CA1043445A

CA1043445A - Infra-red dynamic flame detector

Info

Publication number: CA1043445A
Application number: CA210,218A
Authority: CA
Inventors: Robert Horn
Original assignee: Forney Engineering Co
Current assignee: Forney Engineering Co
Priority date: 1973-12-14
Filing date: 1974-09-27
Publication date: 1978-11-28
Also published as: GB1496666A; AU7437474A; JPS5093491A; NL7414566A; DE2451449C2; IT1025259B; BR7409888A; FR2254756A1; FR2254756B1; ES431404A1; US3903418A; JPS5652148U; DE2451449A1; EG11816A; ZA746673B

Abstract

ABSTRACT OF THE DISCLOSURE
A compact signal transmitter head infrared flame detector unit is mounted in a small opening in the wall of a foasil fuel fired furnace. The unit includes a preamplifier solid state circuit including an infrared sensor which is focused on the combustion zone of the furnace for sensing and transforming flame fluctuations into corresponding preamplified signal current fluctuations, which are conducted by a shielded cable to a transistorized main amplifier filter and rectifier circuit in a remote signal receiver terminal unit. The unit has the capability of distinguishing from the fluctuations in-herent in the flame from the background radiation. An in-tergrated control circuit is associated with the main amplifier circuit for digitally processing the signal fluctuations and automatically operating flame indicators and/or alarms as re-quired by the condition of the flame combustion zone.

Description

~0434~S , INFRA-RED DYNAMIC FLAME DETECTOR
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BACKGROUND OF THE INVENTION
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`!, ~The present invention relates to an infra-red flame detector system for monitoring fossil fuel fired furnaces and controlling indicators as well as the flow of fuel to selected burners in accordance with flame operation. For example, in case of flame failure, it is desirable that such failure be indicated and an alarm be operated and fuel flow be stopped as soon as possible to avoid flooding and possible explosion in the 10 furnace.
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( While the present invention is applicable to both coal ,; .
and oil fired furnaces, it is particularly advantageous where -used with pulverized coal fired burners. The instant invention also finds advantageous use in connection with gas fired furnaces provided with oil fired igniters, since the infra-red -, '~
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1043445 l detector can discriminate between the oil fired ignition and the gas fired combustion. In the case of coal fired burners, the infrared detector is able to see through t~e coal dust and smoke enve~oping the combustion zone, which is not possible with ultra-violet flame detectors which require the flame sighting to take place relatively close to the flame.
The invention provides an infrared dynamic detector system for fuel fired furnaces, in which amplification of the sensed signal is accomplished by a small transistorized pre-amplifier circuit, and a remote solid state main amplifiercircuit. The main amplifier circuit is housed in a signal re-' . ceiver terminal unit that is remote with respect to the pre-; amplifier circuit which is housed in a signal transmitter unit or head. The circuits in such units are connected by a two-conductor shielded cable extending a considerable distance from one unit to the other. The signal transmitter unit is ~ . : .'.-- relatively compact, and is provided with a small optical nipple that fits in an opening provided therefor in the furnace wall, substantially in line with the flame being monitored.
The small solid state infrared signal preamplifier circuit includes an infrared sensor (photosensitive) cell which is aimed at the combustion zone of the monitored flame, with such nipple being located on the head unit so as not to extend into the furnace. Thus, the cell is able to pick up infrared radiation emitted in the combustion zone of the furnace without damage through heat and sparks adjacent there-to, by seeing through any murky atmosphere around such zone.
The infrared sensor transforms the flame fluctuations which are present in the combustion zone under surveillance by the cell into a fluctuation equivalent electrical signa] that ,, ,, 1(~43445 is preamplified by the transistor circuit in the transmitter un~for cor.duction through the long shielded cable to the main amplifier circuit in the remote signal receiver terminal unit.
The main amplifier circuit receives the signal fluctuations from the infrared sensor circùit, and further amplifies such fluctuations. ~he main amplifier circuit com-prises a filter section responsive to an optimum frequency range of from 40 to 60 Hz in the light source frequency, since the primary combustion zone is very rich in such frequencies, and background frequencies fall outside of such range. The output signal db/light source frequence characteristic curve between 9 and 75 Hz rises gradually to a maxi~um of +2 db at ¦-75 Hz (50Hz equals zero db), and then falls somewhat more slowly ¦
at higher Hz values of light source frequencies. Undesirable j--noise is thus eliminated. The main amplifier circuit in combin-ation with an integrated control circuit further amplifies and digitally processes the amplified signal fluctuations for con-trolling flow of fuel to the flame burner being monitored in a f 20 flame safety and alarm system which, for example, operates ~n alarm indicator and shuts off flow of fuel to the burner auto-matically when such flame fails after a suitable delay.
In brief, the system of the present operation opera-tes as follows: As the variable infrared radiation from the :
i, combustion or ignition zone strikes the photosensitive cell, the resistance of the cell changes with the intensity of the infrared source. Such variable resistance in conjunction with 'f a constant current, generates a variable voltage which is amplified. The amplified variable voltage in turn, is con-verted to a variable current which is driven through the /f f , , _ 4f _ , , j; - ff ' ' " : ' "' 11~4~45 shielded cable to the main amplifier. In the main amplifier th~ ariable current flo~s through a resistor, generating a variable voltage which is coupled through.a capacitor to a first stage integrated amplifier. After such first stage of amplification, the AC signal of flame fluctuations is passed to an integrated filter circuit. After the signal is filtered, it is applied to a half-wave rectifier and integrator. The reQultant signal (DC) is compared with a manually pre-set inte-grator signal (DC) of the background, and processed accordingly through time delay and digital circuits as required for control-ing the burner.
The terminal unit is provided for receiving the pre-amplified signals corresponding to the infrared fluctuations sensed by the photosensitive cell viewing the combustion zone in the furnace, from the detector head, which are transmitted through the elongated shielded cable for a considerable distance.
Such terminal unit comprises four (4) major sections consisting of the following: 1) an amplifier and filter section; 2) a rectifier section; 3) a signal level detector section; and 4) a time delay section. The first sectior. comprises a circuit for further amplifying and filtering that part of the signal which is between 45 and 60 cps, and to reject all other frequen-c~es to some extent. The second section comprises a circuit for converting the AC output of the first section to a proportional - DC level. The third section comprises a circuit which then compares the output of the second section with a pre-set limit to determine if a flame is present or not. The four~h sec~ion comprises an adjustable time delay circuit on flame out before the output signals F ~FLAME) and F (NOT FLAME) from the circuit responds to the flame out. There is no delay when-the flame is first detected.
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The present invention provides a frequency response characteristic curve in the light source frequency range of 45-60 Hz which produces a minimum of undesirable noise, i.e., between -1 and +1 db (50Hz equals zero db as standard) in the amplified signal fluctuations. Since the signal transmitter unit or head is mounted in a small opening in the wall of the furnace, it does not suffer from the problems which affect prior sensors in which a lengthy sight tube containing an ultra-violet photosensitive cell, extends into the furnace for monitoring the flame. Such tubes often fail due to the intense heat, and also the severe thermal stress decreases the accuracy of the device. Thus, the present invention is particularly advan-tageous when used in coal fired furnaces.
In gas fired furnaces with oil fired ignitors, the present infrared detector can distinguish between the two, in-dicating when the igniter is in operation. This is not possible in the case of conventional ultra-violet detectors. Also, the invention can be quickly and easily serviced should that become ~; necessary in the field by simply replacing one or more of the ' 20 units or parts thereof. :
SUMMARY OF THE INVENTION
In accordance with an illustrative embodiment demon- - -strating features and advantages of the present invention there is provided an infrared dynamic flame detector including infra-red sensor means responsive to energy of a selected bandwidth ~-and providing an AC output signal in response thereto. Means responsive to the AC signal are provided for amplifying the , , .
' same when the frequency thereof falls between about 45 and 60 CPS and substantially rejecting all other signals. Rectifier ', 30 means convert the amplified AC signal to a proportional DC

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level of the amplifier means output. Circuit means compare the DC output of the rectifier means with a selected limit value and deliver an appropriate output indicative thereof. Time de-lay means respond to the comparitor output to provide an output indicative of a flameout condition for a period longer than a selected delay of the time delay means. When the pro-portional DC signal falls below the selected limit, a flame momentarily affected by fluctuations in intensity may be detected.
BRIEF DESCRIPTION OF THE D~AWINGS
The above brief description, as well as further objects, features, and advantages of the present invention will be more fully appreciated by reference to the following detailed description of a presently preferred but nonetheless illustra-tive embodiment in accordance with the present invention, when taken in connection with the accompanying drawings wherein:
FIG. 1 is a schematic view of an infrared dynamic detector system embodying the invention; ;
;, FIG. 2 is a view mainly in side elevation of a coal fired boiler provided with a flame detector head of the inven-tion located in an opening in the furnace wall;
FIG. 3 is a block diagram of the system of the invention FIG. 4 is a circuit diagram of the entire system;
; FIG. 5 is a characteristic frequency response curve ` of the output signal/light source frequency, provided by the ~ -~
main amplifier; and FIG. 6 is a fragmentary view mainly in side elevation of a gas fired burner having an oil fired ignitor provided with flame detector head comprising the invention~ -DESCRIPTION OF THE PREFERRED EMBODIMENT
As shown in FIG. 1, the infrared dynamic flame de-tector system is composed of a compact head unit 10, an elonga-ted shielded cable 12, and a remote terminal unit 14 which may : .

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` 104~45 be located a considerable distance from the head unit 10. The head, unit 10 has a short tubular projection const ~tuting an optical nipple 16 that fits a small corresponding opening in the furnace wall 18 of a boiler 20, for example, in which a -burner 22 produces a flame 24. Preamplified signals, correspond-ing to the flame fluctuations, are generated and transmitted from the head 10 through cable 12 to the terminal unit 14 where such preamplified signals are further.amplified and digitally processed for control and/or indication of the ON/OFF state of 1~ flame 24, as may be required.
As shown in FIG. 2, the line of sight 26 of the optical system in the short tube 10 or nipple 16 is parallel to burner 22, but within air register 28, and through the throat ? ' 30 of the boiler 20. The flame 24 is surrounded by a conical train 32 of smoke and coal dust, in a combustion zone 34, which i8 seen through the line of sight 26 when the burner 22 is in , operation. Thus, the head 10 is protected from thermal damage while in use.
As shown in FIG. 3, the head 10 comprises a box which `
20 contains an infrared photosensitive cell 36 and a small solid state preamplifier circuit 38. The cell 36 is arranged with the optical system for exposure to radiation emitted from the -. . . !
flame under surveillance. The cell 36 and preamplifier circuit - `
38 transform the flame fluctuations, which are present in the combustion zone, into an equivalent electrical signal which is preamplified for transmission through the cable 12 to the terminal unit 14. The cable 12 is shielded and contains two ` insulated conductors. A main amplifier circuit 40 is provided in the terminal unit 14 as shown in FIG. 3, and may be located 30 some distance fxom the detector head 10.
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lU43445 The terminal 14 utilizes integrated circuits and in-cludes the following four major sections, 1) a main amplifier and filter section 40; 2) a rectifier section 42; 3) a signal level detector section 44; and 4) a time delay section 46 having output signal circuits with F designating FLAME, F designating NOT FLAME and FI desinating instantaneous FLAME. The circuit of section 40 amplifies that part of the signal which is be-tween 45 and 60 cps (Hz), and rejects all other frequencies to a certain extent. The circuit of section 42 converts the AC
output of the amplifier filter section 40 to a proportional DC
level; and the circuit of section 44 then compares the output ; of the rectifier section 44 with a pre-set limit to determine -if a flame is present or not. The output of the level detector ` isection 44 goes to the time delay section 46, the circuit of which provides an adjustable delay on flame out before the F (FLAME) and F (NOT FLAME) signals respond. There is no delay on the flame FI slgnal.
The response curve 50, FIG. 5, of the main amplifier filter section 40 rises to a maximum of +2 db output signal 20 at about 75 Hz of light source frequency, and then falls -: . . .
gradually thereafter, as shown. The output signal is plotted ~n db (50 Hz equals zero db as standard). The primary combus-tion zone is very rich in such frequencies, whereas the tail end or background flame frequencies fall outside of the response ' curve 50. The optimum frequency range is selected from 45 to -~ 60 Hz, and frequencies substantially above and below such range are cut off. This places the useful response on the front portion of the curve 50, which effectively eliminates undesirable noise .
As shown in FIG. 4, the photosensitive cell 36 is pro-,i _ g _ ~ _, lf~4 3 ~ 4 S
vided with pin type terminals A, B and C for quick assembly wi~h_and removal from corresponding sockets which are con-nected to the preamplifier circuit 38. Such circuit comprises a positive supply voltage lead 52, a negative lead 54, and a ground lead 56. Such leads are connected to corresponding leads in the shielded cable 12 by quick disconnect pin and sockQt connectors E, J and H, respectively. Thus, the entire head 10 can be removed quickly for repair and/or replacement in the field. The cell 36 converts variabIe radiation to a :
variable resistance. .
Resistor R5, capacitor C2 and diode D2 are connected to provide a constant voltage source to two constant current sources. Capacitor C2 provides filtering for the constant vol-` . tage source. Resistor R6 and transistor Q4, along with such constant voitage source generate a constant current source for the cell 36. Likewise, resistor R4, transistor Q5 and resistor , R2 are connected in series across leads 52 and 54 to provide a constant current source for biasing a Darlington connected con-figuration of transistors Ql and Q2, also connected across leads 20 52 and 54. Transistor Q3 and resistor R3 are connected in the -circuit 38 to provide an impedance match between the cell circuit ~` (high impedance) and the output transistors (low impedance).
A.capacitor Cl couples only the AC component of the flame signal -to the output transistors Ql, Q2.
In operation, as variable infrared strikes the cell 36, the resistance thereof changes with the intensity of the , infrared source. The constant current and variable cell resist-;l ance generate a voltage at the terminal A of the cell 36. Such variable voltage is then coupled to output transistors Ql, Q2 ,' 30 by the operation of the circuit comprising transistor Q3, re-.

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-1¢~43445 sistor R3 and capacitor Cl. Transistors Ql and Q2 convert th~l~rvariable voltage to a variable current signal which is driven through the shielded cable 12 to the main amplifier resistor R27.
The terminal unit 14 receives the output signal from the cable 12 through quick disconnect pin/socket connectors or .
term~nàls G and I, for repair and/or replacement of the unit 14 .
as may be necessary in the, field. .The main amplifier circuit is provided with a positive voltage lead 58 that is connected to the (+) terminal 60 of a single end 12 volt DC supply. Re-sistor R15 and diode D22 are connected in series across (+) lead 58,and the ground lead 62, which also contains resistor R27, to provide a reference voltage for the operational ampli-fier, since the amplifier operates across the single end supply (~12VDC).
A capacitor C10 is connected in the circuit to provide an AC coupling for the first stage 64 of amplification of the received signal. Undesirable high frequencies are shunted to ground by a capacitor C9 which is connected across resistor R27.
~, 20 The first stage 64 of the signal amplification circuit comprises ~ -a resistor Rl9, an integrated circuit IC2, a resistor R28, a ~ ' capacitor C13, a resistor R22, adjustable resistor Rll, and a -.' aapacitor C12. The voltage gain of the first stage 64 of ampl-... . .
fication is controlled by adjustment of resistor Rll. The main frequency filter circuit 66 comprises a capacito,r C5, a ~ resistor R17, a resistor R16, a resistor R14, a capacitor C3, '< a capacitor C6, an integrated circuit IC3, a capacitor C7, a , resistor R18, a resistor R33, and a capacitor C44. The frequency ,, filter circuit 66 attenuates the high and low requencies of , the flame signal, preferably above and below a selected range of 45 to 6~ Hz. -.~ .
, 1~43~45 A half-wave rectifier-integrator and filter circuit 68Y~s provided in the unit 14, comprising an integrated circuit IC4, a diode D33, a diode D4, a resistor R21, a capacitor C8, a diode D5, a resistor R222, a capacitor Clll, and a resistor R13. The circuit 68 rectifies the filtered flame signal and provides a DC voltage at the base of transistor Q22.
Transistor Q22 and resistor R20 are connected to provide an emitter follower circuit 70 for isolation between the recti-fier integrator circuit 68 and an integrated comparator circuit 10 71 and connections to a meter M.
A voltage divider circuit 72 for background voltage generation is provided comprising a resistor R8, an adjustable resistor R10, and a resistor R9. The desired background setting ~-is obtained by adjusting resistor R10.
The comparator circuit 71 comprises an integrated circuit IC5, a resistor R25 and a resistor R12. The comparator circuit 71 acts to compare the DC flame signal to the background signal, and generates an appropriate output to circuit 79. The pins 76, 78 provide quick disconnect means for the meter M from 20 the unit 14 for repair calibration, and/or replacement.
An (ICl) integrated circuit 79 provides a driver for the signal through the following circuit stages including a time delay circuit 80. The circuit 80 comprises, in addition to the integrated circuit 79, a diode Dll, a resistor R7, an adjustable resistor R66, a resistor R55, a capacitor Cll, a *ransistor Qll, a resistor R44. The ICl integrated circuits 82, 84 and 86, have three (3) flame signal output circuits 88, 90 and 92 for flame ~ F, or no flame F, on a delayed flame loss; and flame FIwithout t a delay.
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The variable current flowing through resistor R27 from the preamplifier circuit 38 generates a variable voltage across the resistor R27, which is coupled through capacitor C10 to the first state of amplification 64. The AC signal is amplified, and passed to the filter section 66. After being filtered the signal is passed on to the half-wave rectifier and integrator section 68. `The resultant DC signal is compared with the back-ground setting by the comparator stage 71 and automatically processed accordingly through digital circuit 79, a NAND gate inverter, the time delay, circuit 80 and digital clrcuits 82, 84 and 86.
As shown in FIG. 5, the main filter section 66 of FIG. 4, is used to attenuate the high and low frequencies above 60 Hz and below 45 Hz of the frequency response characteristic 50 of the infrared cell 36, the preamplifier 38 and main amplifier 64 are utilized for optimum results. Since the best frequency ~ -range is from 45 to 60 Hz, the front portion of the frequency response curve 50 is used, and frequencies substantially above 60 Hz are cut off thus eliminating noise.
As shown in FIG. 6, the head 10 of the present inven-tion is used in a gas fired furnace 21 provided with a main burner 23 that is fueled with gas, and an oil fired flame igniter 25 is provided which extend through openings in furnace wall 19, as does the optical nipple 16 of the head 10. The line of sight 17 of the sensor in this case looks at both the zone - -of the gas flame 29 and that of the oil (igniter) flame 27.
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The present infrared detector can discriminate between the oil fired ignition and the gas fired ignition, and thus determine whether the oil fired igniter is in operation. This is not - 30 possible with conventional ultra-violet flame detectors. - -.. . .
The present invention is also applicable to coal and ~ .
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43g45 oil fired boilers. However, it is particularly advantageous - wh~ used ir a coal fired boiler because the use of infrared radiation is one of the only successful w.ays of flame detection in a coal fired boiler. Due to the smoke and dust created in a coal fired furnace, it is difficult if not impossible to detect -the presence or absence of flame using other methods of detection.
Heretofore, the principal method of flame detection required an ultra-violet sensor which in turn required a rather lengthy tube extending into the furnace, and with such long tube being close to the flame, it was subject to severe thermal stresses which quickly render the optical system inacc~rate. Also, the ultra-violet light, necessary for proper operation of the ultra-violet detector, was masked by the smoke and coal dust which are present in the furnace. The present invention does not suffer from such problems, since the head is substantially flush with furnace wall.
A latitude of modification, change and substitution is intended in the foregoing disclosure and in some instances some features of the invention will be employed without a cor-20 responding use of other features. Accordingly, it is appropriatethat the appended claims be construed broadly and in a manner cons1stent with the spirit and scope of the invention herein.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An infra-red dynamic flame detector comprising:
infra-red sensor means responsive to energy of a selected bandwith and providing an AC output signal in response thereto, means responsive to said AC signal for amplifying same when the frequency thereof falls between about 45 and 60 CPS and substantially rejecting all other signals, rectifier means for converting said amplified AC signal to a proportional DC level of the amplifier means output circuit means for comparing the DC output of the rectifier means with a selected limit value and delivering an appropriate output indicative thereof, time delay means responsive to said comparitor output, for pro-viding an output indicative of a flameout condition for a period longer than a selected delay of said time delay means, when the proportional DC signal falls below the selected limit whereby a flame momentarily affected by fluctuations in intensity may be detected.

2. The infra-red dynamic flame detector of claim 1 wherein said infra-red sensor includes an infra-red responsive cell and an optical nipple for coupling the infra-red energy of the flame thereto.

3. The infra-red dynamic flame detector of claim 2 further including a preamplifier for preamplifying the infra-red signals detected by the cell through said nipple and deliver-ing an output signal therefrom.

4. The infra-red dynamic flame detector of claim 3 further including a remote receiver terminal unit for housing said amplifier means, rectifier means, signal detector means and time delay means and a conducting cable for coupling the output signals of said preamplifier to the amplifier of said remote receiver terminal unit,

5. The infra-red dynamic flame detector according to claim 3 wherein said preamplifier includes regulated DC
voltage supply means and amplifier means responsive to the output of the cell for converting the variable infra-red radiation energy into a signal corresponding to a variable resistance, and output circuit means for delivering only the AC component of the signal corresponding to the variable resistance.

6. The infra-red dynamic detector according to claim 5 wherein said regulated DC voltage supply includes leads of opposite polarity, a resistor and a diode connected in series across said leads, a capacitor connected in a parallel circuit with said resistor for filtering said constant voltage, a resistor and transistor connected in series across said positive and negative leads, means connecting the base of said transistor to the negative side of said capacitor, so that a constant current source is provided for the cell which converts variable infra-red radiation to the corresponding variable resistance, a resistor and a transistor connected in series across said leads for generating a constant current source, two output transistors connected in a Darlington configuration across said leads, and to said last-named transistor for biasing the latter configuration with such constant current output, a transistor and a resistor connected in series across the cell circuit and output transistors to provide an impedance match, and a capacitor coupling only the AC component of the flame signal to the output transistors.

7. An infra-red dynamic flame detector according to claim 4 in which said cell is provided with a ground, positive and negative, leads each provided with quick disconnect terminal pin elements for mating with corresponding external socket elements, so that said cell can be easily removed and replaced.

8. An infra-red dynamic flame detector according to claim 4 including a housing for said optical nipple, pre-amplifier and cell and wherein said conducting cable is equipped with quick disconnect pin elements mating with corresponding circuit socket elements in said housing and remote terminal unit.

9. An infra-red dynamic flame detector according to claim 6 in which said cell resistance changes in accordance with fluctuations in the infra-red source intensity, which variable cell resistance together with the constant current generates a variable voltage at the positive terminal of said cell, such variable voltage then being coupled to the output transistors, thereby causing the latter to convert such voltage to variable current which constitutes the preamplified signal output, which is thereafter transmitted.

10. An infrared dynamic flame detector according to claim 6 in which said cell is provided with a ground, positive and negative, leads each provided with quick disconnect terminal pin elements for mating with corresponding external socket elements, so that said cell can be easily removed and replaced.

11. An infrared dynamic flame detector according to claim 5 in which said cell resistance changes in accordance with fluctuations in the infrared source intensity, which.
variable cell resistance together with the constant current generates a variable voltage at the positive terminal of said cell, such variable voltage then being coupled to the output transistors, thereby causing the latter to convert such voltage to variable current which constitutes the preamplified signal output which is thereafter transmitted.

12. An infrared dynamic flame detector according to claim 1 adapted for use in a coal fired furnace in which formations of smoke and coal dust are formed in the combustion zone of said furnace, said infrared sensor including an optical nipple sized for mounting outside of said combustion zone such that the unit is effectively protected from damage by the intense flame of the burner.

13. An infrared dynamic flame detector according to claim 1 adapted for use in a furnace fired with gas having an oil fired ignitor associated with the gas burner; and said infrared sensor includes a photosensitive cell for converting infrared radiation from said oil fired ignition to fluctua-tions, whereby the means responsive to the AC signal distinguishes between the oil fired ignition and the gas fired combustion, indicating whether said oil fired ignition is in actual operation.

14. An infrared dynamic flame detector according to claim 1 for use with fossil fuel fired burners in a furnace in which the amplifier means amplifies the AC signal correspond-ing in frequency to the flame fluctuations, and wherein the means responsive to the AC signal comprises a filter circuit having the aforementioned frequency response of about 45 to 60 CPS
that is rich in primary combustion zone frequencies, with the background frequencies falling outside of such response range, being attenuated by the filter circuit.