DE102010045198A1 - Cooking appliance with steam generator and method for controlling a steam generator for a cooking appliance - Google Patents

Abstract

The invention relates to a cooking appliance (10) with a cooking chamber (12), a steam generator (14) and a controller (16), the controller (16) having a threshold controller (22) and a continuous controller (24). The invention also relates to a method for regulating a steam generator (14) for a cooking appliance (10), which is characterized in that the steam generator (14) is in an approximation phase in which an actual value to be controlled is located in a cooking chamber (12) of the cooking appliance ( 10) differs greatly from a setpoint, is controlled by a threshold controller (22) and is controlled by a continuous controller (24) in a subsequent holding phase, in which the actual value is close to the setpoint.

Description

The invention relates to a cooking appliance with a cooking chamber and a 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 water vapor generated by the steam generator and introduced into the cooking space 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. Thereafter, the device must replace 10 liters of vapor mixture with 10 liters of pure water vapor to displace the sucked air.

It is therefore an 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 with a cooking chamber, a steam generator and a controller is provided according to the invention, wherein the controller has a threshold controller and a steady controller. 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 this embodiment, the steam generator is in principle still operated with discrete power levels, but in the operating range in which the steam content is to be kept constant, are 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 in order to maintain 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 benefits of a steady regulator can be harnessed without having to infinitely variable steam generator must be used.

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 is also provided for achieving the abovementioned object, which is characterized in that the steam generator differs greatly from a desired value in an approach phase in which an actual value to be regulated in a cooking chamber of the cooking appliance differs. is controlled by a threshold controller and is controlled by a continuous controller in a subsequent holding phase in which the actual value is in the vicinity of the setpoint. This method combines the advantage of a robust threshold controller with the advantage of more precise control by means of a continuous regulator.

Preferably, it is provided that the threshold controller controls the steam generator until the actual value to be regulated in the cooking chamber is at least 90% of the desired value. 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.

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

1 schematically a cooking appliance according to the invention;

2 a diagram for explaining the method according to the invention;

3 schematically on an enlarged scale that with III in 1 marked drain area; and

4 an alternative embodiment to in 3 shown drain area.

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

Furthermore, there is a controller 16 provided as input signals, inter alia, a signal from a temperature and / or humidity sensor 18 that gets inside the cooking chamber 12 is arranged, as well as from a control unit 20 , About the control unit 20 an operator can preselect various cooking conditions inside the cooking chamber or more complex cooking programs.

The control 16 contains a threshold controller 22 as well as a steady regulator 24 , which is implemented here as a PI controller. From the controller 16 will include the steam generator 14 by specifying whether the steam generator is off with a first power level 14.1 or a second performance level 14.2 is operated. In the first performance level 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 space 12 is a drain area 26 provided, the box-like running and with a condensation device 28 is provided. The drain area 26 is with an exhaust pipe 30 connected. Between the connection of the exhaust pipe 30 and the connection to the cooking chamber is a separating plate 32 provided, leaving a steam buffer 34 is formed between the cooking chamber and the separating plate.

Based on 2 will now be explained how in the operating mode "steam" as constant as possible temperature or steam content in the oven 12 is maintained. In this example, a setpoint temperature T _{S is} specified. 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 uses 16 for controlling the steam generator only the threshold controller 22 , 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 nearly 200 seconds have elapsed, the temperature exceeds T _I in the cooking chamber 12 the threshold, when exceeded the steam generator 14 on the first heating stage 14.1 is switched back; the power curve P thus drops to a value of 50%.

After about 230 seconds have elapsed, the actual temperature T _I has approached so far to the target temperature T _S that the control 16 from the threshold controller 22 on the PI controller 24 switches and this for the control of the steam generator 14 used. 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 from the controller 16 treated as a buffer variable by which the steam generator 14 now with the first heating stage 14.1 is operated 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 , thus turns off the controller 16 the first heating stage 14.1 of the steam generator 14 within a given time interval for 75% of the time on and off for 25% of the time when the steam generator is to be operated in the hold stage at 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 point, the PI controller starts 24 to be controlled, 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 the approximately constant value for the integral component P _{I of} the PI controller, it can be seen that the regulator reaches a stable state after approximately 400 seconds.

Basically, each of the PI controller 24 predetermined power for the operation of the steam generator 14 be represented in the holding stage by a plurality of very short heating pulses, between which correspondingly short turn-off 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 3 an embodiment is shown extending through a larger vapor buffer 34 distinguished. This is formed by the fact that the separating plate 32 closer to the exhaust pipe 30 is arranged. The function of the separating plate 32 This involves mixing the vapor with the air in the drainage area 26 to prevent as far as possible. The volume of the divider 32 defined steam buffer 34 is at least as large as the volume of steam between two heating pulses in the oven 12 can condense. This ensures that air is not drawn in between two heating pulses, but steam from the steam buffer 34 (see the arrow D). At least between two heating pulses must therefore also the condensation device 28 be shut off, so in the steam buffer 34 actually steam can be kept.

In 4 an alternative embodiment is shown. In this are flow baffles 36 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 flow baffles 36 act as a vortex breaker, similar to the divider 32 prevent the air in the drain area 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. By the orientation of the flow baffles 36 in the direction of the drainage channel, the flow is hardly affected in the longitudinal direction, 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 (13)

Cooking appliance ( 10 ) with a cooking space ( 12 ), a steam generator ( 14 ) and a controller ( 16 ), whereby the controller ( 16 ) a threshold controller ( 22 ) as well as a continuous controller ( 24 ) having Cooking appliance ( 10 ) according to claim 1, characterized in that the steam generator ( 14 ) at least two performance levels ( 14.1 . 14.2 ) having. Cooking appliance according to claim 2, characterized in that the continuous controller ( 24 ) the steam generator ( 14 ) with a power below the first power level ( 14.1 ) lies. Cooking appliance according to claim 2 or claim 3, characterized in that the first power level ( 14.1 ) about 50% of the second, maximum power level ( 14.2 ) is. Cooking appliance according to claim 4, characterized in that the continuous controller ( 24 ) the steam generator ( 14 operating at approximately 75% of the first ( 14.1 ) is. Cooking appliance according to one of the preceding claims, characterized in that the continuous controller ( 24 ) outputs as a control variable a buffer variable (HS _S ), by means of which the controller ( 16 ) the steam generator ( 14 ) operates pulse width modulated. Cooking appliance according to claim 6, characterized in that the cooking space ( 12 ) with a drain area ( 26 ) is in flow communication, wherein in the drain area ( 26 ) a steam buffer ( 34 ) is formed whose volume is at least equal to the vapor volume, the between two heating pulses in the cooking chamber ( 12 ) condenses. Cooking appliance according to claim 6 or claim 7, characterized in that in the discharge area at least one separating plate ( 32 ) and / or a flow baffle ( 36 ) is provided. Cooking appliance according to one of the preceding claims, characterized in that the continuous controller is a PI controller ( 24 ). Method for controlling a steam generator ( 14 ) for a cooking appliance ( 10 ), characterized in that the steam generator ( 14 ) in an approach phase, in which an actual value to be regulated in a cooking chamber ( 12 ) of the cooking appliance ( 10 ) differs greatly from a setpoint, from a threshold controller ( 22 ) and in a subsequent holding phase, in which the actual value is in the vicinity of the setpoint, by a continuous controller ( 24 ) is controlled. Method according to Claim 10, characterized in that the threshold value controller ( 22 ) the steam generator ( 14 ) controls until the actual value to be regulated in the cooking chamber ( 12 ) is at least 90% of the nominal value. Method according to claim 10 or claim 11, characterized in that the continuous controller ( 24 ) the steam generator ( 14 ) steplessly regulates. Method according to claim 12, characterized in that the steam generator ( 14 ) and the power of the steam generator ( 14 ) is controlled by switching on and off of individual half-waves of the mains frequency.

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