EP0708296B1 - Safety device for gas radiation burners - Google Patents

Description

The invention relates to a safety device for gas radiation burners with an electrical fuse to prevent unused outflow Gas and related devices for igniting the gas or interrupt the gas supply when the flame goes out.

Gas radiation burners are known and for example in DE 24 40 701 C3 described. They are mainly characterized by porous, perforated Burner plates z. B made of ceramic, which is the mixing chamber of the gas burner lock upwards. In operation burn at the top of the Channels of the perforated or perforated burner plates small flames, through which the burner plates are made to glow and as radiant heaters Act.

The temperature of the radiating burner plates depends on the temperature resistance of the burner plate material between about 900 ° C and 950 ° C.

Radiant gas burners are used, among other things. in space heating, in Hot water conditioners, in drying systems and especially in gas cooking devices.

Gas cooking devices with gas radiant burners and glass ceramic hot plates have been described in numerous embodiments, e.g. B. in the DE-OS 26 21 801, DE 33 15 745 A1, US-PS 4,083,355 or in the US-PS 4,201,184.

1. Pilotflamme, die für eine Rückzündung des Brenners sorgt, die wiederum thermoelektrisch überwacht wird (z. B. DE 34 09 334 C2, EP 0 433 209 A1);

2. Ionisationsüberwachung der heißen Abgase über der Brennerplatte (z. B. DE 26 21 801 C2);

3. Thermoelektrische Überwachung des Hauptbrenners (z. B. DE G 86 25 847, U1; DE 37 32 271 C2);

4. Periodische Funkenzündung des Hauptbrenners (z. B. DE 26 33 849 C3).

5. Glühzünder im Dauerbetrieb mit thermoelektrischer Überwachung (z. B. DE 26 41 274 C3).

In the case of a gas cooking device, one or more such gas radiation burners are arranged at a distance below a common glass ceramic plate known per se, a cooking zone being defined by each burner on the upper side of the glass ceramic plate. Each individual gas radiation burner is provided with an ignition device and with an ignition fuse. The ignition fuse is used to monitor the presence of the flame. If this goes out due to a malfunction, measures must be taken to prevent the unused fuel gas mixture from flowing out. The following solutions are currently common:

1. pilot flame, which ensures that the burner reignites, which in turn is monitored thermoelectrically (for example DE 34 09 334 C2, EP 0 433 209 A1);

2. Ionization monitoring of the hot exhaust gases above the burner plate (e.g. DE 26 21 801 C2);

3. Thermoelectric monitoring of the main burner (e.g. DE G 86 25 847, U1; DE 37 32 271 C2);

4. Periodic spark ignition of the main burner (e.g. DE 26 33 849 C3).

5. Glow igniter in continuous operation with thermoelectric monitoring (e.g. DE 26 41 274 C3).

These measures have the disadvantage that they are sometimes technically complex are and that the thermoelectric monitoring, for example, the ignition device or the main burner only an indirect control of the proper Represents operation of the burner.

The object of the invention is to create a cost-effective, reliable Safety device for gas radiant burners that monitor allowed directly at the location of the flame.

This object is achieved with a gas radiation burner with all the features of claim 1.

According to the invention, the burner plate itself is used for ignition protection. The invention takes advantage of the fact that the Usually used burner plate materials, such as. B. ceramics or metals, have a strongly temperature-dependent electrical resistance. The operating state of the burner can thus be simple on the measurement of the electrical resistance of the burner plate material be recorded.

It is sufficient to have the burner plate in the designated places to provide electrical connections with a suitable evaluation circuit are connected (may still need electrical insulation the burner plate is taken care of by the burner's metallic mixing chamber will). Depending on the signals received on the burner plate can then the existing on the burner plate via the evaluation circuit Ignition devices, such as glow igniters, spark igniters, etc. or else valves to interrupt the gas supply to the respective Burner plate can be operated.

All burner plate materials are suitable for the present invention, whose electrical resistance is in the for the present application relevant temperature range of 100 - 900 ° C sufficiently strong and changes sufficiently quickly. For a simple regulation, it is sufficient in the Rule off when the resistance is within the temperature range described changes by at least 10%, but a change by 20% is preferred. The The rate of change in temperature and thus the resistance must be be so large that the ones prescribed in the relevant standards (for Europe EN 30) Safety time between extinguishing the flame and blocking the Gas supply or re-ignition is maintained. Are particularly suitable high temperature resistant materials with strong positive or negative Temperature coefficient.

The invention is preferably used in burner plates made of fiber material, for example made of metal or ceramic fibers because their thermal Mass is very low, resulting in a rapid change in temperature and resistance leads. Burner plates from Global are particularly preferred Environmental solutions made of SiC fibers, such as z. B. in Advanced Gas-Powered Smoothtop, Proceedings of the International Appliance Technical Conference, Madison, Wisconsin, May 10-11, 1994. This Burner plate material not only has a suitable resistance / temperature curve on, it also speaks because of its low thermal Mass very quickly to changes in burner performance. details this is explained in the exemplary embodiment.

Suitable evaluation circuits are known in the prior art and in numerous embodiments. In the simplest case, an evaluation circuit has the following elements: current source, signal amplifier, switching amplifier and solenoid valve. The structure of such a circuit is known to the person skilled in the art. In the above case, the evaluation takes place via the absolute values of the resistance signal. So z. B. a predetermined resistance value that corresponds to a certain current can be used as a threshold value for triggering the re-ignition or switching off the gas supply. After successful ignition, a burner plate made of SiC fibers with the usual dimensions of z. B. 180 mm diameter, for example after 5 sec, a temperature of about 700 ° C. During heating, the specific electrical resistance of a suitable SiC fiber material drops, for example, from 2 × 10 ⁴ to below 10 ³ Ω × cm.

As the resistance value for a particular burner in both cold and is also known in the hot state, these two values, if necessary under Add a security surcharge to take into account a possible Resistance aging, to monitor the burner using a suitable evaluation circuit. The resistance is of the order of magnitude of the upper value, it shows that no flame burns what a new ignition or after several unsuccessful ignition attempts or After a predetermined period of time, the gas supply is switched off. If the resistance is in the range of the lower value, the burner burns perfectly. If the resistance is very small, there is a short circuit and if the resistance is extremely high, there is an interruption, both of which should lead to the gas supply being switched off for safety reasons.

As an alternative to evaluating the absolute values, the change in the resistance signal can also be used used over time. In this case, the Evaluation circuit additionally a differentiator. The change of the signal becomes very large when it is switched on or off (see also Figure 6) and their evaluation represents a procedure that also includes the safety times adheres to, but at the same time from slow material changes (such as aging) or a drift of the evaluation electronics is free. Do not kick when switching on, or when switching off, a change in resistance which is a sign that there is a short or an open circuit, what leads to the safety circuit responding.

For electronic evaluation, DC voltage, AC voltage or a pulse-shaped signal can be used. Since the resistance of the plate in Operation must be in a very specific area, this can be used for detection of short circuits or interruptions in the supply line between the burner plate and electronics are used. The evaluation can also be pulse-shaped Signals used to improve the security of the overall system as there is not only a need to have a voltage here (which may can also occur as a disturbance), but the signal is also correct Frequency must have.

The temperature measuring resistor according to the invention made of burner plate material can also be used to monitor the temperature of the burner plate become.

When using burner plates made of fiber material, e.g. B. SiC fibers, it may be useful as a temperature measuring resistor in the burner plate weave a single fiber or bundle of fibers and thread the ends through to perform the plate up or down. The advantage here is that one has an easier contacting of the (wire-shaped) conductor. Farther is a very well-defined geometry due to the fiber / the fiber bundle given the temperature sensor, so that only small tolerances in the manufacture given are.

Following this model, several fibers can also be laid on a board, so that the central area and the peripheral area are evaluated separately can. In this way it could be analogous to that in DE-PS 40 22nd 844 or US-PS 52 27 610 for glass ceramic cooktops with integrated temperature sensor described method from the different heating deduce the cause of the overheating from the middle and edge area. For example, you could choose between using bad dishes and Distinguish idling without dishes. In a typical arrangement Idle the temperature in the middle 950 ° C and at the edge 930 ° C while when using a bad pot in the middle 980 ° C and at the edge 900 ° C can be measured.

When using the safety device according to the invention, the Burner plates do not consist entirely of SiC; the only important thing is that the heated top layer consists of SiC. The burner plate can For example, for large formats, a sandwich structure from a mechanical Have support structure and the active fiberboard.

The present invention has the particular advantage that security monitoring is made exactly at the point where actually Flames arise or go out. A detour via indirect measurements, such as common in the prior art is not required. This results in greater security. Furthermore, the invention is distinguished Safety device through a simple and inexpensive construction out.

The invention is described below with reference to the figures and an exemplary embodiment explained in more detail:

Figur 1:

in einer graphischen Darstellung den Verlauf des spezifischen elektrischen Widerstandes als Funktion der Temperatur für verschiedene SiC-Modifikationen;

Figuren 2 bis 4:

in einer schematischen Querschnittsdarstellung Gasstrahlungsbrenner mit unterschiedlicher elektrischer Kontaktierung;

Figur 5:

in einer graphischen Darstellung den Verlauf des elektrischen Widerstandes als Funktion der Temperatur in der Aufheiz- und Abkühlphase gemessen an einer SiC-Brennerplatte gem. Ausführungsbeispiel;

Figur 6:

in einer graphischen Darstellung die erste Ableitung der Kurven aus Figur 5.

Show it:

Figure 1:

in a graphic representation the course of the specific electrical resistance as a function of temperature for various SiC modifications;

Figures 2 to 4:

in a schematic cross-sectional view gas radiation burner with different electrical contacts;

Figure 5:

in a graphic representation the course of the electrical resistance as a function of the temperature in the heating and cooling phase measured on a SiC burner plate acc. Embodiment;

Figure 6:

the first derivative of the curves from FIG. 5 in a graphical representation.

One can see in Fig. 1 for three different SiC modifications that of manufactured by Nippon Carbon and are commercially available, the course of the specific electrical resistance with the temperature. This representation was taken from a manufacturer's data sheet. Man recognizes that SiC materials with very different resistance profiles are producible. It is the commercially available SiC materials around materials whose resistance increases sharply with increasing temperature sinks. Because these materials are often used for electrical heating for SiC materials, the course of the resistance with temperature usually specified by the manufacturer. Details about on which way the resistance behavior of these materials is targeted manufacturers can not be influenced, however, announce. See the expert therefore set himself the task for the invention Safety device to find suitable SiC materials, so he will i. a. use the manufacturer's information mentioned above, or the Determine resistance behavior yourself experimentally. He will then choose such materials as burner plate material, their resistance changes as much as possible in the temperature range of interest.

In Figures 2 to 4 are different ways to attach the electrical contacts (1) to the burner plate (2) of a gas radiation burner (3).

In Fig. 2 the burner plate (2) is on the hot top during operation contacted. In this arrangement, the plate (2) must be of the metallic Mixing chamber (4) must be mounted electrically insulated. Alternatively, the 3 on the cold bottom during operation. In this case, the total resistance is made up of the parallel connection the resistors on the bottom and top of the plate (2) together. Is a Flame present, the resistance at the top drops so that the total resistance the arrangement drops. As shown in Fig. 4, the Contact made in the interior of the plate (2) on the underside, the plate (2) must not be installed insulated from the mixing chamber (4) become. The electrical resistance between the two contact points in the hotter interior of the burner plate is much lower than that Resistance in the outside area, so that the current flow towards the edge is negligible is. With this arrangement, the electrical resistance in the Operation a parallel connection of resistance at the bottom of the plate (2) and resistance at the top as in Fig. 3.

Contacting from below is preferred, since the temperature of the contact point is also so low in burner operation (<100 ° C) that many Contacting procedures can be applied, e.g. B. gluing with conductive Epoxy resin. On the top, the temperature can reach over 700 ° C, so that practically only welding processes can be used here.

In all of the arrangements described above, the igniter, such as customary in the prior art, mounted above the burner plate in the exhaust duct.

Embodiment

On the underside (facing away from the flames) of a burner plate made of SiC (α Modification) with a diameter of 14.5 cm and a thickness of 4 mm - 1 cm from the outer edge - two wires made of copper braid glued with conductive epoxy resin adhesive (type Elecolit 323 a + b).

The plate was placed on a plate used for gas cookers with a glass ceramic cover Mixing chamber mounted; the leads were through the chamber floor guided. The plate was connected to 6 V via a series resistor of 470 Ω AC voltage applied. The voltage across the series resistor was measured with a Single channel recorder recorded. The voltage drop became the electrical resistance of the burner plate during operation and its temporal Derivation calculated and shown in Figures 5 and 6. The curves represent the course of the calculated values when the ignition and extinction of the Brenners.

You can see from the fig. 5 and 6 that the resistance when turning on ("B on") drops so much that it is no problem in this experimental setup the flame is recognized within the safety time of 10 s prescribed in EN 30 becomes. Likewise, the flame will go out ("B off") within the Safety time of 60 s recognized.

5 shows the absolute value of the slope; the value is at Switch on ("B on") negative, switch off ("B off") positive. The rise is within a short time from small values so high that it is easy to evaluate. After about 10 s the changes in the signal are only still very low, so that the next switching operation is then already detected could be.

In Fig. 6, instead of the absolute measurands, rates of change are shown based on the further evaluation. The slopes of the curves 5 are very large in the on / off moment, so that controls are implemented with which the above security times not only adhered to, but even fell considerably short of it can be.

Claims (9)

1. Radiant gas burner having a safety device which has an ignition safeguard, integrated in the burner plate of the radiant gas burner, against gas escaping unconsumed, and a device, operationally connected thereto, for igniting the gas or interrupting the gas feed when the flame goes out, characterized in that the ignition safeguard comprises the temperature measuring resistor of the burner plate (2) in a limited subregion of the burner plate (2) which makes contact by means of terminal leads (1).
2. Radiant gas burner according to Claim 1, characterized in that the burner plate (2) consists of ceramic material.
3. Radiant gas burner according to Claim 1 or 2, characterized in that the burner plate (2) consists of fibrous material.
4. Radiant gas burner according to Claim 3, characterized in that the burner plate (2) consists of SiC fibres.
5. Radiant gas burner according to Claim 4, characterized in that continuous SiC fibres or SiC fibre bundles are woven into the burner plate (2) as temperature measuring resistor.
6. Radiant gas burner according to Claims 1 to 5, characterized in that electrical contact is made with the temperature measuring resistor on the underside of the plate.
7. Radiant gas burner according to Claim 6, characterized in that the electrical contact is provided in the inner region of the burner plate.
8. Radiant gas burner according to at least one of Claims 1 to 7, characterized in that mutually independent temperature measuring resistors make contact in each case in the inner region and edge region of the burner plate (2).
9. Radiant gas burner according to one of Claims 1 to 8, characterized in that the temperature measuring resistor is connected to an evaluation unit which detects both interruption and a short circuit from the fact that the ACTUAL value of the resistor or the rate of change of the resistance lies outside a prescribed range.

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