Flame guarding system

Abstract

 The invention relates to a flame monitoring system (1) for a combustion chamber (3) provided with an ionisation sensor (4), comprising a signal generator (8) for supplying a monitoring signal (V_AC) to the ionisation sensor (4) and a signal measuring unit (11) for measuring at least one ionisation signal (V_DC) from the ionisation sensor. The flame monitoring system (1) is further arranged for interrupting the supply of the monitoring signal (V_AC) to the ionisation sensor (4) during at least one interval (to) and measuring a zero signal (V₀) substantially during said interval (t₀).

Description

[0001] The invention relates to a flame monitoring system for a combustion chamber provided with an ionisation sensor, comprising a signal generator for supplying a monitoring signal to the ionisation sensor and a signal-measuring unit for measuring at least a first DC current signal subsequent to the monitoring signal.

[0002] When gases are combusted in combustion devices used for heating purposes, such as central heating installations, water heaters, geysers and furnaces, carbon dioxide and water, among other substances, are formed when air is supplied, e.g. in accordance with the reaction

        CH₄ + 2O₂ → CO₂ + 2H₂O

During such a reaction, free ions and charged particles are released.

[0003] An ionisation sensor utilises said free ions and said charged particles for detecting the presence of a flame. If an AC voltage is supplied to the ionisation sensor, the free ions and the charged particles cause a rectifying effect.

[0004] EP-A-1 300 362 describes a gas burner fitted with a flame monitoring system comprising an ionisation electrode, which extends into the flame. The ionisation electrode oxidises during use, causing the measuring signal from the ionisation electrode to attenuate. To overcome this, a control system is provided which uses an amplification factor in dependence on the attenuation of the measuring signal and which delivers an error signal when the amplification factor reaches a maximum value.

[0005] One drawback of such combustion devices is the fact that the measuring signal may not be sufficiently accurate for a reliable flame monitoring.

[0006] It is an object of the invention to provide a flame monitoring system by means of which a more accurate measuring signal can be obtained.

[0007] This object is accomplished by a flame monitoring system according to the invention, which is characterized in that the flame monitoring system is further arranged for interrupting the supply of the monitoring signal to the ionisation sensor for at least a time interval and measuring a zero signal substantially during said time interval. Such a system provides information on any signals, noise or interference in the system, which information can be used for correcting the ionisation signal.

[0008] In a preferred embodiment of the invention, the system is arranged for measuring the zero signal from the ionisation sensor. It has become apparent that the presence of moisture and/or dirt at the ionisation sensor may lead to a galvanic voltage, which interferes with the ionisation signal. In particular it has become apparent that such a problem occurs in high efficiency boilers, in which a lot of water is produced during the combustion process. Another field of application in which a lot of water is produced is the production of hydrogen from natural gas for use in fuel cells. By suppressing the monitoring signal for a particular interval or interrupting the supply thereof to the ionisation sensor, information about the zero signal caused by the galvanic voltage is obtained in such a case at the ionisation sensor. In that case the zero signal is the signal that is received from the ionisation sensor when no monitoring signal, such as an AC voltage or an AC current, is presented or supplied to the ionisation sensor. The zero signal can be used for providing the corrected ionisation signal as a flame detection signal.

[0009] Other forms of DC interference, such as an offset of an amplifier in the flame monitoring system, can likewise be measured during the space of time in which no monitoring signal is used.

[0010] In a preferred embodiment of the invention, the flame monitoring system furthermore comprises a control unit for making available a flame detection signal, i.e. the corrected ionisation signal. It has become apparent that the flame detection signal can be obtained by subtracting the zero signal from the ionisation signal. Other methods of processing the zero signal are also possible within the framework of the invention. It is also possible, of course, to measure the zero signal first and then the ionisation signal by supplying a monitoring signal.

[0011] In a preferred embodiment of the invention, the signal-measuring unit comprises a low-pass filter. Said low-pass filter makes it possible for both the DC component of the disturbed ionisation signal and the zero signal, which is likewise a DC signal and/or comprises a DC component, to be processed by the signal measuring unit, whilst the AC component of the uncorrected ionisation signal is repressed or eliminated. The signal measuring unit provides an average value of a DC ionisation voltage on the output, e.g. by means of an integrator, on the basis of the measured ionisation current when a monitoring signal is supplied to the ionisation sensor. The signal-measuring unit furthermore provides the DC voltage of the zero signal.

[0012] Preferably, the low-pass filter comprises an input, which is connected to ground via a capacitor. Said capacitor preferably has a large capacity, so that the AC signal being presented is kept out of the low-pass filter in the case of a short-circuit at the ionisation sensor. Said capacitor furthermore helps to ensure that substantially only the DC component of the ionisation signal is supplied to the signal-measuring unit.

[0013] In a preferred embodiment of the invention, the flame monitoring system further comprises a transformer for supplying the monitoring signal. The transformer provides a galvanic separation between the generator of the monitoring signal and the ionisation sensor. The secondary winding of the transformer is preferably connected in series to the ionisation sensor in order to enable a current measurement through the flame.

[0014] In a preferred embodiment of the invention, the interruption of the supply of the monitoring signal takes place by deactivating the signal generator during the interval. This may be effected by means of a switch, for example, which is controlled from a control unit. Such a construction is simple, inexpensive and fast in comparison with, for example, the use of a relay in the signal line for supplying the monitoring signal to the ionisation sensor.

[0015] The invention also relates to a combustion device comprising a flame monitoring system as discussed above. Such a combustion device may e.g. be a gas combustion device as used in central heating installations, a central-heating boiler, a water heater, a geyser or a furnace. The combustion unit may comprise further control means, e.g. for using the zero signal for controlling the combustion device, e.g. turning off, disabling and/or locking the device if it becomes apparent, also after correction of the ionisation signal by means of the zero signal, that no flame is present in the combustion chamber. The combustion device may also be disabled or locked if a flame is present in the combustion chamber, although in fact this should not be the case. The invention is also suitable for monitoring the zero signal in itself.

[0016] The invention also relates to a method for monitoring a flame in a combustion chamber provided with an ionisation sensor, which method comprises a zero measuring step, wherein a zero signal is measured without a monitoring signal being supplied. Such a zero measuring step provides information on signals in the flame monitoring system that may interfere with the ionisation signal. Preferably, the zero measuring signal from the ionisation sensor is measured.

[0017] This information may be used for correcting the ionisation signal, e.g. by carrying out the further steps of

• supplying a monitoring signal to the ionisation sensor;
• measuring an ionisation signal in response to the monitoring signal;
• generating a flame detection signal, comprising the arithmetic step of subtracting the zero signal from the ionisation signal.

[0018] With the method according to the invention, the monitoring signal is preferably an AC voltage having a frequency varying between 1 kHz and 100 kHz, more preferably between 20 kHz and 50 kHz. The lower limit has been selected with a view to keeping the capacity of the transformer in the supply line for the monitoring signal and the value of the input current for the transformer within bounds. Furthermore, a frequency of e.g. 20-50 kHz makes it possible to use a low-pass filter of simpler design than in the situation in which a frequency of e.g. 50Hz is used. Such a simple design is important with a view to enabling a quick measurement of the zero signal by the filter, so that the supply of the monitoring signal can be resumed again. The maximum frequency has been selected on the basis of the consideration that it is preferable not to produce any significant signals in the radio frequency range by means of higher harmonics of the monitoring signal or take measures geared thereto. It should be noted, however, that the method according to the invention can be carried out irrespective of the frequency of the monitoring signal.

[0019] In a preferred embodiment of the invention, the supply of the monitoring signal is interrupted for an interval of 5-50 ms, e.g. 20 ms. The zero measurement preferably takes as little time as possible, after which the measurement of the ionisation signal can be resumed. After all, the ionisation signal is an indicator for the presence of a flame, and if said flame is no longer present, this must be indicated as quickly as possible and be followed by an action such as the turning off of the combustion device. In Europe the requirement is that the combustion device is typically disabled within 1 second after a flame error.

[0020] In a preferred embodiment of the invention, the method further comprises the step of determining the height of the flame on the basis of the flame detection signal. The value of the DC component of the ionisation signal or flame detection signal is a measure of the height of the flame.

[0021] The invention finally relates to an automatic burner for carrying out the above method. The automatic burner is the electrical component in a combustion device that controls elements such as the fan, the valves for the gas supply, the air supply and the like, the flame height, etc.

[0022] EP-A-1 176 364 describes a combustion device and a method for controlling a combustion device, wherein two measurements are carried out with an ionisation sensor, using different gas compositions, for eliminating errors, wherein the relative change of the ionisation signal is used for adjusting the gas-air proportion. Said measurement does not comprise a zero measurement, however, so that effects such as a galvanic DC voltage cannot be observed.

[0023] The invention will be illustrated in more detail hereinafter with reference to the appended figures, which show a preferred embodiment of the invention. It stands to reason that the scope of the invention is by no means limited by this specific, preferred embodiment.

[0024] In the figures:

Fig. 1 shows a flame monitoring system and a combustion device according to the prior art;

Fig. 2 shows a monitoring signal and an ionisation signal according to the prior art;

Fig. 3 schematically shows a flame monitoring system and a combustion device according to a preferred embodiment of the invention;

Fig. 4 schematically shows a signal measuring unit according to a preferred embodiment of the invention; and

Fig. 5 shows an interrupted monitoring signal, an ionisation signal and a zero signal as obtained by using the flame monitoring system as shown in Figs. 3 and 4.

[0025] Fig. 1 schematically shows a flame monitoring system 1 and a combustion device 2 comprising a combustion chamber 3, which is provided with an ionisation sensor 4 that is suspended via a ceramic holder 5. The combustion chamber 3 further comprises a burner bed 6, on which the flames 7 illustrate the combustion process. It will be apparent to those skilled in the art that the combustion chamber 3 will generally comprise other elements (not shown) as well, such as an air inlet and a flue gas outlet. The ionisation sensor 4 is suspended in such a manner that it extends into the flame 7, if such a flame 7 is present.

[0026] The flame monitoring system 1 comprises a signal generator 8 for supplying an alternating monitoring signal, such as an AC voltage V_AC or an AC current I_AC. The description below is based on the use of an AC voltage V_AC as the alternating monitoring signal. The monitoring signal V_AC is supplied to the ionisation sensor 4 via a transformer 9. The secondary winding of the transformer 9 is connected to ground via a signal line 10 and an impedance Z. The signal line 10 is further connected to a signal-measuring unit 11.

[0027] Fig. 2 shows a monitoring signal V_AC as can be supplied to the ionisation sensor 4. If the signal measuring unit 11 is arranged for measuring a DC signal, an ionisation signal is measured in the form of an ionisation current I_DC at the signal measuring unit 11 as a result of the rectifying effect of the flame 7 as discussed in the introduction, and converted into a corresponding voltage V_DC. The ionisation current I_DC typically varies from 0-25µA in dependence on the dimension of the flame.

[0028] The measured ionisation current value I_DC may be inaccurate, due to interference in the flame monitoring system 1. It has become apparent that a galvanic voltage V_o may be locally generated as a result of the presence of moisture and/or dirt at the ionisation sensor 4, which voltage interferes with the ionisation signal I_DC. It has in particular become apparent that such a problem occurs in high efficiency boilers 2, in which a lot of water is produced during the combustion process.

[0029] Figs. 3 and 4 schematically show a flame monitoring system 1 and a combustion device 2 according to a preferred embodiment of the invention. In Fig. 3, components identical or similar to the components that are shown in Fig. 1 are indicated by the same numerals as in Fig. 1.

[0030] Fig. 3 schematically shows a flame monitoring system 1 and a combustion device 2 provided with an ionisation sensor 4. The combustion chamber 3 is connected to ground. The secondary winding of the transformer 9 is connected to the ionisation sensor 4 via a signal line 15 and to an impedance Z, which is connected to ground, so that the signal measuring unit 11 can measured the signal through the flame 7, via the signal line 10. A short signal line 10 in the order of a few mm may be used. The signal-measuring unit 11 is connected to a control unit 13, by means of which processing operations and/or computations can be carried out on the measured signal from the signal-measuring unit, via a signal line 12. The control unit 13 may comprise an AD converter for converting the signal from the signal-measuring unit 11 into a digital form suitable for the control unit. Alternatively, the AD converter may be a separate component.

[0031] The control unit 13 may also drive the signal generator 8. This may take place by controlling the switch 14, with the position of the switch 14 determining the supply of the monitoring signal V_AC to the ionisation sensor. The control unit 13 can interrupt the supply of said control signal V_AC via the switch 14 to carry out a zero measurement.

[0032] A preferred embodiment of the signal-measuring unit 11 is shown in Fig. 4. In this embodiment, the signal-measuring unit 11 comprises a low-pass filter made up of the resistor R2 and the capacitor C1, which performs an integrator function with the operational amplifier 15. The positive input of the amplifier 15 is connected to ground. A capacitor C2 is connected between the positive and the negative input of the amplifier 15. Said capacitor C2 preferably has a large capacity, e.g. in the order of 10-50nF, so that the AC signal V_AC being presented is kept out of the low-pass filter in the case of a short-circuit at the ionisation sensor 4. Said capacitor C2 furthermore helps to ensure that substantially only the DC component of the ionisation signal is supplied to the low-passed filter. Such a simple signal-measuring unit is made possible by the high frequency of the monitoring signal V_AC, which typically ranges from 1-100 kHz, more preferably from 20-50 kHz. As a result of this high frequency, the low-passed filter only comprises a single filter stage, and the signal-measuring unit is quick enough to carry out an accurate zero measurement during the short period that the supply of the monitoring signal V_AC is interrupted.

[0033] The monitoring system 1 may operate in the manner that is shown in Fig. 5, for example. A monitoring signal V_AC is supplied to an ionisation sensor 4 via the transformer 9. If a flame 7 is present in the combustion space 3, the monitoring signal V_AC will be rectified as a result of the rectifying effect as described above. The measurement of said effect takes place through the flame 7, and the measured, uncorrected ionisation signal is supplied to the signal-measuring unit 11 via the signal line 10.

[0034] The signal measuring unit 11 functions to eliminate AC components on the signal line 10 and to allow the DC component of the ionisation signal to pass. The low-pass filter also prevents the monitoring system 1 from delivering signals to the environment or environmental signals from interfering with the operation of the monitoring system 1. The integrator then measures the DC component of the ionisation signal and places a voltage output V_DC on the line 12. Said voltage indicates that a flame 7 is actually present in the combustion space 3. In the absence of a flame 7 in the space 3, no signal should be measured and the combustion unit 2 can be shut off by the automatic burner 1 via a signal line (not shown).

[0035] At a point in time t, the control unit 13 interrupts the supply of the monitoring signal V_AC to the ionisation sensor 4 during an interval to by opening the switch 14, so that the signal generator 8 is deactivated. It will be apparent that there are also other ways of interrupting the supply of the monitoring signal V_AC to the ionisation sensor 4, e.g. by incorporating a relay (not shown) in the signal line 15 between the secondary winding of the transformer 9 and the ionisation sensor 4, which relay can be controlled from the control unit 13. The interval to is preferably as short as possible, e.g. 20 ms, so that the monitoring of the flame by means of the monitoring signal V_AC can take place as much as possible without interference and practically continuously. It will furthermore be apparent that the supply of the monitoring signal V_AC can be briefly interrupted at any point in time t. By way of example, after 0.5 s the monitoring signal V_AC is interrupted for an interval of 20 ms for the zero measurement, after which the supply of the monitoring signal V_AC to the ionisation sensor is resumed.

[0036] The applicant has discovered that a zero signal V₀ is measured at the signal measuring unit 11 during the interval to as a result of the presence of a galvanic voltage V₀ at the ionisation sensor 4. Depending on the electrolytic action at the ionisation sensor 4, said voltage V₀ is either positive or negative and varies e.g. from 0-3 Volt, typically from 0.5-1.0 Volt. In Fig. 5, V₀ is shown to be positive by way of example. The galvanic effect is probably caused by the presence of moisture and dirt at the ionisation sensor 4. In particular in the case of high efficiency boilers 2 a lot of water is produced, which may lead to the presence of moisture at the ionisation sensor 4. According to the invention, the galvanic effect can be measured by carrying out the zero measurement during the interval to. The control unit 13 can compute a flame detection signal V_CDC by subtracting the zero signal V₀ from the ionisation signal V_DC. This compensates for the interference, whilst the measurement of the ionisation signal remains accurate and may e.g. function to provide information on the presence of the flame 7 and the height thereof.

[0037] In a combustion unit 2 in which no flame is present, prior art monitoring systems nevertheless detect a voltage due to the galvanic effect that occurs at the ionisation sensor 4. Said voltage V₀ is incorrectly interpreted as indicating the presence of a flame 7. The invention prevents the occurrence of such incorrect flame detections. As a result, it is also possible, for example, to start a boiler 2 in that condition and/or in a damp environment, since the zero measurement during the interval to makes it possible to compensate for the galvanic effect.

[0038] It will furthermore be apparent that the moisture that is present at the ionisation sensor 4 will evaporate during the combustion process, causing the galvanic voltage V₀ to decrease. The invention makes it possible to monitor this effect by carrying out repeated zero measurements.

Claims

1. A flame monitoring system (1) for a combustion chamber (3) provided with an ionisation sensor (4), comprising a signal generator (8) for supplying a monitoring signal (V_AC) to the ionisation sensor (4) and a signal measuring unit (11) for measuring at least one ionisation signal (V_DC) from the ionisation sensor,
characterized in that
the flame monitoring system (1) is further arranged for interrupting the supply of the monitoring signal (V_AC) to the ionisation sensor (4) for at least a time interval (to) and measuring a zero signal (V₀) substantially during said time interval (to).

2. A flame monitoring system (1) according to claim 1, wherein the monitoring system is arranged for measuring the zero signal (V₀) from the ionisation sensor (4) during said interval (to).

3. A flame monitoring system (1) according to claim 1, which system further comprises a control unit (13) for making available a flame detection signal (V_CDC) by subtracting the zero signal (V₀) from the ionisation signal (V_DC) .

4. A flame monitoring system (1) according to any one of the preceding claims, wherein the signal-measuring unit (11) comprises a low-pass filter.

5. A flame monitoring system (1) according to claim 4, wherein the input of the low-pass filter is connected to a capacitor (C2), which is connected to ground.

6. A flame monitoring system (1) according to any one of the preceding claims, further comprising a transformer (9) for supplying the monitoring signal (V_AC).

7. A flame monitoring system (1) according to any one of the preceding claims, wherein the system is arranged for deactivating the signal generator during the interval (t₀).

8. A combustion device (2) comprising a flame monitoring system (1) according to any one of the preceding claims.

9. A method for monitoring a flame (7) in a combustion chamber (3) provided with an ionisation sensor (4), which method comprises a zero measuring step, wherein a zero signal (V₀) is measured without a monitoring signal (V_AC) being supplied.

10. A method according to claim 8, wherein the zero measuring signal (V₀) from the ionisation sensor (4) is measured during the zero measuring step.

11. A method according to claim 9 or 10, further comprising at least one of the steps of:

- supplying a monitoring signal (V_AC) to the ionisation sensor (4);

- measuring an ionisation signal (V_DC) in response to the monitoring signal (V_AC) ;

- generating a flame detection signal (V_CDC), comprising the arithmetic step of subtracting the zero signal (V₀) from the ionisation signal (V_DC) .

12. A method according to claim 11, wherein the frequency of the monitoring signal (V_AC) varies between 20 and 50 kHz.

13. A method according to any one of the claims 9-12, wherein the supply of the monitoring signal (V_AC) is interrupted for an interval (to) of 5-50 ms.

14. A method according to any one of the claims 9-13, further comprising the step of determining the height of the flame (7) on the basis of the flame detection signal (V_CDC).

15. An automatic burner arranged for carrying out the method according to any one of the claims 8-14.

Drawing