EP4382807A1 - Baking oven and method for operating a baking oven - Google Patents

Abstract

An oven has a muffle and at least two tubular heating elements as heating devices therein, which are arranged next to one another and close together on a ceiling wall of the muffle in an electrically separate and separately controllable manner. The muffle is divided by the heating devices into thermal zones arranged next to one another, with a boundary between two directly adjacent zones running centrally between two directly adjacent heating devices. None of the adjacent zones overlap one another, or the zones are separate from one another, but border directly on one another. The oven has an oven control and a power supply for each of the heating devices, with the oven control being designed to determine or calculate a temperature of at least one heating device and/or in the muffle. The oven control can use these temperatures to determine a cooking process for several different items to be cooked, which are exposed to a different cooking temperature for each of the zones for the same cooking time.

Description

FIELD OF APPLICATION AND STATE OF THE ART

The invention relates to an oven and a method for operating such an oven. The oven has a muffle with an access opening and a door for closing the access opening, as well as at least two heating devices in the muffle.

Baking ovens usually have a so-called top heat radiator and a bottom heat radiator to heat the muffle. The top heat radiator is attached to the inside of the top wall of the muffle, usually runs in a meandering shape and can also be used as a grill for special operating modes. The bottom heat radiator is often under the bottom wall of the muffle, where it cannot be damaged or soiled. There are also so-called hot air and circulating air baking ovens, in which a fan is arranged on the rear wall of the muffle and has a ventilation direction into the muffle. In hot air baking ovens, a fan heater can be provided around the fan so that the fan can blow hot air, usually drawn out of the muffle in a circulating mode, into the muffle. Ultimately, however, the aim is to achieve as uniform or homogeneous a temperature as possible in the muffle.

TASK AND SOLUTION

The invention is based on the object of creating an oven as mentioned above and a method for operating it, with which problems of the prior art can be solved and in particular it is possible to operate the oven in an innovative and diverse manner and with the greatest possible ease of use and benefit for a user.

This object is achieved by an oven with the features of claim 1 and a method for operating such an oven with the features of claims 10, 14, 16 or 20 to 23. Advantageous and preferred embodiments of the invention are the subject of the further claims and are explained in more detail below. Some of the features are described only for the oven or only for one of the methods for operating it. However, they should be able to apply independently and independently of one another both to such an oven and to methods for operating it. The wording of the claims is made part of the content of the description by express reference.

The oven has a muffle, an access opening to the muffle, a door or other closure for closing the access opening and at least two heating devices in the muffle. The heating devices are electrically separated next to one another and arranged close together on a wall of the muffle, where close can mean less than 5 cm apart. This wall can therefore be defined by being the one to which all heating devices are arranged closest. The wall can also run parallel to a plane in which all heating devices are arranged. The wall can essentially run in one surface, preferably it can form the ceiling wall of the muffle. In an oven with several muffles or muffles that can be temporarily separated by cooking chamber dividers, this wall can also explicitly be an intermediate ceiling.

The at least two heating devices are designed as tubular heating elements with a resistance heating conductor, which is advantageously an electrical resistance heating conductor. A tubular heating element can preferably be designed according to the state of the art, for example according to EN 102009048495 A1 . The muffle is divided into zones arranged next to one another, with at least as many heating devices as there are zones, and a boundary between two directly adjacent zones running between the projection of two directly adjacent heating devices. Advantageously, a boundary runs approximately in the middle or exactly in the middle between the projection of two directly adjacent heating devices. This can depend on how strong the adjacent heating devices are or what their rated power is. The existing zones fill the muffle completely, or the entire muffle is divided into these zones. None of the adjacent zones overlap one another, or the zones are preferably separated from one another by radiation heat, although they can be directly adjacent to one another.

Each heating device can be controlled separately, with the oven having an oven control and a power supply for each of the heating devices. The oven control is designed to determine or calculate a temperature of at least one heating device and/or in the muffle. The power control can be designed as usual and have switching means, for example semiconductor switches or relays.

By dividing the oven into zones, it is possible to prepare several items in the muffle at the same time, but in different ways or at least at different temperatures. This enables a new way of preparing meals or dishes that consist of several items, such as roast duck, red cabbage and dumplings. It can be particularly advantageous that all the different items can be placed in the muffle at the same time and cooked for the same time, but at different temperatures. are ready. It is advantageous that the zones are not directly separated from each other, i.e. they have no partitions, walls or anything similar between them. This makes the construction of the oven easier.

In an embodiment of the invention, a reflector can be arranged above at least one tubular heating element, in particular on the aforementioned nearby wall of the muffle. In particular, a reflector is curved, advantageously so that the heat radiation generated by the tubular heating element radiates away from the wall and particularly advantageously within the zone corresponding to this heating device, i.e. it is in a type of channel with a certain width. Preferably, a reflector is arranged above each tubular heating element, which can be designed to fit each tubular heating element exactly.

The heating devices as tubular heaters can have the same nominal power and/or the same external diameter. The same components can be used. But they can also be different, i.e. have different nominal power and/or different external diameters. For example, at least one tubular heater can have a higher nominal power than the others. At least two, preferably three or more, heating devices can be provided next to each other. Either a middle heating device or a side heating device can have a higher nominal power and/or a larger external diameter than the two outer heating devices, which in particular then have the same nominal power and/or external diameter.

Preferably, each heating device defines its own zone in the muffle, whereby this zone can be heated primarily by this heating device. A zone can be defined by the area that a heating device primarily heats with its radiation direction perpendicularly away from the wall on or near which this heating device is arranged. The zones can have a constant width in the direction away from the heating devices.

In an embodiment of the invention, at least one fan can be provided for the muffle, preferably a fan with a fan heater directly associated with it, as is known from hot-air ovens. In particular, a single such fan can be provided for this purpose and arranged in a single zone; alternatively, the fan can generate an air flow that flows directly into more than 50% of only one of the zones. Such a fan can be arranged on a rear wall of the muffle. Other fans with other functions can be provided, for example as extractor fans and/or so-called vapor fans that blow air out at the front above the door. This can be used above all to remove moisture from the muffle.

In an embodiment of the invention, at least one temperature determination means for a temperature in the muffle can be provided, which has at least one temperature sensor. This can be designed to measure the temperature by means of a temperature-variable electrical resistor.

In a first embodiment, the temperature sensor is designed as a discrete temperature sensor separate and apart from the resistance heating conductors of the tubular heaters. It is therefore independent of their design and operation. The temperature sensor is advantageously shielded from the direct effect of a radiant heat flow through a tubular heater by means of a shield, for example arranged in a tube or covered by a half-tube opposite the heating devices. This discrete temperature sensor can advantageously be arranged on a fastening means for one of the heating devices in the muffle, with the temperature sensor standing or protruding from this fastening means into the muffle.

In a second embodiment, the temperature sensor is formed by the heating device or by a resistance heating conductor of a tubular heater, which as usual has a temperature-dependent electrical resistance. From this, when the tubular heater is not heating or is not being operated, the electrical resistance and thus the temperature can be determined in a known manner.

In an embodiment of the invention, the tubular heating elements can be designed with a loop-shaped or meander-shaped course, in particular they can run twice, so to speak. Each tubular heating element can have at least two longitudinal sections that are designed in a loop-shaped manner next to one another and adjoining one another. In a further embodiment of the invention, at least two heating devices can have the same shape or the same course, which in turn can be a simplification.

At least two heating devices can have different shapes or courses, whereby in one embodiment at least three heating devices are provided next to each other and a middle heating device has a different shape or course than the two outer heating devices. These can in particular have the same shape or course.

In an embodiment of the invention, at least one exhaust fan can be provided for an air flow from the muffle to the outside environment, as previously mentioned. An exhaust air duct can be provided from the muffle to the exhaust fan and from there to the outside environment, so the exhaust fan can be provided in between.

In a first embodiment of the invention, a single exhaust fan can be provided for the entire muffle or the entire oven, with the exhaust air duct having an exhaust air branch for each heating device or for each zone, each of which branches off from the wall to which the heating device is closest. In this case, a separate exhaust flap for the exhaust air branch can be provided for each heating device or for each zone, with each exhaust flap being separately operable to open and close. In this way, an exhaust function and an exhaust can be set separately and individually for each zone.

In a second embodiment of the invention, an independent separate extractor fan with an extract air duct can be provided for each heating device or for each zone formed by each heating device. In this case, a separate extract air branch or a separate extract flap for the extract air branch is provided for each zone or for each heating device. In this way, the extract function and extract can also be adjusted separately and individually for each zone.

In a method according to the invention for operating a previously described oven, the heating devices are switched on and off in such a way that at no time are two or more heating devices switched on at the same time or two or more heating devices switched off at the same time. These are advantageously those heating devices that are arranged next to one another and close to or less than 5 cm apart on the common wall of the muffle. In this way, regulations for so-called flickering and network interference can be complied with so that total powers that are not too high are switched on at the same time. Other heating devices, for example for bottom heat, can be provided that can participate in the method according to the invention so that the aforementioned regulations can be complied with, or that work completely independently.

In an embodiment of the invention, certain switching cycles can be carried out for at least two heating devices or for exactly three heating devices, advantageously for all heating devices that are arranged on a wall and that divide the muffle into zones. When a heating device is switched off, another heating device can be switched on at the same time. Additionally or alternatively, when a heating device is switched on, another heating device can be switched off at the same time. It is advantageously provided that a heating device is not switched on more than once and not switched off more than once within a switching cycle.

In a further embodiment of the invention, in at least three zones, the switch-on and switch-off times of the associated heating devices can be determined not only as described above but also in such a way that at no time are all heating devices are switched on. It is advantageous if no more than two heating devices are switched on at any one time, so that the total power supply is limited.

In one embodiment of the invention, a switching cycle can last at least 5 seconds or at least 8 seconds, in particular it can last a maximum of 40 seconds. Preferably, a switching cycle can last longer the more heating devices are switched on and off during this switching cycle. The duration of a switching cycle can be permanently adapted by the control to the number of heating devices that are not permanently switched on or off.

A switching cycle when operating a single heater can be 5 sec to 15 sec. When operating two heaters it can be 15 sec to 25 sec. When operating three heaters it can be 25 sec to 35 sec.

In a further method according to the invention for operating a previously described oven, the steps explained below are carried out. In a step A, the temperature of at least one heating device is determined, preferably several heating devices or all heating devices. This can be done theoretically, so to speak, by recording the duration and power of operation of all heating devices in the oven, with which the heating device is continuously operated on average or which is a nominal power. Based on this, the temperature can be calculated using stored values, which are advantageously stored in a table or in a characteristic curve. So if the energy used over time is known, so to speak, its temperature can be determined.

In a next step B, an air temperature that is relevant for the heat transfer by convection to a food item can be measured in the muffle. This is advantageously done in at least one zone, preferably in all zones of the muffle. For this purpose, at least one discrete temperature sensor is advantageously used in the muffle, which is arranged in the muffle in addition to the heating devices and whose measured temperature is also taken into account. Preferably, several discrete temperature sensors can be used in the muffle, which can increase the accuracy. One temperature sensor can be assigned to each zone. There can be more than one temperature sensor, but fewer temperature sensors than zones, alternatively more temperature sensors than zones. A temperature curve in the muffle can be calculated using the temperature sensors, and from this an assignment of the temperature curve to the zones can be determined. The furnace control thus has the information on the air temperature in each zone. Preferably, the air temperature in the muffle can be measured at exactly two points.

In a next step C, a temperature of the food to be cooked can be calculated on the outside in a zone based on a radiant heat flow emanating directly from the heating device and based on a convective heat flow emanating from the heating device via the air. This temperature can also be seen as the temperature that affects the food to be cooked at this point or in this area. Based on the temperature of a heating device determined in step A, the influence of the radiant heat flow can be determined using stored values. Based on the air temperature measured in step B, the influence of the convective heat flow can be determined. This can be determined in each case by the oven control.

In a further step D, the output of at least one heating device can be determined and adjusted if necessary, preferably of all heating devices in operation. This can be done based on a comparison between the specified temperature and the temperature calculated in the aforementioned step C, in order to bring the calculated temperature closer to the specified temperature by adjusting the output of at least one of the heating devices or of all heating devices. This can preferably be done separately for each zone.

With this variant of the method according to the invention, it is possible to carry out temperature-controlled cooking of food in each zone. This means that a cooking process can be controlled and carried out well.

In an embodiment of the invention, the power of the at least one heating device can be adjusted using PWM with a switching cycle and switching cycle sections or with a method described above. The switching cycle or switching cycle sections can also be adjusted.

In an alternative embodiment of the invention, a power of the at least one heating device can be set or adjusted by a phase control or a phase cut control or by a pulse packet control.

Advantageously, in step A, the temperature of a heating device can be calculated using the following formula: $$\mathrm{T}\left(\mathrm{t}\right)=\mathrm{T}\left(\mathrm{t}-1\mathrm{s}\right)+\mathrm{DF}\left(\mathrm{P}\right)\ast \left[{\mathrm{T}}_{\mathrm{steady}}\left(\mathrm{P}\right)-\mathrm{T}\left(\mathrm{t}-1\mathrm{s}\right)\right]$$

Where T(t) is the temperature of the heater at time t, preferably on its outside, T (t-1sec) is the temperature of the heater 1 sec before, DF(P) is a delay factor, which depends on the current power of the heating device, and T _steady (P) is the temperature in continuous operation or in the steady state or constant state of the heating device, which depends on the current power of the heating device.

Advantageously, in step C, the temperature for each food item in each zone can be calculated using the following formula: $$T_{\mathit{food}}\left(t\right)=a\ast \left(T_{\mathit{THE}}^4-T_{\mathit{food}}^4\left(t-1s\right)\right)+T_{\mathit{air}}$$

Where T _food (t) is the temperature of the food on its outside or the temperature that acts on the food and influences it, T _THE is the temperature of the heating device, T _food (t-1 sec) is the aforementioned temperature of the food 1 sec previously, and T _air is the air temperature previously determined in accordance with step B, and a is an oven-specific factor that has either been determined empirically through tests or that has been determined by simulation, in particular determined and programmed by the manufacturer.

The temperature T _air can advantageously be calculated by measuring the temperature T _sensor of an aforementioned temperature sensor. This temperature T _sensor can be corrected or applied with an offset temperature in order to determine the temperature at the desired location from the location at which the temperature sensor is arranged. This is a method known per se.

Advantageously, in step D, the power can be calculated using the following formula: $$\mathrm{P}=\mathrm{Kp}*\left(\mathrm{T}\_\mathrm{desired}-\mathrm{T}\_\mathrm{actual}\right)$$

Where K _p is the proportional gain, P is the set average power of the heating device, T _desired is the specified temperature for the food in this zone, and T _actual is the calculated temperature of the food in this zone according to step D.

In an advantageous embodiment of the invention, instead of a pure proportional controller, a controller with an I component and/or a D component can be used, preferably as a PID controller. This ensures overshoot-free adjustment.

In a further method according to the invention for operating a previously described oven, a fan is used for the muffle to regulate the temperature in at least one of the zones by switching on the fan or by increasing the fan's operation. This is advantageous if this zone is exposed to more air from the fan than the other zones or if it is currently being exposed to a comparatively higher flow of radiant heat. Alternatively, this can be done to raise the temperature in at least one of the zones by increasing the fan's operation or by switching on the fan if this is deemed desirable for the cooking process set on the oven. This is advantageous if this zone is exposed to more air from the fan than the other zones or if it is currently being exposed to a comparatively lower flow of radiant heat.

A zone with strong radiant heating can be cooled advantageously by switching on the fan or increasing its operation. Then the fan does not have to blow highly heated air onto the food being cooked, but rather just air.

A zone with little or no radiant heating can be heated by turning on or increasing the fan's operation, as heat is blown into it from another zone. Alternatively, the fan can be equipped with its own heater and thus blow hot or highly heated air into the zone, thus heating it up.

In an advantageous embodiment of the invention, a switching cycle for operating the heating devices with a PWM can be divided into switching cycle sections of equal length. These can be 7 to 15 switching cycle sections, in particular 9 switching cycle sections.

In the embodiment of the invention, in the case of operation of exactly two heating devices whose nominal power is the same and for which the added desired power is less than the nominal power of a single heating device, initially only a first heating device can be operated for so many switching cycle sections that it no longer needs to be operated for the remaining switching cycle sections of the entire switching cycle. Then, at the same time as this first heating device is switched off, the other second heating device is switched on and operated for so many switching cycle sections that it no longer needs to be operated for the remaining switching cycle sections of the entire switching cycle. Then the switching cycle starts again from the beginning.

In an advantageous embodiment of the invention, in the case of operation of exactly two heating devices whose nominal power is the same and whose desired power added together is higher than the nominal power of a single heating device, only a first heating device can be operated for so many switching cycle sections that it is sufficient for the remaining switching cycle sections of the entire switching cycle. In this case, before this first heating device is switched off, the other second heating device is switched on, and it can in particular be switched on at such a time that when it is switched off after the number of switching cycle sections has been reached, the first heating device is switched on at the same time. Before the first heating device is switched off, the other second heating device can preferably be switched on. This can take place at such a time that when it is switched off after the number of switching cycle sections has been reached, the first heating device is not switched off at the same time as the second heating device.

In a further development of the invention, in the case of operation of exactly three heating devices, each of which has the same nominal power, at least one first heating device can be switched off when another second heating device is switched on at the same time. They are thus switched on alternately, so to speak. A third heating device is in turn switched on at exactly the time when the second heating device is switched off. Preferably, two or more heating devices cannot be switched on or switched off at the same time at any time. In this way, flicker regulations due to the network feedback can be complied with.

In an advantageous embodiment of the invention, for the operation of a previously described oven or as a further development of a previously described method in the different zones, it can be provided that different items are cooked in the muffle. In this case, different average powers on the heating devices can be controlled by the oven control at different times: At the start of a preparation process, a food carrier with the various items on it can be introduced into the muffle through the access opening once or as a single process. Advantageously, no items are subsequently introduced. At the end of the preparation process, the food carrier with the cooked items on it can be removed, so that preferably all items are cooked with the same operating time of the oven but at different cooking temperatures.

In another method for operating an oven described above, or as a further development of a method described above, different foods can be cooked in the different zones. For this purpose, different average temperatures in the zones can be set by the oven control at different times. Here, too, at the start of a preparation process, a food carrier with the various foods on it can be inserted through the access opening into the muffle, i.e. with all the foods that are to be prepared. After the preparation process has ended, the food carrier with the cooked foods on it can be removed. In this case, all Food can be cooked with the same oven operating time but at different cooking temperatures, or food can be cooked at different temperatures for different periods of time, whereby the total time from the different temperature sections is the same for all zones. This makes the cooking process very simple for the user.

In order to operate an oven as described above, it can be provided that the temperature in the zone associated with the extractor fan or an extractor flap is lowered, i.e. reduced, by means of the at least one aforementioned extractor fan. At the same time, the air humidity is reduced by increased suction or removal of air from this zone. Alternatively, the temperature in the zone associated with this extractor fan or an extractor flap can be increased by means of the at least one extractor fan. At the same time, the air humidity can be increased by reduced suction or removal of air from this zone, whereby this is preferably done by stopping the operation of the extractor fan or closing the extractor flap.

In an embodiment of the invention, the user can select a dish based on the combination of several items to be cooked on the oven control or an external input device. This allows the oven control to prepare and then carry out the operation of the various heating devices. It is possible for the process data for this dish to be loaded into the oven control from a cloud or from the Internet, so it does not have to be stored in the oven control. For this purpose, the oven control has a connection to the Internet, which is easy to implement.

In a further method for operating a previously described oven or as a further development of a previously described method, the user can manually set a temperature separately for each of the zones, i.e. without a previously described method for a cooking process. This eliminates the need for an automated process. This set temperature can be regulated by the oven control as described above. Monitoring a temperature for regulation can be done in one of the ways mentioned above. In a possible further development of the invention, only one of the zones can be operated in order to cook a small amount of food in the most energy-efficient way possible. This is possible, for example, for defrosting a single bread roll or the like.

In an advantageous embodiment of the invention, the food carrier can be placed close to the heating elements, preferably closer than 15cm or even a maximum of 10cm away. This allows the volume to be heated to be significantly reduced, which enables faster and energy-efficient cooking and preparation. The short distance between the food support and the heating elements also means that the intensity of the radiant heat is higher.

At least one heating element can have a surface load of at least 5.0W/cm ² . This is a relatively high surface load. The surface load can be particularly advantageous at up to 8.0W/cm ² , so that a very high proportion of heat radiation can be generated, which can be generated in a very well directed manner.

These and other features emerge not only from the claims but also from the description and the drawings, whereby the individual features can be implemented individually or in combination in the form of sub-combinations in an embodiment of the invention and in other fields and can represent advantageous and protectable embodiments for which protection is claimed here. The division of the application into individual sections and subheadings does not limit the general validity of the statements made under these sections.

Short description of the drawings

Fig. 1

eine Schrägansicht von vorne auf einen erfindungsgemäßen Backofen mit geöffneter Tür zur Muffel,

Fig. 2

eine Ansicht von vorne und von leicht unten in die Muffel hinein, an deren Deckenwandung drei Heizeinrichtungen mit Rohrheizkörpern angeordnet sind,

Fig. 3

eine schematische Ansicht in den Backofen entsprechend Fig. 2 von vorne mit eingebrachtem Backblech mit Unterteilung in drei Zonen unterhalb jeder Heizeinrichtung, in der jeweils ein Gargut angeordnet ist,

Fig. 4

eine Ansicht von unten auf die Deckenwandung der Muffel mit den drei Heizeinrichtungen,

Fig. 5

ein Bedienfeld an dem Backofen, um Temperaturen manuell einstellen zu können,

Fig. 6

eine Darstellung von verschiedenen Verläufen der Temperatur über der Zeit abhängig von der Heizleistung an der Oberfläche einer Heizeinrichtung,

Fig. 7

der Verlauf der auf Dauer erreichten Temperatur links bzw. eines Verzögerungsfaktors rechts abhängig von der dauerhaften Leistung der Heizeinrichtung,

Fig. 8 bis 10

Schaltzyklen mit verschiedenen Schaltzyklus-Abschnitten für zwei oder für drei Rohrheizkörper.

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

Fig.1

an oblique view from the front of an oven according to the invention with the door to the muffle open,

Fig.2

a view from the front and slightly below into the muffle, on the ceiling wall of which three heating devices with tubular heaters are arranged,

Fig.3

a schematic view into the oven according to Fig.2 from the front with baking tray inserted and divided into three zones below each heating device, in each of which a food to be cooked is arranged,

Fig.4

a view from below of the top wall of the muffle with the three heating devices,

Fig.5

a control panel on the oven to manually set temperatures,

Fig.6

a representation of different temperature curves over time depending on the heating power on the surface of a heating device,

Fig.7

the course of the temperature reached over time on the left or a delay factor on the right depending on the permanent output of the heating device,

Fig. 8 to 10

Switching cycles with different switching cycle sections for two or three tubular heaters.

Detailed description of the implementation examples

In the Fig.1 an oven 11 is shown with a housing 12 and a muffle 14 therein. The muffle 14 can be closed as usual by means of an oven door 15. Above the oven door 15 there is an operating device 17, advantageously with a display, which has an internal oven control 19. On a rear wall of the muffle 14 there is a fan 21, which is designed as usual for a circulating air and hot air oven. As a hot air oven it also has a fan heater (not shown here) in order to be able to blow heated air into the muffle 14. This has been explained previously. In front of the fan 21 there is a conventional fan grille 22, here it is round. Several slide-in rails 24 are formed on the side walls of the muffle 14. They can either be produced by appropriate embossing of the metal wall, alternatively they can be separate rails that can be attached by means of screws or hooks.

The front view of the Fig.2 shows how the slide-in rails 24 are arranged horizontally and parallel to one another on the two side walls of the muffle 14 in a conventional manner. The fan 21 with the fan grille 22 is arranged on the rear wall, advantageously at about medium height.

In a manner known per se, the heating devices are arranged just below the top wall of the muffle 14, namely three separate tubular heating elements 26a, 26b and 26c. In the view from below, they are Fig.4 shown in more detail. The tubular heating elements 26a to 26c are formed by tubular heating elements of the same thickness and each run in a double loop. The middle tubular heating element 26b is slightly shorter in terms of overall length and can therefore have a lower electrical output, but this is not mandatory. The tubular heating elements 26a to 26c also all have the same surface output or the same heating output per unit length. As can be seen from the Fig.4 As can also be seen, they are attached together to a standard holding plate 28. This in turn can be attached to the back of the muffle 14 using screws. From the electrical connections protruding from the back, it can be seen that all three tubular heating elements 26a to 26c are electrically contacted separately and can therefore be controlled or supplied with power separately from one another. They are advantageously operated in a cyclical manner, i.e. either switched on or off at full power. This is explained in more detail below.

Dashed lines indicate Fig.3 two zone boundaries ZGab and ZGbc are shown, each of which runs centrally between the tubular heating elements 26a to 26c. They run at least centrally between the largest part of their longitudinal extension, where they are parallel to each other. Their exact However, position depends not only on the geometry and arrangement of the tubular heaters 26a to 26c, but also, or perhaps predominantly, on their heating output. The three zones Za, Zb and Zc formed by the zone boundaries ZGab and ZGbc can be of equal width here. However, if the tubular heaters 26a to 26c have significantly different nominal outputs, they can also be of different widths, or a zone in which a tubular heater with a higher nominal output is arranged will be wider than a zone in which a tubular heater with a lower nominal output is arranged. The zone boundaries ZG and thus also the zones Za, Zb and Zc run evenly and parallel to one another in a direction perpendicular away from the ceiling wall of the muffle 14. The individual zones are directly adjacent to one another, but do not actually overlap, although this is partly a theoretical consideration. In practice, it is not always possible to avoid a tubular heater from one zone giving off or radiating heat into another zone. In the Fig.3 The heat radiation is illustrated by the straight arrows pointing downwards, where the food to be cooked is usually placed. The heat generation through convection, i.e. through the heated air, is shown by the wavy arrows.

In the Fig.2 it can also be seen how a simply shown temperature sensor 30 is arranged in the upper area on the left side wall of the muffle 14. It can have an elongated tube shape, it is advantageously an electrical resistance sensor, for example as an NTC or PT100 or PT1000. It can have a shielding plate 31 which forms a shield against direct radiant heat from the tubular heaters 26a to 26c without having direct contact with it. This temperature sensor 30 is, as explained below, primarily intended to record exclusively the air temperature in the muffle, specifically at the location where it is arranged. From this, as already explained at the beginning, a temperature distribution in the entire muffle 14 can then be calculated on the basis of known values from the furnace control 19 and with a known duration and level of heating operation of the tubular heaters.

An alternative arrangement for a temperature sensor 30' is shown in the Fig.4 shown in dashed lines. This temperature sensor 30' can also protrude from the holding plate 28 and run between the tubular heating elements 26b and 26c. At this point too, the aforementioned shielding plate would be very advantageous.

In the Fig.3 a schematic front view of the muffle 14 is shown, similar Fig.2 . Above the left tubular heating element 26a, for example, a curved reflector 29 is arranged, preferably made of polished stainless steel. This ensures in a known manner that the upwardly directed radiation of the tubular heating element 26a is reflected downwards. Furthermore, by appropriately shaping such reflectors 29 Above all tubular heaters 26a to 26c, an even more precise division of the zones Za, Zb and Zc can be made, which prevents a tubular heater in one zone from radiating into an adjacent zone. This can be avoided.

Also from the Fig.3 It is easy to see, as explained previously, that the zone boundaries ZGab and ZGbc as well as the corresponding zones Za, Zb and Zc run vertically and parallel to one another. The zones do not overlap, but are directly adjacent to one another. As an example, a baking tray 34 placed on slide-in rails 24 is shown here, which is located slightly below half the height. To the left and in the area in zone Za, a product G1 is arranged on it. In the middle of the baking tray 34 in zone Zb, a product G2 is arranged. To the right on the baking tray 34 in zone Zc and below the tubular heating element 26c, a product G3 is arranged. The respective products G1 to G3 are therefore exposed to the convection or air temperature in the respective zone Z as well as to radiant heat, which comes primarily from the tubular heating element 26 arranged above them in their zone.

Due to the side-central arrangement of the fan 21, it is clearly visible that the middle food G2 is mainly in its ventilation direction. This can be used advantageously for a special cooking process. Depending on the presence and, if applicable, operation of a fan heater assigned to the fan 21, even more heat can be brought to the middle zone Zb on the food G2. Alternatively, this middle zone Zb together with the food G2 can be cooled slightly without heating by just operating the fan 21 if this is determined to be appropriate, particularly during the course of the cooking process.

According to the presentation of the Fig.4 The central tubular heating element 26b can be designed to have a nominal power that is 10% to 25% or even 40% lower than that of the other two tubular heating elements 26a and 26c. This could result in a cooking process similar to Fig.3 provide that the lowest temperature should be generated here or the lowest heat flow into the corresponding food G2. This also corresponds to the entered temperatures according to Fig.5 , since the lowest temperature is selected for the middle zone Zb. However, this does not have to be the case; a tubular heater on the side could also have a lower output. Likewise, the middle tubular heater 26b could have a higher output.

In the Fig.5 It is shown how temperatures can be manually specified directly on the control device 17 for the three zones Za, Zb and Zc. These then correspond to a specific operating mode, for example "grill", "baking" or "roasting". Alternatively, a user can select a specific dish with different items to be cooked on the control device 17, whereupon the control device 17 can then display for the user the placement of the various items to be cooked on the left, in the middle or on the right.

From the Fig.5 It can also be seen that the oven control 19 is not only connected to the operating device 17, but also has a memory 20. This is intended for the characteristic curves or tables described at the beginning and explained in more detail below. On the other hand, certain cooking processes for items or dishes can be stored in it and called up as required.

In itself, it is easy to imagine from the above description how the surface temperature of one of the tubular heating elements 26a to 26c is first determined according to the above-mentioned step A. The Fig.6 shows as a guideline the temperature profile of a tubular heater 26 over time t, depending on the average, i.e. the permanent or long-term, power P. The profiles of the Fig.6 obviously show a tubular heater with a nominal power of 1,500 W, since its temperature rises evenly with a flattening out during operation. A maximum temperature reached is around 850 °C after a few minutes, namely when the tubular heater 26 is operated continuously at its full nominal power. This is shown with the solid curve. For the other average powers that are lower than the nominal power, the tubular heater is operated in a clocked manner, advantageously with pulse width modulation, which is explained in more detail below. The lower the average power, the longer it takes for the temperature to rise and the lower the maximum temperature reached over a long period of time. As illustrated by the arrows in the range between 30 sec and 100 sec, a delay factor for a certain power can be determined from this area of the curve. This delay factor is in the Fig.7 applied again. To the right the permanent temperature is reached, also depending on the level of the average power.

In the diagram of the Fig.7 On the left, the continuous temperature T is plotted against the average power P, i.e. on the left vertical axis. On the right vertical axis, the delay factor is plotted against the average power P. While the temperature T is the upper curve, the delay factor is the lower curve. The lower curve is also more of an approximate straight line.

From the diagram of the Fig.7 or a corresponding table, the oven control 19 can determine the temperature that a tubular heating element 26 has itself, at least in continuous operation, during operation of the oven 11, i.e. in particular with a certain predetermined cooking process or a so-called recipe, according to the first step A. The continuous operation is here referred to times of more than three minutes, as a rule even at low average power a constant continuous temperature is reached after five to seven minutes at the latest, see Fig.6 The temperature of the tubular heating element 26 is important in order to determine its radiation output, i.e. the amount of radiant heat that it emits to the food G arranged underneath. The formula for step A explained at the beginning can advantageously be used for this purpose. $$\mathrm{T}\left(\mathrm{t}\right)=\mathrm{T}\left(\mathrm{t}-1\mathrm{s}\right)+\mathrm{DF}\left(\mathrm{P}\right)\ast \left[{\mathrm{T}}_{\mathrm{steady}}\left(\mathrm{P}\right)-\mathrm{T}\left(\mathrm{t}-1\mathrm{s}\right)\right]$$

Here, T(t) is the temperature of the heater at time t, T (t-1sec) is the temperature of the heater 1 sec previously, and DF(P) is the delay factor described above, which depends on the current power of the heater. T _steady (P) is the temperature in continuous operation of the heater, which depends on the current power of the heater.

Next, an air temperature is measured in the muffle according to step B. In general, the air temperature is important because heat is also transferred to the food G by convection. Even if it is generally desired to be able to determine or know the air temperature everywhere in the muffle 14, it may be sufficient to measure the actual air temperature at just one point or in just one zone. This can be done with the temperature sensor 30, which according to Fig.3 in the zone Za is located on the very left. Even if in the Fig.3 If only a single temperature sensor 30 is shown, this may be sufficient. Based on stored values, the air temperature can be determined at any point for the entire muffle 14, especially if the previous operation or the previous average output of the tubular heaters 26 is included. The determination is of course more precise if a separate discrete temperature sensor is provided for each zone Z or if several discrete temperature sensors are provided in general. This naturally means increased effort.

According to the next step C, a radiation heat flow and a convection heat flow for the food to be cooked can be determined from the temperatures determined in steps A and B, in each zone. The temperature of the tubular heating elements 26 is used to determine the extent of the influence of the radiation heat flow on the basis of stored values. The influence of the convection heat flow is also determined on the basis of the air temperature measured in step B in the muffle 14 or in a zone Z. This is advantageously done using the oven control. The oven control 19 then knows which total heat flow reaches the food to be cooked. The formula for step C explained at the beginning can be used advantageously for this.

To determine the air temperature, a certain offset can be added or subtracted from the temperature measured by the temperature sensor 30 in a simple manner. This depends on how the respective locations for the temperature sensor 30 and for the location at which the temperature is to be determined are selected. Such an offset can be determined experimentally for the oven 11 if the location for the temperature sensor 30 is known and can be stored in the oven control 19.

Dabei ist T_food (t) die Temperatur des Garguts an dessen Außenseite bzw. die Temperatur, die auf das Gargut einwirkt, T_THE ist die Temperatur der Heizeinrichtung, T_food (t-1sec) ist die Temperatur des Garguts 1 sec zuvor, T_air ist die Lufttemperatur gemäß Schritt B, und a ist ein backofenspezifischer Faktor, der empirisch durch Versuche ermittelt worden ist oder der durch Simulation ermittelt worden ist, vorteilhaft herstellerseitig. Diese Temperatur T_food (t) kann also vor Beginn des Garprozesses an bestimmt werden, wobei sie dabei zuerst bei Raumtemperatur liegt oder angenommen wird, solange keine anderen Informationen vorliegen oder in die Ofensteuerung 19 eingegeben werden.Then, according to step C, the temperature for each food item G in each zone Z is calculated using the formula from step C. $$T_{\mathit{food}}\left(t\right)=a\ast \left(T_{\mathit{THE}}^4-T_{\mathit{food}}^4\left(t-1s\right)\right)+T_{\mathit{air}}$$

Where T _food (t) is the temperature of the food on its outside or the temperature acting on the food, T _THE is the temperature of the heating device, T _food (t-1sec) is the temperature of the food 1 second previously, T _air is the air temperature according to step B, and a is an oven-specific factor which has been determined empirically through tests or which has been determined through simulation, advantageously by the manufacturer. This temperature T _food (t) can therefore be determined before the start of the cooking process, whereby it is initially at room temperature or is assumed as long as no other information is available or entered into the oven control 19.

Subsequently, according to step D, the power of the individual tubular heating elements 26 can be determined and adjusted if necessary in order to determine a desired heat flow into the food to be cooked for each food item G according to the specified cooking process. The power is calculated or the average power for the respective tubular heating element 26 is determined using the formula: $$\mathrm{P}=\mathrm{Kp}*\left(\mathrm{T}\_\mathrm{desired}-\mathrm{T}\_\mathrm{actual}\right)$$

Where K _p is a proportional gain, P is the set average power of the heating device, T _{desired is} the specified temperature for the food to be cooked in this zone, and T _actual is the temperature of the food to be cooked in this zone calculated according to step C, i.e. T _food (t).

To regulate the temperature, it is best not to use a pure proportional controller, but rather a controller with an additional I component and an additional D component. Such a PID controller has the advantage that it can be regulated to a desired value without overshooting. Such an overshoot of the temperature could be disadvantageous, for example because a good could be heated too much or burned on the surface.

As explained at the beginning, the tubular heaters 26 are to be operated intermittently, i.e. not with a certain set power in continuous operation, but either switched off or switched on with full nominal power. For the operation of two tubular heaters with the same nominal power, Fig.8 A power distribution or switching cycles are shown divided into ten cycles, with a first tubular heater 26a having an average power of 400 W and a second tubular heater 26b having an average power of 600 W. It is shown that first the tubular heater 26a is operated for four cycles or switching cycle sections and then is switched off. The second tubular heater 26b is switched on from the second switching cycle section and then continues to operate for a total of six switching cycle sections. The switched-on states of the two tubular heaters 26a and 26b thus overlap one another for a certain time, while at the same time there is no heating effect at all for a certain time. This switching cycle could therefore also be modified so that when the first tubular heater 26a is switched off or after the last switching cycle section, the first switching cycle section takes place with the second tubular heater 26b being switched on. This would mean that the tubular heaters would be switched on in turn. Alternatively, a switching operation could be carried out here, in which case one tubular heating element 26 is always switched on, but only one.

In Fig.9 a switching cycle for three tubular heaters 26a to 26c is shown. First, the third tubular heater 26c is switched on for six switching cycle sections. One switching cycle later, the first tubular heater 26a is switched on. It remains switched on for three switching cycle sections. When it is switched off, the second tubular heater 26b is switched on, so to speak, replacing it, and also remains switched on for six switching cycle sections. Thus, the tubular heaters 26b and 26c each have an average power of 600 W and the tubular heater 26a has 300 W.

In Fig.10 shows how a switching cycle is divided into nine switching cycle sections. A first tubular heating element 26a is operated at a nominal output of 1000 W with an average, i.e. continuous, output of 667 W, as are the other two. The switching cycle sections are distributed for the individual tubular heating elements 26 that are switched on and off as alternately as possible. Furthermore, at no time are three tubular heating elements switched on at the same time, which reduces the total power requirement at the peak and at the same time evens it out somewhat.

Claims (23)

Oven with a muffle, an access opening to the muffle, a door for closing the access opening and with at least two heating devices in the muffle, wherein: - the heating devices are arranged electrically separated next to each other and close together on a wall of the muffle, the wall extending essentially in one surface, preferably forming the top wall of the muffle, - the at least two heating devices are designed as tubular heaters with a resistance heating conductor, - the muffle is divided into zones arranged next to each other, with at least as many heating devices as there are zones and a boundary between two directly adjacent zones running midway between the projection of two directly adjacent heating devices, - none of the adjacent zones overlap each other or the zones are separated from each other by radiation, but are directly adjacent to each other, - each heating device can be controlled separately, - the baking oven has a baking oven control and a power supply for each of the heating devices, wherein the baking oven control is designed to determine or calculate a temperature of at least one heating device and/or in the muffle. Baking oven according to claim 1, characterized in that a reflector is arranged above at least one tubular heating element, in particular on the nearby wall of the muffle, wherein preferably a reflector is arranged above each tubular heating element, in particular a reflector is curved. Baking oven according to claim 1 or 2, characterized in that at least one fan is provided for the muffle, preferably a fan with associated fan heating, wherein in particular a single fan is provided and is arranged in a single zone or the fan generates an air flow which flows more than 50% into one of the zones. Baking oven according to one of the preceding claims, characterized in that at least one temperature determination means for a temperature in the muffle is provided, which has at least one temperature sensor, preferably for Temperature measurement by means of a temperature-variable electrical resistance, wherein in particular the temperature sensor is designed as a discrete temperature sensor separate and apart from the resistance heating conductors of the tubular heaters, wherein preferably the temperature sensor is shielded from a direct effect of a radiant heat flow through a tubular heater by means of a shield. Baking oven according to claim 4, characterized in that the temperature sensor is formed by the heating device or by a resistance heating conductor of a tubular heating element which has a temperature-dependent electrical resistance. Baking oven according to one of the preceding claims, characterized in that the tubular heating elements are designed with a loop-shaped or meander-shaped course, wherein preferably each tubular heating element has at least two longitudinal sections which are designed in a loop-shaped manner next to one another and adjoining one another. Baking oven according to one of the preceding claims, characterized in that at least one exhaust fan is provided for an air flow from the muffle out to the outside environment, wherein preferably an exhaust air duct is provided from the muffle to the exhaust fan and from there to the outside environment. Baking oven according to claim 7, characterized in that a single exhaust fan is provided for the entire muffle or the entire baking oven, wherein the exhaust air duct has an exhaust air branch for each zone, preferably departing from the wall to which the heating device is arranged closest, and wherein a separate exhaust flap to the exhaust air branch is provided for each zone, wherein each exhaust flap can be operated separately for opening and closing. Baking oven according to claim 7, characterized in that an independent separate extractor fan with an extractor air duct is provided for each heating device or for each zone formed by each heating device, wherein preferably a separate extractor air branch or a separate extractor flap for the extractor air branch is provided for each zone. Method for operating an oven according to one of the preceding claims, characterized in that the heating devices are switched on and off in such a way that at no time are two or more heating devices switched on simultaneously or two or more heating devices switched off simultaneously Preferably, these are the heating devices which are arranged next to one another and close to the common wall of the muffle. Method according to claim 10, characterized in that certain switching cycles are carried out for at least two heating devices or for exactly three heating devices, wherein when one heating device is switched off, another heating device is switched on at the same time and/or when one heating device is switched on, another heating device is switched off at the same time, wherein preferably a heating device is switched on no more than once and no more than once within a switching cycle. Method according to claim 11, characterized in that, in at least three zones, the switch-on times and switch-off times of the associated heating devices are determined in such a way that at no time are all the heating devices switched on, wherein preferably at no time are more than two heating devices switched on. Method according to claim 11 or 12, characterized in that a switching cycle lasts at least 5 seconds, in particular lasts a maximum of 40 seconds, wherein preferably a switching cycle lasts longer the more heating devices are switched on and off during this switching cycle, wherein preferably the duration of a switching cycle is permanently adapted by the control to the number of heating devices that are not permanently switched on or off. Method for operating an oven according to one of claims 1 to 9, characterized by the steps: A determining the temperature of at least one heating device, preferably several heating devices or all heating devices, in particular by recording the duration and power of operation of all heating devices of the oven, with which power the heating device is continuously operated on average, whereby the temperature is calculated on the basis of stored values, B measuring an air temperature in the muffle, using at least one discrete temperature sensor in the muffle, which is arranged in addition to the heating devices in the muffle and whose measured temperature is additionally taken into account, preferably using several discrete temperature sensors in the muffle, one temperature sensor in each zone is assigned or there is more than one temperature sensor but fewer temperature sensors than zones, whereby a temperature profile in the muffle is calculated using the temperature sensors and from this an assignment of the temperature profile to the zones is determined, C Calculating a temperature of the food to be cooked on its outside or a temperature that acts on the food in a zone based on a radiant heat flow emanating directly from the heating device and based on a convective heat flow emanating from the heating device via the air, wherein the influence of the radiant heat flow is determined based on the temperature of a heating device determined in step A using stored values, wherein the influence of the convective heat flow is determined based on the air temperature measured in step B, preferably determined by the oven control, D Determining and, if necessary, adjusting the power of at least one heating device, preferably of all heating devices operated, based on a comparison between the predetermined temperature and the temperature calculated in step C, in order to bring the calculated temperature closer to the predetermined temperature, this preferably being carried out separately for each zone. Method according to claim 14, characterized by adjusting the power of the at least one heating device by means of PWM with a switching cycle and switching cycle sections or with a method according to one of claims 10 to 13. Method for operating an oven according to one of claims 1 to 9, characterized in that a fan is used for the muffle in order to reduce the temperature in at least one of the zones which is subjected to a greater amount of air by the fan than the other zones or which is currently subjected to a comparatively higher radiant heat flow by switching on the fan or by increasing the operation of the fan, or to increase the temperature in at least one of the zones by increasing the operation of the fan or by switching on the fan, if this is to be regarded as desired for the cooking process set on the oven. Method according to claim 15 or 16, characterized in that in the case of operation of exactly two heating devices whose nominal power is the same and whose desired power added together is less than the nominal power of a single heating device, initially only a first heating device is operated for as many switching cycle sections, that it no longer has to be operated for the remaining switching cycle sections of the entire switching cycle, and then simultaneously with the switching off of this first heating device, the other second heating device is switched on and is operated for so many switching cycle sections that it no longer has to be operated for the remaining switching cycle sections of the entire switching cycle. Method according to one of claims 15 to 17, characterized in that in the case of operation of exactly two heating devices whose nominal power is the same and whose desired power added together is higher than the nominal power of a single heating device, initially only a first heating device is operated for so many switching cycle sections that it no longer has to be operated for the remaining switching cycle sections of the entire switching cycle, wherein before this first heating device is switched off, the other second heating device is switched on, in particular is switched on at such a time that when it is switched off after the number of switching cycle sections has been reached, the first heating device is switched on at the same time, wherein preferably before the first heating device is switched off, the other second heating device is switched on, in particular is switched on at such a time that when it is switched off after a predetermined number of switching cycle sections has been reached, the first heating device is not switched off at the same time as the second heating. Method according to one of claims 15 to 18, characterized in that in the case of operation of exactly three heating devices whose nominal power is the same, at least one first heating device is switched off when another second heating device is switched on at the same time, and a third heating device is switched on when the second heating device is switched off at the same time, wherein preferably at no time are two heating devices or more switched on at the same time or switched off at the same time. Method for operating an oven according to one of claims 1 to 9 or method according to one of claims 10 to 19, characterized in that different products are cooked in the different zones, whereby different average powers on the heating devices are controlled by the oven control at different times, so that at the beginning of a cooking process a cooking carrier with the various cooking products on it is introduced into the muffle through the access opening and after the end of the cooking process the cooking carrier with the cooked food is removed from it, whereby preferably all the food is cooked for the same time but at different cooking temperatures. Method for operating an oven according to one of claims 1 to 9 or method according to one of claims 10 to 20, characterized in that different items are cooked in the different zones, whereby different average temperatures in the zones are set at different times by the oven control so that at the start of a preparation process a food carrier with the different items on it is introduced into the muffle through the access opening and after the end of the preparation process the food carrier with the fully cooked items on it is removed, whereby preferably all items are cooked for the same time but at different cooking temperatures. Method for operating an oven according to one of claims 7 to 9, characterized in that by means of the at least one extractor fan, the temperature in the zone assigned to the extractor fan or an extractor flap is lowered and at the same time the air humidity is reduced by increased suction or discharge of air from this zone, or that by means of the at least one extractor fan, the temperature in the zone assigned to the extractor fan or an extractor flap is increased and at the same time the air humidity is increased by reduced suction or discharge of air from this zone, preferably by stopping the operation of the extractor fan or closing the extractor flap. Method for operating an oven according to one of claims 1 to 9 or method according to one of claims 10 to 19, characterized in that the user can manually, preferably without a cooking process as a method according to claim 20 or 21, set a temperature separately for each of the zones, which is regulated in particular by the method according to claim 14 by the oven control.

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