US2431158A - Burner safety control - Google Patents

Description

R. E. YATEs 2,431,158

BURNER SAFETY CONTROL Nav. 18,1947.

original Filed June 28.1941

Patented Nov. 18, 1947 BURNER SAFETY CONTROL Robert E. Yates, Chicago, lll., assignor to-Drying System, 1110- Chime, Ill., a corporation of lllinob original application June as, 194.1, serial No.

1942, Serial No. 449,827

7 Claims. (Cl. 25o-27) This application is a division of copending application Serial No. 400,332, led June 28, 1941.

Heating, baking and drying systems embodying fuel burners .or the like are subject to hazards that arise when fuel introduced into a burner ls notI properly ignited and is thereby enabled to accumulate in an unburned condition in the system wherein the products of combustion from the burner are normally utilized, thereby creating a danger of explosion or other serious damage. It is therefore essential that Systems or installations of the aforesaid character be equipped with means for detecting failure of combustion and which will respond automatically invsuch a contingency to effect a control operation, such as shutting off the supply of fuel to the burner or burners, and thus safeguard the system against risk of explosion or the like.

Numerous types of safety control means or apparatus of the aforesaid character have been heretofore proposed. The earlier forms of such apparatus entailed the use of heat-sensitive devices such as thermostats, thermocouples,pressure diaphragms and the like which were indirectly responsive to the presence or absence of a flame in a burner for effecting whatever control operations were necessary with respect to the burner. Such devices were generally unreliable because an appreciable time lag was entailed in the operation thereof and such devices were readily affected by disturbances such as external air drafts which had no direct relation to combustion conditions in the burner. Later forms of flame detect-ors have operated in dependence upon the iiow of electrical current across a gap between a pair of spaced electrodes that are arranged to be bridged by the flame of the burner. Usually one of these electrodes constitutes what is known in the art as a flame rod which is disposed in the flame zone of the burner so as to be contacted by a flame when one is present, whiley the other electrode may consist of another flame rod similarly situated or it may be aii'orded by a portion of the burner structure itself. Flame detectors of this character may be roughly divided into several classes.

One class of such apparatus entails an arrangemem; in which the value of an ohmic resistance of the electrical path between the electrodes is the determining factor in the operation of the apparatus, the magnitude of this resistance lae-l ing governed by the presence or absence of ame in the burner. This class in turn may be said to comprise at least two varieties. In one of these the electrical current ow through the flame path i to operate a relay or the like. l of this apparatus an alternating-current voltage Divided and this application July 4,

being suilicient to set up a current having a great i enough magnitude (provided a flame is present) In the other form is impressed upon the flame-sensing circuit and an electron tube means is arranged to be governed not only according to the magnitude of the current flow through the flame gap, but also in response to the phase relation of this current with respectto the impressed voltage. An example of the last-mentioned apparatus is disclosed in my Patent No. 2,282,551, patented May 12, 1942, and one of its outstanding advantages is that it operates to detect not only flame failure but also an unsafe condition which may arise when the flame gap is'accidentally short-circuited.

Another of the classes of ame detectors to which reference has been made hereinabove entails an arrangement, in which the conduction of electrical current through the flame is made to depend not so much upon the conductivity of the flame itself as upon the electron-emitting properties of an electrode which is arranged to be heated by the ame. Such flame detectors tend to be unduly sensitive and moreover are unsatisthe burner nozzle).

factory when used on alternating-current because of the fact that the flame itself has a current-rectifying effect, which tends to block the unidirectional flow of current set up bythermionic emission, unless -the electrodes are so arranged that the emitted electrons travel fromr the hot to the cool electrode substantially in the direction of flame propagation (that, is, away from It is disadvantageous to arrange the electrodes in this manner, however, because the cool electrode must then be placed outside of, but very close to, the edge of the flame and any fluctuations of the flame due to crossdrafts or the like may produce corresponding fluctuations in the flow of electron current, with detrimental effects upon the accuracy of operation.

Still another class of flame detecting safety devices is that wherein the arrangement makes use of the substantially unilateral conductivity of a flame without relying upon any thermionic emission characteristics of an electrode. Because of the greater ease with which a current of electrons or negative ions may flow in the direction of :dame propagation than contrary thereto, a substantially unidirectional flow of nega-tive ions or electron currentI takes place between the spaced electrodes in a direction away from the burner nozzle when an alternating-current voltage is impressed across these electrodes. This unidirectional or rectified current is utilized to charge a dielectric body such as a condenser to thereby control the potential applied to the control element or grid of an electron tube and thus regulate the amount of current flow in an output circuit of the tube. If the flame is extinguished or becomes too low, the rectified current flow ceases and the condenser loses its charge, the effect of this being to so condition the electron tube circuit that a control operation is effected or a warning signal given.

The present invention falls in the last mentioned class, but it affords several features which are not realized in prior devices of this character, and particularly it takes into consideration the possibility that the flame-sensing means itself may become defective in several respects and thereby cease to afford protection for the burner system. For example, a flame-sensing rod or electrode may become softened due to the heat of the flame and may therefore so bend that it engages a part of the burner. This ls dangerous because the flame is short-circuited and is therefore no longer included in the input circuit of the electron tube, so that the apparatus is adapted to function safely upon thel occurrence of such a contingency. Then again, instead of there being a direct electrical contact of the flame rod with the burner there may be an accumulation of hygroscopicy carbonaceous or other electrically conductive substance extending between the flame rod and the burner which has a leakage or shunting effect that may simulate a flame even when there is no combustion of the fuel in the burner. It is likewise possible that sufficient leakage may take place through the insulation surrounding the conductors in the flame-sensing circuit to produce a condition simulating the presence of flame in the burner. Prior safety control devices operating on the flame rectification principle were unable to prevent misoperation of the system under at least some of the circumstances enumerated above and therefore an important object of the present invention is to improve such safety apparatus in these respects.

Heretofore it has been proposed to afford means for checking leakage effects such as have just been described in a number of Ways, as by providing one of the flame-sensing electrodes with a sleeve that is intended to intercept any leakage current passing between the electrodes by a path other than through the burner name, and upon the occurrence of excess leakage from an electrode to such a sleeve the apparatus is intended to react as though there had been a flame failure. Such an arrangement is unsatisfactory because obviously the sleeve cannot extend into the flame zone since it would then prevent the proper operation of the device, and hence it is ineffective to detect the leakage paths in this region.

Other prior devices of the character which function in dependence upon the ohmic resistance of the flame path have dispensed with the special sleeve on the flame-sensing electrode, and it has been intended in certain of these devices that they are to react safely whenever there is any substantial leakage from an electrode other than the normal flow of current through the flame, whether this exists by reason of shortcircuiting, insulation failure or carbonization of the electrode in the region of the flame itself. In

these prior devices an all-around leakage check of the flame-sensing circuit is made as a condition precedent to operation of the system. However, once the operation ofthe burners has been initiated, the safety apparatus thereafter functions to detect only flame failure or the existence of a low-resistance shunt path between the flame electrodes, and it will not operate safely upon flame failure if a leakage path having a resistance comparable with that of a normal flame path is.A established between the flame electrodes in theregion of the flame While the burner is in operation. Usually in such burner systems a safe reaction of the control apparatus in response to the detection of current leakage conditions in a burner does not occur until the system has been recycled, that is, disabled due to the operation of some other means such as a room thermostat and then started up again, and if such recycling does not occur soon enough, a risk of explosion is created, Y

Hencey a further object of my invention is to enable the control apparatus to recognize all of the unsafe conditions enumerated hereinabove, and particularly to ascertain whether the electrical path existing between the flame-sensing electrodes while the burner is in operation is afforded by the flame itself or whether the electrodes have become electrically connected in some other manner, especially by a bilaterally conductive shunt or leakage path having an ohmic resistance of a value comparable with that of a normal flame path.

A still further object is'to adapt the safety control apparatus for use in multiple burner systems in a novel manner without unnecessary duplication of parts and at the same time to afford the features and advantages described hereinabove.

A further object of the invention is to afford a control apparatus wherein electron tubes embodied in the apparatus are checked as to conductivity and operativeness prior to the initiation of the operation of the fuel burning system and during the entire time that the system is in operation, and an object related to the foregoing is to render an electron tube conductive in response to the sensing of a flame in a burner and nonconductive when there is a failure of combustion or upon establishment of a short circuit or leakage path through the flame gap, and in this way any internal failure of the tube which causes it to become permanently nonconductive immediately produces the same reaction of the control apparatus as though any of the aforesaid unsafe conditions had been detected. An ancillary object is to so arrange the dielectric body or condenser that it is adapted to be unidirectionally charged during forward half-cycles of the alterhating current, when a flame is present in the burner, and discharged upon flame failure or whenever the normal flame path is shunted.

Other and further objects of the present invention will be apparent from the following description and claims and will be understood by reference to the accompanying drawings which, by way of illustration, show preferred embodiments and the principle thereof and what I now consider to be the best mode ln which I have contemplated applying that principle. Other embodiments of the invention embodying the same or equivalent principle may be used and structural changes may be made as desired by those skilled in the art without departing from the present invention and the pended claims.

In the drawings,

Fig. 1 is a diagrammatic view of the basic form of the invention; and

Fig. 2 is a diagrammaticview similar to Fig. 1 but illustrating one manner in which the invention may be adapted for use in a multipleburner system.

Referring mst to Fig. 1, wherein the basic construction is shown, it will be observed that the apparatus is adapted to controlthe operation of a burner generally designated I2which is supplied with fuel through a pipe I3 that is grounded as indicated at I4. A self-closing fuel valve I5 having an operating solenoid I8 is embodied in the fuel supply pipe I3 to prevent the supply of purview of the ap.

fuel to the burner I2 when the solenoid I6 is in al 4 burner I2, being-grounded, constitutes the other electrode; however, it will be understood that a second rod such as I1 positioned closer to the nozzle of the burner than the first rod I1 may be employed equally well forthis purpose.

The control apparatus constituting the preferred form of the present invention includes an electron tube I8 which is diagrammatically represented in Fig. 1 as being of the tetrode type,

having a shield grid 65 connected directly to the cathode I9'. Other types of tubes such as triodes may be utilized in place of the tube I8, but for the purposes of the present invention a tetrode has been found to be the most satisfactory. I prefer to employ a gas-filled electron tube I8 in order to afford a trigger action, but here again it is to be understood that I dov not limit myself to such a tube since a vacuum tube may be utiiized in lieu thereof if desired.

Alternating current is supplied to the apparatus by a. suitable source such as transformer 53 having a. secondary winding 54 provided with three taps 55, 56 and 51, the tap 56 being electrically intermediate Athe taps 55 and 51. The cathode I9 of the tube I8 is connected by a conductor 59 to the tap 56, and a 4branch lead 59 from the conductor 59.connects the shield grid 65 electrically to the cathode I9. The filament 68 is sup-` energized and which closes when the relay is energized to complete a .circuit for energizing the fuel valve solenoid I 6.

The burner I2 is equipped with a flame-sensing electrode I1 which is connected by a conductor 30 to a resistor 1I and thence through this resistor to the conductor 62' leading to the transformer tap 55. A protective sheath 13 surrounding and electrically insulated from the conductor 30 is electrically connected by conductor 14 to a conductor 15 leading to the transformer tap 51. A condenser 52 is connected on one side to the conductor 15 and on its other side to ground as 6 indicated at 16. A resistor 11 is connected in parallel with the condenser 'I2 so as to afford a leakage path for the condenser. The tube I8 has a control grid 18 which is connected through a protective resistor 19 and a conductor 80to ground as indicated at 1G.

The voltage induced in that section of the transformer secondary 54 between the taps 56 and 51 affords a negative bias for the control grid 18 of the tube I8 during the forward halfcycles of `the alternating current when the anode 22 is at a positive potential with respect to the cycles.

cathode I 9, and this negative bias tends to render the tube I8 non-conductive.

It has been explained hereinabove that a flame -as F in a fuel burner as I2 has the property of rectifying alternating current; that is, during those haii-cycles in which the burner I2 is negative With respect to the electrode I1, electrons or negative ions may flow from the burner I2 to the electrode I1 in the direction of flame propagation, provided a flame as F is present, thereby producing current flow from the electrode I1 to the burner I2. However, during the intervening half-cycles substantially no current flow can take place through the flame F between the electrode I1 and the burner I2.

As the apparatus is arranged in Fig. l the tube I8, insofar as the bias between anode and cathode is concerned, tends to be conductive in those half-cycles of the alternating current in which the anode 22 is positive with respect to the cathode I9, or in other words, during the forward half- It will be understood, however, that the normal negative bias on the control grid 18 overcomes this tendency of the tube t'o be conductive in such forward half-cycles of the alternating current unless this normal negative bias of the control grid is reduced in response to the pres` ence of a flame F, as will be hereinafter described, During such forward half-cycles of the a1ternating current the electrode I1 is positive with respect to the burner I2, and therefore electrons or negative ions may flow from the burner to the electrode in the direction of flame propagation, provided a flame is present. However, during the intervening or inverse half-cycles of the alternating current the electrode I1 is negative with. respect to the burner I2 and therefore the flame F is effective to block the flow of electron current between the burner I2 and the electrode I1. Thus, when a flame F is present in the burner I2, the current flows through the flame-sensing circuit during forward half-cycles of the alternating current, this circuit extending from the transformer tap through the resistor 1I, the conductor 30, the electrode I1, the flame F,

, the 'burner I2, the condenser 52 and the resistor 11 in parallel, and the conductor 15 to the transformer tap 51. Due to the rectlfying effect of the flame F, this current is of a non-bilateral character so that the condenser 52 assumes a substantially unidirectional charge to thereby produce a direct-current voltage drop between points 51 and 16 which tends to counteract the negative bias on the control grid 18. So long as the flow of rectified current continues in the flamesensing circuit this voltage drop will be of such a direction and magnitude that the potential of the control grid 18 with respect to the cathode I9 is more positive than is the firing voltage of the tube I8. Under these circumstances the tube I8 is rendered conductive during the forward half cycles of the alternating current, and a pulsating current is passed through vthe winding of 7 the relay 20 to maintain this relay energized, and the relay contact 24, therefore, remains closed and maintains the fuel valve solenoid I6 energized to thereby enable fuel to be supplied to the burner I2.

In the event that the flame F in the burner I2 is accidentally extinguished, the flow of rectified current through the aforesaid flame-sensing circuit ceases and the charge on the condenser 52 leaks off through the resistor 11. Since there is Vno longer any substantial voltage difference between the points 51 and 16 to counteract the negative bias applied to the control grid 18, the tube I8 ceases to conduct current and the relay 20 deenergizes to open its contact 24. 'I'his results in deenergization of the solenoid I8 and consequent closure of the fuel valve I5 tovprevent emission of unburned fuel from the burner I2.

If the flame 1electrode I1 accidentally becomes grounded as by engaging the burner I2, alternatingcurrent is passed through the condenser 52, and under these circumstances substantially all of the voltage between the transformer taps 55 and 51 is absorbed in the resistor 1|, the circuit being so designed that on alternaitng current the condenser 52 acts as a low-impedance shunt around the resistor 11. Because of the decreased voltage drop between the points 51 and 15, the potential of the control grid 18 relative to the cathode I9 becomes more negative than the firing voltage of the tube during the forward half-cycles of the alternating current, thereby rendering the tube I8 non-conductive.

Similarly, if there is a bilateral leakage path existing between the electrode I1 and ground, so that alternating current is passed through the flame-sensing circuit, the condenser 52 serves to shunt the resistor 11, causing the tube I8 to become non-conductive. Failure of the insulation between the conductor 30 and the sheath 13 producing excessive leakage currents has a shunting effect upon the condenser 52 which likewise renders the condenser ineffective to maintain the tube I 8 in a conductive state, and the same is true if the sheath 13 becomes grounded.

For purposes ofl initiating operation of the burner I2 after it has been disabled by the safety apparatus, a normally open switch 40 may be connected in parallel with the relay contact 24 in the fuel valve energizing circuit. This switch is momentarily closed by the operator to energize the solenoid I6 independently of the relay 20 to enable fuel to be supplied to the burner I2. The fuel is ignited lby a suitable ignition means (not shown) which'is temporarily rendered operative for the purpose of starting the burner in operation, such ignition means preferably being under the control of the starting switch 40. When the operator ascertains that the fuel has been properly ignited, he releases the starting switch 40 and thereafter the energizing circuit for the fuel control valve solenoid I6 is maintained closed through the contact 24 of the relay 20 (the tube I8 having been rendered conductive as hereinbefore described upon the establishment of a flame F in the burner I2) unless and until a hazardous condition'again arises requiring a. shut down of the system.

It has been stated hereinabove that the present invention may be readily adapted for use in multiple burner systems, and one such arrangement and which is operatively associated with one of thev fuel burners I2 included in a bank of burners that are supplied with fuel by a single pipe Il having the necessary branch connections to the various burners. Corresponding parts of the structure shown in Figs. 1 and 2 are designated by like reference characters and it will be understood that the operation of the master control unit MU is substantially the same as that of the apparatus shown in Fig. 1.

In addition to the master control unit MU there are provided a plurality of secondary control units designated SU which are respectively associated with the other burners I2' of the system in much the same manner as the master unit MU is associated with the burner I2. Thus, each burner I2 has an electrode I1' which is connected by a shielded conductor 30' to the control grid 18 of the electron tube I8' in the respective secondary.

unit SU. The shielded conductors 8D and 5B' may be passed through individual grounded conduits or it may be preferred to run these conductors through a common conduit for at least a portion of the respective lengths, The secondary units are substantially identical in construction with the master unit MU except that the relays as 25', have their relay contacts conneced in series. as will be presently described, to jointly govern the operation of the solenoid I8 of the normally closed fuel valve I5 which controls the fuel supply to all of the burners I2, and hence corresponding parts cf the master and secondary control units bear similar reference characters which differ only in the prime sumxes. The anodes 22' of the tubes I8' in the several secondary units SU are connected through the coils of their respective relays 20 to the taps 55 of their respective transformer secondaries 54 in the same manner as in the master control unit MU. In'like manner the cathode I9 of the tube I8 and the cathodes I9 of the tubes I8 are all electrically connected by wires 59 and 58", respectively, the taps 58 of their respective transformer secondaries 54 and 54. Hence, the electron tubes I8 and I8 in the various control units are connected and operate in the same manner as in the embodiment of the invention shown in Fig. l, and anode current of these tubes passes through the windings of their respective relays 20 and 20' to close the contacts 24 and 24' thereof in response to the presence of a flame at their respective burners I2 and I2. A wire 8I connects a line wire I1 to the stationary contact of the relay 20, a wire 82 connects the relay contact 24 to the stationary contact of the relay 20' of the Afirst secondary unit SU, a wire 83 connects the relay contact 24 of this secondary unit SU to the stationary contact of the relay 20 of the next unit SU, and so on until the last of the unit SU has been thus connected, and the contact Yi4 of the last unit SU is connected by a wire 85 to one terminal of the solenoid I5; and since the other terminal of the solenoid I8 is connected to the other line v wire L2, the `solenoid I8 is controlled by a series circuit which may -be broken by opening of any one of the relay contacts 24 or 24'.

It will be recalled that the tubes I8 are normally biased so as to be nonconductive, and that when a flame as F is present in a burner as I2 the associated tube as I8 is rendered conductive. Similarly, the tubes I8 are normally biased so as to be nonconductive and when the flames are present in all of the other burners I2' the corresponding tubes I8 are all conductive, and hence the relays 20 and 25' are energized when a flame 9V is present at all of the burners. However, if a flame in any one of the burners becomes extinguished, or too low, the tube I8 or I8 associated with such burner is rendered non-conductive so that current flow stops place in the associated relay coil, which relay thereupon deenerglzes and opens its contact 24. The fuel supply pipe I3 has a valve I5 controlled by the solenoid I6 which receives its energizing current through the relay 'contacts 24 and 24' of all of the relays 20 and 20' in series, and when a relay 20 or 20' deenergizes to open the Contact 24 or 24 thereof the solenoid I8 deenergizes and enables the fuel valve i5 to close and thereby interrupt the supply of fuel to all of the burners in the system.

The other safety features described hereinabove in connection with the circuit of Fig. 1 are also afforded in the case of each and every control unit in the multiple system as shown in Fig.` 2. Thus, if there is a, shunting of the flame gap in any particular burner due to grounding out of the flame-sensing electrode or the building up of a non-rectifying resistance path across the flame gap as a result of carbonization and the like, the electron tube in the control unit associated with that burner is rendered nonconductive so as to deenergize its relay and thereby break the circuit to the fuel valve solenoid and shut off the supply of fuel. The same is true if there is an insulation failure which causes excessiveleakage in the cable including a conductor 30 or 30' leading to the flame electrode of a particular burner. Hence, a dependable safety control apparatus for a multiple burner system is afforded.

From the foregoing description it will be apparent that the invention provides a safety control apparatus of novel and economical construction and which is more reliable in operation than the control devices heretofore employed to govern fuel burner systems and the like. Thus, the present apparatus acts to protect the burner system against hazards which arise not only when the fuel admitted to a burner or burners is improperly ignited resulting in a failure of combustion, but also when the safety control apparatus itself becomes defective in any of a number of different ways. For example, where the apparatus is adapted to respond to the flow of electrical current through a gap between spaced electrodes positioned in the region of a flame in a burner, as is done in the present instance, there is always the possibility that the aforesaid electrical path through the flame may be accidentally shunted due to a flame-sensing electrode coming into electrical contact with the other electrically conductive substances such as carbon accumulating on a l0 currence of any of the undesirable contingencies mentioned above, and the arrangement is such that an electron tube is rendered conductive when a fiame is present in the burner and nonconductive if the flame is extinguished or the electrical path through the flame is shunted so as to numfy the current-rectirymg @neet or the to the burner. However, if there is a failure of combustion or the llame path becomes shunted, the condenser loses its unidirectional charge and the electron tube is then rendered nonconductive. Means under control of the electron tube thereupon operate to interrupt the supply of fuel to the burner so that unburned fuel cannot be emitted by the burner. It will be understood, of course,

-that any other or additional control operations with respect to the fuel burner may be performed by the apparatus when an unsafe condition exists, such as operating an alarm or automatically operating an ignition means to reignite the fuel.

flame-sensing electrode or on the surface of the insulators supporting such electrode, and in some instances the ohmic resistance of this shunt path may be of a value comparable with that of a normal flame path so that the gap between the electrodes has substantially the same conductivity (except that it is of bilateral rather than unilateral character) in the absence of a flame as would normally be the case when a flame is present. The present invention is adapted to guard against dangers attending such improper conditions in the burner and is particularly useful and advantageous in that all of these safety features are afforded in the same apparatus without unduly complicating the construction thereof.

In the present invention the unilateral electrical characteristics of flames are utilized to control the operation of electron tubes for the pur pose of producing a safe response upon the oc-i The present burner control is so l arranged that the electron tube is rendered conductive when a flame is present in the burner and nonconductive whenever there is a flame failure or shunting of the flame gap due to a short circuit or leakage between the flame electrodes, and this feature is important because it enables the apparatus to react safelywhenever a tube assumes a permanently nonconductive state as a result of internal failure. Such an apparatus has been found/to be particularly applicable to single burner installations. Another feature of the present invention is that it may be readily adapted to use in connection witlrmultiple burner systems as well as with single burners. In multiple burner operation unnecessary duplication of fuel valves is avoided by providing a single master control unit to control a common fuel supply means, and a plurality of secondary control units each operatively associ-A ated with a respective burner, each of said control units responding to conditions in its burner to govern the common fuel supply means.

Still another advantage of the present invention is that if the insulation surrounding the conductors in a flame-sensing circuit becomes faulty so as toenable'excessive leakage of current to take place through the insulation, the apparatus is able to detect such conditions and respond in a safe manner. This is particularly important where the apparatus is to be utilized in chemical plants or the like where the insulation is subject to damage from corrosion or other causes.

While I have illustrated and described selected embodiments of my invention, it is to be understood that these are capable of variation andv modification and I therefore do not wish to be limited to the precise details set forth but desire to avail`myself of such changes and alterations as fall within the purview of the following claims.

I claim:

1. In a flame detecting device for detecting the presence of a flame in a fuel burner or the like, an electron tube having a. control element normally biased to render said tube nonconductive,

a condenser effective when charged unidirectionally in a given sense to nullify the bias on said control element and thereby render said tube conductive, and means providing a circuit extending from an alternating current source and including a pair of electrodes insulated from each other and adapted to be positioned in the path of a flame of substantially unilateral conductivity and through said condenser whereby said condenser tends to become charged unidlrectionally for rendering said tube conductive when a flame is present, said circuit also embodying a first resistor connected in parallel with said condenser for causing said condenser to lose its unidirectional charge and thereby become ineffective to prevent said tube from assuming a nonconductive state when said circuit is interrupted due to flame failure, and a second resistor connected in series with said condenser and effective when said flame path is short-circuited to so limit the magnitude 'of the charge on said condenser as to render said tube nonconductive.

2. In a flame detector operating in dependence upon the property of a flame to conduct electrical current between two spaced electrodes more readily when the first of said electrodes is positive with respect to the second electrode than when the relative polarities of said electrodes are reversed, the combination of an alternating current transformer having three taps, the potential of the second tap being intermediate the potentials of the first and third taps; an electron tube having an anode, a cathode and a control grid; means electrically connecting said anode to said rst tap, means electrically connecting said cathode to said second tap, a first resistor, a condenser; a pair of electrodes insulated from each other and adapted to be located in the path of a name with one of said electrodes spaced from the other in the direction of propagation of the name, circuit means electrically connecting said condenser in circuit from said third transformer tap to said other electrode, means electrically connecting said one electrode through said first resistor to said first transformer tap, said first electrode being intermediate said first tap and said second electrode, a second resistor connected in parallel with said condenser, and means electrically connecting said control grid to said circuit means between said condenser and said other electrode, whereby said tube is rendered conductive only in response to the establishment of a substantialy unidirectional charge on said condenser whenever said electrodes are electrically interconnected solely through the unidirectlonally conductive medium of a flame, and said tube becomes nonconductive upon flame failure as well as whena bilateral electrical connection of any kind or\afny resistive value exists between said electrodesfrsV` 3. In a flame detector operating in dependence upon the property of a flame to conduct electrical current between two spaced electrodes more readily when the first of said electrodes is positive with respect to the second electrode than when the relative polarities of said electrodes are reversed, the combination of an alternating current transformer having a secondary provided with three taps, the potential of the second tap being intermediate the potentials of the first and third taps; an electron tube having an'anode, a cathode and a control grid: means electrically connecting said anode to said first tap, means electrically connecting said cathode to said second tap, means including a. first resistor connecting said first electrode to said first tap, a condenser through which said second electrode is connected to said third tap, a second resistor connected in parallel with said condenser, and a third resistor connecting said control grid to said second electrode, whereby said tube is rendered conductive only in response to the establishment of a substantially unidirectional charge on said condenser whenever said electrodes are electrically interconnected solely through the unidirectlonally conductive medium of a flame, said tube becoming nonconductive upon flame failure as well as when a bilateral electrical connection of any kind or any resistive value exists between said electrodes.

4. In a name responsive device, an electronic discharge device having an anode, a cathode and a control grid, a pair of spaced electrodes insulated from each other and adapted to be positioned in the path of a flame with a particular one of said electrodes displaced from the other of said electrodes in the direction of propagation of the flame, a source of alternating electric current having a first, a second and third tap, a control circuit connected between said first tap of said source and said anode and from said second tap to said cathode, circuit means connecting said third tap to said cathode and said control grid to normally render said discharge device nonconductive during those cycles of the alternating current when said anode is positive with respect to said cathode, said circuit mea'ns including a capacitance and a resistance arranged in parallel and disposed in and forming a part of said circuit means, a flame sensing circuit including means affording an alternating current path from said other electrode to the terminal of said capacitance that is electrically adjacent to said grid, and said flame sensing circuit also means affording an alternating current path from said particular electrode to said third tap of said source, and said flame sensing circuit further including resistance means operable in the event of grounding of said particular electrode or in the event of bilateral conductivity of said flame sensing circuit to limit the instanta neous charge impressed on said capacitance to an amount insufficient to render said electron discharge device conductive.

5. In a flame responsive device, an electronic discharge device having an anode, a cathode and a control grid, a pair of spaced electrodes insulated from each other and adapted to be positioned in the path of a name with a particular one of said electrodes displaced from the other of said electrodes in the direction of propagation of the flame, a source of alternating electric current having nrst and second taps, a control circuit including means connecting said rst tap of said source to said anode, means including a cathode connection and a control grid connection associating said cathode and grid with said second tap to normally render said discharge device nonconductivevduring those cycles of the alternating current when said anodeais positive with respect to said cathode, circuit means affording an alternating current path from said particular electrode to said one tap 0f said source, means affording an alternating current path from said other electrode to said control grid connection at a particular point therein, said control grid connection intermediate said particular point and said second tap including a 13 capacitance and a resistor in Parallel, whereby bridging of said electrodes by a name propagated in a direction extending from said other electrode-to said particular electrode charges said condenser in such a sense that positive bias is applied to said control ,electrode to nullifythe normal negative bias thereon and render said discharge device conductive, and a limiting resistor included in said circuit means and operable when said particular electrode is grounded to so limit the instantaneous charge on said capacitance as to render said capacitance ineiective to nullify the normally negative bias on said grid.

6. In a ame responsive device, a pair of electrodes insulated from each other and adapted to be positioned in a iiame with one of said electrodes spaced from the other of said electrodes in the direction of propagation of the llame, an electron discharge device having an anode, a cathode and a control grid, a source of alternating electrical energy having first and second taps, means connecting said anode to said rst tap of said source, circuit means including a cathode connection and a control grid connection associating said cathode and said control grid with said second tap of said source to normally render said discharge device nonconductive during those cycles of the alternating current when said anode is positive with respect to said cathode, a conductor including a protective resistor connecting said one electrode to said first tap of said source, a sheath protecting at least a portion of said conductor and arranged in insulated relation to said conductor, said control grid connection including a capacitance and a resistance in parallel in said connection, means connecting said sheath to said'control gridconnection between said capacitance and said second tap oi' said source, and means a'ording an alternating current path between the other side of said condenser and said other electrode,

7. In a device for detecting the presence of a flame in a fuel burner or the like, an electron tube having an anode, a cathode and a control grid, an alternating current source connected to said anode and cathode to aord an anode-cathode cirv cuit, means including a condenser and a resistance in parallel and connected between said grid 14 and said source to normally bias said control grid negative with respect to said cathode during that portion of each alternating cycle when said anode is positive with respect to the cathode to thereby normally block said tube and render the same non-conductive, said condenser being effective when charged unidirectionally in a given sense and in a predetermined amount to nullify the normal blocking action of said grid and thereby render said tube conductive, and means providing a flame-sensing circuit including said alternating current source and extending through a path in said flame of substantially unilateral conductivity and through said condenser in such a sense in respect to the anode-cathode circuit of said tube and with respect to the direction of propagation of said llame that said condenser tends to become charged unidirectionally for rendering said tube conductive when a flame is present, and means in said llame-sensing circuit operable to limit the magnitude of the instantaneous alternating current charge impressed on said condenser and said control grid in each cycle of the alternating current to a value insufflcient to render said tube conductive whereby said control grid is ineiective to render said tube conductive when said flame-sensing circuit is bilaterally conductive.

' ROBERT E. YATES.

REFERENCES CITED -The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,313,943 Jones Mar. 16, 1943 2,127,977 Lamb Aug. 23, 1938 2,245,730 Sparrow June 17, 1941 2,224,119 Harrison Dec. 3, 1940 2,260,977 Jones Oct. 28, 1941 2,112,736 Cockrell Mar. 29, 1938 2,343,001 Cohen Feb. 29, 1944 2,361,294 Jones Oct. 24, 1944 2,299,501 Schneider Oct. 20, 1942 2,243,071 Crago May 27, 1941 2,379,871 Beam et al. July 10, 1945 2,404,001 Smith July 16, 1946

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