US2898981A - Flame detection apparatus - Google Patents

Description

Aug. 11, 1959. L. v. WESTBROOK FLAME DETECTION APPARATUS Filed Dec. 24, 1954 5 E E V M M V, 6 M? w 48 VIVVVV United States Patent FLAME DETECTION APPARATUS Lowell V. Westbrook, Rockford, 111.;assignortotBarber- Colman --Company, Rockford, 111., a corporation 'of Illinois This invention relates generally to apparatus for detecting the presence of a flame and giving a signal such as actuation of a switch in response thereto. More particular'ly, the invention relates to flame detection apparatus of the rectifier type which responds to rectifying impedance'between two electrodes contacting a flame or "of a rectifying photocon'duct'ive cell exposed to the light of a flame and which'distinguishes such impedance from a hilaterally conductive impedance. Hereto'fore, such apparatus -has required complicated electronic circuits including electron tubes.

'One object or the :invention is to provide apparatus of the above character which completlyavoids the use of electron tubes while still providing the safeguards obtainable with ;prior apparatus, which iso'f low cost, and which requires little maintenance.

Another object is to provide a novel relation of circuit elements which enables the apparatus to'respond toismall currents'through the rectifying impedance while still operating quickly'to signalthe absence .of the -llame after thelatter is'extinguished.

Other objects and advantages of 'theinvention'will become apparent from the following 'deteiile'd description taken in connection with'the accompanying -'tlrawing 'in which Figure '1 isa-schematic wiring diagram of liame detection apparatus embodying the novel features of ith'e present invention, shown during the fflame out condi-' .tion.

Fig. '2 is 'a schematic diagram similar to "a ,part of Fig. '1 but showing the 'flame on" -condition.

Fig.3 isaifragmentary sch'ematic'iiiagram or modified apparatus.

"Iurningnow to the drawing, -the invention willbe described in connectionwi'th fa gas burner '10, such 'as'usetl in a gas furnace, gashot'waterheaterorthelike. "Such burner is supplied from a, gas line 11 from a source .12,

the fiow'beingunder the control o'f'a solenoid'valve 13 which preferably "is arranged .to 'perm'it'the How of gas when the solenoid .is energized. Connected ahead of thersolenoid is a pilot burner :14 having a normally lighted "flame occupying a position 1'5 adjacent "the burner '10. Posi'tioned within the flame space is an electrode 16.

In accordance with the :present invention, .a novel 'fl'am'e detector icircuit is provicled,.inluding acapacitor which is charged by the "rectifying action of "thellame "15 acting in conjunction'withtheilame electrode1'6 and the pilot burner 14, jthe capacitor being so .constructed and "arranged-as todeform" in response to :the charge and "thereby turn oit .thefflowofl gas tothe mainburnerwhen the tpilotfflame is extinguished. 'The capacitor indicated in the drawing at 20 has a maimplate'fll "andan'aux'ilia'ry plate "22, the plates "beingnseparated by a'layer of "dielectric 123 having piezoelectricproperties. TFor the dielectric "23,1 prefer to 'usebariumtitanate which, in addition to'being piezoelectticfhasan "exnemely high dielectric -constant-sothat'the capacitor'plates 21;22 have 8,898,981 Patented Aug. 11, 1959 ice a high capacity in relation .to their size, a capacity which may be on the-order of L015 of a microfarad.

In thepresent embodiment, a series detector circuit'is employed, the capacitor plates 21, 22 being connected by conductors 48 and '49 respectively to the pilot burner '14 and to one terminal of a secondary .w'indingf25 ofa transformer '26 providing a source of alternating current when its primary winding 25 is connected to suitable ..alternating current supply lines (not shown). 'To complete the detector circuit, the flame electrode 16 'is connected through a series resistor 27 and aaconductor 5D "to the other terminal of Lthesecondary '25. .The circuit thus extends 'in series from theiilame electrode 1'6 to the flame 15 through the series resistor '27, the conductor 50, the secondary winding 25, the conductor 49,, the capacitorl'O, the conductor 48 and .the pilot burner '14. The transformer secondary should be.capable,of producing approximately ,500 volts .r.m.s. 'The series resistor 27 may have a resistance on .the order of one megohm to protect the transformer 26 .and to limit the current through the capacitor 120 when the flame electrode 1'6 and the burner 14 are short-circuited.

In carrying out the invention, the mainplatc 21 of the capacitor ZlLis formed of a relatively stilt plate of metal engaged at its ends .in resilientbracketsill, exerting endiwise pressure on the plate. The second or auxiliary "plate 2.2.ispreterably somewhat shorter, as shown, and

Inoreilexi'ble .so as to offer much less resistance to "flexingor htwving. Thepreferred dielectric material, barium '"titanate, "is a polycrystalline ceramic material which is per'se known in the-artand the use of which'is discussed, for example, in an article .entitled 'Titanate Ceramics ffor Electromechanical Purposes appearing in the February, 1950 issue of Industrialand Engineering 'Cheniistry published by the American Chemical Society. 'When subjected to 'an electric field of proper polarity, polycrystalline barium titanate vtends to expand physically in a vdirection parallel .to the ifiel'cl and to contract 'stronglyin all directions at right angles to the new. 111 the-present device, the field .between .the plates 221,12 resultingfrom the unidirectional charge "built up thereon causes the capacitor to bodily bow or 'flex in the direction of the stronger plate .21 and it is such Iflexure which isutilized to control the'fiowlof gasinthe preserit device. The flexure ofthe capacitorZQ, ,greatly exaggerated, is shownin'FigJZ.

In order to .control 'theioperation of the-solenoid, the center portion of the capacitor 20 'is coupled to amov- "able'contact 31 which, together with a stationarily supporte'dcontact 32,"forms a pair of normally open contactsthat*are'arrangedin series .withthe valve and with "a source or valve operating voltage 33. For this series arrangement, "the movable contact is connected'by a iflexiole conductor 51 to one terminalof'the valve, the Ifixeil contact 32 is connected by a'conductorf52 to one .ter-

' .minal ozfithe source and the other terminals or the valve 'anflthe source ;are;connec,ted by a conductor 53. f-llnterposed between the contacts and the plate 21 or the capacitor is a snap action mechanism including .the snap action lea'f springi35 'having.a,pair of supporting legs '36, 37 and a central or compressionleg 3,8"ywhich Iis'engaged by a bracket "39 suitably secured to theplate 21. Thus, when the capacitorhows upwardly, sasshown MB; 2," the central or compression leg 38is raisediahove :the level of'the'two supportinglegs causing themovahle contact 31'to snap downwardly against the contact 132. This completes the energizing circuit for the valve-13 and the'valve'stays open aslong as'thercapacitorplates are bowed. When the plates straighten as shown 'in Figs. 1 and 3,the-movable contact 31is shifted overcenter "and against another stationary contact 1'54 spaced tfrom the'lirst contact 32.

To insure that the contacts 31 and 32 open promptly sistor 40 is correlated with the capacitance between the plates 21,22 to'dis'sipate the charge on the latter quickly to the shutdown value when the'charging circuit is interrupted while still delaying such dissipation for a definite time'interval long enough to maintain the contacts 31 and 32 closed and to avoid a nuisance shutdown when the flame merely flickers but is not extinguished.

In carrying out the invention the leakage resistor 40 is not in the circuit when the contacts 31, 32 are open,

i.e., under flame off conditions. This is accomplished by connecting the resistor 40 eflectively in a series circuit including the capacitor 20 and the contacts 31, 32, al-

though it will be apparent that contacts auxiliary to the To provide this circuit, the resistor is connected at opposite ends to one capacitor plate 22 through the conductor 49 and to the first fixed contact 32 through the conductor 52. Also, the movable contact 31 is connected by another flexible conductor 55 and the conductor 48 to the other capacitor plate 21. When the contacts 31 and 32 are closed, the resistor is connected across the capacitor by the conductors 49, 52, 55 and 48. When the contacts are open, this circuit is interrupted.

Since the resistor 40 is effectively out of the circuit when the contacts 31 and 32 are open, all of the unidirectional current flowing from the electrode 16 is available to charge the capacitor 20. Accordingly, the sensitivity of the device is extremely high and suflicient bowing of the capacitor plate 21 takes place so as to positively throw the switch-operating mechanism over center when the pilot flame is turned on. Sensitivity" as used herein is the ratio of deflection of the capacitor and the unidirectional current flow. Once the contacts 31, 32 have closed the resistor 40 is connected in parallel with the capacitor by the conductors 49, 52, 55 and 48 so that a portion of the current in the detector contacts 31, 32 and operated as an incident to movement 'of the leaf spring 35 may be employed for this purpose.

circuit through the flame 15 flows through the resistor and thus is no longer effective for charging purposes. Furthermore, during the idle half cycles of the current wave, the charge is partially leaked from the capacitor plates in the circuit through the parallel resistor.

The net effect of this is to reduce the amount of bowing v of the capacitor 20, causing the snap switch mechanism to be slightly retracted toward the point of changeover. Stated in other words, the effect of the leakage resistor 40, connected as shown, is to condition the switch to turn 0 Thus, while the sensitivity of the device is initially high so as to cause positive switching action when the flame is initially turned on, the sensitivity is substantially reduced after closure of the contacts 31, 32 to insure that the switch turns off quickly upon failure of the pilot flame. Using the present shunt resistor arrangement, it is possible to employ snap switch mechanisms of the type having a large mechanical differential and hence an extremely positive switching action while keeping the electrical differential within reasonable bounds. In a practical case, it was found that with a leakage resistor on the order of 40 megohms resistance, the closure of the contacts occurred at a capacitor voltage of 125 volts and opening of the contacts occurred at approximately 75 volts. Changing the value of the resistance provides a convenient way of adjusting the voltage to which the capacitor charges after closure of the contacts 31, 32, decreasing the value of the resistance serving to decrease that voltage and thus reduce the operating voltage differential.

In the operation of the apparatus, let it be assumed that no flame is present at the pilot burner 14 and that the capacitor 20 is discharged. Under these conditions,

the plates 21 and 22 are substantially straight in their relaxed shapes and the switch contacts 31 and 32 are open to interrupt both the energizing circuit for the fuel valve 13 and the leakage path through the resistor 40. Should a short circuit develop between the flame electrode 16 and the burner 14, the series resistor 27 limits the current flow to the plates to instantaneous values insuflicient to produce bowing of the plates for closure of the contacts 31 and 32. The plates also fail to become charged to a value high enough to close the contacts 31 and 32 when any other bilaterally conductive impedance bridges the conductors connecting the flame electrode and the burner into the circuit, for example, distributed capacitance between the conductors.

As soon as a flame 15 appears at the burner 14 to provide a rectifying impedance, a larger current flows from the flame rod 16 to the burner 14 in the series detector circuit during alternate half cycles of the source 25 than flows in the opposite direction during the intervening half cycles. Such current impulses of greater magnitude are integrated by the capacitor 20 which thus becomes charged to apply a field to the dielectric body 23 causing the latter to deform and the main plate 21 to bow outwardly. The compression leg 38, moving with the main plate, shifts the movable contact 31 overcenter and into engagement with the normally open fixed contact 32 when the charge on the plates reaches the higher valve energizing value. Closure of the contacts completes the energizing circuit for the fuel valve 13 through the conductors 51, 53 and 52 and the leakage path through the resistor 40 through the conductors 49, 52, 55 and 48.

Upon completion of the leakage path through the resistor 40, the charge on the plates 21 and 22 drops. However, by a proper selection of the value of this resistor, the charge remains higher than the shutdown value as long as the impedance between the flame electrode and burner conductors is rectifying. When the flame is extinguished and the capacitor charging circuit is opened, the charge on the plates is dissipated through the leakage resistor until the shutdown value is reached. As an incident to such dissipation, the dielectric body 23 tends to return to its original shape and the main plate 21 tends to straighten, the movable contact 31 shifting overcenter at the shutdown value to open the contacts 31 and 32 and interrupt the leakage path and the energizing circuit for the fuel valve, thereby shutting 011 the fuel. Since the shutdown time interval is determined by the resistance-capacitance relation between the leakage resistor and the capacitor, this interval remains substantially constant. Upon interruption of the leakage path, the remainder of the charge on the plates is dissipated through the internal leakage resistance of the capacitor.

To summarize the operation, the capacitor 20 is charged when the flame 15 is present to complete the charging or detector circuit which extends from the flame electrode 16 through the resistor 27, the conductor 50, the transformer secondary 25, and the conductor 49 to the auxiliary plate 22 of the capacitor, from the main plate 21 of the capacitor to the burner 14 through the conductor 48, and from the burner through the flame 15 to the electrode 16. Such charging results in deformation of the dielectric body 23 and bowing of the capacitor plates for closure of the contacts 31, 32 to complete the energizing circuit for the valve 13 and, also, the shunt circuit through the leakage resistor 40. Upon completion of the energizing circuit which extends in series through the contacts 31, 32, the conductor 52, the source 33, the conductor 53, the valve 13 and the conductor 51, the valve opens to permit fuel to flow to the main burner 10. The shunt circuit extending from the fixed contact 32 through the conductor 52, the leakage resistor 40, and the conductor 49 to the auxiliary capacitor plate 22 and f om the main plate 21 through escapes the c nductors as and "5 5 to "thefinovabl'e c'onta'ct "3-1 provides a leakage path through which the ca acitcr is discharged when the fla'r'neis extinguished and the charging 'circuit is interrupted. As an incident 'to "such discharge, the dielectric body 23 and the capacitor plates resume their original "shape in which the " contacts 31 and 32 are open "to interrupt both the leakage path through the resistorj lo and the valve energizing {circuit thereby permitting the valve "to "close and shut off the fuel supply to the main burner 10.

It will be apparent that the novel apparatus described above responds only when a rectifying impedance bridges the flairle electrode and burner conductors and :fails to give a false indication of a flame either when theconductors are bridged by a bilaterally conductive imp'edance or when an open circuit condition develops at any pointinthe circuit. The plates 21 and 22, inaddition to applying a field to the dielectric body '23 fordefofmingthe =l'atter, also cooperate therewith to provide a capacitance for integrating the impulses of unidirectional current resulting from rectifying impedance. Since the leakage path through the resistor 40 is open during initial charging of the capacitor, the latter will become charged sufficiently to close the contacts 31 and 32 in response to small impulses of current resulting from a small rectitying ratio of the impedance to current flow from the burner 14 to the flame rod 16 to the impedance to current flow in the opposite direction. Since the dielectric constant of the preferred piezoelectric material, barium titanate, is extremely high as compared to conventional piezoelectric materials, the capacitance is high, for example, 0.015 mfd., and thus even a small D.-C. component is capable of building up a large charge on the plates and resulting in substantial deformation thereof. The apparatus may be constructed easily of few parts excluding vacuum tubes and therefore is inexpensive not only to manufacture, but also, to maintain in service use.

While the invention has been described in connection with a rectifying electrode 16 located within the flame, it will be apparent to one skilled in the art that the invention is not so limited but would include the use of other sensitive means for providing a unidirectional component of current under flame on conditions. Such means may include a photoconductive cell with rectifying properties, for example, a photoelectric vacuum tube 45 having an anode 46 and a cathode 47 with the anode 46 taking the place of the electrode 16. When a light impinges upon the cathode 47 electrons are given off from the cathode surface, thereby producing a limited current flow during alternate half cycles of the exciting alternating current wave and the capacitor is thereby charged, in the presence of flame, just as outlined above.

While the invention has been described in connection with controlling flow of gas to a gas burner, it will be understood that it is not by any means limited thereto and may be used to control the flow of fuel to oil burners and other types of burners regardless of the fuel employed.

I claim as my invention:

1. In apparatus for detecting the presence of a flame at a burner in a combustion control system, the combination of, a body of dielectric material having the property of changing dimension when subjected to an electric field, a pair of plates of conducting material straddling said body to create an electric field therein and cooperating with the body to form a capacitor, means constructed to include the flame for producing a rectifying impedance when a flame is present at said burner, a source of alternating voltage connected in a circuit with said impedance means and said plates for building up a charge on the latter and deforming said body when a flame is present at said burner, a switch having a part connected to said body and actuated in B 0 one sense by deformation of the body when said-charge is higher than a first predetermined value and -in the opposite sense by restoration of the body resulting from dissipation of the charge below a second predetermined value, and a normally open circuit shunting said -c'apacitor and closed in response to actuation of said switch in said one sense, said shunting circuit including a resistor cooperating With said capacitor to reduce the dis- :charge time of the latter after extinguishrnent of the flame and having a resistance highenough'fo'r the maintenance of said charge above said second value when the shunting circuit is closed and said impedance is rectifying.

'2. In "apparatus for detecting the presence of :a flame at "a burner in a combustion control system, the combination of, a body of dielectric material having the property'of changing dimension when subjected 'to an electric field, a pair of plates of conducting material straddling said body to create an electric field therein and'cooperatingwith the body 'to form a capacitor, means constructed to include the flame for producing a rectifyi n'g impedance when a flame is present at said burner,

a source of alternating voltage connected in a circuit with said impedance means and said plates for building up a charge on the latter and deforming said body when a flame is present at said burner, a switch having a part connected to said body and actuated in one sense by deformation of the body when said charge is higher than a first predetermined value and in the opposite sense by restoration of the body resulting from dissipation of the charge below a second predetermined value, a resistance element connected in series with said source and limiting current flow to said capacitor to an instantaneous value insufficient to actuate said switch when said impedance means is short circuited, and a second resistance element connected in a shunt around said capacitor and cooperating therewith to determine the time interval required for dissipation of said charge to said second value after extinguishment of the flame and cessation of the rectifying action of said impedance.

3. In apparatus for detecting the presence of a flame at a burner in a combustion control system, the combination of, a body of dielectric material having the property of changing dimension when the material is subjected to an electric field, two plates of conducting material straddling said body to create an electric field through the body when a voltage is applied to the plates, said plates and said body cooperating to form a capacitor, means constructed to include the flame for producing an impedance which is rectifying when a flame is present at said burner, a source of alternating voltage connected in a circuit with said impedance means and said plates to provide a rectified current for building up a charge on said plates when a flame is present at said burner, a switch having a part connected to and movable with said body for actuation of the switch in response to the change in dimension of the body when the charge on said plates reaches a predetermined value, and a resistance element connected in said circuit in series with said source and limiting current flow to said capacitor to a safe value insuflicient to produce actuation of said switch when said impedance means is short circuited.

4. In apparatus for detecting the presence of a flame at a burner in a combustion control system, the combination of, a body of dielectric material having the property of changing dimension when subjected to an electric field, two plates of conducting material strad dling said body to create an electric field therein and cooperating with the body to form a capacitor, means constructed to include the flame for producing an impedance which is rectifying when a flame is present at said burner, a source of alternating voltage connected in a circuit with said impedance means and said plates for building up a charge on the latter to deform said body when a flame is present at said burner and said irnpedance is rectifying, a switch having a part connected to said body for actuation of the switch in one sense by the deformation of the body when the charge on said plates is higher than a first predetermined value and for actuation of the switch in an opposite sense when said charge is below a second predetermined value, and a resistance element connected in a shunt around said plates and cooperating with said capacitor to control the time interval required for dissipation of said charge to said second predetermined value following extinguishment of said flame and cessation of the rectifying action of said impedance.

5. In a combustion control system including a burner, the combination of, a source of fuel for said burner, a body of dielectric material having the property of changing dimension when the material is subjected to an electric field, two spaced plates of conducting material sandwiching said body to create an electric field through the body when a voltage is applied to the plates, said plates and said body cooperating to form a capacitor, means including the flame for producting an impedance which acts as a rectifier when a flame is present at said burner,

References (Iited in the file of this patent UNITED STATES PATENTS 2,074,637 Ballentine Mar. 23, 1937 2,201,879 Blattner et a1. May 21, 1940 2,313,943 Jones Mar. 16, 1943 2,386,648 Aubert Oct. 9, 1945 2,387,108 Arndt, et al. Oct. 16, 1945 2,684,115 Cairns July 20, 1954 2,715,939 Smith Aug. 23, 1955 2,800,551 Crownover July 23, 1957

Images