EP2550484A2

EP2550484A2 - A method and device for controlling the combustive air of a burner in general

Info

Publication number: EP2550484A2
Application number: EP11721107A
Authority: EP
Inventors: Bruno Giordano
Original assignee: IDEA SpA
Current assignee: IDEA SpA
Priority date: 2010-03-23
Filing date: 2011-03-22
Publication date: 2013-01-30
Anticipated expiration: 2031-03-22
Also published as: EP2550484B1; IT1399076B1; WO2011117810A2; WO2011117810A8; WO2011117810A3; ITVR20100056A1; ES2720449T3; TR201905478T4

Abstract

Method of controlling a burner (BR) provided with combustion chamber (CC), combustive air supply means (F) driven by a motor, at least one fuel supply duct (DTI) interceptable by a respective electric valve (EV) means, and at least one exhaust duct (DT3) for the combustion gases, comprising the following steps: - predetermining a minimum and a maximum value for the current absorbed by said motor - establishing, for a plurality of flow rate values of fuel supplied to the burner, the minimum supply voltage of said motor for ensuring a combustive air flow rate suitable for ensuring correct combustion; - supplying fuel to said burner at a predetermined flow rate; - detecting the supply voltage of said supply means and the current absorbed by said motor - checking that, for the supplied fuel flow rate, the supply voltage is greater than a predetermined threshold value and that the value of the current absorbed does not diverge from a predetermined threshold value, and - generating an output control signal when at least one of the values of voltage and current does not comply with the respective thresholds.

Description

A METHOD AND DEVICE FOR CONTROLLING THE COMBUSTIVE AIR FLOW RATE OF A BURNER IN GENERAL.

The present invention regards a method and a device for controlling the combustive air flow rate of a burner in general, especially for use in atmospheric boilers with closed chamber of so-called "C"-type.

As is known, for controlling the combustive air flow rate of C-type atmospheric boilers, an air pressure switch is used which requires the prearrangement of mechanical securing components to the boiler, the electrical connection with a circuit board, the pneumatic connection with the fan, and a Venturi tube for the fan. As is known, problems of condensate formation occur in the Venturi apparatus, which compromise its functioning.

On the other hand, an air pressure switch in an atmospheric boiler, once a predetermined air pressure threshold is reached, reacts in a manner entirely independent of the flow rate of the combustible gas.

Hence, with conventional atmospheric boilers, problems often occur that are connected with the generation of signals, so-called "false signals", not in keeping with a specific actual situation.

A system of fault detection in a warm air furnace is is disclosed in US- 2005/0284463 and comprises a sensing circuit designed to measure a level of current which depends on the amount of AC loading (e.g. the ignition element, the fan, the inducer and/or other loadings, such as a low voltage transformer or the like) applied thereto. In such known system the measured level of current consumption is measured at different times during the operation sequence of the furnace and is compared with an expected value. If the measured current level exceeds the expected value by a threshold amount, a fault in the furnace can be detected and an indication of at least one warm air furnace component that is most likely to have caused the fault can be provided. The measured current level is a peak value that requires to be offset by a correction factor correlated to voltage variations, such variations being caused by power fluctuations or noise that naturally occur in a power delivering system and are detectable by a suitable voltage sensing circuit. Such a system has the inconvenience of not being always sufficiently responsive to changes in the working conditions of the warm air furnace components, and thus it reacts too slowly to malfunctions of the system components.

US-5 906 440 teaches a method of controlling an induced fan for use in gas furnaces. Such a method requires measurement of the current absorbed by the fan and the electromotive force produced thereby. The flow rate of the combustive air is then controlled by varying the fan speed in response to the fan measured current and electromotive force. The drawback of this method is that the control of the fan speed is quite complicated to be carried out.

In EP-1 331 444 a method of controlling a gas burner is disclosed. A sensor means at the exhaust duct of the gas burner monitors the carbon monoxide concentration in the exhaust gas and generates a respective electrical signal. A calibration process can be triggered wherein the gas/air mixture is varied until the exhaust sensor signal matches a given threshold value. In order to do so the fan speed can be varied.

The main object of the present invention is that of providing a method and a device for controlling a burner, which is reliable and which can be used in an effective manner even in different functioning conditions of the burner, especially if employed in atmospheric boilers with closed chamber of so-called "C"-type.

Another object of the present invention is that the method and the device mentioned above are simple to make and maintain.

Another object of the present invention is that of providing a device for controlling a burner which can be calibrated as a function of the real functioning conditions of the burner.

According to a first aspect of the present invention, there is provided a method of controlling a burner including combustion chamber, combustive air supply means driven by a motor, at least one combustible fluid supply duct interceptable by a respective electric valve means, and at least one exhaust duct of the combustion gases, characterized in that it comprises the following steps:

- predetermining a minimum value and a maximum value for the current absorbed by the motor of said supply means;

- establishing, for a plurality of flow rate values of combustible fluid supplied to the burner, the minimum supply voltage of the motor of the supply means for ensuring a combustive air flow rate suitable for guaranteeing correct combustion;

- supplying combustible fluid to said burner at a predetermined flow rate; - detecting the supply voltage of said supply means and the current absorbed by the motor of said supply means;

- checking that, for the supplied combustible fluid flow rate, the supply voltage V is greater than a predetermined threshold value and that the value of the current absorbed does not diverge from a predetermined threshold value, and generating an output control signal when at least one of the values of voltage and current does not comply with the respective thresholds.

According to another aspect of the present invention, there is provided a device for controlling a burner according to the method referred to above, having a combustion chamber, combustive air supply means driven by a motor, at least one combustible fluid supply duct interceptable by a respective electric valve means for adjusting the combustible fluid flow rate, at least one exhaust duct for the combustion gases, said device including a control circuit board comprising

- sensor means designed to detect the actual ionization current value of the flame in said combustion chamber and to produce output electrical signals correlated thereto;

- detecting means arranged to detect actual values of supply voltage and current absorbed by the motor of said supply means and to generate respective output signals;

- a microprocessor designed to produce output control signals in response to input signals correlated to said output electrical signals transmitted by said sensor means and said detection means;

characterized in that

said control circuit board also comprises

- second signal converter means designed to acquire output control signals from said microprocessor and convert them into respective output signals representative of threshold values of supply voltage and current absorbed by the motor of said supply means; and

- second and third signal comparator meansdesigned to receive in input, on one side, said output electrical signals transmitted by said detection means and, on the other, said output signals of said second signal converter means and to compare them in order to generate output signals to applied as respective input signals to said microprocessor.

Further aspects and advantages of the present invention will better appear from the following detailed description of specific embodiments thereof, such description being made with reference to the accompanying drawings, in which:

- Figure 1 is a block diagram of a device for controlling a gas burner incorporated in an atmospheric boiler with closed chamber according to the present invention;

- Figure 2 shows a diagram showing characterization curves of a suction fan for supplying combustive air to the burner of Fig. 1 ;

- Figure 3 is a flow diagram which illustrates the functioning of the device according to the present invention;

- Figure 4 shows a curve which represents the ratio between the ionization current and the excess air index in the burner of Fig. 1 ;

- Figure 5 shows curves representative of the ratio between the combustible gas flow rate and the ionization current in the burner of Fig. 1 ;

- Figure 6 is a flow diagram which illustrates the steps for calibrating the burner of Fig. 1 ; and

- Figure 7 is a flow diagram which illustrates the steps for monitoring the ionization current in the burner of Fig.1.

In the accompanying drawings, equivalent or similar parts or components were marked with the same reference numbers.

First, with reference to Fig. 1 , a control device 1 according to the present invention is schematically illustrated. Such device is applied to a burner BR, e.g. a gas burner for use in an atmospheric boiler CA, so-called of type C, i.e. with closed combustion chamber CC that can be supplied with forced combustive air by means of combustive air supply means, such as suction means, typically an electric fan F driven by a motor, e.g. an electric motor (not shown and of any suitable type), preferably with fixed speed. The burner BR is supplied both with combustible fluid, preferably a combustible gas through a supply duct DT1 interceptable by an electric adjustment valve EV, and with combustive air through a combustive air supply duct DT2. Associated with the burner is a duct, e.g. with heating coil HC mounted in the combustion chamber CC of the boiler CA, in which water to be heated is made to flow through. The fan F is advantageously arranged in the upper zone of the combustion chamber CC, from which it sucks the combustion fumes and discharges them outside through a duct DT3; the fan thus creates reduced pressure in the combustion chamber CC which is open at the bottom due to the presence of a bell-shaped, insulating partition wall PI for isolating the combustion chamber. Due to such reduced pressure, combustive air is sent through the duct DT2 and enters the top of the boiler where it contributes to cooling the fan F and is pre-heated before entering the lower part of the combustion chamber by impacting with the burner BR. Preferably, a combustion fumes analyzer FA is also provided for, placed in the duct DT3, which sends detected concentration value signals to a display DS.

The device 1 is composed of a control circuit board 2 provided with a microprocessor 3, preferably formed by two separate functional units: one 4 for acquiring and processing signals and another for managing and adjusting 6, particularly intended for controlling possible variations of the modulation level (opening/closing) of the electric valve EV and, consequently, the thermal flow rate of the burner BR, by sending suitable output control signals via cable or wireless network 5. Preferably, a signal can be provided for that is designed to open/close the electric valve, and another signal can be provided that, when the electric valve is open, determines the degree of opening of the electric valve itself.

The device 1 also comprises signal converter units or circuits 7 and 8, which receive, in input, the output control signals from the microprocessor 3, and in particular from the management and adjustment unit 6 and directed towards the electric valve EV, and they convert them in order to determine the limit thresholds, as explained below. The first converter circuit 7 is set to convert the acquired signals into corresponding threshold values correlated with respective flame ionization current values and to send them, in input, to a first comparator circuit or means 9, while the second converter circuit 8 is set to convert the output signals acquired in input from the microprocessor into respective threshold values of supply voltage V of the fan and of current AC absorbed by the motor of the electric fan F and to transmit them in input to a second 10 and a third 1 1 comparator circuit or means.

The device 1 also comprises sensor means, such as an electrode 12 of any suitable type, suitably arranged in the combustion chamber CC of the burner BR so as to be immersed in the flame after the lighting of the burner itself; it is thus able to detect the actual value of the ionization current of the flame Fl in the combustion chamber and to produce output electrical signals. The electrode 12 is in external communication with a transducer circuit or means 13 (typically a microammeter or very sensitive ammeter of any suitable type), which is set to transform the thermal stresses on the electrode 12 into corresponding electrical signals; such signals are sent in input to the first comparator circuit or means 9. The comparator circuit 9 will compare, in use, the signals received in input coming from the transducer circuit 13, representative of the actual ionization current of the flame Fl, with threshold signals coming from the converter circuit 7 in order to produce corresponding output signals to be transmitted in input to the acquisition and processing unit 4.

Preferably, the voltage applied on the electrode can be between 170 Volt AC and 1000 Volt AC, such that the detection of flame ionization current is not affected by the aging of the electrode or by dirt accumulated on the electrode itself, since the measuring error deriving from such phenomena would not be perceptible by the measurement instrument.

Making up part of the device 1 are also two detection or measurement units or means of any suitable type in electrical connection with the motor of the fan F: one 14 set to measure the supply voltage V of the motor of the fan and the other 15 set to measure the current absorbed AC by the same motor. The measurement units 14 and 15, in use, emit output signals which are applied in input to a respective comparator circuit or means 10 and 1 1. The comparator circuit 10 is set to carry out the comparison between supply voltage signals V coming from the measurement unit 14 and threshold signals received from the converter circuit 8 in order to produce output signals that are applied to an input of the acquisition and processing unit 4, whereas the comparator circuit 1 1 compares, in use, absorbed current signals AC coming from the measurement unit 15 in order to produce output signals which are applied to another input of the acquisition and processing unit 4.

According to the present invention, the detected absorbed current can be alternating current or direct current. More particularly, it is possible: a) to detect the alternating current and, e. g. by means of a current transformer, also obtain direct current; b) to detect the alternating current by means of a shunt device; or c) to detect the alternating current and the displacement between current and voltage.

When the supply voltage V of the motor of the fan F and the current absorbed by the same motor are alternating electrical signals, their effective values are advantageously calculated. The time cycle considered for this calculation is half the cycle of the mains frequency (e.g. 50 Hz).

More particularly, the supply voltage V of the motor of the fan F is processed by a voltage divider block (not illustrated in the drawings) so that the output signal of the voltage divider block is a voltage V_out proportional to the supply voltage V of the fan motor. V_out is then converted into a digital signal by an A/D converter (also not illustrated in the drawings) and its effective value V_eff is determined.

In so far as the absorbed current signal is concerned, the latter is proportional to the voltage drop at a shunt device that is connected in series with the fan motor. The current signal is converted into a digital signal by an A/D converter and then the effective value l_eff is calculated.

It will be noted that the effective current value l_eff thus obtained depends on the motor winding temperature and the frequency. Such a dependence is reduced to a negligible effect by correcting l_eff with respective correction factors. Such correction factors are defined during the "characterization" procedure of the fan F which is carried out at a design stage. The "characterization" procedure of fan F comprises the steps of determining a specific correction factor for any temperature that the motor winding can reach, and a correction factor for any frequency.

More particularly, the motor windings temperature can be obtained by applying a temperature sensor means directly to the motor winding or, for example, by measuring the electrical resistance of the winding. In the latter case, a known voltage can be applied to the series formed by the motor winding and a known resistance. An electrical divider is thus obtained and, by measuring the voltage drop at the winding, the resistance value of the winding can be derived. Given the resistance, the temperature of the motor winding can be calculated in a known way.

The frequency correction factor can be stored in a table or calculated in any suitable way starting from the number of samples used for calculating the effective current and voltage values.

If the acquisition and processing unit 4 detects an irregular functioning condition, i.e. if, on the basis of the signals received from one of the comparator circuits 9, 10 or 1 1 , it detects that the flame ionization current Fl or the supply voltage V of the fan F or the current absorbed AC from the motor of the fan F exceeds the respective working threshold value established by one of the converter circuits 7, 8, it produces an output signal which is applied in input to the management and adjustment unit 6, which will produce a corresponding output control signal to be sent to the electric valve EV.

Such control signal can be a complete closure signal of the electric valve EV, so as to stop the functioning of the burner, or it can be a modulation signal of the opening/closing of the electric valve or, alternatively, it can also be a control signal for an acoustic or light alarm device, or for a remote warning device of any suitable type.

A method for controlling a burner according to the present invention will be described in more detail below. Such method is preferably carried out by the device 1 , typically with the use of a gas burner BR for atmospheric boiler CA of type C, i.e. with closed combustion chamber, with forced suction of combustive air by means of fixed-speed electric fan F.

One such evaluation is carried out by means of:

- the monitoring or calculation of the supply voltage V and the current absorbed AC by the motor of the fan F; and/or

- the monitoring or calculation of the ionization current of the flame Fl of the burner BR.

Preferably, a so-called "safety" cycle is initially carried out, aimed to verify the correct functioning of the motor of the fan F and of the fan F itself while the burner is turned off. A prewashing step is achieved of the combustion chamber CC of the burner BR, in order to avoid that an unburned gas saturation situation occurs at the time of burner lighting.

The verification of the correct functioning of the electric fan F is grounded on the correlation that exists between the value of the supply voltage V of the motor of the fan F and the value of the current absorbed AC by the same, such current depending on the functioning conditions of the fan F, its value is unique for each fan F model.

During the calibration or "characterization" procedure of the fan F, characterization curves are obtained of the fan by varying the supply voltage (V) and detecting the corresponding current absorbed (AC) by the motor of the fan F (see Figure 2 where the absorbed current AC is illustrated as a function of the supply voltage V).

A first curve, the curve A, is obtained in normal functioning conditions, which means that the motor of the fan F is power supplied, the air intake and the exhaust ducts of the burner are cleared and there are no mechanical malfunctions, such that the combustive air flow rate is sufficient to obtain good combustion. In general, a linear progression curve (a straight line) is obtained. In Fig. 2, V_n indicates the nominal functional voltage.

A second curve is obtained, curve B, by blocking the rotation of the wheel of the fan F, such that the flow rate of combustive air is equal to zero, since the fan F does not function. In such case, given the same supply voltage V_n of the motor of the fan F, the corresponding current absorbed AC by the motor is greater than that of the preceding case. A linear curve progression is obtained.

A third curve, the curve C, is obtained by disconnecting the wheel of the fan F from the shaft of its motor. Also in this case, the combustive air flow rate is equal to zero and the current AC absorbed by the motor of the fan F is less than that absorbed in normal functioning conditions (curve A). The progression of the curve C is also linear.

It will be noted that other working conditions can be taken into account in order to characterize the fan F. Thus, for example, a curve G (not shown in the drawings) can be obtained when the fan F is on, and the duct DT3 is completely or partially clogged.

The characterization procedure of the fan F then provides for a second step during which, with the burner on, among the possible flow rates of combustible gas supplied to the burner BR, the minimum supply voltage is identified that ensures a flow rate of combustive air sufficient to guarantee correct combustion.

Once the burner is turned on, and at the termination of the characterization procedure of the fan, the device 1 according to the present invention acquires, as specified above, by means of suitable circuit means such as the units 14 and 15, the value of the supply voltage V and the current absorbed AC by the motor of the fan F.

Once such values are acquired, the device 1 carries out two controls:

- it verifies that, for a given flow rate of combustible gas supplied to the burner, the supply voltage to the motor of the fan is greater than the minimum threshold value evaluated during the second step of the characterization procedure (values which are compared by the comparator circuit 10); and

- it verifies that the current absorbed by the motor of the fan does not diverge from the nominal functioning value by an interval greater than a predetermined quantity (curves of the diagram of Fig. 2).

If, after one of such controls, it results that the supply voltage V is less than the threshold value or that the current absorbed AC diverges too much from the value corresponding to the nominal value, the device 1 (or better yet its signal acquisition and processing unit 6) produces an output signal which either activates a signaling device (typically alarm), also remote, and/or interrupts the functioning, i.e. turns off the burner BR.

In Fig. 3, a block diagram is reported that illustrates the operating sequence of the control of the voltage V and the current AC applied to the motor of the fan F. Initially, with burner BR turned off (block 301 ), the lighting of the burner is requested (block 302), giving a start control of the motor of the fan F (block 303). The measurement unit 14 at this point measures the supply voltage V of the motor of the fan F, and this is compared in the comparator circuit 10 with a minimum threshold value (block 305). If the supply voltage V measured results less than a minimum threshold value, i.e. an anomaly is verified (block 306), the signal acquisition and processing unit 6 produces an output error signal, which in the embodiment illustrated with reference to an atmospheric boiler blocks the lighting procedure of the burner BR. If the measured voltage is greater than a pre-established threshold value, then the current absorbed AC by the motor of the fan F is detected (block 307) and the detected value is compared in the comparator circuit 1 1 in order to establish if it falls within the pre-established working range (block 308), i.e. if it diverges from the nominal functioning value. If the absorbed current AC diverges from the nominal functioning value by an interval greater than the working range, the signal acquisition and processing unit 6 produces an output error signal which prevents the lighting of the burner BR (block 306); otherwise, the lighting of the burner BR (block 309) is commanded.

Once the burner BR is lit and that, therefore, the flame is present, the device 1 can ensure the control of the burner and more particularly that the flow rate of combustive air is sufficient for guaranteeing that the percentage of unburned gases (like CO) is less than a threshold value. It can accomplish this in different ways, i.e. by monitoring:

1 ) the supply voltage V of the motor of the fan and the current absorbed AC by the motor of the fan F;

2) the flame ionization current Fl; or 3) the supply voltage V of the motor of the fan, the current absorbed AC by the motor of the fan F and the flame ionization current Fl.

The method for monitoring or controlling the supply voltage V and the current absorbed AC by the motor of the fan is similar to that described above with reference to the characterization procedure of the fan (see blocks 304 and 307, in particular).

It therefore provides for: predetermining a minimum value and a maximum value for the current absorbed by the motor of the supply means F; establishing, for a plurality of flow rate values of combustible gas supplied to the burner BR, the minimum supply voltage of the motor of the supply means F for ensuring a combustive air flow rate suitable for ensuring correct combustion; supplying combustible gas to the burner at a predetermined flow rate; detecting the supply voltage V of the supply means F and the absorbed current AC of the motor of the supply means F; verifying that, for the flow rate of supplied combustible gas, the supply voltage V is greater than a predetermined threshold value and that the value of the absorbed current AC does not diverge from a predetermined threshold value, and if at least one of the voltage V and current AC values does not respect the respective thresholds, generating an output control signal.

More particularly, if the supply voltage V measured is less than a minimum threshold, or if the supply voltage V measured is greater than the threshold value but the absorbed current AC diverges from the nominal functioning value by an interval greater than the working interval or range, then the signal acquisition and processing unit 6 produces an output error signal that commands the closing of the electric valve. According to the method 2) in accordance with the present invention, the value of the flame ionization current is evaluated or monitored, in particular the detected value is used for controlling if there is a sufficient combustive air flow rate for ensuring good combustion (the closest possible to that stoichiometric).

As is known, there exists a very precise correlation between the temperature of the flame and the quality of the combustion and, in particular, between the temperature and the excess air index λ, which gives a precise indication with regard to the combustive air present. In addition, the temperature of the flame is correlated to the flame ionization current Fl, according to Richardson's law. Therefore, there is a connection of proportionality between the flame ionization current Fl and the excess combustive air index λ.

According to the method 2), the flame ionization current Fl is then measured by means of the transducer means 13, evaluating the excess air index λ.

The relation between the ionization current and the excess air λ is illustrated in Fig. 4, which shows a curve L with a maximum, where λ=1 in correspondence with stoichiometric combustion. During the normal functioning of an atmospheric boiler with closed chamber, the index λ is never less than 1.2 - 1.3 and the normal functioning area Al is indicated with a dotted area.

When the device 1 functions in work conditions corresponding to the area

Al, if the combustive air flow rate decreases, the excess air index λ decreases and the flame ionization current Fl correspondingly increases. When the value of the flame ionization current detected by the sensor means 12, 13 exceeds a specific threshold, this means that the air flow rate through the duct DT2 has become insufficient for a correct and safe combustion, so that the signal acquisition and processing unit 6 closes the electric valve EV and turns off the burner BR. Such threshold is determined during the characterization process or routine of the burner BR carried out during construction. Its value is corrected during periodic calibration processes in order to adapt it to the changing operating conditions of the device 1.

In Figure 5, the qualitative progression is illustrated of the curve obtained by correlating the flame ionization current Fl and excess air index λ data, obtainable as indicated above for a specific burner model and for a specific boiler structure.

Then, the characterization procedure is carried out of the burner in order to determine the threshold curve corresponding to every possible combustible gas, curve which identifies the correct functioning of the burner BR.

During such procedure, three curves D, E and F are determined, which are a function of the combustible gas flow rate and the flame ionization current Fl.

The curve D represents the expected value of the ionization current in normal functioning conditions, i.e. the link between the flame ionization current Fl and the excess air index λ for the particular burner-boiler model. In order to obtain such curve, an optimal excess air index value is set, then for each combustible gas flow rate the corresponding flame ionization current Fl is measured. In such a manner, corresponding pairs of combustible gas flow rate values and ionization current values Fl are obtained, reported in Figure 5.

The curve E instead represents the threshold value for each modulation level of the combustible gas flow rate. In order to obtain the curve E, one can, for example, determine a combustible gas flow rate value, decrease the combustive air flow rate and simultaneously detect, by means of the analyzer FA, the percentage of polluting substances such as carbon monoxide (CO) in the burner exhaust gases. When the percentage of polluting substances (e.g. carbon monoxide) reaches a threshold value, the corresponding flame ionization current value Fl is measured and the point corresponding to the threshold ionization current and combustible gas flow rate is reported in the graph of Figure 5.

The threshold values of the carbon monoxide must be such to ensure that the quantity of carbon monoxide (CO) in the exhaust fumes remains less than 0.1 % in any functioning condition, as established by current laws on the matter.

The curve F instead represents the minimum flame ionization current capable of allowing the detection of the flame presence.

The curves that are identified during the characterization procedure of the burner are related to optimal functioning conditions. Nevertheless, with the passage of time, phenomena can be verified (such as the aging of the electrode, the deposition of soot, the presence of impurities in the air or the formation of condensate) which can bring the aforesaid curves to "drift" with respect to the nominal values, i.e. that given the same combustible gas flow rate, lower flame ionization current values are obtained.

In order to modify the reference curves, so as to adapt them to the actual functioning conditions of the burner BR which account for the abovementioned phenomena, a periodic calibration procedure of the device 1 is provided for.

Such calibration procedure (see Fig. 6) provides for - after a suitable start calibration command (block 600) and after the lighting of the burner (block 601 ) - having a minimum combustible gas flow rate (block 602) and maintaining such fuel supply condition for a time sufficient for obtaining the stabilization of the device 1 (block 603), after which it proceeds to detect (block 606) the corresponding value of the flame ionization current Fl. It compares the detected value for the purpose of checking if it falls within a pre-established interval or range (block 605). If the flame ionization current value Fl diverges by a value greater than a threshold interval from the nominal values obtained during the characterization step, the management and adjustment unit 6 produces an output control signal (block 606) and the end of the calibration is commanded with the stopping of the burner BR (block 607).

Otherwise, the device 1 is supplied with a maximum combustible gas flow rate (block 608), such supply condition is maintained for a time sufficient for obtaining the stabilization of the device 1 (block 609) and then the corresponding value of the flame ionization current Fl is detected (block 610). If the value falls within a predefined range, i.e. if the value detected with maximum gas flow rate diverges by a value smaller than a threshold interval from the nominal values obtained in characterization step (block 61 1 ), the two ionization current values thus acquired are used for suitably calibrating the reference curves (block 612); otherwise the management and adjustment unit 6 produces an alarm signal (block 606), and in both cases the calibration procedure terminates (block 607).

The calibration procedure of the system described above can be programmed so that it typically takes place:

- at the start of the first start request of the burner BR, after having connected the device 1 to the electrical power supply network;

- 24 hours the preceding calibration procedure;

- after a specific number of lightings of the burner BR; or

- each time that the measured flame ionization current Fl diverges from that expected by more than a specific threshold. During the functioning of the burner BR, the flame ionization current Fl is continuously detected and acquired (see Fig. 7). After the lighting of the burner BR, the flame is certainly present (block 700), and its value is compared (block 701 ) with a threshold value connected with the supplied combustible gas flow rate (block 702).

If the detected ionization current value exceeds the threshold value, the management and adjustment unit 6 of the device produces an alarm or anomaly signal (block 703) which in the illustrated embodiment turns off the burner BR, or alternatively reduces the opening of the electric valve EV so as to supply a reduced flow rate of combustible gas. In such second case, the device preferably emits a functioning anomaly signal.

In addition, according to the present invention, it is continuously verified that there is monotonicity in the flame signal-gas flow rate ratio, i.e. that with the increase of the excess air index λ, the flame ionization current Fl decreases. Such control serves to ensure that the device 1 operates within the correct work zone of Fig. 6, and therefore not in the zone related to values of λ less than 1 .

More particularly, if the flame ionization current signal Fl is less than the threshold value, it is evaluated if the modulation level or better yet the combustible gas flow rate is decreased (block 706) and, if it has, it is verified if the ionization current signal Fl has also decreased (block 705). If both values are diminished, the described cycle is repeated starting from block 701 , otherwise the management and adjustment unit 6 produces an error control signal (block 703), since with the decrease of the gas flow rate, an increase of the flame ionization current value Fl would have occurred and the burner BR would be functioning in an incorrect manner (see Figures 5 and 7). If, instead, the combustible gas flow rate is not decreased, according to the method according to the present invention, it is evaluated if the flow rate is increased (block 706). If it is not, the cycle is repeated starting from the acquisition of the flame ionization signal Fl (block 701 ), otherwise it is evaluated if also the flame ionization current signal Fl is increased (block 707). If both the flow rate values of the combustible gas and the flame ionization current Fl are increased, the burner BR functions regularly and the described routine is repeated starting from the block 701 , otherwise the management and adjustment block 6 produces an anomaly or block signal, since the burner BR is clearly functioning in an incorrect manner (Figures 5 and 7).

According to the present invention, the control of the burner can be ensured and more particularly it can be ensured that the combustive air flow rate is sufficient for guaranteeing that the percentage of polluting substances in the exhaust (such as CO) is less than a threshold value by means of a combination of the above-described methods, i.e. by monitoring the supply voltage V of the motor of the fan, the current absorbed AC by the motor of the fan F and the flame ionization current Fl (method 3); such method allows making a more accurate control with respect to the above-described methods.

In accordance with a method variant according to the present invention, during normal functioning, when the fan is driven at maximum speed, a variation or modulation (diminution) is commanded of the fan speed, so as to reduce the combustive air flow rate, and the flame ionization value is controlled until it coincides with the value corresponding with an excess air value λ equal to or corresponding with 1 ; at such value, as said, there is a stoichiometric ratio between the combustible fluid and the combustive air, and, therefore, there is correct combustion. If the speed of the fan was not modulated, it would not be possible to reach such functioning condition only by operating (opening) the electric valve EV. By varying the combustible fluid flow rate via the opening/closing of the electric valve (EV), the functioning area of the burner, as said above, is the area Al of Fig. 4, whereas for conditioning a functioning in the zone to the left of such area, it is necessary to operate (decrease) the combustive air flow rate.

The use of the device 1 according to the present invention in an atmospheric boiler of type C with forced suction allows avoiding the use of an air pressure switch for monitoring the combustive air. No air pressure switch is therefore required - and likewise no mechanical fixing components of the same to the boiler, no electrical connection, no pneumatic connection with the fan nor a Venturi tube for the fan are required. Such tube, as is known, presents serious condensate formation problems in the Venturi apparatus, which affect its functioning. The device 1 therefore also allows reducing the number of components of the boiler with respect to the conventional solutions.

A device 1 according to the present invention therefore has mainly electronic and not mechanical functioning, and correctly functions independent of the stack length or of the structure of the burner BR or boiler CA, so that no problems occur with regard to the generation of signals not in keeping with the actual operative situation, or "false signals", as is not infrequently verified in conventional systems provided with pressure switch.

Moreover, it will be noted that the detection of actual effective values of supply voltage V_eff and current l_eff absorbed by the motor of the fan F enables one to acknowledge any change in the functioning of device 1 more rapidly and more accurately than with the devices disclosed in US-2005/0284463, US-5 806 440 and EP-1 331 444.

An air pressure switch, then, operates upon the attainment of a predetermined air pressure threshold and in a manner entirely independent of the combustible gas flow rate. According to methods 2) and 3) (i.e. the methods based on the monitoring of the flame ionization current) according to the present invention, however, it is possible to vary the threshold values at which an alarm or error message is produced, such threshold values being dependent on the actual combustible gas flow rate. This allows and ensures a correct functioning both of the burner BR and of the boiler CA also in the presence of low thermal flow rates, with partial obstruction of exhaust ducts, a condition in which a conventional air pressure switch would command functioning interruption.

The device and the method according to the present invention can be set in such a manner so as to respect the requirements dictated by current law on the matter and allow continuously checking the combustive air flow rate, both at the start, before the lighting of the burner BR, i.e. in the absence of flame, and after the lighting of the burner.

The device according to the present invention therefore allows obtaining feedback from the fixed speed fan F of an atmospheric boiler CA of type C, obtaining a precise indication on its functioning status and, thus, on the combustive air flow rate.

The method and the device described above are susceptible to numerous modifications and variants within the protective scope defined by the contents of the claims.

Claims

1. A method of controlling a burner (BR) provided with combustion chamber (CC), combustive air supply means (F) driven by a motor, at least one combustible fluid supply duct (DT1 ) interceptable by a respective electric valve means (EV), and at least one exhaust duct (DT3) for the combustion gases, characterized in that it comprises the following steps:

- predetermining a minimum value and a maximum value for the current absorbed by the motor of said supply means (F);

- establishing, for a plurality of flow rate values of combustible fluid supplied to the burner (BR), the minimum supply voltage of the motor of the supply means (F) for ensuring a combustive air flow rate suitable for ensuring correct combustion;

- supplying combustible fluid to said burner at a predetermined flow rate;

- detecting the supply voltage (V) of said supply means (F) and the current absorbed (AC) by the motor of said supply means (F);

- checking that, for the supplied combustible fluid flow rate, the supply voltage (V) is greater than a predetermined threshold value and that the value of the current absorbed (AC) does not diverge from a predetermined threshold value, and

- generating an output control signal when at least one of the values of voltage (V) and current (AC) does not comply with the respective thresholds.

2. A method of controlling a burner (BR) provided with combustion chamber (CC), combustive air supply means (F) driven by a motor, at least one combustible fluid supply duct (DT1 ) interceptable by a respective electric valve means (EV), and at least one exhaust duct (DT3) of the combustion gases, characterized in that it comprises the following steps:

- supplying combustible fluid at a predetermined flow rate;

- checking that the actual flame ionization current value (Fl) in said combustion chamber (CC), which value is correlated to the quality of the combustion, does not exceed a predetermined threshold value, and

- generating an output control signal when the detected flame ionization current value (Fl) is greater than said threshold value.

3. A method according to claim 2, characterized in that it comprises a characterization process or routine of the burner (BR), during which, for each combustible fluid flow rate, said predetermined threshold value is evaluated for a flame ionization current (Fl).

4. A method according to claim 3, characterized in that said characterization process of the burner comprises a step of evaluating a curve (E) representative of the flame ionization current (Fl) threshold value for each combustible fluid supply flow rate, which is obtained by means of the following operations:

- determining a combustible fluid flow rate value;

- decreasing the combustive air flow rate;

- detecting the percentage of polluting substances in the exhaust gases of the burner (BR), until the percentage of said polluting substances reaches a threshold level; and

- obtaining the flame ionization current (Fl) threshold value.

5. A method according to claim 3 or 4, characterized in that said characterization process provides for the determination of a curve (D) representative of the expected value of the flame ionization current (Fl) as a function of the combustible fluid flow rate.

6. A method according to claim 5, characterized in that said curve (D) is obtained by setting an optimal value of excess air index (λ) and, for a plurality of values of combustible fluid flow rate, detecting the corresponding flame ionization current value (Fl).

7. A method according to claim 5 or 6, characterized in that it comprises the following operative steps:

- continuously checking that there is monotonicity between the combustible fluid flow rate and the flame ionization current, i.e. that with the increase of the excess air index (λ), the flame ionization current (Fl) decreases or vice versa and,

- in the absence of monotonicity, turning off the burner (BR) or adjusting the opening/closing of said electric valve means.

8. A method according to any claim 5 to 7, characterized in that said excess air index (λ) is used as an indicator of the percentage of carbon monoxide (CO) present in the current of the combustion fumes of the burner (BR).

9. A method according to any claim 1 to 8, characterized in that it comprises a characterization procedure of said supply means (F), and in that during a first step of the characterization procedure of said supply means (F), three curves are obtained as a function of the supply voltage (V) and the current (AC) absorbed by the motor of said supply means (F), by supplying said supply means (F) with multiple predetermined voltages (V) and detecting the corresponding absorbed current (AC): - a first curve (A) being obtained in normal functioning conditions, in which the combustive air flow rate is sufficient for obtaining good combustion;

- a second curve (B) being obtained by blocking the rotation of the wheel of said supply means (F), in order to calculate maximum values of absorbed current for each supply voltage, and

- a third curve (C) being obtained by disconnecting the wheel of said supply means (F) from the shaft of its motor, in order to calculate minimum values of absorbed current for each supply voltage.

10. A method according to any claim 2 to 9, when dependent upon 2, characterized in that it comprises during normal functioning, when said supply means are driven at maximum speed, the following step:

- decreasing the supply speed of said supply means (F) so as to reduce the sucked combustive air flow rate until an excess air value (λ) is obtained equal to or corresponding with 1 .

1 1 . A method according to any claim 1 to 10, characterized in that it comprises a calibration procedure of the burner (BR), comprising:

- supplying combustible fluid at a minimum flow rate to the burner;

- maintaining such supply condition for a sufficient time for attaining the stabilization of the burner (BR);

- detecting the flame ionization current value (Fl);

- checking if the detected value falls within a pre-established range or interval;

- if the flame ionization current value (Fl) is within the pre-established range, proceeding with a subsequent step; or commanding the end of the calibration and generating an error signal; - supplying a maximum combustible fluid flow rate;

- maintaining such supply condition for a time sufficient for obtaining the stabilization of the burner (BR);

- detecting the corresponding flame ionization current value (Fl);

- checking if said detected value falls within a pre-established range or interval; and

- when the flame ionization current value (Fl) is within the range, the two ionization current values are used for calibrating the reference curve; when it is outside such range, the end of the calibration is commanded and an error signal is produced.

12. A method according to any claim 1 to 1 1 , characterized in that said detected supply voltage (V) of said supply means (F) and said current absorbed (AC) by the motor of said supply means (F) are effective values.

13. A method according to claim 12, characterized in that said effective current value (l_eff) is corrected by two correction factors, one correlated to the temperature of the motor winding and one correlated to the frequency.

14. A method according to claim 13, characterized in that said correction factors are defined during the "characterization" procedure of said fan (F).

15. A method according to claim 13 or 14, characterized in that said correction factor correlated to the winding temperature comprises a plurality of correction factors one for each temperature of the motor winding.

16. A control device for implementing the method of controlling a burner (BR) according to any claim 1 to 15, the burner having a combustion chamber (CC), combustive air supply means (F) driven by a motor, at least one combustible fluid supply duct (DT1 ) interceptable by a respective electric valve means (EV) for adjusting the combustible fluid flow rate, at least one exhaust duct (DT3) for the combustion gases, said device including a control circuit board (2) having

- sensor means (12, 13) designed to detect the actual ionization current value of the flame (Fl) in said combustion chamber (CC) and to produce output electrical signals correlated thereto;

- detection means (14, 15) set to detect actual values of supply voltage (V) and current (AC) absorbed by the motor of said supply means (F) and to generate respective output signals;

- a microprocessor (3) designed to produce output control signals in response to input signals correlated to said output electrical signals transmitted by said sensor means (12, 13) and said detection means (14, 15);

characterized in that

said control circuit board (2) also comprises

- second signal converter means (8) designed to acquire output control signals from said microprocessor (3) and convert them into respective output signals representative of threshold values of supply voltage (V) and of current (AC) absorbed by the motor of said supply means (F); and

- second and third signal comparator means (10, 1 1 ), which designed set to receive in input, on one side, said output electrical signals transmitted by said detection means (14, 15) and, on the other side, said output signals of said second signal converter means (8) thereby comparing them and generating output signals to be applied as input signals to said microprocessor (3).

17. A device according to claim 16, characterized in that it further comprises: - first signal converter means (7) designed to generate threshold signals correlated with respective flame ionization current values and to operate in response to said output control signals of said microprocessor (3); and

- first comparator means (9), which are set to receive in input both threshold signals generated by said first signal converter (7) and said output electrical signals transmitted by said sensor means (12, 13), said output electrical signals transmitted by said sensor means (12, 13) being correlated to the quality of the combustion in said combustion chamber (CC), thereby comparing them and generating output signals to be applied as input signals to said microprocessor (3).

18. A device according to claim 16 or 17, characterized in that said output control signal from said microprocessor (3) is a closure control for said electric valve means (EV).

19. A device according to claim 16 or 17, characterized in that said output control signal from said microprocessor (3) is a command for controlling the opening/closing of said electric valve means (EV).

20. A device according to any claim 16 to 19, characterized in that it comprises a fumes analyzer (FA) located in said exhaust duct (DT3) for the combustion gases.

21 . A device according to any claim 17 to 20, characterized in that said microprocessor (3) comprises a unit (6) for acquiring and processing signals received in input from said first, second and third comparator means (9, 10, 1 1 ) and a management and adjustment unit (6) designed to command possible variations of the opening/closing level of said electric valve means (EV).

22. A device according to any claim 17 to 21 , characterized in that said microprocessor (3) is set:

to verify that, for the combustible fluid flow rate corresponding with a specific opening of said electric valve means (EV), the supply voltage (V) of said supply means (F) is greater than a predetermined threshold value and that the value of the absorbed current (AC) of said supply means (F) does not diverge from a predetermined threshold value,

to check that the actual flame ionization current value (Fl) does not exceed a predetermined threshold value, and

if at least one of the values of voltage (V) of said supply means (F), of current (AC) of the motor of said supply means (F) and of flame ionization current (Fl) does not respect the respective thresholds, to generate an output control signal.

23. A device according to any claim 16 to 22, characterized in that said supply means (F) are suction means.

24. A device according to any claim 16 to 23, characterized in that said burner (BR) is a component of a boiler.