NL1024388C2 - Flame monitoring system. - Google Patents

Description

VI monitoring system

The invention relates to a flame monitoring system for a combustion chamber provided with an ionization sensor, comprising a signal generator for supplying a monitoring signal to the ionization sensor and a signal measuring unit for measuring at least a first direct current signal following the monitoring signal.

When gases are burned in incinerators for heating purposes, such as central heating (central heating), boilers, geysers and furnaces, carbon dioxide and water, for example according to the CH4 + 202 CO2 + 2H20 reaction, occur when air is supplied.

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

An ionization sensor uses these free ions and charged particles to detect the presence of a flame. When an alternating voltage is applied to the ionization sensor, the free ions and charged particles cause a rectifying effect.

EP-A-1 300 632 describes a gas burner with flame monitoring provided with an ionization electrode which protrudes into the flame. The ionization electrode oxidizes during use, so that the measurement signal from the ionization electrode decreases.

To that end, a control system is provided which, depending on the decrease of the measurement signal, applies a gain factor and issues an error signal when the maximum gain is reached.

A disadvantage of such combustors is that the measurement signal may not be sufficiently accurate to provide reliable flame monitoring.

It is an object of the invention to realize a flame monitoring system that provides a more accurate measurement signal.

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This purpose is achieved by a flame monitoring I

system according to the invention, characterized in that the I

flame monitoring system is further adapted to supply the I

monitoring signal at the ionization sensor for at least I

5 to interrupt a time interval and to measure a zero signal I

during essentially the time interval. Such a system

system provides information about I present in the system

signals, noise or disturbances which information can be given I

used to correct the ionization signal. I

In a preferred embodiment according to the invention

The system is arranged to receive the zero signal from the I

ionization sensor measured. It has been found that due to moisture I

and / or contamination at the ionization sensor a galvanic voltage

may cause interference with the ionization signal. In the

15 it has been found that such a problem arises I

with high-efficiency boilers where a lot of water is produced I

during the combustion process. Another area of application I

which releases a lot of water concerns the production of hydrogen I

from natural gas for use in fuel cells. Through the storage I

20 signal to be suppressed during a time interval or the I

interrupt its supply to the ionization sensor, I

in such a case, information was obtained about the zero signal

sew as a result of the I present at the ionization sensor

galvanic voltage. The zero signal is then the signal that I

25 comes from the ionization sensor when there is no monitoring signal

Sew, such as an alternating voltage or alternating current, becomes I

offered or supplied. The zero signal can be used I

for the corrected or pure ionization signal as flame de

supply signal. I

Other forms of DC disturbance, such as an offset I

of an amplifier in the flame monitoring system may also be I

measured once during the time interval where no I

monitoring signal is applied. I

In a preferred embodiment according to the invention

The flame monitoring system further comprises an I

control unit for making a flame detector available

detection signal, i.e. the corrected ionization signal. I

It has been found that this flame detection signal can be obtained I

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3 by subtracting the zero signal from the ionization signal. Other methods of processing the zero signal also belong to the invention. It is of course also possible to first measure the zero signal and then sew the ionization signal by applying a monitoring signal.

In a preferred embodiment according to the invention, the signal measuring unit comprises a low-pass filter. This low-pass filter ensures that both the dc component of the disturbed ionization signal and the zero signal, which is also a dc signal and / or has a dc component, can be processed by the signal measuring unit, while the ac component of the impure ionization signal is banned or eliminated. The signal measuring unit gives, for example by means of an integrator, an average value of a dc ionization voltage on the output based on the measured ionization current when a monitoring signal is applied to the ionization sensor. The signal measuring unit also gives the dc voltage of the zero signal.

The low-pass filter preferably has an input 20 which is grounded via a capacitor. This capacitor preferably has a large capacity, so that in the event of a short circuit at the ionization sensor the alternating signal offered is kept outside the low-pass filter. This capacitor also contributes to the fact that substantially only the dc component of the ionization signal is supplied to the signal measuring unit.

In a preferred embodiment of the invention, the flame monitoring system further comprises a transformer for supplying the monitoring signal. The transformer 30 ensures a galvanic separation of the generator from the monitoring signal and the ionization sensor. The secondary winding of the transformer is preferably connected in series with the ionization sensor to enable flow measurement through the flame.

In a preferred embodiment according to the invention, the supply of the monitoring signal is interrupted by deactivating the signal generator during the time interval. This can for example take place 1 024388

4 I

by a switch which is switched from a control unit I

controlled. Such an embodiment is simple, inexpensive I

and fast compared to, for example, applying an I

relay in the signal line for supplying the monitoring I

5 signal to the ionization sensor. I

The invention also relates to a burner

device comprising a flame monitoring system such as I

discussed above. Such a combustion device om

for example, gas combustion devices as applied I

10 for a central heating (central), a central heating boiler, a boiler, I

a geyser or an oven. The combustion unit can further I

include control means for applying the zero I, for example

signal for controlling the combustion device, I

for example switching off, blocking and / or locking I

15 of the device when also after correction of the I

ionization signal with the zero signal shows that there is no flame in I

the combustion chamber is present. The incinerator I

can also be blocked or locked when I

a flame is present in the combustion space although this is not I

20 may be the case. The invention also lends itself there

for the zero signal. I

The invention also relates to a working method

method for monitoring a flame in a combustion chamber I

provided with an ionization sensor comprising a zero measurement step I

25 wherein a zero signal is measured without applying an I

monitoring signal. Such a zero measurement step provides I

information about signals in the flame monitoring system that the I

ionization signal. Preferably, it becomes zero

ionization sensor signal measured. I

This information can be applied to the ionization

correction signal, for example by further steps I

from I

- applying a monitoring signal to the ionization I

sensor; I

- measuring an ionization signal following the monitoring

king signal; I

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- applying a flame detection signal comprising the calculation step of subtracting the zero signal from the ionization signal.

In the method, the monitoring signal is preferably an alternating voltage with a frequency between 1 kHz and 100 kHz and more preferably between 20 kHz and 50 kHz. The lower limit is chosen from the consideration that the transformer in the supply line for the monitoring signal does not become too large and the input current for the transformer remains limited. Furthermore, with a frequency of, for example, 20-50 kHz, the low-pass filter can be of simpler design than for a frequency of, for example, 50 Hz. Such a simple embodiment is important to enable the filter to quickly measure the zero signal so that it is possible to switch back to supplying the monitoring signal. The maximum frequency has been chosen on the basis that it is preferable not to cause any notable signals in the radio area by having to take higher harmonics of the monitoring signal or to take appropriate measures. However, it is worth noting that the method according to the invention works regardless of the frequency of the monitoring signal.

In a preferred embodiment according to the invention, the supply of the monitoring signal is interrupted for a time interval of 5-50 ms, e.g. 20 ms. The zero measurement step is preferably as short as possible, after which the measurement of the ionization signal can be continued. After all, the ionization signal is an indicator for the presence of a flame and when this flame is no longer present, it must be signaled as quickly as possible and followed by an action such as switching off the combustion device. In Europe, the requirement is that the combustion device is typically blocked within 1 second after a flame error.

In a preferred embodiment according to the invention, the method further comprises the step of determining the height of the flame on the basis of the flame detection signal. The 1024388

I 6 I

I value of the dc component of the ionization signal or flame failure

The detection signal is a measure of the height of the flame. I

The invention finally relates to an I

Burner control unit for carrying out the above discussed I

Method. The burner control is the electrical component at I

I a combustion device that takes care of the control I

I of elements such as the fan, the valves for the gas supply

I feed, air supply and the like, the flame height etc. I

EP-A-1 176 364 describes a combustion device I

10 and method for controlling a combustion device, I

I in which two measurements are made with an ionization sensor I

I with different gas compositions to eliminate errors, I

I wherein the relative change of the ionization signal I

I is used to adjust the gas-air ratio

I thing. However, this measurement does not work with an I

I zero measurement, so that effects such as a galvanic dc voltage are not I

I can be observed I

The invention will be further illustrated below

I on the basis of the attached figures, which is a preferred I

I show an embodiment according to the invention. Naturally I

The invention is in no way limited by this I

I specific and preferred embodiment. I

I In the figures: I

FIG. 1 a flame monitoring system and a combustion I

A device according to the prior art; I

FIG. 2 a monitoring signal and an ionization signal I

I according to the prior art; I

FIG. 3 is a schematically illustrated flame monitoring system

System and a combustion device according to an I

Preferred embodiment of the invention; I

FIG. 4 is a schematically illustrated signal measurement unit

I according to a preferred embodiment of the invention, I

I and I

FIG. 5 an illustration of an interrupted surveillance

I signal, an ionization signal and a zero signal as I

I obtained by applying the flame monitoring system according to I

FIGs. 3 and 4. I

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7

FIG. 1 schematically shows a flame monitoring system 1 and a combustion device 2 comprising a combustion chamber 3 which is provided with an ionization sensor 4 suspended via a ceramic holder 5. The combustion chamber 3 further comprises a burner bed 6 on which the flames 7 represent the combustion process. It will be clear to those skilled in the art that the combustion chamber 3 can furthermore often comprise other elements (not shown), such as for example an air inlet and a flue gas outlet. The ionization sensor 4 is suspended such that it protrudes into the flame 7 in the presence of the flame 7.

The flame monitoring system 1 comprises a signal generator 8 for supplying an alternating monitoring signal, such as an alternating voltage VAC or an alternating current IAc. In the following, an alternating voltage VAC is always assumed as an alternating monitoring signal. The monitoring signal VAC is added to the ionization 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.

Furthermore, the signal line 10 is connected to a signal measuring unit 11.

FIG. 2 shows a monitoring signal VAC as applied to the ionization sensor 4. When the signal measuring unit 11 is arranged to measure a dc current signal, at signal measuring unit 11 an ionization signal in the form of an ionization current IDc is measured as a result of the rectifying effect of the flame 7 as discussed in the introduction and converted into a corresponding voltage VDc · The ionization current IDC typically varies in the range 0-250μΑ depending on the size of the flame.

The measured ionization current IDC can be inaccurate due to disturbance in the flame monitoring system 1. It has been found, for example, that as a result of moisture and / or contamination at the site of the ionization sensor 4 a galvanic voltage V0 can arise which disrupts the ionization signal loc. In particular, it has been found that such a problem occurs with high-efficiency boilers 2 in which much water is produced during the combustion process.

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8 I

FIGs. 3 and 4 schematically show a flame monitoring system

system 1 and a combustion device 2 according to an I

preferred embodiment of the invention. Identical or ge I

similar components as shown in FIG. 1 are shown in FIG. 3 I

5 indicated by identical reference numerals. I

FIG. 3 schematically shows a flame monitoring system 11

and a combustion device 2 provided with an ionization sensor

sor 4. The combustion chamber 3 is grounded. The secondary winding I

of the transformer 9 is connected via a signal line 15 I

10 with the ionization sensor 4 and via the signal line 10 with an I

impedance Z which is grounded, so that the signal measuring unit I

11 can measure the signal through the flame 7. The signal line 10 I

can have a small length in the order of a few mm. The I

signal measuring unit 11 is connected via a signal line 12

15 with a control unit 13 with which operations on and / or work

accounts with the measured signal from the signal measuring unit I

can be executed. The control unit 13 can be an analog

contain lye-digital (AD) converter for converting I

the signal from the signal measuring unit 11 to one before the be

20 control unit suitable digital form. The AD converter can I

alternatively be a separate component. I

The control unit 13 can also control the signal generation

control tower 8. This control can be done by the I

controlling the switch 14, the position of the switch I

25, the supply of the monitoring signal VAc to the I

ionization sensor. The control unit 13 can be connected via the I

switch 14 during a time interval the supply of this I

interrupt monitoring signal VAC to perform a zero measurement

run. I

A preferred embodiment of the signal measurement unit

11 is shown in FIG. 4. The signal measuring unit 11 comprises I

in this embodiment, a low-pass filter formed by the I

resistor R2 and capacitor C1 which have an integrator function I

fulfills with the operational amplifier 15. The positive input I

35 of the amplifier 15 is grounded. Between the positive and I

The negative input of the amplifier 15 is a capacitor C2 I

included. This capacitor C2 preferably has a large I

capacitance, e.g. of the order of 10-50nF, so that the offered I

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9 AC signal VAC in the event of a short circuit at the ionization sensor 4 is kept outside the low-pass filter. This capacitor C2 also contributes to substantially only supplying the dc component of the ionization signal to the low-pass filter 5. Such a simple signal measuring unit is possible due to the high frequency of the monitoring signal VAc, which typically lies in the domain 1-100 kHz and more preferably in the domain 20-50 kHz. Because of this high frequency, the low-pass filter comprises only a single filter stage and the signal measuring unit is fast enough to carry out an accurate zero measurement during a brief interruption of the supply of the monitoring signal VAC.

The operation of the monitoring system 1 can be, for example, as shown with reference to FIG. 5. A monitoring signal VAC is supplied via the transformer 9 to an ionization sensor 4. If a flame 7 is present in the combustion space 3, the monitoring signal VAC will be rectified as a result of the rectifying effect already described. The measurement of this effect takes place through the flame 7, the measured impure ionization signal being supplied via the signal line 10 to the signal measuring unit 11.

The function of the signal measuring unit 11 is to eliminate ac components on the signal line 10 and to pass the dc component of the ionization signal. The low-pass filter also prevents the monitoring system 1 from issuing signals to the environment or from environmental signals disrupting the operation of the monitoring system 1. The integrator then measures the dc component of the ionization signal and gives a voltage output vDc on the line 12. This voltage indicates that indeed a flame 7 is present in the combustion space 3. If there is no flame 7 in the space 3, no signal should be measured and the combustion unit 2 can be shut off by the burner control 1 via a signal line (not shown).

The control unit 13 interrupts at a time t the supply of the monitoring signal VAC to the ionization sensor 4 for a time interval t0 by opening the switch 1024388

I 10 I

I 14 so that the signal generator 8 is deactivated. The I

It will be clear that the supply of the monitoring signal VAC I

I can also be connected to the ionization sensor 4 in other ways

I interrupted, e.g. by including a relay (not shown) in I

5 the signal line 15 between the secondary winding of the I

I transformer 9 and the ionization sensor 4, which relay can be I

Iden controlled from the control unit 13. The I

I time interval t0 is preferably as short as possible, e.g.

I ms, so that the flame monitoring by means of the monitoring signal I

10 VAC can be undisturbed and practically continuous as much as possible I

I take place. It will further be clear that the supply of I

I the monitoring signal νΜ at any time of time t short I

I can be interrupted. As an example, the I

I monitoring signal VAC after 0.5s during a time interval of I

I15 20ms interrupted for the zero measurement, after which the supply of the I

I monitoring signal VAC at the ionization sensor is resumed. I

I The applicant has observed that during the I

I time interval t0 a zero signal V0 is measured at the signal

signal measuring unit 11 due to a galvanic voltage V0 I

I 20 at the ionization sensor 4. This voltage V0 is dependent on I

The electrolytic effect at the ionization sensor 4 positive I

I or negative and varies, for example, in the range of 0-3 Volts and I

I typically in the range between 0.5 and 1.0 Volts. In Pig. 5 is V0 I

I took a positive example. The galvanic effect becomes I

I 25 probably caused by moisture and dirt in the ionizer

I tion sensor 4. Especially with high-efficiency boilers 2, a lot of I comes

I water free which can give rise to moisture at the ionizer

The sensor according to the invention is the galvanic effect I

I measurable by means of the zero measurement during the time I

I terval t0. The control unit 13 can have a flame detection signal I

I Vcdc calculate the zero signal I from the ionization signal VDc

I deduct V0. This compensates for the disturbance

While the ionization signal measurement is accurate I

I remains and may, for example, serve as information about the application

I 35 presence and height of the flame 7. I

I In a combustion unit 2 in which there is no flame I

I is, according to the monitoring systems, out of the position of I

In the prior art, a voltage was detected due to the I

I 1024388

11 galvanic effect at the ionization sensor 4. This voltage V0 is incorrectly interpreted as the presence of a flame 7. The invention prevents such an incorrect flame detection. This also makes it possible, for example, to start a boiler 2 in a wet condition and / or in a moist environment, since the zero measurement during the interval t0 makes it possible to compensate for the galvanic effect.

It will further be clear that the moisture at the ionization sensor 4 will evaporate during the combustion process, so that the galvanic voltage V0 will decrease. The invention makes it possible to follow this effect by making repeated zero measurements.

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Claims (15)

A flame monitoring system (1) for a combustion chamber (3) provided with an ionization sensor (4), comprising an II signal generator (8) for supplying a monitoring signal II sew (Vac) to the ionization sensor (4) and a Signal measuring unit (11) for measuring at least one ionization signal (Vdc) from the ionization sensor II, characterized in that II the flame monitoring system (1) is further adapted to supply the II of the monitoring signal (VAC) to the ionization sensor (4) interrupting at least one time interval (t0) and II measuring a zero signal (V0) during substantially the time interval II (t0) · I Flame monitoring system (1) according to claim 1, wherein the monitoring system is adapted to measure the zero signal I (V0) from the ionization sensor (4) during the time interval (t0). I 3. Flame monitoring system (1) according to claim 1 or II 2, which system further comprises a control unit (13) for II making available a flame detection signal (Vcdc) by II subtracting the zero signal (V0) from the ionization signal II (VDC ). I The flame monitoring system (1) according to any of the preceding claims, wherein the signal measuring unit (11) comprises an I low-pass filter. I 25 The flame monitoring system (1) according to claim 4, wherein the input of the low-pass filter is connected to a capacitor (C2) connected to ground. I The flame monitoring system (1) according to any of the preceding claims, further comprising a transformer (9) for supplying the monitoring signal (VAc). The flame monitoring system (1) according to any of the preceding claims, wherein the system is adapted to deactivate the I signal generator (8) during the time interval (t0). I 1024388 A combustion device (2) comprising a flame monitoring system (1) according to any of the preceding claims. 9. Method for monitoring a flame (7) in a combustion chamber (3) provided with an ionization sensor (4) comprising a zero measurement step in which a zero signal (V <>) is measured without applying a monitoring signal (VAC). The method of claim 8, wherein during the zero measurement step, the zero signal (V0) from the ionization sensor 10 (4) is measured. The method of claim 9 or 10 further comprising at least one of the steps of: - applying a monitoring signal (VAC) to the ionization sensor (4); 15. measuring an ionization signal (Vdc) following the monitoring signal (VAC); - generating a flame detection signal (Vcdc) comprising the calculation step of subtracting the zero signal (V0) from the ionization signal (VDc) The method of claim 11, wherein the monitoring signal (VAc) has a frequency between 20-50 kHz. A method according to any of claims 9-12, wherein the supply of the monitoring signal (VAC) is interrupted during a time interval t0 in the domain 5 - 50 ms. A method according to any of claims 9-13, further comprising the step of determining the height of the flame (7) based on the flame detection signal (Vcdc). 15. Burner control device adapted to carry out the method according to any one of claims 8-14. 1024388