EP2428735B1 - Cooking device with steam generator and method for regulating a steam generator for a cooking device - Google Patents

Description

The invention relates to a cooking appliance with a cooking chamber, a steam generator and a controller that controls the steam generator. The invention further relates to a method for controlling a steam generator for a cooking appliance.

In cooking appliances for temperature or humidity control threshold regulators are usually used because they can be used usually without specific adaptation to the device size, the type of heating (gas or electric) and the load in the oven. In steam mode, however, threshold controllers have serious disadvantages if a high steam content in the cooking chamber is to be maintained. A threshold controller controls between an upper threshold and a lower threshold. If the upper threshold has been exceeded, the steam generator is switched off. Subsequently, the temperature is reduced by condensing the water vapor inside the cooking chamber (for example, to be cooked products). At the same time, the volume of the gas contained in the interior of the cooking chamber is reduced, so that air has to be sucked in, usually from a condensation or drainage region of the cooking appliance. When the temperature in the cooking chamber has finally reached the lower threshold value of the threshold controller, the steam generator is switched on again. Thereafter, water vapor is again produced until the temperature in the cooking chamber has again reached the upper threshold.

However, since the sucked-in air mixes with the steam in the cooking chamber, the steam generated by the steam generator and introduced into the cooking chamber must displace a mixture of water vapor and sucked-in air in order to increase the steam content and the temperature in the cooking chamber. As a result, much more steam must be produced than air has been sucked. As an example, it is assumed that at 90% humidity, 1 liter of water vapor condenses in the oven, which is replaced by air. Thereupon the device must Replace 10 liters of steam mixture with 10 liters of pure water vapor to displace the sucked air.

From the DE 42 23 653 A1 a device for controlling cooking, roasting and cooking processes in the cooking chamber of a baking oven is known, wherein a control circuit is provided with an electronic controller, the sensor-controlled adjusts a corresponding oven energy level depending on cooking properties and their associated temperature-time profiles with gargutspezifischen fluctuation ranges is adjustable.

The generic WO 2009/065197 A1 discloses a gas-powered home oven that also provides the option of steaming. In this case, a controller is provided which, in addition to other alternatives mentioned, may have a two-point controller or a PID controller.

The EP 1 761 111 A2 and the EP 1 724 530 A1 also describe home-baking ovens with an integrated steam generator, the ovens having centralized control comprising a PID controller or other suitable controller.

From the US 2006/0081592 A1 Again, a steamer is known, which is based on natural convection and has a drainage area with a condensate.

It is the object of the invention to provide a cooking appliance and a method for controlling a steam generator, with which, especially at high steam content, a better control is possible to the effect that the intake of air is largely avoided.

To achieve this object, a cooking appliance according to the invention with the features of claim 1 is provided. The invention is based on the idea of controlling the steam generator in an operating range in which the steam content is to be kept constant, with the signal of the continuous controller. This prevents the steam generator from being switched off until air has been sucked in.

According to a preferred embodiment of the invention it is provided that the steam generator has at least two power levels. According to In this embodiment, the steam generator is in principle further operated with discrete power stages, but in the operating range in which the steam content is to be kept constant, be controlled by the continuous controller. By using two or more power levels, an optimal adaptation to the respective power requirement can be made.

Preferably, it is provided that the continuous controller operates the steam generator with a maximum average power, which is below the first power level. The steam generator then works with a holding stage, which can be integrated into the threshold controller like an additional smaller heating stage.

It is preferably provided that the first power level is in the range of about 25% to about 50% of the second, maximum power level. It has been found that with such a spreading of the heating stages, the desired performance can be well represented. In addition, these power levels result constructively in an advantageous manner. In an electrically operated steam generator halving the power in star or delta connection can be achieved with a relay or SSR. In the case of a gas-driven steam generator, on the other hand, the smallest heating level will have a capacity of approximately 25% of the maximum power, even if lower powers could be advantageous.

Preferably, it is provided that the continuous controller operates the steam generator with a power that is about 75% of the power of the first power level. In other words, assuming a power in the first heating stage of 50% of the maximum power) in the holding stage, the steam generator operates at a power of 37.5% of the maximum heating power, which has proved to be advantageous for maintaining a constant steam content in the cooking space ,

According to the preferred embodiment, it is provided that the continuous controller outputs as a control variable a buffer variable, by means of which the controller operates the steam generator in pulse width modulation. In this way, the advantages of a continuous controller can be used without having to use a continuously variable steam generator.

It is preferably provided that the cooking chamber is in flow communication with a drain region, wherein in the drain region, a steam buffer is formed whose volume is at least equal to the vapor volume condensed between two heating pulses in the cooking chamber. This embodiment takes into account that due to regulatory requirements in some countries, the frequency of the permitted switching operations per unit time is limited. As a result, certain waiting times must be maintained during operation of the steam generator in the holding stage, before it can be switched on again after switching off the steam generator. This does not prevent a certain amount of steam from condensing during the waiting period, the volume of which must be compensated for by sucked gas. The use of a steam buffer in the discharge area ensures that no air is sucked in, but steam.

According to the preferred embodiment, it is provided that the continuous controller is a PI controller. Such a controller is sufficient to maintain the setpoint (temperature or humidity) to be maintained in the cooking chamber during the holding phase. A more elaborate PID controller can be dispensed with.

According to the invention, a method for controlling a steam generator for a cooking appliance with the features of claim 10 is provided to solve the above object. This method combines the advantage of a robust threshold controller with the advantage of more precise control by means of a continuous regulator.

The threshold controller controls the steam generator until the actual value to be controlled in the cooking chamber is at least 90% of the setpoint. It simplifies the design of the continuous controller if it only has to take over control in the vicinity of the setpoint and the approach to the setpoint is taken over by the robust threshold controller.

According to one embodiment, it is provided that the continuous controller continuously regulates the steam generator. In this way, the quality of control can be increased and the energy consumption can be reduced.

• Figur 1 schematisch ein erfindungsgemäßes Gargerät;
• Figur 2 ein Diagramm zur Erläuterung des erfindungsgemäßen Verfahrens;
• Figur 3 schematisch in vergrößertem Maßstab den mit III in Figur 1 gekennzeichneten Ablaufbereich; und
• Figur 4 eine alternative Ausgestaltung zum in Figur 3 gezeigten Ablaufbereich.

The invention will be described below with reference to an embodiment shown in the accompanying drawings. In these show:

• FIG. 1 schematically a cooking appliance according to the invention;
• FIG. 2 a diagram for explaining the method according to the invention;
• FIG. 3 schematically on an enlarged scale that with III in FIG. 1 marked drain area; and
• FIG. 4 an alternative embodiment to in FIG. 3 shown drain area.

In FIG. 1 schematically a cooking appliance 10 is shown, which is intended for commercial kitchens and is known as a combination damper. It contains a cooking chamber 12 into which products to be cooked can be introduced. It is a steam generator 14 is provided which can generate water vapor, which can be passed into the interior of the cooking chamber 12. Of course, the cooking appliance 10 also has a heater in order to introduce heated air into the cooking chamber. However, since this heater in the context of the present application without meaning, it is not shown here for the sake of clarity.

Furthermore, a controller 16 is provided, which receives as input signals, inter alia, a signal from a temperature and / or humidity sensor 18, which is arranged in the interior of the cooking chamber 12, and by an operating unit 20. About the operating unit 20, an operator can different cooking conditions in Inside the cooking chamber or select more complex cooking programs.

The controller 16 includes a threshold controller 22 and a continuous controller 24, which is designed here as a PI controller. Among other things, the controller 16 controls the steam generator 14 by specifying whether the steam generator is switched off, is operated with a first power stage 14.1 or a second power stage 14.2. In the first power stage 14.1, the steam generator has a power that is about 50% of the second, maximum power level. The power levels are specified here by the controller, for example, by the interconnection of three radiators in star or delta connection. Depending on the requirements, other or more power levels may be used.

Below the cooking chamber 12, a drain region 26 is provided, which is designed like a box and provided with a condensation device 28. Of the Drain area 26 is connected to an exhaust pipe 30. Between the connection of the exhaust pipe 30 and the connection to the cooking chamber, a separating plate 32 is provided, so that a steam buffer 34 is formed between the cooking chamber and the separating plate.

Based on FIG. 2 will now be explained how in the operating mode "steam" as constant as possible temperature or steam content in the cooking chamber 12 is maintained. In this example, a target temperature T _s is given. However, the method works in the same way, if instead of the temperature of a certain humidity in the oven is specified as the setpoint.

At the beginning of the process, the actual temperature T _I in the cooking chamber is significantly below the setpoint temperature T _S , which is around 95 ° C here. Therefore, the controller 16 uses only the threshold controller 22 for the control of the steam generator. This operates the steam generator 14 in the second heating stage 14.2, ie with a power of 100% (see the power curve P in the diagram). After almost 200 seconds have elapsed, exceeds the temperature T _I in the cooking chamber 12, the threshold above which the steam generator 14 is switched back to the first heating stage 14.1; the power curve P thus drops to a value of 50%.

After about 230 seconds have elapsed, the actual temperature T _I has approached the setpoint temperature T _S so far that the controller 16 switches from the threshold value controller 22 to the PI controller 24 and uses this to control the steam generator 14. Accordingly, the value for the setpoint power HS _{S of} the steam generator in the holding stage increases to a predetermined value of 75% of the power of the first heating stage (see curve HS _S ). This value HS _S is treated by the controller 16 as a buffer variable, based on which the steam generator 14 is now operated with the first heating stage 14.1 pulse width modulated. Since the maximum power of the steam generator is set here with 75% of the power of the first heating stage 14.1, the controller 16 switches on the first heating stage 14.1 of the steam generator 14 within a given time interval for 75% of the time and for 25% of the time, if the steam generator should be operated in the holding stage with maximum power. This is the case here as long as the setpoint H _S for the holding stage remains at the value of 75%, that is to say just under 250 seconds. At this time The PI controller 24 begins to regulate, as can be seen from the fluctuating values for the setpoint HS _{S of} the holding stage.

In this way, a continuous regulator in combination with a steam generator, which is switched between two discrete power levels, can be used to keep the steam content in the cooking chamber comparatively constant. At approximately constant value for the integral portion _I P of the PI controller will be appreciated that the controller reaches a steady state after about 400 seconds.

In principle, the power given by the PI controller 24 for the operation of the steam generator 14 in the holding stage can be represented by a multiplicity of very short heating pulses, between which correspondingly short switch-off times are used. In electrically operated steam generators could be adjusted quasi-continuous by switching on and off of individual half-waves of the mains frequency, the heating power. In this way one comes very close to an actually stepless control of the power, which would be advantageous in terms of control quality and energy consumption.

However, for both practical and regulatory reasons, it is not possible in all countries to choose the switch-off time between two consecutive heating pulses as short as desired. The switch-off time or waiting time between two successive heating pulses therefore leads to steam condensing in the cooking chamber. The corresponding volume of steam must be sucked into the oven.

In FIG. 3 an embodiment is shown, which is characterized by a larger steam buffer 34. This is formed by the fact that the separating plate 32 is arranged closer to the exhaust pipe 30. The function of the separating plate 32 is to prevent a mixing of the steam with the air in the discharge region 26 as far as possible. The volume of the vapor buffer 34 defined with the separating plate 32 is at least as great as the vapor volume which can condense between two heating pulses in the cooking chamber 12. This ensures that air is not sucked in between two heating pulses, but steam from the steam buffer 34 (see the arrow D). Accordingly, the condensation device 28 must also be switched off at least between two heating pulses, so that steam can actually be stored in the vapor buffer 34.

In FIG. 4 an alternative embodiment is shown. In this flow guide plates 36 are used, which extend in the longitudinal direction of the drain. The process can either have a circular cross section (see detail D1) or a box-shaped cross section (see detail D2). The Strömungsleitbleche 36 act as a vortex breaker, similar to the separating plate 32 prevent the air in the drain region 26 mixed with the existing steam there. Thus, as pure as possible vapor can be sucked back into the cooking space in the phases between two heating pulses. Due to the orientation of the flow guide plates 36 in the direction of the drainage channel, the flow in the longitudinal direction is hardly affected, but strongly suppressed in a direction transverse thereto.

According to one embodiment, the flow baffles extending in the flow direction can all be mounted on the top of the drain. This makes it easier to rinse out soiling, so that no impurities can accumulate on the sheets.

LIST OF REFERENCE NUMBERS

10:

Cooking appliance

12:

oven

14:

steam generator

14.1:

first heating stage

14.2:

second heating stage

16:

control

18:

sensor

20:

operating unit

22:

threshold value controller

24:

PI controller

26:

drainage area

28:

condensation device

30:

exhaust pipe

32:

Divider

34:

steam buffer

36:

flow baffle

Claims (12)

1. A cooking device (10) comprising a cooking chamber (12), a steam generator (14), and a control unit (16) which drives the steam generator (14), the control unit (16) including a threshold controller (22), characterized in that the control unit (16) additionally includes a continuous controller (24),
   wherein in an approximation phase in which an actual value to be closed-loop controlled in a cooking chamber (12) of the cooking device (10) approximates a desired value, the steam generator (14) is controlled by the threshold controller (22) until the actual value to be closed-loop controlled in the cooking chamber (12) amounts to at least 90% of the desired value, and
   in an operating range which corresponds to a holding phase which follows the approximation phase and in which the steam content is to be kept constant, the steam generator (14) is driven with the signal of the continuous controller (24).
2. The cooking device (10) according to claim 1, characterized in that the steam generator (14) has at least two power stages (14.1, 14.2).
3. The cooking device according to claim 2, characterized in that the continuous controller (24) operates the steam generator (14) at a power that is below the first power stage (14.1).
4. The cooking device according to claim 2 or claim 3, characterized in that the first power stage (14.1) amounts to about 50% of the second, maximum power stage (14.2).
5. The cooking device according to claim 4, characterized in that the continuous controller (24) operates the steam generator (14) at a power that amounts to about 75% of the power of the first power stage (14.1).
6. The cooking device according to any of the preceding claims, characterized in that the continuous controller (24) outputs a buffer variable (HSs) as a controlled variable based on which the controller (16) operates the steam generator (14) using pulse width modulation.
7. The cooking device according to claim 6, characterized in that the cooking chamber (12) is in fluid communication with a drainage region (26), the drainage region (26) having a steam buffer (34) formed therein the volume of which is at least equal to the steam volume that condenses in the cooking chamber (12) between two heating pulses.
8. The cooking device according to claim 6 or claim 7, characterized in that at least one separating plate (32) and/or one flow baffle plate (36) is/are provided in the drainage region.
9. The cooking device according to any of the preceding claims, characterized in that the continuous controller is a PI controller (24).
10. A method for closed-loop controlling a steam generator (14) of a cooking device (10) according to any of the preceding claims, characterized in that in an approximation phase in which an actual value to be closed-loop controlled in a cooking chamber (12) of the cooking device (10) approximates a desired value, the steam generator (14) is controlled by a threshold controller (22) until the actual value to be closed-loop controlled in the cooking chamber (12) amounts to at least 90% of the desired value and, in a subsequent holding phase in which the actual value is near the desired value, the steam generator (14) is controlled by a continuous controller (24).
11. The method according to claim 10, characterized in that the continuous controller (24) controls the steam generator (14) in a continuously variable manner.
12. The method according to claim 11, characterized in that the steam generator (14) is electrically operated and the output of the steam generator (14) is closed-loop controlled by switching individual half-waves of the mains frequency on and off.

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