat loss through the water too low and the risk of overheating of the heater too large. This can therefore also be evaluated as a signal, so that the controller 29 reduces the heating power or the heater 11 or 111 turns off, including appropriate signaling to an operator. In the example shown this may be after about 10 to 20 hours.

The dash-dotted line E is to show schematically how, at a certain point in time, the heating conductor current drops significantly more rapidly or faster when there is no water for heating and for removing the heat on the medium side of the carrier 13 or 113. This is the above-described case of empty cooking or dry-running in a PTC heating conductor. Then, the threshold value G _H falls below quickly, which can be recognized by the controller 29 again. Since the drop of the Heizleiterstroms then but still significantly faster than the course D, just this special case of the reduced Heizleiterstroms can be determined. If at the same time the leakage current increases, for example, similar to the course C accordingly Fig. 4 , the controller 29 can not evaluate this as a case of a sudden localized or small scale calcification, also not as a case of large-scale calcification, but just as an event of empty cooking or drying. This can then be displayed in a special signaling to an operator. Furthermore, the controller 29 then turns off the heating device 11 or 111 in any case completely, since on the one hand there is otherwise the risk of damage and on the other hand heating makes no sense anyway.

In the case of such dry cooking or dry running, the heating conductor current falls, for example, within less than one minute, for example within 10 seconds to 30 seconds, so much that it falls below the limit value G _H. Correspondingly quickly then the signal increases accordingly Fig. 4 ,

In the Fig. 6 is shown how at a water temperature of 50 ° C, the leakage current, represented here by a corresponding voltage of the measuring device 30, in the second range behaves when switching on the heater 11 or 111 or the single heating element 17 or the two heating elements 117a and 117b. The solid curve is a normal operation, so that it can be seen that after one to two seconds the leakage current reaches a value that appears to be constant per se, with a course essentially corresponding to the curve A of FIG Fig. 4 equivalent. Is a hot spot or a localized or small scale calcification given already when switching on the heater 11 or 111, the leakage current increases according to the dashed curve to three times. However, if this calcification or hot spot does not become larger or worse, then a relatively stable state is also achieved, which manifests itself in the substantially constant course. Here, the leakage current for the rise takes about 10 seconds, so it is also a very fast process.

In Fig. 7 It is shown how the Heizleiterstrom I behaves over time t (in minutes) when growing a large scale calcification. The Heizleiterstrom I is recorded on the left y-axis, on the right y-axis, the power P is recorded. In addition, the voltage U and the temperature T are recorded, these two without scaling, but with correct relative course. The basic characteristics of Heizleiterstrom I and power P over 100 hours of operation exemplify the growth of a large-scale calcification in the operating conditions of U = 230V and a temperature of T = 65 ° C. This can also add up over a large number of operating cycles. The time axis is not scaled in the area to the left of the double-dashed line as in the right, but within the two areas each already linear.

After switching on at t = 0, the Heizleiterstrom I increases with the rising voltage to a maximum value, the power P also, for example within a few seconds, then both fall off. The temperature T rises rather slowly until it reaches 65 ° C. This is here after about 18 minutes. Since the heating of the heating element due to the heating of the water now drops at a constant water temperature and thus the resulting proportion of change in the resistance of the heating element and thus the Heizleiterstroms I also eliminated, the waste is weaker or less. This is where the large-scale calcification begins. This starts even at a temperature of 65 ° C, well below that of boiling water. As a result of this large-scale calcification, the heating conductor current I drops further, in the example about 6% in 100 hours or 6000 minutes. The heating power drops accordingly, since the voltage U remains clearly identifiable.

Claims (13)

1. Heating device (111) for heating water (5), wherein

   - the heating device (111) has a carrier (112),

   - at least one heating element (117) is applied to the carrier,

   - the heating element (117) has precisely one heating conductor or a plurality of heating conductors which are connected one behind the other,

   - the heating device (111) has at least one flat dielectric layer (119) which substantially covers the at least one heating element (117),

   - an electrically conductive connection area (121) is in each case provided on both sides of the dielectric layer (119),

   - at least one of the connection areas (121) is connected to a controller or measuring device for detecting a leakage current as current flow through the dielectric layer (191),
   characterized in that

   - the at least one heating element (117) is connected to measuring means for monitoring a heating conductor current through the heating element,

   - at least two heating elements (117a, 117b) which are electrically separated and can be operated independently of one another are applied to the carrier (112),

   - a single flat dielectric layer (119) is provided on one side of the heating elements (117a, 117b) for the purpose of connection to the controller or measuring device for detecting a leakage current,

   - the dielectric layer (119) substantially covers the heating elements (117a, 117b).
2. Heating device according to claim 1, characterized in that an insulating layer (115) is applied to the carrier (112), a heating element (117) being applied to the said insulating layer, wherein the flat dielectric layer (119) is applied over the heating element and covers, in particular, a closed area, such as substantially a rectangle, wherein an electrically conductive connection area (121) is applied to the dielectric layer, once again with substantially the same area, wherein the other electrically conductive connection area is formed by the heating element (117).
3. Heating device according to claim 1 or 2, characterized in that the power density of the heating element (117) is at least 30 W/cm², preferably at least 100 W/cm².
4. Heating device according to any of the preceding claims, characterized in that the heating elements (117a, 117b) engage one in the other or are arranged in an interleaved manner, preferably with heating conductors as sections of the heating elements which run in a straight line and parallel in relation to one another, wherein at least one heating conductor of another heating element (117b) runs between two parallel heating conductors of a heating element (117a), in particular parallel in relation to the said heating conductors.
5. Method for operating a heating device (111) according to any of the preceding claims for heating water (5), characterized in that, during operation of the heating device, both a heating conductor current through the heating elements (117a, 117b) or the heating conductors and also a leakage current through the dielectric layer are monitored over time, wherein

   - in the case of a PTC heating conductor, limescale formation over a large surface area of a medium side of the carrier (112) is identified if there is an excessively slow drop in the heating conductor current of at least 2% in 100 hours,

   - in the case of an NTC heating conductor, limescale formation over a large surface area of a medium side of the carrier (112) is identified if there is an excessively slow increase in the heating conductor current of at least 2% in 100 hours,

   - locally limited limescale formation on or limescale formation over a small surface area of or a hotspot on a medium side of the carrier (112) is identified when there is an excessively rapid increase in the leakage current by at least 30% in less than 20 hours.
6. Method according to claim 5, characterized in that the absolute maximum value is 200%, preferably at most 300%, of the leakage current at the beginning of operation of the heating device (111) without any limescale formation on a medium side of the carrier (112).
7. Method according to claim 5 or 6, characterized in that, after limescale formation over a large surface area of a medium side of the carrier (112) is identified, a signal is sent to an operator that cleaning or limescale removal should be performed.
8. Method according to any of claims 5 to 7, characterized in that in the case of locally limited limescale formation on or limescale formation over a small surface area of the medium side of the carrier (112) being identified, the heating power of that heating element (117a, 117b) in the region of which the locally limited limescale formation or limescale formation over a small surface area occurs is reduced, wherein, in particular, this heating element (117a, 117b) or the heating device (111) is immediately switched off and, after a waiting period of 2 seconds to 20 seconds, this heating element or the heating device is switched on again, wherein switching off and switching on of this heating element or of the heating device are preferably repeated several times in order to chip away the locally limited limescale or limescale over a small surface area due to a rapid change in temperature.
9. Method according to any of claims 5 to 8, characterized in that, if neither monitoring of the heating conductor current indicates a slow drop or increase nor monitoring of the leakage current through the dielectric layer (119) indicates a rapid sharp increase, but both a rapid drop or increase in the heating conductor current and a rapid sharp increase in the leakage current occur at the same time at a specific point in time, this is assessed as a container which is provided with the heating device (111) having boiled dry.
10. Method according to any of claims 5 to 9, characterized in that, when a limit value for the leakage current is exceeded and/or when there is an excessively sharp increase in the leakage current, a fault search is started and, to this end, the heating elements (117a, 117b) are operated individually one after the other, and the leakage current at the at least one dielectric layer (119) above the operated heating element is detected in each case, wherein

    - in the case of the leakage current in each case being the same, in particular differing by at most 10%, during individual operation of the heating elements (117a, 117b), limescale formation over a large surface area of the medium side of the carrier (112) is identified,

    - in the case of the leakage current differing by at least 10%, in particular differing by at least 30%, during individual operation of the heating elements (117a, 117b), locally limited limescale formation on or limescale formation over a small surface area of the medium side of the carrier (112) in the region of the heating element with the higher leakage current is identified.
11. Method according to claim 10, characterized in that, in a case of the leakage current in each case being the same and in each case indicating a rapid sharp and approximately identical increase during individual operation of the heating elements (117a, 117b), in particular by at least 20% in less than 1 minute, this is identified as a container which is provided with the heating device (111) having boiled dry.
12. Method according to claim 10 or 11, characterized in that, in a case of locally limited limescale formation on or limescale formation over a small surface area of the medium side of the carrier (112) in the region of a heating element (117a, 117b) being identified, the power of this heating element is reduced or switched off and at least one further heating element is further operated at an unchanged power, wherein, in a further case of limescale formation over a large surface area of the medium side of the carrier being identified, this heating element is connected in series with at least one further heating element in order to be further operated at a reduced power.
13. Method according to any of claims 5 to 12 in a dishwasher, wherein a controller of the dishwasher lowers a setting for operation of a water-softening arrangement in the dishwasher for a lower level of water softening until limescale formation over a large surface area of a medium side of the carrier (112) is identified, preferably by means of a slow drop or a slow increase in the heating conductor current, wherein the controller automatically increases or intensifies the level of water softening again in response to the said limescale formation being identified.

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