EP1002997B1 - Method for controlling a fuel/air ratio of full premix gas burner - Google Patents

Abstract

The method involves measuring ionisation signals in the flame region using an ionisation electrode, detecting the fan speed, deriving a signal representing the current air ratio from the current ionisation signal and comparing it with a demand value, whereby the fan speed is taken into account in deriving the first signal and selecting the demand value, and deriving a control signal for the gas regulating valve from the comparison. A signal representing the current power is detected and compared with a defined value to obtain information about the burner's operating state.

Description

The invention relates to a method for air quantity control of a provided with a fan and with a gas control valve at least teilvormischenden, preferably vollvormischenden gas burner, being measured in the flame ionization signals using an ionization electrode, the fan speed is detected from the current ionization signal for the current air ratio representative first signal derived and this is compared with a predetermined setpoint, the fan speed in the derivative of the first signal and / or in the selection of the setpoint is taken into account, and from the comparison, a control signal for the gas control valve is derived.

The air quantity control of gas burners is becoming more and more important in practice. With the help of air flow control, it is possible to operate gas burners in the optimum working range, in which the pollutant emissions, in particular the CO and NO _x emissions are low, the thermal load of the gas burner is very uniform and both the burning behavior and the efficiency of the gas burner are optimal. It has been found that the optimum working range is between 1.15 and 1.3 for an air ratio. With a Luftzahlregelung also the susceptibility of the gas burner can be reduced and a safe and quiet burner operation can be ensured.

Instead of a one-time set of air numbers, air-flow control is required because the composition of the fuel gas supplied by the supply network can vary greatly. Accordingly, the gas quality varies greatly, in particular the Wobbe index of the fuel gas. If the gas composition of the fuel gas changes, then the air ratio control intervenes and changes the gas supply with the aid of the gas control valve in such a way that the gas burner continues to operate at the desired air ratio.

For the air number control, the air ratio can be determined with the aid of various measured variables. However, it has proved useful to determine the air ratio via the ionization signal detected with the aid of an ionization electrode (cf DE-C2-196 27 857). The ionization electrode provides a stable, easy to maintain and simultaneously low-priced air-fuel sensor, which can also be installed with very little effort, if it is not already available for flame monitoring anyway. In addition, the ionization signal allows a very reliable and accurate determination of the air ratio.

From EP-A2-0 770 824 a method and for controlling a gas burner with the aid of an ionization electrode is known in which a calibration cycle is forcibly traversed at intervals.

DE-U1-296 12 014 discloses a gas burner for atmospheric premixing operation having at least two flame detection elements mounted at a different distance from the burner surface and controlling combustion with their signals via a regulator.

If the fan speed is known, the current air ratio can be determined from the ionization signal and controlled with a characteristic curve characteristic of the respective fan speed.

Alternatively, from the ionization signal and the fan speed, using appropriate evaluation circuits, a signal representative of the current air ratio can be derived, which in a specific power range is essentially independent of the burner output. This has the advantage that the air ratio over this power range can be controlled with a single characteristic. The representative of the current air ratio signal is compared with the setpoint for the desired air ratio setpoint of the characteristic, and from this comparison, a control signal for the gas control valve is derived.

However, it has been shown that over time changes in the supply air or exhaust air system of the gas burner can occur, which cause the gas burner, despite the above Luftzahlregelung no longer works in the optimal working range.

The object of the invention is therefore to enable in the aforementioned method, the verification of the operating state of the gas burner.

This object is achieved in that a representative of the current burner power second signal is detected and this is compared with a predetermined value, being derived from this comparison information about the operating state of the gas burner.

The invention is based on the finding that, if changes occur in the supply air or exhaust air system of the gas burner, the burner output set via the fan speed no longer corresponds to the actual burner output. In this case, the missing correlation between fan speed and burner power can be detected and compensated by detecting another signal representative of the current power. If the second signal representative of the current power deviates from the value specified for the set power, the gas burner does not produce the desired power.

In this case, z. For example, if the deviation exceeds a predetermined threshold, a service indicator will be activated. In the case of a particularly large deviation, a shut-off mechanism can also intervene, which automatically shuts off the gas burner. Alternatively, the fan speed can be varied until the representative of the current power second signal corresponds to the predetermined value. In this embodiment, the gas burner then has to be operated in consideration of the changed correlation between fan speed and firing power. In this way, a gas burner can be operated safely and with the desired performance over a long period of time.

A development of the invention is characterized in that the first signal is used as the predetermined value for the comparison with the second signal.

Advantageously, the power consumption of the fan or the temperature level of the boiler or the air mass flow is detected by the fan as a second signal.

A particularly preferred alternative embodiment is characterized in that the second signal is derived from a current ionization signal, the second signal being representative of both the current power and the current air ratio. This embodiment is based on the finding that the ionization signal itself can be used to check the current power. Since the ionization signal is power-dependent, the current operating performance in a very wide power range can be checked according to customer requirements. If the second signal derived from the ionization signal and representative of the current air ratio and the current power deviates from the value specified for this air ratio and this power, the gas burner does not produce the desired power. The required steps can then be initiated.

A development of the invention is characterized in that a signal representative of both the current power and the current air ratio is used as the first signal, wherein the first and the second signal have a different dependence on the air ratio and / or the power. Advantageously, the ionization signals themselves can be used as the first and / or second signal.

The method according to the invention can be implemented particularly simply by measuring the current ionization signal, from which the first signal is derived, with the aid of a first supply voltage, and the current ionization signal, from which the second signal is derived, by means of a second supply voltage becomes.

Advantageously, the ionization signals are measured by applying an AC voltage, preferably of 230V, to the ionization electrode. The polarity effect of the flame causes that only in each case a half-wave, an ionization current flows. As a result, the ionization signal can be obtained from a DC component of the tapped voltage derived. A measured in this way ionization signal can be evaluated very reliable and accurate. For evaluation, the signal is usually first applied to a low-pass filter.

Alternatively, the ionization signals may be measured by applying a triangular voltage or a square wave voltage to the ionization electrode.

The ionization signal measured to derive the signal representative of the current air ratio and the ionization signal measured to derive the signal representative of the current air ratio and current power need not be measured by applying the same voltage to the ionization electrode. For example, For example, the ionization signal for determining the signal representative of the current air number can be measured with the aid of an alternating voltage and the ionization signal for determining the signal representative of the current air number and for the current power with the aid of a triangular voltage or a rectangular voltage or vice versa.

Preferably, the ionization signals are used alternately to derive the first signal and to derive the second signal.

The gas burner can be kept in the long term in the optimum operating range that the second signal at regular intervals, for. B. once a minute, is detected.

To calibrate the system, it is advantageous that at the start of operation of the gas burner reference measurements are carried out in which reference signals for different powers and different air numbers are detected and stored as a predetermined value for comparison with the second signal. If the second signal is derived from a current ionization signal, reference measurements are preferably carried out at the start of operation of the gas burner, in which detected reference ionization signals for different fan speeds and different air numbers and these are stored as a predetermined value for comparison with the second value.

Further advantageous embodiments of the invention are characterized in the subclaims.

• Fig. 1 ein erstes erfindungsgemäßes Ausführungsbeispiel veranschaulichendes Diagramm; und
• Fig. 2 zwei ein zweites erfindungsgemäßes Ausführungsbeispiel veranschaulichende Diagramme.

In the following the invention will be explained in more detail with reference to two illustrated in the drawings embodiments. In the drawing show:

• Fig. 1 shows a first embodiment of the invention illustrative diagram; and
• 2 shows two diagrams illustrating a second embodiment according to the invention.

Fig. 1 shows a diagram in which the voltage of a measuring signal against the air ratio λ is plotted. Six different measured signal curves are shown. The signal waveforms labeled Signal 1 are the signals representative of the current air ratio. These were derived from the measured at an alternating voltage of 230V ionization signal and the fan speed using a special evaluation circuit. The signals are shown for different powers. As can be seen, the curves lie almost completely on top of each other, d. H. These signals are actually power-independent.

The signal waveforms labeled Signal 2 are the signals representative of the current air ratio and current power. For measurement, an AC voltage of 230 V was again applied to the ionization electrode and the ionization signal then passed through a low-pass filter, bypassing the special evaluation circuit. It is noticeable that the curves, which in turn are recorded for different performances, differ greatly from one another. The voltage differences between the Meßsignalkurven are particularly large at a given air ratio in the lower power range. Thus, a very large power range of the gas burner can be reliably monitored, that the air ratio control is carried out in normal operation using the superimposed signal 1 characteristics and to check the power, the AC voltage of 230V is applied to the ionization electrode and the special evaluation circuit is bypassed. By comparing the measured value obtained in the latter measurement with the various predetermined signal 2 curves, it can be determined whether the gas burner actually provides the desired performance.

Fig. 2 shows two diagrams in which the ionization signal is plotted against the fan speed for a second embodiment of the invention. The measured values shown were recorded at a constant air ratio λ of 1.3. In both diagrams, the ionization signals are shown at a supply voltage of the ionization electrode of 50V and 230V. The upper diagram shows the normal operating condition of the boiler. At the set gas supply results at a speed of 2000min. ^{-1 is} an ionization signal at a supply voltage of 50V of 109. This is the setpoint for controlling the air ratio of 1.3. To check the system, the supply voltage of the ionization electrode is switched at regular intervals to the control voltage of 230V. As can be seen from the above diagram, in this case the ionization signal is only approximately 102. The difference between the two signals is thus approximately 7. As long as the difference between these two detected ionization signal values is in the range of 7, the operation of the gas burner is in the optimum operating range secured.

If the air-exhaust system is clogged, z. B. by a disturbance in the chimney, the entire workspace moves towards higher speeds, as can be seen in the lower diagram in Fig. 2. In the case of the given gas supply, the difference between the two ionization signals at 2000 min. ^-1 about 14. By comparing this deviation with the deviation of 7 in the normal operating condition can be easily determined that the gas burner is not the desired performance he brings. While in the first embodiment, the evaluation circuit has been switched, two ionization signals are generated in this second embodiment by switching the supply voltage, which have a different dependence on the current performance of the gas burner. In both cases, it is possible to operate a gas burner for a long time safely and with the desired performance.

Numerous modifications are possible within the scope of the invention. For example, For example, the ionization signal can be detected by applying a voltage of any shape to the ionization electrode. Likewise, the ionization signal can be measured by means of a DC voltage. For sampling the ionization signal from the ionization electrode, the same transducer can be used to derive the signal representative of the current air ratio and the signal representative of the current air ratio and current power. Alternatively, two transducers can be assigned to the ionization electrode or even two separate ionization electrodes can be arranged in the flame region of the gas burner. Finally, the reference measurements can be carried out by the manufacturer before the start of operation.

Claims (14)

1. Method for controlling the fuel/air ratio of an at least partially premixed gas burner provided with a fan and a gas control valve, whereby ionisation signals are measured in the flame zone with the aid of an ionisation electrode,
   the fan speed is recorded,
   a first signal which is representative for the current fuel/air ratio is derived from the current ionisation signal and this is compared with a set value, whereby the fan speed is taken into account when deriving the first signal and/or selecting the set value, and
   an actuating signal for the gas control valve is derived from the comparison,
   characterised in that
   a second signal representative for the current burner output is recorded and compared with a preset value,
   whereby information on the operating condition of the gas burner is derived from this comparison.
2. Method according to claim 1, characterised in that the first signal is used as the preset value for comparison with the second signal.
3. Method according to claim 1 or 2, characterised in that the power consumption of the fan or the temperature level of the boiler or the air mass flow through the fan is recorded as the second signal.
4. Method according to claim 1 or 2, characterised in that the second signal is derived from a current ionisation signal, the second signal being representative both for the current burner output and for the current fuel/air ratio.
5. Method according to claim 4, characterised in that a signal which is both representative for the current burner output and for the current fuel/air ratio is used as the first signal, the first and the second signal exhibiting a different dependence on the fuel/air ratio and/or the output.
6. Method according to any one of claims 1 through 5, characterised in that the ionisation signals themselves are used as the first and/or second signal.
7. Method according to any one of claims 4 through 6, characterised in that the current ionisation signal from which the first signal is derived is measured with the aid of a first supply voltage, and the current ionisation signal from which the second signal is derived is measured with the aid of a second supply voltage.
8. Method according to any one of claims 1 through 7, characterised in that the ionisation signals are measured by applying an AC voltage to the ionisation electrode.
9. Method according to any one of claims 1 through 7, characterised in that the ionisation signals are measured by applying a delta voltage or a square wave voltage to the ionisation electrode.
10. Method according to any one of claims 4 through 9, characterised in that the ionisation signals are used alternately to derive the first signal and to derive the second signal.
11. Method according to any one of claims 1 through 10, characterised in that the second signal is recorded at regular intervals, e.g. once per minute.
12. Method according to any one of claims 1 through 11, characterised in that when the gas burner is started up reference measurements are taken during which reference signals for various outputs are recorded and stored as preset values for comparison with the second signal.
13. Method according to any one of claims 1 through 12, characterised in that, when the gas burner is started up, reference measurements are taken during which reference ionisation signals for various fan speeds and various fuel/air ratios are recorded and stored as preset values for comparison with the second signal.
14. Method according to any one of claims 1 through 13, characterised in that the gas burner is switched off or recalibrated or a fault is displayed when the second signal deviates from the preset value by more than a preset threshold value.