EP2017531B1 - Method for monitoring an ionisation electrode signal in burners - Google Patents

Description

The invention relates to a method for checking the ionisationselektrodensignals in burners.

Ionization electrodes are used to detect the presence of a flame. In a flame, ions can move freely. If a voltage is applied to two electrodes located in the flame area, a current flows in the flame. When the flame goes out, the current flow comes to a standstill. If the measured ionization current falls below a certain limit value, the regulation of the burner locks the gas supply in order to avoid uncontrolled gas leakage.

The ionization current depends on several factors. For example, the ionization current decreases when the surface of the electrodes is covered by a deposition layer due to the influence of the flame.

In the worst case, it may come to a fuel gas shutdown despite the presence of a flame, if the ionization electrodes are too heavily coated with deposits.

In addition, the ionization current of the fuel gas-air ratio λ is dependent. For stoichiometric combustion, the ionization current is maximum.

A method for controlling a gas-fired burner of a heating system by means of the measurement of carbon monoxide emission in the exhaust gas is from DE 103 00 602 A1 known. Here, the fuel gas-air mixture of the burner is enriched, whereby the air ratio decreases. An exhaust gas sensor measures the carbon monoxide emission in the exhaust pipe and forwards the signal to a control. If the excess of air falls below a certain level, usually around 8% of excess air, carbon monoxide emissions rise steeply. If the regulation that the carbon monoxide emission has exceeded a predetermined threshold value, the mixture is not further enriched. The mixture is then defined as lean to achieve optimum combustion.

The EP 770 824 A2 discloses a method for controlling a fuel gas-air mixture of a burner, in which the ionization current or the ionization voltage is detected. During the calibration procedure, the fuel gas-air mixture is enriched and the ionization voltage is measured. If the latter reaches a maximum, the combustion is stoichiometric. The mixture is then deliberately emaciated. The absolute value of the ionization voltage may vary due to wear, contamination or bending. If the voltage maximum does not reach a certain value, a fault signal is triggered and the burner is switched off.

Out AT 411 189 B It is known that with near-stoichiometric combustion, the carbon monoxide emissions in the exhaust gas rise very sharply. To control the fuel gas to air ratio of a burner, the mixture is enriched until high carbon monoxide emissions are measured; the mixture is then deliberately emaciated. Regarding the ionization current measurement teaches the AT 411 189 B that a plausibility test can be carried out. It is known from the prior art that in the field 1.0 <λ <1.3 when enriching both the ionization current, as the carbon monoxide emissions increase steadily. Therefore, the sees AT 411 189 B For example, in the case where the ionization current increases as carbon monoxide emissions fall, there must be a fault, so the calibration process is aborted and the control continues to operate with the old setpoints. If the ionization current falls during enrichment, the combustion should be substoichiometric; the carbon monoxide emissions would then have to be extremely high. If no such emissions are measured, according to the teaching of AT 411 189 B there is a mistake.

The invention has for its object to provide a method that detects a change in the ionization electrode signal early on to initiate countermeasures before the failure can.

According to the invention this is achieved according to the features of the independent claim, characterized in that in a gas burner with a device for separate control of the fuel gas and air quantity and an exhaust gas sensor for measuring the carbon monoxide concentration or concentration of unburned hydrocarbons, the fuel gas-air mixture is enriched until the exhaust gas sensor detects a signal which corresponds to a predetermined or calculated threshold value, for this state the ionization electrode signal of an ionization electrode is detected and compared with a reference value, wherein in the case in which the ionization electrode signal falls below the reference value a warning is issued.

Advantageous embodiments of the invention will become apparent from the features of the independent claims.

Thus, the mean value of the at least two last ionisationselektrodensignale can be formed to give instead of single influences trends greater weight. If a second reference value, which is lower than the first reference value, falls below, then the heater is switched off to avoid unsafe conditions.

• Figur 1 eine Vorrichtung zur Durchführung des erfindungsgemäßen Verfahrens und
• Figur 2 den Verlauf des Ionisationsstroms und der Kohlenmonoxidemissionen während der Durchführung des erfindungsgemäßen Verfahrens.

The invention will now be explained in detail with reference to FIGS. Hereby show:

• FIG. 1 an apparatus for carrying out the method according to the invention and
• FIG. 2 the course of the ionization and the carbon monoxide emissions during the implementation of the method according to the invention.

A heating system according to Fig. 1 has a burner 1 with a surrounding heat exchanger 10, to which an exhaust pipe 9, in which an exhaust gas sensor 6 is connected. The burner 1, a fan 2 is connected upstream. On the input side of the blower 2 is an air intake line 13, in which also a fuel gas line 12, which is separated by a gas valve 4 from the fuel gas supply 11, extends. The gas valve 4 has an actuator 5. The fan 2 has a drive motor 7 with speed detection 8. Actuator 5, drive motor 7, speed detection 8 and exhaust gas sensor 6 are connected to a controller 3, which has a memory module 31 and computing module 32. Also with the control is an ionization electrode 14, which is positioned just above the burner 1, connected.

When burner operation of the controller 3, for example due to a not shown room thermostat in conjunction with a flow temperature detection, also not shown in the computing module 32, a target power of the burner 1 is calculated. In the memory module 31 is to the desired power a target signal for the fuel gas and Amount of air deposited. With these desired signals, the blower 2 is driven with its drive motor 7 and its speed detection and the gas valve 4 with its actuator 5, whereby a fuel gas-air mixture flows into the blower 2 and from there to the burner 1. The mixture is burned on the outer surface of the burner 1, flows through the heat exchanger 10 and then flows through the exhaust pipe 9 into the open air.

Fig. 2 shows the relationship between carbon monoxide concentration CO, ionization current I and combustion air ratio λ. Here, three different ionization current lines for different states of the ionization electrode are shown. In order to achieve complete combustion, a combustion air ratio λ of 1.0 is theoretically necessary. $$\mathrm{\lambda }=\frac{{\mathrm{m}}_{\mathrm{L}}}{{\mathrm{m}}_{\mathrm{L}.\min }}$$

Here, m _{L is} the actual air flow and m _{L, min is} the stoichiometric air flow. The combustion of hydrocarbons into carbon dioxide always produces carbon monoxide as an intermediate. Due to the limited reaction time in the heat affected zone and insufficient mixing of fuel gas and air, in practice, however, a certain excess air is necessary to ensure complete burnout. Therefore, a CO value of well over 1000 ppm is usually reached at just over-stoichiometric combustion. Only with an excess of air of about 10%, the carbon monoxide emissions in the fully reacted exhaust gas fall significantly and reach in conventional burners values well below 100 ppm. As the air ratio increases, however, the combustion temperature drops because of the proportion of inert gases; the combustion reaction is slowed down and the reaction at the heat exchanger stops. Therefore it is off an air surplus of about 80%, a significant increase in carbon monoxide emissions.

At the beginning of the process according to the invention is any fuel gas to air ratio. The control 3 continuously controls the actuator 5 of the gas valve 4 in such a way that more and more fuel gas passes into the blower 2 at the same amount of air. As a result, the mixture is enriched; the air ratio drops. The exhaust gas sensor 6 measures the carbon monoxide emission in the exhaust pipe 9 and forwards the signal to the control 3. If the controller 3 registers that the carbon monoxide emission has reached or exceeded a threshold CO _limit specified in the memory module 31, then the mixture is not further enriched. It is known that such carbon monoxide emissions are achieved at an air ratio of about 1.08. First, it is assumed that a new ionization electrode; the ionization electrode signal is therefore not reduced. The ionization electrode signal I _{1 of} the ionization electrode 14 at the predetermined threshold CO _limit is measured and compared in the calculation module 32 of the controller 3 with a first reference value I _limit from the memory module 31. Since the ionization electrode signal I _{1 is} greater than the first reference value I _limit , no further measures are necessary.

In a later implementation of the method according to the invention, the ionization electrode is already somewhat provided with deposits; the ionization electrode signal is lower. At the predetermined threshold CO _limit , the ionization electrode signal I _{2 of} the ionization electrode 14 is lower than at the beginning. Since the ionization electrode signal I _{2 is} still greater than the first reference value I _limit , no further measures are necessary.

In an even later implementation of the method according to the invention, the ionization electrode is provided with strong deposits; the ionization electrode signal is significantly lower than at the beginning. At the predetermined threshold CO _limit , the ionization electrode signal I _{3 of} the ionization electrode 14 is smaller than the first reference value I _limit . Therefore, the control 3 gives an indication of maintenance. This indication can be made, for example, in the form of a warning light or via a remote data connection to a specialist tradesman.

If the ionization electrode signal falls below a second reference value I _switch-off , the fuel gas supply to the burner 1 is switched off.

According to the invention, instead of the predetermined threshold CO _limit , a gradient (Δ CO / Δ λ) _{limit can also be} preset. Furthermore, instead of a single measurement, averaging can take place over several measurements. It can be compared both with a given reference value and with the measurements of previous measurements.

Claims (3)

1. Method to test the ionisation electrode signal of an ionisation electrode (14) of a gas burner (1), in particular having bellows (2), having an electronic control (3), which produces a target signal for the quantity of combustible gas and the quantity of air for a predetermined burner capacity, a device to regulate the quantity of combustible gas (4, 5) and an exhaust gas sensor (6), which produces a signal equivalent to the carbon monoxide concentration or the concentration of unburnt hydrocarbons, wherein the mixture of combustible gas and air is enriched until the exhaust gas sensor (6) detects a signal, which corresponds to a predetermined or calculated threshold value alone or in connection with at least one further signal, characterised in that in this state the ionisation electrode signal of an ionisation electrode (14) is detected and is compared to a reference value or to at least one measured value from earlier tests, wherein in the case in which the ionisation electrode signal falls below the reference value or substantially falls below at least one earlier measured value, a warning is issued.
2. Method to test the ionisation electrode signal of an ionisation electrode (14) of a gas burner (1) according to claim 1, characterised in that the average value of the at least two last ionisation electrode signals is formed as a reference condition and is compared with the reference value or the average value of the at least two preceding ionisation electrode signals as a reference condition.
3. Method to test the ionisation electrode signal of an ionisation electrode (14) of a gas burner (1) according to claim 1 or 2, characterised in that in the case of a second reference value being fallen below, which is lower than the first reference value, or in the case in which at least one earlier measured value is significantly fallen below, the gas burner (1) is switched off.

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