EP1837599B1 - Method for automatically ending a pyrolysis cleaning process in an oven - Google Patents

Description

The invention relates to a method for automatically ending a pyrolysis cleaning process in an oven.

From the US 4,481,404 For example, there is known a method of automatically stopping a pyrolysis cleaning operation in an oven using a gas sensor responsive to the fumes produced during pyrolysis. As soon as a sufficiently low concentration of a flue gas is detected by means of the gas sensor, the pyrolysis cleaning process is automatically terminated. The gas sensor used and the evaluation of its output signals must be adapted to the respective type of oven.

The US 4,954,694 also shows a method for automatic completion of a Pyrolysereinigungsvorgangs in an oven, wherein after switching on the heat source, the oxygen concentration is measured in a Abluftweg and from the course of the measured values, the duration of the pyrolysis process is determined. In order to facilitate the cleaning of the cooking chamber, in the known method, either a fixed or dependent on the pyrolysis time interval is additionally heated after completion of the actual pyrolysis process.

The invention thus has the problem of providing a method for automatic completion of a Pyrolysereinigungsvorgangs in an oven, in which the quality of Pyrolysereinigung while reducing the energy consumption during pyrolysis is improved.

According to the invention, this problem is solved by a method having the features of patent claim 1. Advantageous embodiments and further developments of the invention will become apparent from the following subclaims.

The advantages achievable with the invention are, in particular, that the quality of the pyrolysis cleaning is improved while reducing the energy consumption during pyrolysis.

In principle, the limit value GW for the oxygen concentration can be selected within wide suitable limits. Conveniently, the limit value GW for the oxygen concentration is in a range of about 20.5 mol% to 21.0 mol%.

An advantageous development provides that the pyrolysis cleaning process comprises three phases, namely a heating phase until reaching a predetermined or automatically determined during the heating pyrolysis temperature T _{P of} the cooking chamber, a holding phase whose duration is dependent on the measured oxygen concentration and during the pyrolysis T _{P is} kept substantially constant by means of a temperature control, and a cooling phase to achieve a predetermined final temperature of the cooking chamber T _E , with T _E <T _P , said under claim 1 process steps a) to d) performed only during the holding phase I. become. This makes it possible to restrict the monitoring of the oxygen concentration to the holding phase, so that the energy consumption is further reduced.

Figur 1

eine Frontansicht eines Backofens, in dem das erfindungsgemäße Verfahren verwendet wird,

Figur 2

eine Schnittdarstellung des Backofens aus Fig. 1 und

Figur 3

ein Diagramm der Garraumtemperatur und der Sauerstoffkonzentration in Abhängigkeit der Zeit.

An embodiment of the invention is shown purely schematically in the drawings and will be described in more detail below. It shows

FIG. 1

a front view of a baking oven, in which the method according to the invention is used,

FIG. 2

a sectional view of the oven Fig. 1 and

FIG. 3

a diagram of the cooking chamber temperature and the oxygen concentration as a function of time.

In Fig. 1 an oven is shown in which the inventive method is used. The oven has a control panel 2 with a display 4 and 6 controls. The cooking chamber 8 can be closed by a door 10, wherein the door 10 has a see-through disk 12.

Fig. 2 shows the oven in a sectional view from the side. The oven has an electrical control 14 behind the control panel 2. The resulting in the cooking chamber 8 during pyrolysis flue gases 17, so-called fumes, can escape via an exhaust path 16 in the free environment. The path of the flue gases 17 is symbolized by arrows 18. In the cooking chamber 8 designed as an electric radiant heating heat source 20 for heating the cooking chamber 8 is arranged. Between the cooking chamber 8 and the exhaust path 16, an oxidation catalyst 22 is arranged, on which the flue gases 17 discharged through the exhaust air path 17 are converted in a manner known to those skilled in the art. Downstream of the oxidation catalyst 22, an oxygen sensor 24 for detecting the oxygen concentration is arranged in the exhaust path 16. The aforementioned components are connected in a manner known to those skilled in the art to the electrical controller 14 in a signal-transmitting manner.

Notwithstanding the embodiment described here, the oxygen sensor 24 may also be arranged at another suitable location in the oven. The same applies to the oxidation catalyst 22, which may for example also be arranged in the exhaust path 16.

• Fig. 3 zeigt den Verlauf der Garraumtemperatur a und der Sauerstoffkonzentration b in Abhängigkeit der Zeit, die auf der Abszisse in Minuten, abgekürzt min., aufgetragen ist. Auf der linken Ordinate ist die Garraumtemperatur a in Grad Celsius, kurz °C, aufgetragen. Die absolute Sauerstoffkonzentration b ist auf der rechten Ordinate in Mol-% angegeben.

The inventive method will now be described with reference to Fig. 3 explained in more detail:

• Fig. 3 shows the course of the cooking chamber temperature a and the oxygen concentration b as a function of time, which is plotted on the abscissa in minutes, abbreviated min. On the left ordinate is the cooking space temperature a in degrees Celsius, short ° C, applied. The absolute oxygen concentration b is given on the right ordinate in mol%.

The oven has heavy contaminants 26 in the embodiment. Please refer Fig. 2 , The user starts the pyrolysis cleaning process by means of the controls 6. He is guided and informed about the display 4 in a manner known to those skilled.

The cooking chamber temperature a is at the beginning of the pyrolysis cleaning process, time 0 min., Room temperature, ie about 20 ° C. The oxygen concentration b corresponds to the oxygen concentration b in the atmosphere, ie about 21 mol% or 0 mol% deviation.

During a heating phase, the cooking chamber 8 is now heated by means of the heating source 20 with a previously set and stored in a memory 141 of the electric controller 14 speed, namely about 10 ° C per 1 min., Heated. The heating takes place by means disposed in the cooking chamber 8 and with the electrical control 14 signal transmitting connected temperature sensor 30 and a known temperature control by the electric controller 14, which turns on or off the heat source 20 according to the above-mentioned heating curve.

If too much flue gas 17 arises, this can be detected on the basis of the oxygen concentration b. Too much flue gas 17 impedes the conversion of the flue gases 17 to the oxidation catalyst 22, so that the flue gases 17 can no longer be fully implemented. For this reason, the heating of the cooking chamber 8 is always interrupted during the heating phase when the oxygen concentration b at the oxygen sensor 24 falls below a predetermined minimum value, here 18 mol% or 3 mol% deviation. This happens at the in Fig. 3 pictured example a total of four times. Thereafter, the heating is continued as already explained, since the permissible minimum value for the oxygen concentration b is not exceeded.

In the present exemplary embodiment, the heating phase described above is followed by a holding phase during which a predetermined pyrolysis temperature T _{P is} kept essentially constant by means of the temperature sensor 30 and the temperature control already explained in a manner known to the person skilled in the art.

The pyrolysis temperature T _P during the holding phase I is fixed here. The pyrolysis temperature T _P here is 460 ° C, see Fig. 3 , Alternatively, it would also be conceivable to automatically determine the pyrolysis temperature T _P during the heating phase, for example as a function of the time profile of the oxygen concentration b, or to select it as a function of inputs by the user on the control panel 2.

The duration of the holding phase 1 depends on the measured oxygen concentration b. In the present embodiment, the limit value GW for the oxygen concentration b is set to 20.5 mol%.

As already explained, the oxygen concentration b has dropped to values below the limit value GW during the pyrolysis cleaning process. As long as the oxygen concentration b remains below the limit value GW, the holding phase I continues and the pyrolysis temperature T _P is further regulated to 460 ° C.

With progressive pyrolysis cleaning the contaminants 26 are implemented and thus less. As a result, the amount of flue gas 17 also decreases and the oxygen concentration b at the oxygen sensor 24 increases again. Please refer Fig. 3 , About 160 min. After the start of the pyrolysis cleaning process, the oxygen concentration b first exceeds the limit value GW for the first time, ie 20.5 mol%. As a result, a first counter, which is integrated in the evaluation circuit 142, is _reset to a time interval l _{Zsuatz_1} = 30 min. is set and in the evaluation circuit 142 integrated second counter, the time interval l _{Zsuatz_2} = 60 min. is set, started at the same time.

Since the temperature in the cooking chamber 8 during the time interval l _{Zsuatz_1} = 30 min., So up to 190 min. after the start of the Pyrolysereinigungsvorgangs, not lower than 445 ° C and the oxygen concentration b does not fall again below the limit GW during this period, the heat source 20 is switched off after the l _{Time_ Zusatz_1} . The cooking chamber temperature a decreases.

Although, as already explained, the cooking chamber temperature a is controlled during the entire holding phase I to the pyrolysis temperature T _P from here 460 ° C by means of temperature control, it is usual during pyrolysis that the cooking space a due to the continuous reaction of the flue gases 17 at the Oxidation catalyst 22 fluctuates. Therefore, it is necessary that the aforementioned minimum value for the oven temperature a during the time interval l _{Zsuatz_1 is chosen} to be correspondingly lower than the pyrolysis temperature T _P , here 445 ° C.

Due to the aforementioned switching conditions ensures that the pyrolysis is sufficient to clean the cooking chamber 8 and possibly located therein and also contaminated Gargutträger satisfactorily. A renewed decrease in the oxygen concentration b below the limit value GW during the time interval l _{Zusatz_1} would mean that too much flue gas 17 is generated. In this case, the cooking chamber 8 would still be too dirty, so the contaminants 26 not yet implemented satisfactorily. The same applies to a decrease in the cooking chamber temperature a to values below 445 ° C during the time interval l _{Zusatz_1} , since at such low oven temperatures a effective pyrolysis does not take place.

If the aforementioned switching conditions are not met, the heat source 20 would be here after expiration of the time interval l _{Zusatz_2,} ie after 60 min. from the time the limit value GW for the oxygen concentration b is exceeded for the first time, it is switched off automatically.

At the conclusion of the Pyrolysereinigungsvorgangs the cooking chamber temperature a in the present embodiment during a cooling phase to a final temperature T _E is reduced from 70 ° C here. On the one hand, this is achieved by the above-described switching off the heating source 20. On the other hand, to speed up the cooling, the flushing of the cooking chamber 8 with fresh air can be increased by means of a blower not shown in the figures. If the cooking chamber temperature a has dropped to the final temperature T _E , the door 10, which is automatically locked during the start of the pyrolysis cleaning process, is unlocked again, so that it can be opened by the user without danger. The cooling phase is in Fig. 3 not fully illustrated.

While the oven temperature a steadily decreases during this phase to the previously set final temperature of 70 ° C, the oxygen concentration b no longer changes.

The inventive method is not limited to the illustrated embodiment. For example, it would also be conceivable to use the method only during the holding phase. Also, the time intervals l _{Zusatz_1} , l _{Zusatzu_2} and the pyrolysis temperature T _P and the final temperature T _{E can be selected} within wide suitable limits.

Claims (3)

1. Method for automatically ending a pyrolysis cleaning process in an oven, comprising the following method steps:

   a) switching on the heat source (20) for the pyrolysis cleaning process;

   b) measuring, by means of an oxygen sensor (24), the oxygen concentration (b) in the cooking chamber (8) or in an exhaust air channel (16) for conducting vapour (17) out of the cooking chamber (8);

   c) comparing, in an evaluation circuit (142) of an electric control means (14) of the oven, the currently measured oxygen concentration (b) with a predetermined threshold value GW which is stored in a memory (141) of the electric control means (14);

   d) if the measured oxygen concentration (b) has fallen below the threshold value GW during the pyrolysis cleaning process and the oxygen concentration (b) has reached or exceeded the threshold value GW for the first time, switching off the heat source (20)

   d1) after a predetermined time interval I_{Zusatz_1} has elapsed after the threshold value GW has been reached or exceeded, as long as the cooking chamber temperature (a) measured by means of a temperature sensor (30) does not fall bellow 445 °C during the time interval I_{Zusatz_1} and the oxygen concentration (b) does not fall bellow the threshold value GW within the time interval I_{Zusatz_1}, or

   d2) after a predetermined time interval I_{Zusatz_2} has elapsed after the threshold value GW has been reached or exceeded, where I_{Zusatz_2} > I_{Zusatz_I}.
2. Method according to claim 1, characterised in that the threshold value GW for the oxygen concentration (b) is in a range of from approximately 20.5 mol.% to 21.0 mol.%.
3. Method according to either claim 1 or claim 2, characterised in that the pyrolysis cleaning process comprises three phases, namely a heating phase until a pyrolysis temperature T_P of the cooking chamber (8) which is predetermined or automatically determined during the heating phase is reached, a holding phase, the duration of which depends on the measured oxygen concentration (b) and during which phase the pyrolysis temperature T_P is kept substantially constant by means of temperature regulation, and a cooling phase for reaching a predetermined final temperature of the cooking chamber T_E, where T_E < T_P, the method steps a) to d) mentioned under claim 1 only being carried out during the holding phase.

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