EP2439451B1 - Device for recognising the presence of a flame - Google Patents

Description

The invention relates to a device for detecting the presence of a flame and a method for detecting the presence of a flame according to the preamble of claim 1 or 13.

Devices for detecting the presence of a flame are used as a flame detector in the monitoring of incinerators and as a flame detector in the field of fire protection.

The goal of any incineration plant operator is to increase overall efficiency in terms of safety advances and optimal availability improve its combustion, reduce pollutant emissions and safely monitor the combustion process.

For safe monitoring, radiation detectors are provided which convert radiations into a measurable electrical quantity according to a fixed law. If a detectable threshold for the measured variable is undershot, a "flame-off" signal can be generated, whereupon the fuel supply can be switched off for safety reasons.

In the case of the radiation detectors, a distinction is made between photoelectric and thermal detectors which have different radiation sensitivities and, in accordance with the task set out, are used in relation to the parameters to be detected.

The photoelectric detectors respond to the energy quanta of a radiation and are dependent on the wavelength in their spectral sensitivity.

In particular, for price reasons, UV tubes are still used for flame monitoring as a flame sensor or flame detector, but there is basically the problem that in these components so-called "percussion" may occur. It can take place without external UV radiation, a glow discharge, which can not distinguish the electronics connected to the UV tube electronics from a normal flame signal.

There are several known solutions, as percussion can be detected and processed safety-related.

Out DE 1 256 828 A For example, it is known to apply the photocell only periodically with the originating from a flame UV radiation by a UV-sensitive element is exposed by a rotating perforated disc only periodically to the radiation. With a monitoring circuit, the capacitors and transistors, can be detected in connection with the rotating perforated disk a light-through of the UV-sensitive element.

In principle, therefore, in order to detect ignition, the incident radiation is periodically interrupted by a diaphragm mechanism. If further discharges occur in the tube in this dark phase, this is detected by the electronics connected to the UV tube, that is, a flame relay is switched off.

A mechanically designed aperture mechanism, which periodically interrupts the incoming radiation, has a limited lifetime due to wear. If the times between two successive closures of the mechanical diaphragm are lengthened, the switching complexity becomes less, to determine flashes between the two shutters of the diaphragm; the detector element should be redesigned and secure.

Reducing the time between two consecutive shutters of the shutter mechanism increases safety with respect to the detection of a bleeder, but also increases the mechanical wear with respect to the shutter mechanism. In addition, deviations from the adjustment of the diaphragm and, for example, soiling due to abrasion can lead to failure of the flame monitoring.

Out DE 1 293 837 A is a device for monitoring a UV tube having a pulse generator for errors of the UV tube is known in which a threshold value at the output of the pulse generator is such that it responds only to those pulses that occur when working properly UV tube. In this case, certain signal shapes and values can be assumed, which lead to a faulty detection of Durchzündern or extraneous radiation.

The DE 1 955 338 B which discloses the preamble of claims 1 and 13, it is known to use two UV photovoltaic cells monitoring the same flame, which are followed by relay circuits consisting of at least two relays. The relay circuits only have a switching state which allows a fuel supply when the sum of the signals - a voltage drop occurring at a series resistor - of the UV photocells exceeds a certain value, and the difference between the two voltage drops falls below a certain value. It is described that a detection of Durchzündens is of no importance, as long as the second UV photocell is working properly. This is disadvantageous for burners that are in operation for half a year or more, so that it can not be ruled out that both UV photocells will also ignite during this time. The DE 1 955 338 B Therefore, the path is taken to design a UV flame detector with a single UV photocell and a downstream channel without the use of mechanical elements. The ignition of a UV photocell can be recognized by the fact that a constant gas discharge at the series resistor generates a DC voltage, which is utilized as a signal for the faulty state of the UV photocell. This requires a (foreign) radiation.

Out DE 26 29 321 A1 a device for flame monitoring in burners in furnaces is known in which an electronically controllable liquid crystal panel is mounted in front of the optical flame sensor, passes through the optical signal from the flame alone on the flame sensor and periodically controlled by the transparent state in the darkened state is. The modulated light signal from the flame sensor is evaluated via a frequency filter for flame monitoring. The DE 26 29 321 A1 Thus, the way is taken to use for shadowing the flame sensor, a liquid crystal panel, which requires no (fine) mechanical components. In practice, this application leads to a high attenuation of the UV radiation and to an insensitivity which is no longer sufficient for the desired measurement purposes. From the foregoing, it will be apparent that several solutions have been proposed to devise a safe flame detection apparatus or method for detecting the presence of a flame that reliably detects faucets. However, the proposed solutions all have a drawback which leads to high wear and / or costly construction of mechanical components or electronic circuits, compromises being accepted.

The object of the invention is therefore to provide a device for detecting the presence of a flame or a method for detecting the presence of a flame, with the or the presence of a flame with little effort and a long service life is provided with constant reliability and availability.

This object is solved by the features of claim 1 and 13, respectively.

In this way, a device for detecting the presence of a flame or a method for detecting the presence of a flame is provided, wherein at least two UV tubes arranged in this way are provided which have the substantially same field of vision. That is, with the UV tubes, the substantially same flame area of the flame can be monitored. The UV tubes can be supplied with DC voltage via an operating resistor. Via a controller, the two UV tubes are switched on and off in succession within a predetermined time interval. That is, one of the two UV tubes is supplied in each case via the operating resistance with the DC voltage, that is turned on, while the other is turned off. After switching off the previously supplied with the DC voltage UV tube, the other of the two UV tubes is turned on. There is no time range in which both UV tubes are switched on together at the same time. In the predetermined time interval falls both turning on and off the two UV tubes as also a time that elapses between turning off one UV tube and turning on the other UV tube. This time can be specified via the controller. It is therefore via the controller, a time or a distance between the switching off of a UV tube and switching on the other UV tube predeterminable. The UV tubes are turned on for a predeterminable period of time. When UV radiation occurs, an ignition takes place in the UV tube, and a current flows through the tube through the operating resistance with a voltage drop below the burning voltage. As a result, the ignition in the UV tube must tear off immediately. Thereafter, the operating voltage returns to its original value, which is above the ignition voltage, with a re-ignition starts when UV radiation occurs. This process is repeated rapidly in succession, so that pulses per period, in which the UV tube is turned on, arise, the number of which is dependent on the intensity of the UV radiation. These pulses are recorded for each of the two UV tubes and compared with each other. Between the switching off and on of the UV tubes, the anode of the respective UV tube is grounded to extract an ionization in the discharge area. The anode of the switched-off UV tube is set to ground potential. If differences of the signals contained by each UV tube are detected, they can be used for any necessary alarm messages or shutdowns of the burner. Should flash pulses occur in a UV tube during possible continuous operation of the torch, these spark pulses add up to the pulses originating from the UV radiation to be monitored. The jets are thus detected and arrive with the evaluation. Furthermore, uneven aging of the UV tubes is detected by comparing the detected signals of the two UV tubes. The presence of a flame is certainly detected redundantly fully electronic without mechanical wear. The detection of the flame is safe, as a self-examination of the UV tubes is constantly. The self-test is independent of whether a flame is present or not. Through a further double protection in the evaluations analog and parallel to the digitally switched evaluation, you get the demanded high security with good availability. The additional use of the number of pulses results in the use of a more reliable magnitude than the discharge current or voltage heretofore recognized in the art, as the current or voltage to be measured may still have a negative influence on the anneal ionization. The possibly occurring due to prolonged operation and prolonged storage fuses are detected safely.

Preferably, the controller is designed for the programmable definition of turn-on and turn-off, so that even strong influences due to the aging of the tubes are detected due to Glühzündungen. Sudden ionization clouds, which lead to pulse ignition, can initiate the recordable ignitions. Again, this would be recognized within a short time.

The self-test according to the invention, even if there is no flame, leads, for example, to better availability in the case of stand-by gas blocks in power plants. Even if the gas blocks are not fired, the self-test takes place. Even before the gas block is to be fired, it can be seen that the device is defective or occur in the UV tubes. The device can then be replaced immediately. In previously known methods and devices takes place only shortly before firing a (Vorbelichtungs-) control, which can lead to the fact that the burner or the gas block can not be put into operation, since previously the device must be replaced. Since the exchange is made on request for firing the gas block, the availability has been reduced so far. The device and the method are used as a flame detector.

With regard to fire protection, continuous self-testing for uneven aging of the UV tubes results in self-diagnosis of when the device needs to be replaced or changed. The device and the method are used as a flame detector.

The previous approaches to detecting the malfunction of UV tubes in detecting the presence of a flame went a different way. Despite the age of the proposed solution options, the inventor was the first to recognize that the device according to the invention or the method according to the invention overcomes the disadvantages of the devices or methods known from the prior art.

Preferably, the required high voltage generation for the UV tubes via a Villard cascade circuit is generated with a charge pump for the frequency, wherein a control voltage in the low voltage range is present at one to five volts DC. The Villard cascade circuit can be operated with a supply voltage of 24 V DC as in the usual and common switchgear. By the chosen design of the high voltage generation by means of cascade connection, the disadvantages of the power supply solutions known on the market are avoided. For example, a switching transformer or a mains transformer would also be a very expensive and also more space-intensive solution.

Preferably, the amount of DC voltage can be preselected by the controller to operate the UV tube with a predetermined sensitivity setting and to undergo a self or self-test. Thus, very easily to a pre-selectable time applicable to the respective UV tube DC voltage for operation of the UV tube can be automatically changed to a predetermined value, for example, an increase of about 15% according to EN 298 or TÜV regulation of for example, be provided 236 volts to 271 volts. The overall sensitivity and number of pulses in UV irradiation is strongly dependent on the DC voltage. The number of pulses increases sharply with the increase of the DC voltage. An increase of about 10% of the DC voltage will increase the relative sensitivity by about 50%. As a rule, the sensitivities change with an increase the DC voltage from about -10% to about + 10% by about 100%. If the expected or pre-calculable number of pulses is not determined as part of the increase in the operating voltage, the UV tube is defective. With the operating voltage change of the UV tube thus takes place a self-examination by the controller. The greater the increase in the operating voltage is selected, the more sensitive is the setting with regard to a self-test of the UV tube.

Preferably, the increase of the DC voltage for each UV tube is periodically, wherein in particular with a successive switching on of the two UV tubes does not take place at the same time in both an increase in the DC voltage during operation of the UV tube.

Preferably, the UV tubes are rotatable about a rotatable latchable unit to the flame. As a result, the alignment of the tubes can be made very precisely rotatable and lockable on a housing. It is possible that the UV radiation radiates axially or radially to the unit with the UV tubes. With a radial irradiation on the unit, a small longitudinal extent in the direction perpendicular to the monitoring flame or a sudden flame is possible.

Preferably, the UV tubes in the housing or the unit via plug connections with secured locking in the unit can be fastened, so that in case of service, the tubes are easily interchangeable in the block. The replacement can be done by replacing a complete unit or a unit separable section, which simplifies maintenance and / or repair.

The control is preferably in the form of an SMD, that is to say a surface-mounted component or flat component. The permissible ambient temperature can be increased to a maximum of 120 ° C depending on the data of the selected UV tubes. Preferably, the time interval is in the range of one second and the time that the UV tubes are on, in the range of a few hundred milliseconds. The time that the UV tubes are each set to ground potential is in the range of a few milliseconds, so that an error in the range of within one second is recognized immediately. Safe monitoring is guaranteed. A timely shutdown or error message, especially during continuous operation and long unattended operation of the burner over 72 hours, is thus guaranteed as well as the readiness at standstill.

• Fig. 1 zeigt schematisch UV-Röhren einer erfindungsgemäßen Vorrichtung;
• Fig. 2 zeigt schematisch die in Fig. 1 gezeigten UV-Röhren eingebaut in einen Brennraum eines Brenners;
• Fig. 3 zeigt schematisch ein Blockschaltbild einer erfindungsgemäßen Vorrichtung;
• Fig. 4 zeigt schematisch ein Blockschaltbild eines Monitorkanals der in Fig. 3 gezeigten Vorrichtung;
• Fig. 5 zeigt schematisch eine Auswertung von der als Flammenwächter verwendeten erfindungsgemäßen Vorrichtung ermittelten Signalen bei korrekt arbeitenden UV-Röhren sowohl bei vorhandener Flamme und nicht vorhandener Flamme; und
• Fig. 6 zeigt schematisch eine Auswertung von der als Flammenwächter verwendeten erfindungsgemäßen Vorrichtung ermittelten Signalen bei einer defekten UV-Röhre mit nicht vorhandener Flamme.

The invention will be explained in more detail with reference to the embodiments illustrated in the accompanying drawings.

• Fig. 1 shows schematically UV tubes of a device according to the invention;
• Fig. 2 schematically shows the in Fig. 1 shown UV tubes installed in a combustion chamber of a burner;
• Fig. 3 schematically shows a block diagram of a device according to the invention;
• Fig. 4 schematically shows a block diagram of a monitor channel of in Fig. 3 shown device;
• Fig. 5 shows schematically an evaluation of the signals used as a flame detector according to the invention used device in correctly operating UV tubes both in the presence of flame and no flame; and
• Fig. 6 shows schematically an evaluation of the device used as a flame detector device according to the invention in a defective UV tube with no flame.

Fig. 1 shows UV tubes 1, 2 of a device according to the invention. The device has at least the two UV tubes 1, 2, which can be supplied via an operating resistor with a DC voltage. The two UV tubes 1, 2 have substantially the same field of view, so that they capture the same flame area of a flame. The two UV tubes 1, 2 are arranged close to one another and can be exposed in the direction of a flame to be monitored or possibly occurring.

There is a unit 4 is provided, on which the two UV tubes 1, 2 are arranged or attached. The two UV tubes 1, 2 are releasably secured by means of plug connections with secured locking in a cylindrical portion 20 of the unit 4. The cylindrical portion 20 of the unit 4 has a radially aligned window 21, which exposes the UV tubes 1, 2 with its front region relative to the flame, so that the two UV tubes 1, 2 in particular to the flame root to be monitored or . are aligned with a possibly occurring flame. The unit 4 is securely held in a mounting bracket 3. For a rotatable locking for aligning the UV tubes 1, 2 on the flame, the cylindrical portion 20 of the unit 4 has an external toothing 22. The mounting bracket 3 has a recess into which the cylindrical portion 20 can be inserted and which in turn has one of the external teeth 22 corresponding or complementary teeth 23. In Fig. 1 is the mounting bracket 3 as from two mounting bracket sections 3a, 3b configured receptacle for the unit 4 shown. The two mounting bracket sections 3a, 3b receive the cylindrical unit 20 with its external teeth 22 in the recess with the toothing 23. The mounting bracket 3 is pivotally mounted so that the arranged in the cylindrical portion 20 of the unit 4 UV tubes 1, 2 can be aligned with the flame to be monitored or a possibly occurring flame.

The unit 4 is locked in the mounting bracket 3 with a balance. When optimally aligned, the two UV tubes 1, 2 are aligned with the flame root, since there the UV content is highest. In the optimized case, the UV tubes 1, 2 each capture half the flame root, that is, one of the two UV tubes 1, 2 detects the "right" and the other of the two UV tubes 1, 2 the "left" area of the flame root. With optimal alignment and at identical behavior of the two UV tubes 1, 2 is measured at an existing flame an identical number of pulses at the same time unit. By a possibly set different DC voltage for the operation of the UV tubes 1, 2, different detected pulse counts of the two UV tubes 1, 2 can be compensated for a balance.

In Fig. 2 is the device according to the invention according to Fig. 1 shown in front of a combustion chamber of a burner arranged as a flame detector. In the Fig. 2 Two flame guards are provided, which are aligned with the flame root (the area of the flame designated w). The units 4 are locked in the mounting brackets 3. The area of the flame designated m is the combustion zone. Below the flame, the pressure is indicated relative to the burner center axis, which represents the x-axis. In a flame evaluation, the frequency, the amplitude and the wavelength can be evaluated. For the flame evaluation, a sensor 24 (see. Fig. 1 ) be provided as he, for example, in EP 2105669 A1 is disclosed.

For controlling and operating the UV tubes 1, 2, a control is provided which may be present as an SMD. The controller for operating the UV tubes 1, 2 can also control the sensor 24 and evaluate the detected signals.

Fig. 3 schematically shows the control with other elements. The controller has a microcontroller or microprocessor 5 which is connected to the UV tubes 1, 2 via a high voltage switching and tube discharge unit 6. With the microprocessor 5, the controller also controls a cascade circuit 7, which is designed as a Villard cascade circuit. The cascade circuit 7 and the high voltage switching and tube discharge unit 6 may be implemented as part of the controller. The cascade circuit 7 can the Hochspannungsumschaltungs- and tube discharge unit 6 with Supply voltage. The microprocessor 5 and the cascade circuit 7 are connected bidirectionally. As a result, a regulation of the high voltage is possible.

The cascade circuit 7 shown schematically as a Villard cascade circuit has a charge pump, which adjusts the high voltage via a frequency, wherein the control voltage is in the low voltage range. The cascade voltage can be operated with a DC voltage, in particular 24 V DC.

The DC voltage generated by the cascade circuit 7 can be fed to the UV tubes 1, 2 for the operation of the UV tubes 1, 2 via the high voltage switching and tube discharge unit 6. By means of the microprocessor 5, the DC voltage used for the operation of the UV tubes 1, 2 can be preselected. Thus, the operating voltage of the UV tubes 1, 2 can be selected via the control. Exemplary DC voltages for the operation of the UV tubes 1, 2 are 325 volts, 345 volts, 365 volts and 385 volts.

The signal to the UV tubes 1, 2 in the form of pulses due to a detected existing flame is supplied to both the microprocessor 5 of the controller and a safety-related monitor channel 8. The output of the monitor channel 8 is connected to an input of a safety-related relay drive 9, the is also bidirectionally coupled to the microprocessor 5 of the controller. This also allows monitoring of the relay stage or relay control 9.

The monitor channel 8 checks for the presence of a gap in the pulsating UV tube signal. The periodically occurring gap arises when switching the UV tube voltage between the UV tubes 1, 2 and is checked for compliance with their characteristic values. The characteristic values are the minimum and maximum width of the gap as well as their minimum and maximum distance. Thus, it is ensured in the safety-oriented monitor channel 8 that every writable component failure in the UV tube circuit is detected. The monitor channel 8 itself is such constructed safety-oriented, that each component failure in the monitor channel 8 leads to a safe shutdown. In addition, the temporal behavior of the signal generated in the monitor channel 8 is checked by the microprocessor 5 for plausibility.

In Fig. 4 is a block diagram of the monitor channel 8 with the two UV tubes 1, 2 shown. There are two re-triggerable monostable flip-flops 14, 15 are provided. The first flip-flop 14 detects the actual signal gap in the flame signal, which is assumed to be at least 50 ms. The second flip-flop 15 detects the minimum period of occurrence of the signal gap in the signal, which is assumed to be about 800 ms. A downstream high-pass filter 16 filters fewer and more frequent gaps, such as a flame that is too weak or a defective tube if there is no flame. A high pass 16 downstream rectifier 17 finally generates a sawtooth-shaped analog signal, which is checked by the subsequent safety-related relay stage 9 for compliance with a voltage window.

The monitor channel 8 operates only with dynamic signals of a specific timing, so that an occurrence of a static signal inevitably leads to a (safety-related) shutdown. The timing of the monitor channel 8 can be designed so that here a flame break leads within a second to shutdown.

The microprocessor 5 of the controller carries out a signal evaluation of the signals detected by the UV tubes 1, 2 for flame monitoring, which allows a test of the UV tubes 1, 2 with reliable detection of a non-existing flame. If, as described below, a weak flame or a flame break is detected, the fuel supply is interrupted via the safety-related relay control 9. It is also possible, in addition to the safety-related channel to control a evaluation relay 10 by the microprocessor 5 of the controller.

Via the LED (s) 11 or the LED arrangement 11 which can be controlled by the microprocessor 5, contact and wireless remote data transmission is possible bidirectionally to the control. Data and signals of the microprocessor 5 can be read out for maintenance purposes and / or in an error case and preselected via a data bus.

In addition, a current driver 12 is provided for small currents in the range of 4 to 20 milli-ampere, which can be controlled by the microprocessor 5; the current driver 12 may provide a signal representative of the qualitative flame evaluation in the form of a current. The circuit according to Fig. 2 is operated with the voltage or the current of the power supply 13.

In Fig. 5 is the inventive process of switching on and off of the two UV tubes 1, 2 with the detection of pulses to the UV tubes 1, 2 with existing flame and correctly operating UV tubes 1, 2 shown. The time t is plotted on the x-axis.

The topmost curve in Fig. 5 indicates the voltage applied to the UV tube 1 voltage. The middle curve indicates the voltage applied to the UV tube 2. In the illustrated embodiment, the voltage applied to the UV tubes 1, 2 varies between the voltage levels 0 volts, 325 volts and 380 volts. The controller switches the two UV tubes 1, 2 on and off in succession at a distance of a predetermined time. The switching on and off of the two UV tubes 1, 2 takes place successively within a predetermined time interval, wherein the two UV tubes 1, 2 are turned on for a predeterminable period of time. The values of the predetermined time interval and the predetermined time interval as well as the distance are stored in the variable memory of the microprocessor 5. In a periodic control or a periodic operation of the UV tubes 1, 2, it may also be provided that the time period for each UV tube 1, 2 stored in the memory of the microprocessor 5 is the same as the distance between the direct succession switching one and the same UV tube 1, 2. The following comparison and the self-examination of the UV tubes 1, 2 and the consistency check against each other is simplified when the time of operation for both UV tubes 1, 2 is identical. Furthermore, the distance between the adjacent switching one and the same UV tube 1, 2 for both UV tubes also be the same.

According to Fig. 5 results in the predetermined time interval between the indicated times t ₁ and t. ₅ The distance between turning off the UV tube 1 and turning on the UV tube 2 is determined by the in Fig. 4 defined times t ₂ and t ₃ defined. The period of time, the two UV tubes 1, 2 are turned on, resulting from the in Fig. 4 indicated times t ₂ and t ₁ for UV tube 1 and t ₄ and t ₃ for UV tube 2. Between the off and on of the UV tubes 1, 2, the anode of the respective UV tube 1, 2 on Ground potential placed for the extraction of ionization in the discharge area, ie the UV tube 1 is placed between t ₂ and t ₅ and the UV tube 2 between t ₄ and t ₇ to ground potential.

The predetermined time interval is preferably about one second such that t ₅ -t ₁ = 1s. The distance between turning off the UV tube 1 and turning on the UV tube 2 is preferably in the range of a few hundred milliseconds, in particular t ₇ -t ₆ = t ₅ -t ₄ = t ₃ -t ₂ = 200 ms; and the time that the two UV tubes 1, 2 are turned on is preferably t ₄ -t ₃ = t ₂ -t ₁ = 300 ms. With the preferred values, a periodic switching on and off of the two UV tubes 1, 2 is ensured, in which the two UV tubes 1, 2 are driven with the same periodicity, which is the control and comparison of the number of pulses obtained, as subsequently is described simplified. However, it is also possible to control the UV tubes 1, 2 differently by first dividing them by the duty cycle of the respective UV tube 1, 2 in order to compare the number of pulses.

The lower curve represents the number of pulses detected by each of the two UV tubes 1, 2 during their operation by the microprocessor 5.

In the Fig. 5 the case is shown that the flame is not present until time t ₁ , and is ignited only at t ₁ . Until time t ₁ , the pulses detected or counted by the microprocessor 5 on the UV tubes 1, 2 are zero. From the time t ₁ pulses due to the existing flame on the UV tubes 1, 2 detected by the microprocessor and counted during operation of the UV tubes 1, 2nd

Since the two UV tubes 1, 2 have the same field of view on the flame, the number of pulses counted relative to a unit of time and at the same operating voltage is equal or within a tolerance range of approximately 5% -10%. Therefore results in correctly functioning flame detector and existing flame that each of the quotient of number of pulses and pre-definable period of time, the UV tubes 1, 2 are turned on, ie here t ₆ -t ₅ or t ₄ -t ₃ , is equal to or within the tolerance is equal for the same operating voltage. If this is not the case, it is possible to draw conclusions about an error or a defective or aged UV tube 1, 2. This will be with reference to Fig. 6 explained.

If the operating voltage of the UV tubes 1, 2 varies, the number of pulses detectable on the UV tube 1, 2 also varies. According to the Fig. 5 The two UV tubes 1, 2 at two different operating voltages, namely 325 volts and 380 volts, operated. At higher operating voltage more pulses from the microprocessor to the UV tubes 1, 2 are counted. If this is not the case or if the expected number of pulses does not result, it is possible to draw conclusions about an error or a defective or aged UV tube 1, 2. This is in relation to Fig. 6 explained.

As Fig. 5 can be seen, the increase of the operating voltage for the two UV tubes 1, 2 from 325 volts to 380 volts to an increase in the number of pulses from 1000 to 2000, in the considered embodiment, the difference between t ₅ and t ₁ , ie the predetermined Time interval is one second.

In the self-test of the flame monitor, which is performed by the microprocessor 5, a self-consistency test is performed for each of the UV tubes 1, 2. The detected number of pulses must be higher at elevated operating voltage than at the lower operating voltage at incident UV radiation or existing flame. In addition, the detected number of pulses must be in a predictable range. Thus, the detected pulses of a UV tube 1, 2 are compared against each other. Furthermore, the detected pulse counts for the two UV tubes 1, 2 are compared with each other. Identical UV tubes 1, 2 must provide the same or within a tolerance range equal pulse counts at the same operating voltage. Furthermore, for the UV tubes 1, 2, threshold values can be stored in the memory of the microprocessor 5, which form a lower limit and an upper limit for the value of the number of pulses at the respective operating voltage of the UV tube 1, 2. These thresholds may also be used for testing the UV tube 1, 2.

Fig. 6 shows the in Fig. 5 According to the invention, the process of switching on and off the two UV tubes 1, 2 with the detection of pulses to the UV tubes 1, 2 in the explanation of assumed defective UV tube 2. There are two different cases in the period a and Period b in Fig. 6 shown.

For the purpose of explanation, a flame exists in the period a and there is no flame in the period b.

As in Fig. 5 the time t is plotted on the x-axis. The uppermost curve indicates the voltage applied to the UV tube 1. The middle curve indicates the voltage applied to the UV tube 2. The voltage applied to the UV tubes 1, 2 varies between the voltage levels 0 volts, 325 volts and 380 volts.

The defect of the UV tube 2 in the time interval a is recognized by the fact that the number of pulses detected for the UV tube 2 is compared with the number of pulses detected for the UV tube 1. The number of pulses determined by the UV tube 2 is increased with respect to the number of pulses detected by UV tube 1 at the same operating voltage. The UV tube 2 is identified as defective.

Since no flame is present in the period b, no pulses are counted on the UV tube 1, neither at 325 volts operating voltage nor at 380 volts operating voltage. In the case of the UV tube 2 no pulses are counted at 325 V operating voltage, but pulses are counted at an increased operating voltage of 380 V. The behavior of the UV tube 2 allows the conclusion that the UV tube 2 has aged and needs to be replaced. In the UV tube 2 occur so-called flash-on. With the varying operating voltage of the same UV tube, a self-test is possible. By comparing the pulses of the same UV tube 1, 2 at varying operating voltages can thus be determined whether the UV tube 1, 2 is still working correctly. In addition, a comparison with the pulses of the second of the two UV tubes 1, 2 is possible.

A self-examination of the flame guard also takes place when there is no flame, ie in the idle mode of the burner, as are detected even in the absence of flame igniter, since the presence of two UV tubes 1, 2 allows each of the two UV tubes. 1 To compare 2 determined pulse numbers. If only one of the two UV tubes 1, 2 shows pulses, then it is possible to conclude that there is a defect or a puncture in the UV tube 1, 2 detecting the pulses. As described also due to Deviations in the number of pulses in the presence of a flame, that is detected in the operating mode of the burner, the fuse.

In addition, another self-examination, as it is for example in Fig. 5 is described with respect to the period b, possible by the operating voltage of the UV tubes 1, 2 is changed. Although the flame is not present, the consistency of the detected signals is checked. The test for consistency includes both the comparison of the signals of the same UV tube 1, 2 with the same or changed operating voltage and the comparison of the signals of the two UV tubes 1, 2 with each other.

So far not recognizable as such, so-called Durchzünder be reliably detected and a reliable statement whether a flame is present or not, is possible.

Claims (15)

1. Apparatus for detecting the presence of a flame with a UV tube which can be supplied with a DC voltage via an operating resistor, in which at least two UV tubes (1, 2), which are arranged in this manner and have substantially the same field of vision, and a controller are provided, characterized in that the controller is configured in such a manner that the two UV tubes (1, 2) can be switched on and off in succession with an interval of a predefined time within a predetermined interval of time, with the result that the UV tubes (1, 2) are switched on for a predeterminable period of time, the number of pulses received by each UV tube (1, 2) being able to be detected and compared, the anode of the respective UV tube (1, 2) being able to be connected to earth potential between the switching-off and switching-on of the UV tubes (1, 2) in order to suction ionization in the discharge area.
2. Apparatus according to Claim 1, characterized in that the predeterminable period of time is the same for each of the two UV tubes (1, 2).
3. Apparatus according to Claim 1 or 2, characterized in that the predetermined interval of time is one second.
4. Apparatus according to one of Claims 1 to 3, characterized in that the DC voltage is a high voltage which can be produced by a cascade circuit (7), in particular a Villard cascade circuit, having a charge pump.
5. Apparatus according to one of Claims 1 to 4, characterized in that the magnitude of the DC voltage for operating the UV tubes (1, 2) can be preselected by the controller for a self-test of the UV tube (1, 2).
6. Apparatus according to one of Claims 1 to 5, characterized in that the DC voltage for operating the UV tubes (1, 2) is increased for predeterminable periods of time in order to increase the sensitivity of a self-test of the respective UV tube (1, 2).
7. Apparatus according to Claim 6, characterized in that a period of time with an increased DC voltage for operating the UV tube (1, 2) is followed by a period of time with a non-increased DC voltage for operating the other UV tube (1, 2).
8. Apparatus according to Claim 6 or 7, characterized in that periods of time with an increased DC voltage are periodic for a UV tube (1, 2).
9. Apparatus according to one of Claims 1 to 8, characterized in that the UV tubes (1, 2) can be oriented with respect to the root of the flame to be monitored by means of a rotatable latchable unit (4).
10. Apparatus according to Claim 9, characterized in that the UV tubes (1, 2) can be fastened in the unit (4) by means of plug connections with secure locking.
11. Apparatus according to one of Claims 1 to 10, characterized in that the controller is in the form of an SMD.
12. Apparatus according to one of Claims 1 to 11, characterized in that the time for which the UV tubes (1, 2) are each switched on is in the region of a few milliseconds, and the interval of time is approximately one second.
13. Method for detecting the presence of a flame with a UV tube which can be supplied with a DC voltage via an operating resistor, in which at least two UV tubes (1, 2), which are arranged in this manner and have substantially the same field of vision, are used, characterized in that the two UV tubes (1, 2) are switched on and off in succession with an interval of a predefined time within a predetermined interval of time, with the result that the UV tubes (1, 2) are switched on for a predeterminable period of time, the number of pulses received by each UV tube (1, 2) being counted and compared, the anode of the respective UV tube (1, 2) being connected to earth potential between the switching-off and switching-on of the UV tubes (1, 2) in order to suction ionization in the discharge area.
14. Method according to Claim 13, characterized in that the operations of switching on the UV tubes (1, 2) and counting and comparing the pulses and connecting the anode to earth potential are carried out periodically and continuously.
15. Method according to Claim 13 or 14, characterized in that the method is carried out without interruption.

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