EP2860458B1 - Method for operating a cooking appliance in the form of a combi-steamer and cooking appliance for carrying out the said method - Google Patents

Description

The present invention relates to a method for operating a cooking appliance in the form of a combination steamer and such a cooking appliance for carrying out the method.

Modern combi-steamers have a cooking chamber, a heating device, at least one fan for circulating the cooking chamber atmosphere present in the cooking chamber, at least one temperature sensor, a control or regulating device and a device for introducing water with an adjustable volume flow, which is directed onto the fan and by means of finely divided into steam.

Combi steamers are convection ovens with an additional steam generation unit. As a rule, the device actuators, such as the heating device, the fans, the water feeding device, etc., are controlled or regulated with the aid of electronic controls. There are two different technologies and embodiments for the steam generating units or devices. A first embodiment consists of a steam generator arranged outside the cooking chamber. The steam is generated outside the cooking chamber and fed into the cooking chamber via a pipe. The amount of steam is regulated by means of suitable measuring equipment.

The other, second embodiment includes the so-called direct steam generation in the cooking chamber. In this case, liquid water is introduced into the cooking chamber via a line and then fed to the fan or its fan wheel in the cooking chamber. The rotation of the fan wheel breaks up the water droplets that hit it so much that they evaporate in the hot oven air.

When steam is generated directly in the cooking chamber, the amount of steam produced is lower than when steam is generated outside of the cooking chamber. As a result, there can be quality problems with the cooking medium at the start of a cooking process or cooking process.

In the embodiment of direct steam generation, a continuous volumetric flow of water must take place onto the fan wheel or fan wheel during the cooking process in the "cooking/steaming" operating mode. For devices intended for use in canteen kitchens, this volume flow is between 10 and 30 liters per hour, depending on the size and type of device. This volume flow value is a compromise between the desired cooking quality and energy consumption that is reduced as much as possible. A lower volume flow automatically means lower energy consumption, since the water volume flow fed in has to be heated to 100 °C, which corresponds to the usual temperature value in the cooking chamber in the "Cooking/Steaming" operating mode.

An additional energy requirement results from the conversion of the aggregate state of the water from the liquid state to the vapor state, due to the vaporization enthalpy of the water. If the water volume flow is too low, the steam content in the cooking atmosphere in the cooking chamber increases too slowly in relation to the rising temperature. This has the negative effect that sensitive items to be cooked, such as cauliflower, broccoli, etc., are damaged by drying out, which leads to an unsatisfactory cooking result.

If you increase the water flow rate to avoid the problem, a lot of energy has to be expended on heating and evaporating the water. Everyone knows how long it takes and thus how much energy is required to cook a large quantity of spaghetti in a 10 liter saucepan, for example, whereby the energy requirement for the enthalpy of vaporization is still not taken into account due to the "liquid" state of aggregation.

Although the energy expenditure at the beginning of the cooking process has to be accepted, the energy input is unnecessary in the further course of the cooking process, since the cooking chamber atmosphere is saturated and the cooking products have absorbed sufficient water vapor. Devices of this prior art waste energy because water is unnecessarily heated, which then runs unused through the device and is routed to the sewage system.

Although modern combi-steamers have humidity measurement devices to determine the humidity in the cooking chamber, they cannot be used in the "steaming" operating mode because measurement errors occur in this operating mode that are too great for this type of cooking. With the "Steaming" cooking method, the atmosphere in the cooking chamber must be 100% saturated with water vapor because even the smallest amounts of air that remain can result in a loss of quality in the cooked product. The existing measuring systems for humidity, which can be operated with high levels of soiling and temperatures of up to 300 °C and are still economical, cannot determine the humidity in the range of 98% to 100% saturated cooking chamber atmosphere with sufficient accuracy. For this reason, water is continuously added to the combi-steamers of the prior art in the "steaming" operating mode.

In order to overcome these disadvantages, the patent applicant above patent application has developed a water metering technology, which according to EP 1 687 570 B1 was protected. This technology makes it possible to reliably and easily change the volume flow independently of the line pressure. The patent applicant has used this to introduce a higher volume flow at the beginning of the cooking process and to reduce it again at a later point in time. The at the time of registration EP 1 687 570 B1 Existing prior art was not able to change a volumetric flow rate during the cooking process.

However, it has been found that the time-fixed switchover from a high volume flow to a lower volume flow and possibly again to an even lower volume flow has considerable advantages, but also has disadvantages. According to this prior art, the gradation or reduction over time can only be optimized for one product and, above all, only for one load state of the cooking appliance. In the case of other deviating loading states, the volumetric flow is changed either too early or too late. This leads to the result that if switching was done too early, there was a quality problem with the food, and if switching was too late, there was a corresponding waste of energy.

An obvious solution to the above problem such that the operator enters the load amount into the electronic device control is not practical. The risk of incorrect operation is too great, so the quality problem will not be solved.

Alternatively, it could be considered to integrate a weighing system for the food in the device. However, devices of this type are technically very complex and therefore correspondingly expensive, since temperatures of up to 300° C. prevail in the cooking chamber and the cooking chamber must be vapor-tight in relation to its surroundings. The sealing of the lines to the weighing system and its sensors is also correspondingly complex. In addition, such additional devices can break, not least because the devices are also handled roughly. Such robust devices are therefore correspondingly expensive.

the EP 2 468 101 A1 describes a method for operating a cooking appliance, a control or regulating device determining a period of time as a function of a cooking chamber temperature detected by means of a temperature sensor, after which a water volume flow is reduced.

The present invention is therefore based on the object of creating a method for operating a cooking appliance in the form of a combi-steamer, with which the above-mentioned disadvantages are overcome and both the quality requirements for cooking in the "cooking/steaming" operating mode and a corresponding reduction in the Energy expenditure for generating steam is achieved in the cooking chamber.

This object is solved by the features of claim 1.

The method according to the invention is characterized, among other things, by the fact that at the beginning of the cooking process in the "Steaming" operating mode, a predetermined water volume flow, which is dependent on the respective appliance type, is fed into the cooking chamber onto the fan wheel, and that the control or Control device determines a period of time after which the water volume flow is lowered or reduced.

This ensures that, on the one hand, there is sufficient water vapor in the cooking chamber at the start of the cooking process and the food to be cooked is prevented from drying out, and, on the other hand, when the desired saturated cooking chamber atmosphere is reached the water volume flow is lowered or reduced, so that a corresponding energy saving is realized.

When a cooking program starts, the temperature is usually lower than the target temperature. This is also the case when the cooking appliance is preheated. When the cooking chamber door is opened, the hot air in the cooking chamber suddenly escapes during the loading process and the cold food also lowers the temperature in the cooking chamber. Since the output of the heating device is known, it can be determined empirically through tests, but also theoretically, what degree of loading is present in the cooking chamber when the temperature rise is determined. More precisely, a heat capacity of the food to be cooked in the cooking chamber is determined, and since the food to be cooked usually consists mainly of water, this heat capacity corresponds to the load. A lower load means a lower heat capacity of the added food. The actual temperature rises to the setpoint more quickly than with a higher load. In conclusion, the heat capacity of the food to be cooked can be determined from the increase, and therefore, if the type of food to be cooked is known, the quantity of food to be cooked can be determined.

The present invention thus includes an embodiment in which it is specified in the device control or device software that the slope of the temperature increase is determined and certain slope values (the time derivative of the temperature dT/dt) correspond to a loading state. The following table is given as an example. Temporal temperature increase dT/dt Category Degree of loading Duration for input water flow rate Duration of high flow rate 0.5K/s - 0.4K/s 1 2 minutes 0.4K/s - 0.3K/s 2 4 minutes 0.3K/s - 0.2K/s 3 6 minutes

The table above shows how the load level is determined as a function of the temperature increase in the cooking chamber at the start of the cooking process.

According to the invention, the length of time is determined as a function of the load quantity in such a way that the change in temperature over time is determined over a certain period of time dT/dt in the cooking chamber. Knowing the degree of loading present, this determination can be established for specific cooking goods by means of appropriate cooking tests.

Alternatively, the load quantity can also be determined by how long it takes until the target temperature is reached again in the cooking chamber. With an almost empty cooking chamber, this may be achieved again in a few seconds after loading, whereas it can take several minutes with a full load. The prerequisite is that the temperature sensor is placed in the cooking chamber near the food to be cooked, e.g. on the oven wall near the food to be cooked.

The present invention also includes the alternative solution that the period of time corresponds to the time that elapses after the cooking space has been loaded with food to be cooked until the target temperature of the cooking process is reached. When comparing the target temperature with the measured actual temperature, the agreement can then be determined and from there the water volume flow is reduced.

Furthermore, it is advantageous that the water volume flow is not reduced in one stage or in one step, but in several steps. The extent of the reduction in the case of small steps is correspondingly lower, with ideally a linear reduction in the water volume flow, at least in sections, being conceivable.

Advantageously, according to the method according to the invention, the type of food to be cooked or its physical condition can be determined on the basis of the recorded temperature profile and the result can be taken into account in the calculation of the duration for the reduction of the water volume flow. This is because there are considerable differences in the case of an item to be cooked at room temperature in contrast to a frozen item to be cooked.

After determining a frozen item to be cooked, which can be determined, among other things, due to the characteristic temperature profile in the cooking chamber, the water volume flow is particularly greatly reduced. This is possible because when Defrosting the frozen food to be cooked releases a significant amount of water from the food itself.

As stated above, when the cooking chamber door is opened, it cannot be prevented that the hot cooking atmosphere in it suddenly leaves the cooking chamber, so that according to the invention after such an opening of the cooking chamber door, the method according to one of claims 1 to 4 is repeated, specifically with the previously determined values.

Advantageously, the steps of reducing the water volume flow can be of different lengths and can be determined as a function of the measured cooking chamber temperature.

Furthermore, the extent of the reduction in the water volume flow is determined by the control or regulating device as a function of the temperature profile in the cooking chamber.

Furthermore, in the case of a large load quantity, ie a low value dT/dt or a long period of time until the setpoint temperature is reached, the extent of the reduction can advantageously be made less than in the case of a low load. Furthermore, it makes sense to switch over the volume flows depending on the type of cooking product. It is particularly advantageous if the device itself can recognize which product type it is. For example, in the case of deep-frozen foods, the volume flow can be reduced very early on, because the products bring water with them from the ice that has stuck to them. This adhering water protects the food from an initially dry oven atmosphere. Deep-frozen cooked products have a characteristic temperature curve, which is determined by the fact that large quantities of product have a particularly strong temperature drop, which is very low for a considerable cooking time and only increases more quickly when the ice has melted. Physically, this is the effect of the enthalpy of fusion. With these products in particular, it is particularly advantageous to reduce the energy consumption of the heating device, since the heating output is required to heat the food to be cooked.

The present invention also includes a cooking appliance capable of and suitable for carrying out a method according to any one of claims 1 to 5.

According to the invention, the temperature sensor is advantageously arranged at a large distance from the heating device, preferably on the wall of the cooking chamber opposite the heating device, so that, contrary to the prior art, the disadvantageous effects of a neighboring heating device on the temperature sensor, which then essentially measures the temperature of the heating and does not record the temperature in the cooking chamber can be avoided. In this way, the actual temperature in the cooking chamber can be recorded.

Advantageously, the control or regulating device records the time course of the temperature rise in the cooking chamber after loading and uses this to determine or determine the length of time until the water volume flow is reduced.

Fig. 1a

ein Temperatur-Zeit-Diagramm mit einer Temperaturkurve im Garraum am Beispiel von gefrorenem Gargut;

Fig. 1b

ein Volumenstrom-Zeit-Diagramm mit identischer Zeitachse zu Fig. 1a und dem Kurvenverlauf des zugeführten Wasservolumenstroms;

Fig. 2a

ein Temperatur-Zeit-Diagramm mit dem Temperaturkurvenverlauf 1 für geringe Beladung und dem Temperaturkurvenverlauf 2 für große Beladung;

Fig. 2b

ein Volumenstrom-Zeit-Diagramm mit einem Kurvenverlauf 1 für geringe Beladung und einem Kurvenverlauf 2 für große Beladung mit den entsprechenden Volumenstromreduzierungen zu bestimmten Zeitpunkten; und

Fig. 2c

ein Volumenstrom-Zeit-Diagramm mit gegenüber Fig. 2b linearisierten Verläufen der Volumenstromreduktion der beiden Kurvenverläufe 1 und 2.

Alternatively, there is also a great advantage that the control or regulating device determines from the comparison between the current cooking chamber temperature and the target temperature of the cooking process when the water volume flow is reduced and sends a corresponding signal to a water supply valve. Further details, features and advantages of the present invention result from the following description with reference to the accompanying drawings. It shows:

Fig. 1a

a temperature-time diagram with a temperature curve in the cooking chamber using the example of frozen food;

Fig. 1b

a volume flow-time diagram with an identical time axis Fig. 1a and the curve of the supplied water volume flow;

Figure 2a

a temperature-time diagram with the temperature curve 1 for a small load and the temperature curve 2 for a large load;

Figure 2b

a volume flow-time diagram with a curve 1 for low loading and a curve 2 for high loading with the corresponding volume flow reductions at specific times; and

Figure 2c

a volume flow-time diagram with opposite Figure 2b linearized courses of the volume flow reduction of the two curves 1 and 2.

In Fig. 1a is a temperature-time diagram shown with a curve 10, which represents the temperature in the cooking chamber over time. The curve 10 forms the temperature profile in the cooking chamber when loading with frozen food to be cooked. It is preheated to 100°C and water is injected at a rate of 25 l/h before the food is placed in the cooking chamber.

At time t ₁ , the cooking chamber door is opened for the loading of food to be cooked, so that the temperature in the cooking chamber drops suddenly, as shown by curve section 12 . In the example of Fig. 1a frozen food to be cooked is introduced between t ₁ and t ₂ and the cooking chamber door is closed. Due to the frozen food to be cooked, the temperature according to curve segment 14 then continues to drop in the time period t ₂ to t ₃ despite further heating by the cooking appliance. At time t ₃ , the temperature measurement in the cooking chamber establishes that there is no further drop in temperature and that the temperature begins to rise slowly due to the thawing process of the frozen food to be cooked, as curve section 16 shows. As of time t ₄ , the thawing process is clearly complete and the cooking chamber is heated much more quickly in curve section 18 between times t ₄ and t ₅ . The temperature profile then quickly approaches the desired setpoint value T set of 100° C. _, as curve section 20 shows. This is retained until the end of the cooking process at time t _END .

In the case of an item to be cooked that is used at ambient temperature or only slightly cooled, the curve 10 is shorter in the range t ₂ to t ₃ and significantly steeper in the range t ₄ to t ₅ .

In Fig. 1b , to which reference is now made and in connection with Fig. 1a is shown, a diagram is also shown that indicates the water volume flow V̇ in l/h over time t in minutes, with the time axes from Figures 1b and 1a are identical. This results in a corresponding curve 30 of the volume flow V - for generating a corresponding amount of steam in the cooking chamber when the frozen food to be cooked is introduced.

The water volume flow V̇ is interrupted at time t ₁ by opening the cooking chamber door and after loading and closing the cooking chamber door at time t ₂ the initially large water volume flow of 25 l/h is continued until time t ₃ . Since the cooking appliance has recognized that the food to be cooked is frozen, the water volume flow V is significantly reduced according to the invention at time t ₃ , in the example from 25 l/h to 15 l/h. This amount is continued until shortly after time t ₅ , time t ₆ , when the cooking chamber temperature has reached the setpoint temperature T set _, after which the water volume flow V is further significantly reduced, in the example to 10 l/h. The supply of water volume flow is interrupted at the end of the cooking process at time t _END .

The following is referred to the two diagrams Figures 2a and 2b referenced. the Figure 2a shows a temperature-time diagram with curves 1 and 2 and the Figure 2b also shows two curves 1 and 2 in a water volume flow-time diagram.

At time t ₁ the cooking chamber door is opened and the temperature in the cooking chamber drops abruptly during charging, with differences in curves 1 and 2 occurring at time t ₂ when heating is resumed for the following reasons.

Curve 1 shows loading with a small amount of food to be cooked, whereas curve 2 shows the temperature profile when loading a large amount of food to be cooked. After loading, curve 2 of the water volume flow V̇ drops below curve 1 because the larger load further reduces the cooking chamber temperature.

In the period t ₂ to t ₃ with a small load, according to curve 1, the cooking chamber temperature rises to the desired temperature T set _, whereas with a large load, curve 2 shows a flatter rise and takes until time t _{5 to} reach the temperature T set.

From the time change dT / dt both curves 1 and 2, it is possible for the cooking appliance to determine the quantity of food and, for example, characteristic periods Δ t ₁ and Δ t ₂ to determine in which the maximum volume flow, as from Figure 2b according to the 1 and 2 is supplied.

Since the temperature T set point _is reached at the point in time t ₃ with a low load, the water volume flow is reduced at this point in time, as shown in the curve in FIG Figure 2b displays. Correspondingly later is only at time t ₅ according to curve 2 from Figure 2b reduces the amount of water supplied.

According to the invention, it is possible to carry out a further reduction at a point in time t ₄ with a low load and at a point in time t ₆ with a large load, down to a minimum value according to FIG Figure 2b is 12 l/h. Of course, the supply of water ends at the end of the cooking process at time t _END .

In Figure 2c an idealized curve of a substantially linear reduction in the amount of steam is shown. The amount of food to be cooked is determined as described. By means of a steaming unit that can be easily dosed, the amount of water supplied is reduced in small steps from the maximum to the minimum volume flow. The essentially linear behavior minimizes the disruptive influences on the control of the cooking process, so that compared to curves 1 and 2 in Figure 2b substantially linear reduction profiles of curves 1 and 2 between t ₃ and t ₄ or t ₅ and t ₆ result.

In contrast to the alternative automatic determination of whether the food to be cooked is frozen, the present invention has a simple input option, for example in a touch-sensitive field, so that the user can select frozen food.

Reference character list:

1 -

curve progression

2 -

curve progression

10 -

curve progression

12 -

curve section

14 -

curve section

16 -

curve section

18 -

curve section

20 -

curve section

30-

curve progression

Claims (8)

1. A method for operating a cooking appliance in the form of a combi-steamer which has a cooking compartment, a heating device, at least one fan for circulating a cooking compartment atmosphere present in the cooking compartment, at least one temperature sensor, a control or regulating device and a device for introducing water of an adjustable volume flow into the cooking compartment, the water being directed towards the fan and being converted into steam by means of fine dispersion,

   wherein a period of time depending on the amount of load is determined in such a manner that the temporal change of a temperature over a certain time segment dT/dt in the cooking compartment is determined,

   characterized in that

   frozen foods can be selected by a user using an input option,

   wherein, if frozen foods are not selected in a operating mode of "cooking/steaming" at the beginning of a cooking process, a predetermined volume flow of water, depending on the respective type of appliance, is guided into the cooking compartment to the fan, wherein, depending on the cooking compartment temperature sensed by means of the at least one temperature sensor, the control or regulating device determines the period of time which corresponds to a time that elapses after loading the cooking compartment with food to be cooked until a desired temperature of the cooking process is reached and then the volume flow of water is decreased or reduced,

   wherein, after input of the frozen foods selection at the beginning of the cooking process in the operating mode of "cooking/steaming", a predetermined volume flow of water depending on the respective type of appliance is guided into the cooking compartment to the fan impeller, and wherein, depending on the cooking compartment temperature sensed by means of the temperature sensor, the control or regulating device determines the period of time which corresponds to the time at which, after loading of the cooking compartment with frozen foods, no further dropping of the temperature in the preheated cooking compartment takes place and then the volume flow of water is greatly decreased or reduced in comparison to the cooking process when frozen foods are not selected.
2. The method according to Claim 1, further characterized in that the volume flow of water is decreased in a plurality of steps.
3. The method according to Claim 1 or 2, further characterized in that, after opening of a cooking compartment door, the method according to Claim 1 or 2 is repeated, specifically with the previously determined values for the volume flow of water.
4. The method according to one of Claims 1 to 3, further characterized in that the steps for reducing the volume flow of water differ in length and are determined depending on the cooking compartment temperature measured.
5. The method according to one of Claims 1 to 4, further characterized in that the size of the reduction of the volume flow of water is determined by the control or regulating device depending on the temperature variation in the cooking compartment.
6. A cooking appliance in the form of a combi-steamer, having a cooking compartment, a cooking compartment door closing the latter, a heating device, at least one fan for circulating a cooking compartment atmosphere present in the cooking compartment, at least one temperature sensor, a control or regulating device and a device for introducing water of an adjustable volume flow into the cooking compartment, said water being directed towards the fan and being converted into steam by means of fine dispersion,

   wherein the control or regulating device senses a temporal variation of a temperature rise in the cooking compartment after loading and from this determines a period of time until reduction of the volume flow of water,

   characterized in that

   said cooking appliance is configured to carry out the method according to one of Claims 1 to 5, wherein it has an input option for selecting frozen foods by a user,

   wherein, if frozen foods are not selected in the operating mode of "cooking/steaming" at the beginning of the cooking process, the predetermined volume flow of water, depending on the respective type of appliance, is guided into the cooking compartment to the fan, wherein, depending on the cooking compartment temperature sensed by means of the at least one temperature sensor, the control or regulating device determines the period of time which corresponds to the time that elapses after loading the cooking compartment with food to be cooked until the desired temperature of the cooking process is reached and then the control or regulating device decreases or reduces the volume flow of water, and

   wherein, upon input of the frozen foods selection in the operating mode of "cooking/steaming", the control or regulating device more greatly reduces the volume flow of water in comparison to the cooking process of non-frozen foods after the control or regulating device has determined the time from which no further reduction in the temperature in the preheated cooking compartment takes place after loading with frozen food.
7. A cooking appliance according to Claim 6, further characterized in that the temperature sensor is arranged at a large distance from the heating device, preferably on a wall of the cooking compartment which is opposite the heating device.
8. A cooking appliance according to Claim 6 or 7, further characterized in that the control or regulating device determines from a comparison between current cooking compartment temperature and desired temperature of the cooking process when the reduction in the volume flow of water takes place and transmits a corresponding signal to a water supply valve.

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