Method of Cleaning a Cooking Appliance and Cooking Appliance

Description

CROSS-REFERENCE TO RELATED APPLICATIONS AND PRIORITY

This patent application claims priority from German Patent Application No. 10 2024 138 947.8 filed Dec. 19, 2024. This application is herein incorporated by reference in its entirety.

The present invention relates to a method of cleaning a cooking appliance comprising a cooking chamber and a heating device having heating pipes. The invention also relates to a cooking appliance which performs the method.

BACKGROUND OF THE INVENTION

Cooking appliances having heating devices are commonly used for cooking food in the gastronomy. Depending on the use and when cooking certain foods, a large amount of impurities may occur in the cooking chamber, which are distributed throughout the cooking chamber and adhere to the surfaces.

Common cleaning methods are based on the chemical cleaning of surfaces. This involves adding water and chemicals to form a washing liquor, which can additionally be heated and is then usually circulated in the cooking appliance for a certain period of time or flushed through the cooking chamber at least once.

However, chemical cleaning is in some places not sufficient. Especially a heating device having one or more heating pipes cannot always be completely cleaned with chemical cleaning, as the heating pipes partly have areas that are difficult for the washing liquor to access. In addition, the heating pipes are heated to a different degree during cooking, which also affects the cleaning result. In unheated sections, which usually reach at most the ambient temperature in the cooking chamber, impurities (such as grease and other residues from cooked food) adhere less strongly, so that they can be easily removed by chemical cleaning. In actively heated sections, the significantly higher temperatures cause the impurities to decompose chemically, so that the impurities do not adhere strongly, either. However, between actively heated sections and unheated sections, there are areas where the temperatures during cooking are at a level therebetween. There, the thermal effect generates a particularly strong adhesion of the impurities, which is accompanied by a chemical transformation of the impurities and sticking thereof by burning. As a result, the impurities can no longer be sufficiently removed by chemical cleaning.

Over time, an increasingly thick layer of various impurities can accumulate in the so-called burnt-on areas. This may lead to smoke development in the cooking chamber, which can alter the taste of the food, severely impair the air quality in the kitchen, and damage the cooking appliance.

The object of the present invention is to provide a method of cleaning a cooking appliance which overcomes the disadvantages described above and reliably cleans the cooking chamber of the cooking appliance.

BRIEF DESCRIPTION OF THE INVENTION

According to the invention, the object is achieved by a method of cleaning a cooking appliance comprising a cooking chamber and a heating device having a heating pipe. The heating pipe has a first, actively heatable section and a second section adjacent to the first section which is not actively heatable. During normal operation of the cooking appliance, impurities deposit and burn onto a bunt-on area in the second section. The cleaning method comprises the following steps. First, a cleaning program is selected. Then the heating device is switched on so that the first section of the heating pipe is heated. The heating device is then operated over a cleaning period at a power such that a temperature is reached over a burning period of at least 1 minute and preferably at least 5 minutes by heat transfer in the burnt-on area, so that the impurities adhering in the burnt-on area are detached. At the same time, it is prevented that the temperature in the cooking chamber exceeds a maximum permissible cooking chamber temperature.

The invention is based on the fundamental idea that by heating the first section of the heating pipe more intensely and/or for longer, the burnt-on area of the second section of the heating pipe is heated to such an extent that the impurities adhering thereto are thermally oxidized. In other words, the burnt-on area is temporarily shifted during the cleaning process from the actual burnt-on area resulting from normal operation and is located at a different point on the heating pipe during the cleaning process than during normal operation. However, it is important that the maximum permissible temperature in the cooking chamber is not exceeded to protect temperature-sensitive components in the cooking chamber, such as the cooking chamber seal, from damage. With the method according to the invention, the heating device is thus heated particularly strongly, while the cooking chamber remains relatively “cold.” Neither the walls nor the cooking chamber door and/or the sensors installed in the cooking chamber are heated to such an extent that any impurities adhering thereto are oxidized. Therefore, there is no cleaning of the entire cooking chamber, including the cooking chamber walls, the bottom, and the cooking chamber door; instead, only the heating device and, if necessary, any supports and/or nozzles located in the immediate vicinity thereof are cleaned.

The method thus ensures that heavily soiled areas of the heating pipes are effectively cleaned without damaging temperature-sensitive components in the cooking chamber.

Even if a temperature of, for example, 400° C. is exceeded in places in the burnt-on area during normal operation, the impurities there are not sufficiently removed. For sufficient removal, the temperature of 400° C. must be exceeded for at least 5 minutes, for example.

In principle, the burnt-on area may also slightly extend into the actively heated first section of the heating pipe. This depends on how the temperature transition from the first section to the second section develops during normal operation of the cooking appliance. Due to heat conduction, a temperature drop may already occur in the second section at the edge of the first section compared to the area of the first section, which is not affected by heat conduction due to the greater distance to the second section. Also, the burnt-on area may not directly adjoin the first section, so that a short portion of the second section is sufficiently cleaned during normal operation. The exact formation of the temperature transition and thus the location of the burnt-on area depends on several factors, e.g., the cooking chamber temperature used, the typical temperature in the actively heated area, possibly the composition of the dirt, etc.

The heating device may be a heating device having an electric resistance heater if an electrically operated cooking appliance is involved. If the cooking appliance is gas-powered, the heating device includes a gas burner which heats the first section of the heating pipes. Usually, an electric resistance heater has a plurality of heating pipes arranged approximately parallel to each other, while a heat exchanger has a long, multi-coiled heating pipe.

In electric resistance heaters, the heating pipes are usually only heated at a defined distance after passing through the cooking chamber wall inside the cooking chamber to prevent overheating of the connections and the cooking chamber wall and thus higher heat losses. The unheated sections, especially those near the passage through the wall of the cooking chamber, form the second section. In addition, there is a plurality of supports along the heating pipes for fastening the heating pipes to each other and also to the wall of the cooking chamber, which are in contact with the first section of the heating pipes but are not heated themselves. In principle, it is conceivable that the method according to the invention could also be used to clean further areas and components in the cooking appliance which either adjoin the first section or are located in spatial proximity thereto. Cleaning is then carried out by convective heat transfer and/or radiation.

In gas-heated cooking appliances having heat exchangers, depending on the exhaust gas temperature, the internal flow velocity, and the cooling (in the sense of “heat dissipation”) on the outside of the heating pipe, it is impossible to avoid significant temperature differences along the heating pipe. This results in such low temperatures in areas located in front of the gas burner and in areas located far downstream of the gas burner that it is impossible to prevent the impurities from sticking by burning.

In the following, a combustion time refers to the length of time which is usually sufficient to decompose the adhering impurities by thermal oxidation. The combustion time is at least 5 minutes. After this time, the impurities in the burnt-on area begin to glow on their own.

The cleaning time refers to the length of time that the heating device must be operated until the desired temperature of, for example, at least 400° C. is reached. This also includes the subsequent combustion time. The cleaning time is therefore the time from the start of the process to the end of cleaning. The cleaning time varies depending on the severity of the contamination, the type of impurities, and the temperature achieved in the burnt-on area. For example, a cleaning time of 15 minutes is sufficient to achieve a good cleaning result for average contamination. The combustion time may, for example, be half the cleaning time.

In the following, actively heated refers to heating by means of a heat source. Non-active or passive heating occurs when the second section is also heated solely by heat transfer from the first section or because it is heated by the hot medium flowing therethrough. The second section then has a lower flow velocity and/or a lower exhaust gas temperature, which may result in temperatures in the critical range. The flow around the second section and cooling thereof also play a role here. The second section does not have its own heat source.

During normal operation, the cooking appliance performs any cooking process for cooking food. Normal operation represents typical cooking operation. During normal operation, the temperature in the burnt-on area is below 400° C. The range between 300° C. and 400° C. is referred to as the medium temperature range or critical temperature range, as this range causes particularly strongly adhering, burnt-on impurities on the heating tube.

The temperature in the burnt-on area set in step c) may be at least 400° C. From this temperature onwards, the burnt-on area is cleaned effectively and particularly reliably.

According to one aspect of the invention, step c) is followed by step d), in which the cooking chamber is rinsed with a cleaning solution or clear water. The rinsing step removes the impurities that have fallen from the burnt-on area and deposited, for example, on a floor of the cooking chamber. This prevents the impurities from burning thereon again. The cleaning solution may be, for example, a lye or an acid.

If a cleaning step at high temperature is integrated into the middle of a chemical cleaning program, then this cleaning step must usually be followed by cooling, e.g., in the form of an accelerated cooldown. This is because further cleaning steps usually follow, in which water or a chemical washing liquor is used again in the cooking chamber. The high temperature in the cooking chamber would cause sudden evaporation with further critical consequences. Therefore, the cooking chamber and the heating element must first be cooled down again before the next steps can begin. This cooling must therefore also be additionally integrated into such a combined cleaning process. In other cases, e.g., when using thermal-oxidative cleaning at the end of the cleaning process, cooling does not necessarily have to be integrated.

According to a further aspect of the invention, the heating device is operated at maximum power in step c). The cleaning time is in particular at least 15 minutes. The combination of maximum power and a sufficiently long cleaning time leads to an optimal cleaning result. However, it should be noted that the maximum cooking chamber temperature must not be exceeded. If necessary, the heating device is therefore only operated at maximum power if, for example, a thermal load is introduced into the cooking chamber or the cooking chamber is actively or passively cooled. Basically, the heating device must be operated long enough at maximum or, alternatively, medium-high heating power.

In step c), the temperature in the cooking chamber may be a maximum of 300° C. and the temperature in the first section of the heating pipe may be between 500° C. and 850° C. The maximum cooking chamber temperature depends on the thermally sensitive components installed in the cooking chamber, such as the cooking chamber seal or sensors, which must not be damaged by the cleaning process. The maximum temperature in the first section of the heating pipe must also be within a temperature range which does not cause damage to the heating pipe.

The heating device may either be an electric heater, wherein the first section is actively heated electrically and the second section is not actively heated and is heated indirectly via heat conduction, convection, and/or heat radiation. Alternatively, the heating device may also be a heat exchanger, wherein the first section is actively heated via a burner, in particular a gas burner, and the second section is located downstream of the first section and is not actively heated by combustion, but “only” by the combustion gases. The cleaning method according to the invention may thus be applied to both electrically operated cooking appliances and gas-operated cooking appliances. In other words, hot exhaust gas is generated at the burner by combustion and flows through the heat exchanger. In the first section, which is located in the area of the burner, the heat transfer is sufficient to remove impurities from the heating pipe. In the second section, the heat transfer is insufficient, for example due to low flow velocities or low exhaust gas temperatures, to generate sufficiently high temperatures for the thermal oxidation of the impurities during normal operation.

According to a further aspect of the invention, the cooking chamber may be actively cooled during the cleaning process, in particular by directing steam from a steam generator into the cooking chamber and/or introducing water from a nozzle into the cooking chamber and/or opening a cooking chamber ventilation system in a further step. Active cooling has the advantage that the heating power of the heating device can be maximized without the temperature in the cooking chamber exceeding the maximum permissible cooking chamber temperature. In this way, the temperature in the burnt-on area can be increased to such an extent that the impurities are thermally oxidized and, at the same time, the cooking chamber temperature in the cooking chamber remains below the maximum permissible cooking chamber temperature. The faster, longer, and further the temperature in the burnt-on area is above 400° C., the faster the cleaning program is completed. Active cooling (i.e., heat dissipation) thus leads to a better cleaning result and a shorter cleaning duration.

The cooking appliance or cooking chamber may additionally also be cooled by an open cooking chamber door during the cleaning process. Alternatively or additionally, the thermal load in the cooking chamber may be increased and/or the speed of a rotating fan wheel in the cooking chamber may be reduced. Due to the open cooking chamber door, hot air can escape from the cooking chamber, cooling it down. A thermal load arranged in the cooking chamber causes the cooking chamber to heat up more slowly. A slower-rotating fan wheel reduces the air velocity near the heating pipes, so that the heat released there is used more to heat the second sections and is not dissipated into the cooking chamber. In combination with other ways of cooling the cooking chamber, such as an open cooking chamber ventilation system, the optimum fan wheel speed may be in the middle range. Under certain circumstances, a higher fan wheel speed may even lead to higher temperatures in the burnt-on area, improving the cleaning result. In addition to a changed fan wheel speed and a high cooking chamber temperature, cooling can be reduced by limiting a reverse operation of the fan wheel to the direction of rotation which generates the higher temperatures at the points to be cleaned.

The power of the heating device in step c) may be temporarily reduced, resulting in a heating pause. The heating power of the heating device may then be increased again, creating a heating peak. With a longer heating pause than during normal operation of the cooking appliance, a longer phase with maximum heating power (the heating peak) can then take place without reaching impermissibly high temperatures in the cooking chamber, as the cooking chamber has cooled down again slightly. Due to the high temperatures reached on the heating pipes during this process, it is possible to reliably remove any deposited impurities. The use of heating pauses and heating peaks generally results in the cooking chamber being heated less intensely, so that the cooking chamber does not need to be cooled, or needs to be cooled less, and no or only a low thermal load needs to be introduced into the cooking chamber.

According to a further aspect of the invention, the first section of the heating pipes is heated to such an extent that impurities located on one or more supports of the heating pipe and/or on a steaming nozzle and/or on other indirectly heated components in the cooking chamber are detached. Both the support and the steaming nozzle are heated by heat transfer from the first section to such an extent that any burnt-on impurities decompose by thermal oxidation.

The process may be an independent cleaning program which can be selected separately by an operator of the cooking appliance or which the cooking appliance performs automatically and independently after a certain period of time. Alternatively or additionally, the cleaning program may also be combined with further cleaning programs. Cleaning programs cleaning the cooking chamber with chemical cleaning agents are suitable for this purpose, for example. If the burnt-on area, the support, and the steaming nozzle are first heated to such an extent that the burnt-on impurities fall off, the remaining impurities can be easily removed with the chemical cleaning agent in the subsequent chemical cleaning step. It is also possible to create completely new combined cleaning sequences in which the cleaning program is integrated. If the process is integrated into an existing cleaning program, it is preferably carried out after a cleaning step with lye and before a cleaning step with acid. The cleaning step with acid then corresponds to the optional step d), in which the cooking chamber is rinsed with a cleaning solution.

According to the invention, a cooking appliance comprising a cooking chamber and a heating device having at least one heating pipe is also provided. The heating pipe has a first, actively heatable section and a second section adjacent to the first section which is not actively heatable and has a burnt-on area on which burnt-on impurities can deposit. The cooking appliance has a control unit which is configured and set up to operate the cooking appliance and the heating device such that they carry out the method according to any of the preceding claims. The control unit not only controls the heating device itself, but can also additionally control a steam generator, the steaming nozzle, or the cooking chamber ventilation system to cool the cooking chamber. In addition, the control unit may also receive signals from various sensors, such as a temperature sensor, and thus control the heating device and other components of the cooking appliance accordingly. The control unit may also prompt a user via a user interface to place a thermal load in the cooking chamber, for example a container filled with water.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and characteristics of the invention will become apparent from the following figures and the associated description, in which:

FIG. 1 shows a schematic representation of a cooking appliance according to the invention with a schematically represented heating device;

FIG. 2 shows a detail of a cooking appliance according to the invention with a fan wheel and an electric heating device;

FIG. 3 shows a detail of a cooking appliance according to the invention with two fan wheels and an electric heating device;

FIG. 4 shows a heating device according to the invention for a gas-powered cooking appliance;

FIG. 5 shows a diagram with two curves showing the temperature distribution along a heating pipe, curve A showing the temperature distribution during normal operation of a cooking appliance according to the invention and curve B showing the temperature distribution during a cleaning method according to the invention;

FIG. 6 shows a diagram with two curves showing a temperature profile at a burnt-on area of a heating device according to the invention, curve A showing the temperature profile during normal operation of a cooking appliance according to the invention and curve B showing an ideal temperature profile during a cleaning method according to the invention;

FIG. 7 shows a diagram with two curves showing a temperature profile at a burnt-on area of a heating device according to the invention, curve A showing the temperature profile during normal operation of a cooking appliance according to the invention and curve B showing a temperature profile with heating pauses during a cleaning method according to the invention; and

FIG. 8 shows a schematic representation of the method for cleaning the cooking appliance from FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a cooking appliance 10 for professional use in restaurants, canteens, and large-scale gastronomy. The cooking appliance 10 has a cooking chamber 12 which is delimited by walls 14. A fan wheel 18 is attached to a side wall 16 of the cooking chamber 12. The fan wheel 18 is mounted on the side wall 16 so as to be rotatable about an axis of rotation by means of a motor (not shown). An air baffle plate 20 is provided to shield the fan wheel from the cooking chamber 12.

A heating device 22, which is shown only schematically, is provided in the area of the fan wheel 18, to the left of the air baffle plate 20 in FIG. 1. FIGS. 2, 3, and 4 show the respective heating device 22 in detail.

The heating device 22 has a heating pipe 24.

When food is cooked in the cooking chamber 12, the generated impurities are distributed throughout the cooking chamber 12, so that impurities also deposit on the heating device 22 and the heating pipe 24.

Depending on how strongly the heating pipe 24 is heated during a cooking process, impurities adhere to the heating tube 24 to different degrees.

Strongly adhering impurities do not deposit in a first section 26 of the heating pipe 24, which is actively heated, because the impurities that hit it are already thermally oxidized during normal operation and fall off the first section 26. Depending on whether the cooking appliance 10 is an electrically operated cooking appliance 10 or a gas-operated cooking appliance 10, the first section 26 is heated either by an electric resistance heater or by a gas burner.

The first section 26 is followed by a second section 28. However, this is not actively heated, but is only partially heated by heat transfer from the first section 26 to the second section 28 (i.e., in the case of an electric heater by heat conduction or in the case of a gas heater by the combustion gas). The second section 28 does not reach temperatures as high as the first section 26, and impurities are not removed by thermal oxidation during normal operation as in the first section 26.

Two different areas can be identified on the second section in terms of temperature distribution and, consequently, in terms of impurities.

A first area 29 of the second section 28, which is far away from the first section 26, has essentially the same temperature as that prevailing in the cooking chamber 12, i.e. between 250° C. and 300° C., as there is no or only negligible heat transfer from the first section 26 to the first area 29 of the second section 28 due to the spatial distance. As a result, impurities adhere only slightly and can therefore be removed very easily by chemical cleaning.

A second area of the second section 28, hereinafter referred to as burnt-on area 30, has an average temperature in the range between 300° C. and 400° C. during normal operation of the cooking appliance.

The temperatures that occur in the burnt-on area 30 during normal operation cause the impurities to adhere particularly strongly (“burn on”) to the surface of the burnt-on area 30. Thermal oxidation of the impurities does not yet occur, and the impurities are difficult to remove completely with chemical cleaning agents.

The spatial extent of the burnt-on area 30 on the heating pipe 24 depends on the cooking processes or programs typically performed with the cooking appliance. Depending on how long and at what temperature the first section 26 is heated during a cooking process, the burnt-on area 30 shifts on the second section 28. Generally speaking, the hotter the first section 26 is, the greater the distance between the burnt-on area 30 and the first section 26. It can also happen that the burnt-on area 30 extends partially into the first section 26. In any case, however, it is not possible to completely clean the burnt-on area 30 of impurities during normal operation.

The cooking appliance 10 has a control unit 32 which is set up, among other things, to operate the heating device 22.

If the cooking appliance 10 is switched to a cleaning mode, the control unit 32 controls the heating device 22 such that the first section 26 is heated to such an extent that, by means of heat transfer, the second section 28 and thus also the burnt-on area 30 are heated to such an extent that the impurities present on the burnt-on area 30 are removed from the heating pipe 24 by thermal oxidation.

The control unit 32 is additionally set up to monitor the cooking chamber temperature and to control the cooking appliance such that the cooking chamber temperature always remains below a maximum cooking chamber temperature of, for example, 300° C., even during the cleaning operation. If necessary, the control unit 32 ensures that the cooking chamber 12 is cooled so that in the optimal case, the heating device 22 can be operated at maximum power for a desired cleaning time.

To this end, the control unit 32 can, for example, drive the fan wheel 18 accordingly and reduce the speed thereof so that the heater is cooled less by air circulation. Alternatively or additionally, the control unit 32 can also operate and open a cooking chamber ventilation system, control a steam generator so that it generates steam which is introduced into the cooking chamber, and/or activate a steaming nozzle.

FIG. 2 shows in detail a first embodiment of a cooking appliance 10 having a single fan wheel 18 around which several heating pipes 24 are arranged.

The heating pipes 24 are introduced into the cooking chamber 12 in the area of a feedthrough 34 from a technical room. The heating pipes 24 are not heated in this area to prevent overheating of connections, undesirable heating of the wall of the cooking chamber and correspondingly higher heat losses. The second section 28 is therefore located in the area of the feedthrough 34, more precisely, the first area 29 of the second section 28 is located at the feedthroughs, and the burnt-on area 30 is located near the first section 26.

Adjacent to the second section 28 is the first section 26, which is actively heated by means of an electric resistance heater.

On the side of the heating pipes 24 diametrically opposite the feedthrough 34 is a support 36 which secures the heating pipes 24 to the side wall 16 of the cooking appliance 10.

In the area of the feedthrough 34, there is also a steaming nozzle 38, by means of which water can be sprayed into the cooking chamber 12. Neither the support 36 nor the steaming nozzle 38 are directly heated, so that, similar to the burnt-on area 30, adhering impurities can deposit there.

FIG. 3 shows a second example embodiment of a cooking appliance having two fan wheels 18, around which two heating pipes 24 are also arranged. The heating pipes 24 emerge from the feedthrough 34 and are fixed to the corresponding side wall 16 of the cooking appliance 10 by means of supports 36. These two heating pipes also have the unheated second section 28 in the area of the feedthrough 34, with the first area 29 near the feedthrough and the burnt-on area 30 near the first section 26, while the first section 26 is electrically heated by means of a resistance heater.

In this example embodiment of the cooking appliance 10, strongly adhering impurities also occur in the burnt-on area 30, in the area of the feedthrough 34, on the support 36 and on the steaming nozzle 38.

FIG. 4 shows a heating device 22 having a heat exchanger 40, which is installed in a gas-powered cooking appliance 10.

A gas burner 42 which actively heats the first section 26 is arranged in the first section 26 of the heating device 22. In the present heating device 22, a fuel gas-air mixture is conveyed from a blower (not shown) into the gas burner 42 via a heating pipe 24. The hot combustion gases produced during combustion then flow through the heating pipe 24, which forms the second section 28. The gas burner 42 is supplied with a fuel gas-air mixture via the blower, which exits at the gas burner 42 and is burned there. The exhaust gas produced in this way flows through the interior of the heating pipe 24 and transfers the heat thereto. However, the heat transfer is not equally strong everywhere, as not all areas are flowed through at the same speed and the exhaust gas temperature decreases towards the end of the heating pipe.

Both upstream of the first section 26 with the gas burner 42 and downstream of the first section 26, the heating pipe 24 has burnt-on areas 30. The burnt-on areas 30 are located at the points on the heating pipe 24 where the heating pipe 24 is either just introduced into the cooking chamber 12 or just before it leaves the cooking chamber 12 again. The exact position of the burnt-on area 30 depends on the flow velocity and the exhaust gas temperature as well as the heat dissipation on the outside of the heating pipe 24.

In electric heating devices, the temperature in the first section 26 is usually similar because the heating pipes 24 are generally heated everywhere with approximately the same power density.

In the heat exchangers of a gas-powered cooking appliance 10, much greater temperature differences are achieved, particularly due to the downwardly decreasing exhaust gas temperature. However, there are also areas with locally stronger heat transfer (e.g., stronger turbulence, elbow flow, or similar flow effects) or locally weaker heat transfer (e.g., dead zones), which increases or decreases heat transfer locally. Depending on the heating pipe layout and other boundary conditions, this can result in the formation of additional burnt-on areas 30.

To detach the strongly adhering impurities in the burnt-on areas 30 of all embodiments shown above, the heating device 22 is operated over a cleaning period of, for example, 15 minutes with such power that, due to heat transfer in the burnt-on area 30, a temperature of, for example, at least 400° C. is established over a combustion period of, for example, at least 5 minutes. At the same time, a temperature of 300° C. is not exceeded in the cooking chamber.

FIG. 5 schematically shows two curves which illustrate a temperature distribution along a heating pipe 24 during a normal cooking program in a cooking appliance 10 in normal operation (curve A) and during the cleaning process (curve B), respectively. The length of the heating pipe 24 is plotted on the x-axis and the temperature reached there is plotted on the y-axis.

The two curves each represent a snapshot, as the distribution is not constant during a cooking or cleaning cycle. The decisive factor is that the distribution in the cooking mode usually corresponds more to the distribution of curve A most of the time, and in the cleaning mode, especially during the combustion period, it corresponds more to the distribution of curve B.

Below the graph, two schematic heating pipes 24 are additionally shown, wherein the upper heating pipe 24a and the positions of the sections and areas marked therein correspond to a heating pipe 24 operating in normal operation, and the sections and areas of the lower heating pipe 24b correspond to a heating pipe 24 during the cleaning process.

The temperatures set on the two heating pipes 24a, 24b can basically be divided into three areas:

One area of the heating pipe 24a, 24b has a temperature in a temperature range T1, which essentially corresponds to the temperature in the cooking chamber. This part is the first area 29 of the second section 28 of the heating pipe 24a, 24b.

The adjoining area is the burnt-on area 30, which has a temperature in the temperature range T2. The temperature range T2 is shown hatched for clarification and represents the critical or medium temperature range in which strong adhesion of impurities occurs.

The first section 26 adjacent to the second section 28 has a temperature above the temperature range T2 and is thus in the temperature range T3, in which thermal-oxidative decomposition of the impurities takes place.

When comparing the two curves A and B and the heating pipes 24a, 24b shown below, it is noticeable that the temperature at the first section 26 during the cleaning process (curve B) is significantly higher than during the cooking process (curve A).

This leads to a spatial shift of the burnt-on area 30 on the heating pipe 24b compared to the heating pipe 24a further in the direction of the first area 29. The burnt-on area 30 is located further to the left on the heating pipe 24b than on the heating pipe 24a. As a result, the actual burnt-on area 30 on the heating pipe 24b has a temperature in the temperature range T3, and thermal oxidation therefore takes place there.

At the same time, the same temperature is present in the first area 29 of the second section 28 on both heating pipes 24a and 24b. The cooking chamber temperature is therefore just as high during the cleaning process (curve B) as during normal operation (curve A).

By raising the temperature in the original burnt-on area 30 for a sufficiently long period of time, for example over a cleaning time of at least 15 minutes, thermal oxidation of the impurities adhering there occurs, causing them to fall off. The necessary cleaning time depends on the cooking system used and the amount and strength of the impurities adhering.

FIG. 6 shows the temperature curve in the burnt-on area 30 of a heating pipe 24 and compares the temperature curve during a normal cooking process (curve A) with the temperature curve at the burnt-on area 30 during the cleaning process (curve B).

At the start of the cooking process (curve A), when the cooking chamber temperature is below a desired temperature, the cooking chamber 12 is still cold. Therefore, the heating device 22 is initially continuously active and heats very strongly. This causes the burnt-on area to heat up, so that it is initially in the temperature range T1 and then in the temperature range T2. The temperature range T2 is also exceeded briefly, so that the burnt-on area 30 has briefly a temperature in the temperature range T3. However, a temperature peak in the temperature range T3 does not necessarily occur in every cooking process. Depending on the cooking process, there are also processes in which the peak is only in the range T2. Only T1 is also possible, but then there is no strong adhesion.

Once the desired temperature in the cooking chamber 12 has been reached, the heating device 22 switches to cyclic operation. The control unit 32 controls the heating device 22 such that the cooking chamber temperature is kept as close as possible to the desired temperature. When the desired temperature is reached for the first time, the heating device 22 is deactivated, causing the temperature in the heating pipe 24 to drop. Similarly, the cooking chamber temperature drops after a short time delay, so that the heating device 22 is reactivated when a lower threshold value for the cooking chamber temperature is reached. The temperature of the heating pipe 24 rises again. This results in repeated small heating peaks.

As long as the desired temperature of the cooking chamber 12 is not changed, for example by a user, the cyclic operation continues, since maintaining the desired temperature in the cooking chamber 12 usually does not require the heating device 22 to operate at maximum heating power continuously.

The highest temperature of the heating pipe 24 in the burnt-on area 30 occurs once when the desired temperature of the cooking chamber 12 is reached for the first time (first peak of curve A on the far left). The temperature level then drops more or less sharply, depending on the cooking process, until it stabilizes at an approximately constant level. The temperature level at which the temperature of the heating pipe 24 stabilizes is precisely in the critical temperature range T2, in which increased adhesion of impurities occurs. In principle, processes are also conceivable in which the temperature does not remain in the temperature range T2 the entire time, but briefly falls below or exceeds it. Even then, strong adhesion is possible. The decisive factor is that the curve remains in the temperature range T2 for a longer period of time and only rarely, and above all never for long, enters the temperature range T3.

The reason for the gradual drop in temperature after the first higher peak is that even when the cooking chamber temperature has already reached the desired temperature, the temperature in the heating pipe 24 continues to rise slightly due to the inertia of the cooking appliance until the entire cooking appliance is heated through.

Curve B shows the idealized curve of a temperature profile of the heating pipe 24 in which the heating device 22 is permanently active. For example, the thermal load in the cooking chamber 12 may have been increased by placing a container of water in the cooking chamber 12, for example. This requires a constantly high power of the heating device 22 to achieve the same (high) desired temperature in the cooking chamber 12.

The temperature of the heating pipe 24 initially rises more slowly due to the higher load in the cooking chamber 12, but then exceeds the highest temperature of curve A (first peak) and, after reaching the desired temperature in the cooking chamber 12, remains at approximately its maximum temperature in the temperature range T3, as the heating device 22 is permanently active.

If the heating power of the heating device 22 is constantly at its maximum, the temperature of the heating pipe 24 can be maintained permanently above the temperature range T2, and the burnt-on area 30 can be cleaned by thermal oxidation.

The curve shown here represents the highest possible temperature curve during cleaning. Cleaning can also work if the heating device 22 is not permanently active, i.e., if it switches into a cyclic mode, but on average is active for a significantly longer time (higher average power) than in normal operation. The decisive factor is that the temperature remains in the temperature range T3 for a longer period of time, even if it is a cyclic operation. In such a hotter cyclic operation, the individual heating phases are longer and the heating pauses shorter than during normal operation.

It may even make sense to set the temperature as close as possible above the temperature range T2. Ideally, a smaller thermal load or less cooling of the cooking chamber is thus sufficient, which is usually easier to achieve. In addition, the temperature load on some components is then not quite as high, which avoids an unnecessary reduction in the service life of these components.

Alternatively or in addition to increasing the thermal load in the cooking chamber 12, a cooking chamber door of the cooking appliance 10 can be opened so that the heat introduced into the cooking chamber 12 can be released into the environment. A cooking chamber ventilation system can also be opened so that cool air flows into the cooking chamber, and the cooking chamber can be actively cooled with a water source such as steam from a steam generator and/or water from a steaming nozzle. In addition, it is advisable to utilize the maximum possible temperature of the cooking chamber so that the heating device 22 is set to always strive for the maximum permissible cooking chamber temperature.

In addition, the speed of the fan wheel 18 can be reduced. In addition to a lower fan wheel speed and a high cooking chamber temperature, cooling can be reduced by limiting a reverse operation of the fan wheel 18 to the direction of rotation generating the higher temperatures at the points that are to be cleaned.

The highest possible temperature at the heating pipe 24 is achieved when the thermal load corresponds exactly to the maximum power of the heating device 22. The thermal load is the consumer of the heat generated. By controlling the heating device 22, on average only as much heat is generated at the heating pipe 24 as is consumed by the consumers, otherwise the cooking chamber temperature would in most cases rise significantly above the maximum cooking chamber temperature. When the cooking chamber 12 is empty, only the air and the components of the cooking appliance 10 need to be heated initially. Once the cooking appliance 10 is heated up, only the energy for the heat losses of the cooking appliance 10 needs to be supplied, so that on average very little power is required. By increasing the thermal load, the cooking appliance 10 must on average be supplied with more heat on a permanent basis. The type of consumer is irrelevant for the required heating power. With an open cooking chamber ventilation system, the fresh cold air must additionally be heated. When using a water container, the water must also be heated and may need to be evaporated later. With an open cooking chamber door, air is exchanged with the cold ambient air, so that the incoming cold air must also be heated.

If the thermal load is lower than the maximum heating power, the cooking appliance 10 enters the cyclic operation described above with reference to FIG. 6. If the load is even higher, the heating power is not sufficient to reach the desired temperature of the cooking chamber 12. This increases heat dissipation, and the temperature of the heating pipe 24 is lower despite the heating device being permanently active.

In addition to the generation of a permanently high thermal load, the required temperature in the burnt-on area 30 can also be achieved by alternately heating and cooling the cooking chamber 12.

This is shown in FIG. 7, where curve A again shows the temperature profile at the burnt-on area 30 during normal operation and curve B shows the cyclic operation of a possible cleaning process.

During the cleaning process, after heating and when the desired temperature in the cooking chamber 12 is reached, there is a heating pause and thus a longer cooling of the cooking chamber 12, which also causes the temperature of the heating pipe 24 to drop more sharply (curve B) compared to the temperature during normal operation (curve A). Subsequently, when reheating to the desired temperature of the cooking chamber 12 in curve B, the heating device 22 must heat for longer again, resulting in a higher heating peak compared to the heating peak occurring during normal operation.

The sum of the peaks allows the temperature for the thermal-oxidative decomposition of the impurities in the burnt-on area 30 to be reached for a sufficiently long time. This method is particularly suitable when a thermal load cannot be permanently introduced into the cooking chamber 12. However, this results in a longer cleaning time.

FIG. 8 shows an overview of the cleaning method according to the invention.

In a first step S1, a cleaning program is selected.

In a second step S2, the heating device is then switched on so that the first section 26 of the heating pipe 24 is heated.

In the third step S3, the heating device 22 is operated at a power for a cleaning time of, for example, 15 minutes such that, due to heat transfer in the burnt-on area 30, a temperature of at least 400° C. is established over a combustion time of at least 5 minutes, so that the impurities adhering to the burnt-on area 30 are detached. At the same time, the maximum permissible cooking chamber temperature of, for example, 300° C. is not exceeded. In step S2, the power of the heating device 22 can be temporarily reduced so that one or more heating pauses occur, wherein after the heating pauses the power of the heating device 22 is increased again and a heating peak occurs.

Parallel to or following the heating of the cooking chamber 12 with the heating device 22, the cooking chamber 12 can be actively cooled in an additional step S4, for example by directing steam from a steam generator into the cooking chamber 12, and/or by introducing water from a steaming nozzle 38 into the cooking chamber 12 and/or by opening a cooking chamber ventilation system. In addition, the process can be carried out with the cooking chamber door open and/or the thermal load in the cooking chamber 12 can be increased and/or the speed of a rotating fan wheel 18 in the cooking chamber 12 can be reduced.

In a further step S5, the cooking chamber 12 is rinsed with a cleaning solution such as a lye or acid.

The cleaning method can either be carried out as an independent cleaning program or can be combined with an existing cleaning program of the cooking appliance 10, which is used, for example, to clean the cooking chamber 12 using chemical cleaning agents.