US2410524A - Burner safety control - Google Patents

Description

1946. D. E. RICHARDSON ETAL 2,410,524

BURNER SAFETY CONTROL Filed June 28, 1941 6 Sheets-Shoot 1 whiz f h 26 L .Lg V

S ZIPPL Y 51 4, J- a91L f Z' 37 I L, 4/ 42 l WMbI/PS.

Mel ll: aes (7.31 'Mztlf fimaaww 6 Sheets-Sheet 2 Nov. 5, 1946. D. E RICHARDSON ET AL BURNER SAFETY CONTROL Fild June 28, 1941 worrr gai 5, 1946. D. E. RICHARDSON ETAL 2,410,524

BURNER SAFETY CONTROL Filed June 28, 1941. 6 Sheets-Sheet 3 110 1/- was lPt'cizardsvz/z M NW 4 D. E. RICHARDSON ET AL 2,410,524

BURNER SAFETY CONTROL Fil ed June 28, 1941 s Sheets-Sheet 4 Nov. 5, 1946.

D. E. RICHARDSON ET AL I BURNER SAFETY CONTROL Filed June 28, 1941 6 Sheets-Sheet 5 Nov. 5, 1946. D. E. RICHARDSON ET AL 2,410,524

BURNER SAFETY CONTROL Filed June 28, 1941 a Sheets-Slieet e Patented Nov. 5, i946 BURNER SAFETY CONTROL M Donald E. Richardson and Robert E. Yates, cm-

111., assignors to. Drying Systems, Inc.,

' Chicago, 111., a corporation of Illinois Application June 28, 1941, Serial No. 400,332

I 16 Claims.

This invention relates to fuel burners and like heating devices and particularly to safety con- ,trol apparatus for use with such devices.

Heating, baking and drying systems embodying fuel burners or the like are subject-to hazards that arise when fuel introduced into a burner isnot properly ignited and is thereby enabled to accumulate in an unburned condition in the system wherein the productsoi combustion from the burner'are normally utilized, thereby create ing a danger of explosion'or other serious damage. It is therefore essential that systemsor installations of the aforesaid character be equipped with means for detecting failure of combus ion and which will respond automatically in such a contingency to eflect a control operation, such as. shutting ed the supply'of' fuel to i 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, pres-- ,sure' diaphrasms and the like which were 'indirectly responsive to thepresence or absence of a flame in a burner for eifeotingwhatever con- .troi operations were necessary-with respect'to the burner. Such devices were generally unreliable because an appreciable time lag was en- .tailed in the operation thereof and such devices were readily affected by disturbances such as external air draitsflwhiohhad no direct relation Later 1 forms of flame detectors have operated in deto combustion conditions in the burner.

pendence ;upon' the flow of electrical current across a gap-between a pair of spaced electrodes I 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, while the other electrode may consist of another time rod similarly situated or it may be afforded by a portion of the burner structure itself. Flame detectors of this character roughly divided into several classes. a

may be Oneclass'of such apparatus entails anarrangement in which the value ofan ohmic resistanc of the electrical path betweenfthe electrodes is the determining factor in the operation of the apparatus, the magnitude of this resistance being governed by the presence or absence of flame in the burner. This class in turn may be said to comprise at least two varieties. In one of these theelectricalcurrent flow through the flame path is Produced by impressing a relatively high voltage across the flame-sensing electrodes,

this'voltage being suflicient to set up a current 5 having a great enough magnitude-(provided a flame is present) to operate a relay Or the like. In the other form ofthis apparatus an alterhating-current voltage is impressed upon the flame-sensing circuit and an electron tube means is arranged to b 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 respect to the impressedfvoltage. An example of the last-mene tioned apparatus is disclosed in the, copendin'g application of Robert Yates; Serial No. 245,-

173, flied December 12, 1938, now Patent No. 2,282,551,- patented May 12, 1942, and one of its outstandingadvantages 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. I

Another-of the classes of flame detectors to whioh'reierenoe hasbeen made hereinabove entails anarrangement 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 tobe 3 heated by the flame. Buch'flame detectors tend to be unduly sensitiveand moreover are,unsatisfaotorywhen used on alternating-current because of the fact that the flame itself has a current-rectifying effect which tends to block the a unidirectional now of current set up by thermionic -emissiOn 1m1ess the electrodes are so arranged that the emitter electrons travel from the hot-to the cool electrode substantially in .direction of flame propagation (that is, away 40 from the burner nozzle). It is disadvantageous 1 to arrange theelectrodes in this manner, how ever, 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 cross-drafts .or the like may produce corresponding fluctuations in the'flow of electron current, with detrimental effects upon the accuracy of operation.

Still another classof 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. Be-

cause of the greater ease with which a current pl; of electrons or negative ions may flow in the direction of flame propagation than contrary thereto, a substantially unidirectional flow of negative ions or electron current 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 rectifled 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 is 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 furnishes a false indication of thepresence of a flame unless it is adapted to function safely upon the 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 hygroscopic, 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 suflicient 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 apath other than through the burner flame, 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 none since it would then prevent the proper operatlon of the device, and hence it is ineffective to detect 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-sensingelectrode, and it has been intended in certairf nf .these devices that they are to react safely whenever there is any the normal flow of current through the flame, whether this exists by reason of short-circuiting, 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 flamesensing circuit is made as a condition precedent to operation of the system. However, once the operation of the burners has been initiated, the

safety apparatus thereafter functions to detect only flame failure or the existence of a lowresistance shunt pathbetween 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 established between the flame electrodes in the region 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.

' Hence, a further object of our invention is to enable the control apparatus to recognize all of the unsafe conditions enumerated hereinabove. and particularly to ascertain whether the electrlcal path existing between the flame-sensing electrodes while the burner is in operation is afforded by the flame itself or whether the electrodes have been 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.

In flame-detecting devices which embody electron tubes, particularly those of the gas-fllled type, it is desirable to have the tube perform as little work as possible so as to prolong the llie of the tube. Normally the control apparatus may well be in an idle condition and needbe called upon to function only on comparatively rare occasions, as in the event of flame failure or upon the occurrence of some other hazardous condition against which the apparatus is designed to guard, and even then it need operate for only a short time. Hence, it is advantageous to so arrange the tubes that they remain nonconductive while the burner system is in normal operation, or in other words during the greater part of the time, and become conductive only in an emergency requiring operation of the control apparatus, and so to do is an object of a preferred form of the present invention.

Electronic safety control apparatus of the character in which an electron tube is adapted to be rendered nonconductive when the presence of a flame in the burner is sensed may become subsubstantial leakage from an electrode other theta.

Ject to misoperation due to internal failure of the tube. For example, the tube may become nonconductive due to burning out of the filement or because of other circumstances tending to decrease its-conductivity so that the tube tends to give a false indication that a flame is present in the burner even though this may not actually be the case. In the aforesaid copending application, Serial No. 245,173, there is disclosed means for causing the apparatus to react safely in the event one or more electron tubes become defective due to filament failure. However, inasmuch as this is only one of the possible causes of tube failure, we have in the present instance provided means forautomatically checking eaehelectron tube to determine whether or system;

not it is capable of operating properly and to detect failure of the tube due to any of a number of causes. Preferably such checking of the tubes should take place prior to the initiation of operation of the heater system, and so to do constitutes another object of the invention.

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 afiord the features and advantages described hereinabove.

A specific object of a preferred form of the present invention is to so arrange an electron tube and a condenser in relation to an alternating current source and to the flame-sensing electrodes that the substantially unilateral conductivity of the electrical path through the flame normally enables the condenser to become unidirectionally charged during those half-cycles of the alternating current in which the tube isinoperative (that is, in the inverse half-cycles).

An ancillary object is to so control the tube by the condenser that the tube remains continuously nonconductive if and.when a. substantial unidirectional charge exists on the condenser, whereby, if this charge is dissipated due to the establishment of a bilateral electrical connection between the flame-sensing electrodes, such as may be caused by a short circuit ora. leakage shunt, or if this charge leaks of! without being replenished because of flame failure, the tube becomes effective to conduct current during forward half-cycles of the alternating current.

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 we now consider to be the best mode in which we have contemplated applying that principle. th-

Fig. 2 is a diagrammatic view similar to Fig. 1

but illustrating one manner in which the invention may be adapted for use in a, multiple burner system; i

Fig. 3 is a. schematic view of a master control unit embodyingvarlous improvements over the apparatus shown in Fig. 1;.

Fig. 4 is a. schematic view similar to Fig. 3 and showing another manner in which the apparatus may be incorporated in a multiple burner Fig. 5 is a simplified wiring diagram of the apparatus illustrated in Figs. 3 and 4;

Fig. 6 is a diagrammatic view showing the manner in which spare control units may be provided;

Fig. '7 is a plan detail view of a tube socket as it is mounted in a control unit;

Fig. 7A is a sectional detail view taken sub stantially on the line IA-IA on Fig. '7;

Fig. 7B is a partly sectional detail view of a screw shown in Fig. 7A; and

Fig. 8 isa diagrammatic view of a modified form of the invention.

Referring first to Fig. 1, wherein the basic construction is shown, it will be observed that the apparatus is adapted to control the operation of a burner generally designated I2 which is supplied with fuel through pipe I3 that is grounded as indicated at H. A self-closing fuel valve I5 having an operating solenoid i6 is embodied in the fuel supply pipe ii to prevent the supply of fllel to the burner l2 when the solenoid i6 is in a deenergized condition. The burner I2 is equipped with a flame-sensing rod or electrode II which is insulatedly mounted in such a manner as to extend into the flame F of the bumer when such a flame is present, the electrode I! being maintained in predetermined spaced relation with the tip or nozzle of the burner l2.

as H positioned closer to the nozzle of the burner than the first rod i! may be employed equally well for this purpose.

The control apparatus constituting the preferred form of the present invention includes an electron tube l8 which is diagrammatically represented in Fig. 1 as being of the tetrode type, having a shield grid 65 connected directly to the cathode l9. Other types of tubes such as triodes may be utilized in place of the tube 18, but for the purposes of the present invention a tetrode has been found to be the most satisfactory. We prefer to employ a gas-filled electron tube III in order to afford a trigger action, but here again it is to be understood that we do not limit ourselves to such a tube since a vacuum tube may be utilized in lieu thereof if desired. The cathode l9 ofthe tube I8 is electrically-connected to a line wire Li which leads to. one pole of a suitable source of alternating current. A line wire L2 leading from the other pole of said source of current is electrically connected to one end of'the winding of a relay 20, the other end of this relay winding being connected by a conductor 2| to the anode 22 of the tube ii. The heater orfilament 23 of the tube I8 is electrically connected to a suitable filament voltage supply source,

The relay 2|! has a contact 24 which is normally closed when the relay is in a deenergized condition so as to establish an electrical connection from a conductor 25, which leads from one end of the winding of the fuel valve solenoid IE, to another conductor 26 that leads to one pole of a suitable source of voltage. The other pole of this source of voltage is electrically connected by a conductor 21 to the other end of the winding of the solenoid i6. Thus the solenoid i6 is maintained energized so long as the relay contact 24 is closed, and while it is in this condition it holds the fuel valve ii open to enable the supply of fuel to the burner l2. However, if the relay contact 24 opens upon energization of the relay 20,

the solenoid I6 is deenergized and the fuel valve 7 tive under normal conditions when there is combustion of the fuel in the burner, so that the relay 20 remains deenergized and thereby enables the fuel valve solenoid it to remain operative for maintaining the fuel valve l open.

Since the tube I8 is to be responsive to combustion conditions in the burner l2, means have been provided to control the operation of this tube in accordance with the presence or absence of a flame as F in the burner, Thus, the control element or grid 28 of the tube It is connected through the medium of a protective resistor 29 to a conductor 30 which is electrically connected to the flame-sensing electrode l1. This conductor 30 is also electrically connected by a resistor Ii to a conductor II which is connected by the filament of a lamp 3! to the wire 32 leading to the cathode i9 and line wire Ll, In order to prevent stray electrical disturbances from affecting the tube II, the conductor 30, which in ordinary practice is a t to be of considerable length, is enclosed in a metallic sheath II which is insulated therefrom throughout its length, this sheath 34 being electrically connected to the conductor I. Usually the dielectric insulating medium between the conductor 30 and the sheath 8 aflords a slight amount of electrostatic capacity, which has been diagrammatically represented in Fig. 1 as being lumped in a single capacitance 8|. hereinafter referred to as a condenser.

- In order to complete the flame-sensing circuit the line wire L2 is grounded as indicated at It so that whenever a flame F is present in the burner I! an electrical circuit is established from line wire L2 through the burner l2, flame F, electrode I I, onductor Ill, and through the resistor II and condenser 35 in parallel to the conductor ll, thence through the lamp 33 to the cathode 8 F between the electrode I1 and the burner it. As the apparatus is arranged in Fig. 1, the tube It is conductive only during those half-cycles of the alternating current in which the line-wire L2 is positive with respect to the line wire Ll, or in other words during the forward half-cycles. Under these conditions the electrode I1 is negative with respect to the burner 12 and therefore the flame F is effective to block the flow of electron current between the burner l2 and electrode I1. 'During the inverse half-cycles, in which the polarities of the line wires Li and L2 are reversed, the tube i8 is nonconductive but the flame F then conducts current. Under the latter circumstances, inasmuch as the line wire Ll is at positive potential with respect to the line wire LI, the condensers "and 31 are so charged that the grid 28 of the tube I8 assumes a negative potential with respect to the cathode la. The full charge on these condensers may be built up throughout a series of inverse half-cycles, the length of time required to impart a substantially steady charge to these condensers being determined by the values of capacity and resistance in the circuit. Once the condensers have assumed their full charge, the grid 28 is imparted sufficient negative bias to prevent conduction of current through the tube l8. The resistor Ii return wire 32 which is electrically connected to the line wire Ll. To augment the path afforded by the condenser 35 and resistor 8!, another condenser 81 may be electrically connected across the conductors 30 and 4| in parallel with the resistor ll. Ordinarily the amount of current has a high enough resistance so that the condenser charge is not appreciably diminished during the forward half-cycles so long as a flame P is maintained in the burner l2. Under these conditions the relay 20 remains in a deenersized state to thereby maintain an energizing circuit to the fuel valve solenoid l6, and hence fuel continues to be supplied to the burner l2.

If for some reason the flame F should become extinguished or unduly low, however, the above described flame-sensing circuit is broken and the condensers l5 and I1 discharge through the resistor 3|, causing the grid 28 to lose its negative bias, and thereupon the tube is is rendered conductive during forward half-cycles of the alternating current. Hence, a pulsating rectiflowing through this flame-sensing circuit is so fled current passes through the winding of the minute as to be incapable of lighting the lamp ll, except when certain kinds of short-circuits occur, as will be explained.

For the purpose of further protecting the derelay 20, causing this relay to energize and open its contact 24. Preferably the relay" is of the slow-to-release type so that it maintains its contact 2| open so long as pulsating current is device from stray electrical disturbances as well as livered to this relay winding by the tube ll.

to prevent accidental injury by electrical shock to persons working with the apparatus, the cable comprising the conductor 30 andsheath 84 may be passed through a conduit 38 which is grounded The energizing circuit for the fuel valve solenoid It is thereby broken; causing the fuel valve II to close and prevent any further supply of fuel to the burner it.

as indicated at 38, the sheath ll being suitably 58 For the purpose of initiating operation of the insulated from this conduit. The presence of the sheath 3| intermediate the conductor I0 and grounded conduit 38 has a tendency to prevent excessive leakage through the insulation surburnerafter it has been disabled by the safety apparatus a normally open starting switch ll may be connected in parallel with the relay contact 24 in the fuel valve energizing circuit. This rounding the conductor 10, inasmuch as this 00 switch is momentarily closed 'by the operator to sheath lslmaintained at substantially the same potential as the line wire Li and therefore tends to divert any leakage current proceeding from the line wire L2 by way of ground away from the condoctor I0.

It has been explained herelnabove that a flame energize the solenoid l6 independently of the 'relay 20 to enable fuel to be supplied to the burner II. The fuel is ignited by suitable ignition means (not shown) which is temporarilyrendered 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 operatorascertains that the fuel has been properly ignited he releases the switch 40 and thereafter the valve energizing circuit is maintained closed through the contact 24 of the. relay Ill (the tube I! having been rendered nonconductive upon the establishment of a no current flow can take place through the flame 15 name F in the burner it) unless and until a hazardous condition again arises requiring a vshut-down of the system.

There are occasions in practice when the safety control apparatus is rendered partially defective by unwanted leakage or short-circuit conditions in the flame-sensing circuit. For example, the electrode ll may accidentally come into contact with the burner l2 or to some other grounded part of the structure, or there may be a leakage path existing between the electrode I! and burner l2 due to accumulation of carbon deposits or I line wire L2 so that the tube I3 is rendered conductive during forward half-cycles. Similarly, if there is a leakage path between the electrode i1 and burner |2,then because of the non-rectifying or bilateral character of this leakage .path the condensers as 35 and 31 do not assume any appreciable unidirectional charge. Any charge which the condenser receives during the inverse half -cycle is generally lost during the succeeding forward half-cycle so that the grid 23 has no opportunity to retain its negative bias and therefore the tube I8 is rendered conductive to interrupt operation of the burner I2.

It should be noted in this connection that safe operation of the apparatus is insured even though the leakage resistance in the flame gap is of a value approximately equal to the ohmic resistance of a flame as F, and this is true even though the false flame resistance comes into being while the burner is in operation. In most instances where bilateral conduction through a leakage path occurs, the response of the apparatus is practically instantaneous. In other instances, where the leakage resistance is of a critical value, the apparatus may wait until there'is an actual flame failure (if and when this occurs) before operating to shut off the fuel supply to the burner. In any event, however, the emission of unburned fuel is prevented, and it is not necessary as in the case of prior types of control apparatus to go through the process of recycling the system (that is, effecting a shutdown by other means and then starting the system inoperation again) in order to so condition the apparatus that it will disable the fuel supply means upon detecting current leakage in the flame path.

Another undesirable condition may exist when there is excessive leakage or a direct short-cir-' cuit between the conductor 30' and the sheath, such as may be caused by a failure of the insulation. In this case the amount of resistance in parallel with the condensers 35 and 31 is greatly reduced so that these condensers discharge practically instantaneously during forward 'halfcycles of the alternating current, thereby render ing the tube I8 conductive with the effect aforesaid. There is also a possibility that the sheath 34 may become grounded, causing the filament of the warning lamp 33 to be connected directly across the line wires LI and L2. This produces a relatively heavy flow of current through this filament, causing the lamp 33 to light. The tube l3 also becomes conductive due to the shunting of the condensers 35 and 31. and the relay 20 is thereupon energized to shut off the fuel supply.

It has been stated hereinabove that the present invention may be readily adapted for use in multiple burner systems, and one such arrangement for achieving this object is diagrammatically illustrated in Fig. 2.v Thus, there is provided a master control unit designated MU which is substantially identical with the device shown in Fig. l, and which is operatively associated with one of the fuel burners l2 included in a bank of bumers that are supplied with fuel by a single pipe I3 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. l.

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 01' the system in much the same manner as the master unit MU Thus, each is associated with the burner l2. burner I2 has an electrode I1 which is connected by a shielded conductor 30" to the control grid 28' of the electron tube ill in the respective secondary unit SU. The shielded conductors 30 and 30' may be passed through individua1 grounded conduits such as 38, Fig. 1, or it may b preferred to run these conductors through a common Y conduit for at least a portion of their respective lengths. The secondary units are substantially identical in construction with the master unit MU except that they do not include any relays.

the cathodes I! of the tubes III are all electrically connected by wires 32 and 32', respectively, to a common conductor 46 leading to the line wire Ll. Hence, the electron tubes [8 and I8 in the various control units are all connected in parallel and the combined anode current of these tubes passes through the winding of the relay 20. The conductors 30' are connected through the medium of resistors 30 and condensers 31' to conductors 4|, which are also connected to the sheaths 34', and from the conductors 4| electrical connections with the respective cathode return wires 32' are established through the fllaments of lamps 33 embodied in the units SU.

It will be recalled that when a flame as F is present in a burner as I2 the associated tube as I8 is rendered nonconductive at all times. Similarly, when the flames are present in all of the other burners l2 the corresponding tubes I8 are all nonconductive, and under these circumstances the relay 20 remains deenergized. However, if

a flame in any one of the burners becomes ex- .tinguished, or too low, the tube associated with such burner is rendered conductive so that current flow takes place in the common anode circuit including the windings of the relay 20, which relay thereupon energizes and opens its contact i 24. The fuel supply pipe I 3 has a valve l5 controlled by the solenoid I6 which receives its enersizing current through the relay contact 24. but

when the relay 2!! is energized to open the contact 24 the solenoid 1-6 deenergizes and enables the fuel valve It to close and thereby interrupt the supply of fuel to all of the burners in the system. One of the advantages of arranging the electron tubes to be nonconductive under normal combustion conditions is that the relay 2') need merely be sensitive to any substantial current flow whatsoever in the common anode circuit. On the other hand, if the tubes were arranged to be conductive when flames were present in their corresponding burners, and nonconductive in the absence of flame, the relay 20 would be required to detect a proportional decrease in total anode current for each tube rendered nonconductive. In a multiple burner system having, say, sixteen burners and associated flame-detecting tubes, the operation of the relay 20 might become quite critical and on that account easily influenced by external factors such as fluctuating line voltages having no relation to the combustion conditions in the burners. Hence, we prefer that the electron tubes should be adapted to pass current only under abnormal conditions.

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 conductive to shut off the supply of fuel. The same is true if there is an insulation failure which causes excessive leakage 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 with a minimum duplication of parts, it being noted that only one relay 20 and fuel valve l need be employed. The other portions of the apparatus such as electron tubes, resistors and condensers are relatively inexpensive so that the construction is quite economical in addition to being reliable.

It has been mentioned that another object of the present invention is to provide means for automatically checking the conductivity of the electron tube or tubes embodied in the safety control apparatus as a condition precedent to initiating operation of the fuel burner system, and to thereby insure that if any tube is defective so that it is unable to give an indication of flame failure, the system will be prevented from operating until a good tube has been substituted for the defective one. It is also desired to insure that the system will not function unless tubes which are especially adapted to be used in a safety control apparatus of this character are utilized in making tube replacements. A further object is to adapt the control apparatus for governing the operation of a burner system having both pilot burners and main burners. For the purpose of attaining these and other objects the type of apparatus illustrated in Figs. 3, 4 and 5 may be conveniently employed.

Referring first to Fig. 3, the control apparatus is there represented as comprising a master unit MU which is operatively associated with a burner system including a pilot burner 50 and a main fuel burner 5|, these burners being supplied with fuel through separate supply pipes 52 and 5 respectively.- The pilot burner 50 is so positioned with respect to the main burner 5| that a pilot flame originating in the burner 50 may ignite fuel issuing from the nozzle of the main burner 5|. A flame-sensing electrode I1 is mounted in association with these burners in such a manner that it extends into the region of the pilot flame, and if gaseous fuel is being utilized in the main burner the electrode I] may also pass through the main burner flame zone. However, if atomized liquid or pulverized solid fuel is being used in the main burner it may be preferable to arrange the electrode l1 so that it barely skirts the fringe of the main flame as it enters the pilot flame zone, thus minimizing the possibility of fuel gathering on the electrode and forming carbon accumulations thereon, while at the same time insuring that the pilot flame must be at least high enough to ignite the atomized or pulverized fuel in order that it may contact the electrode. The pilot and main burner fuel supply pipes H and 53 are respectively provided with fuel valves 54 and I! which are operated by solenoids I8 and 51, respectively. When these solenoids are in a deenergized condition the fuel valves 54 and II are closed so that fuel is not supplied to the burners 50 and ii.

The application of the control apparatus shown in Fig. 3 to a burner system having a plurality of main burners and pilot burners will be explained subsequently in connection with Figs. 4 and 5, and for the present it will be assumed that the control apparatus comprises merely a master unit MU controlling a single main burner 5i and associated pilot burner 50. Electrical current is supplied to the apparatus by a suitable source of alternating-current voltage whenever the doublepole switch S is closed. One of the poles of the switch 8 is connected directly by a wire L0 to a terminal 58 on the control unit MU. The other pole of the switch 8 is connected to a wire L! which serves as one of the line wires for the electrical system when the switch S is closed. The pole of the switch S to which the wire Ll is connected is likewise electrically connected to another line wire Ll through the medium of a preliminary control apparatus indicated by the rectangle designated 59, Fig. 3. Such preliminary control apparatus may comprise several forms of contact-making devices such as flow switches, time delay devices and the like which all serve the function of insuring that the heater in which the products of combustion from the burner 5| are to be utilized is first thoroughly scavenged and otherwise conditioned for operation before any combustion can tak place. After the apparatus' 59 has performed this function it causes an electrical connection to be established between the line wire LI and the switch pole connecting with the source of electrical current.

The line wires Li and L2 are respectively connected to terminals 60 and 61 on the control unit MU, and conductors G2 and 53 lead from these terminals to the primary winding of a transformer 64 included in the unit MU. The secondary of the transformer 6i supplies voltage to a filament 23 of the tube l8, this tube being similar to and performing the same function as the tube I8, Fig. l. The tube 18 is provided with a shield grid that is directly connected to the cathode IQ of the tube, and the cathode is is also electrically connected to the center tap on the secondary of the transformer 64.

An electron tube such as [8 of the character which we contemplate utilizing in the circuit of 13 Fig. 3 is customarily provided with eight prongs in its base, these prongs being adapted to be received in correspondingly positioned contacts af forded in a tube socket as 9, Figs. 7 and 7A. The various elements of the tube are internally connected to respective prongs, and electrical connection between these tube elements and the portions of the circuits external of the tube are established through the prongs of the tube and the contacts of the tube socket in which these prongs are received, the various external circuit conductors being soldered or otherwise suitably secured to the aforesaid contacts. In Fig. 3 the various tube socket contacts are diagrammatically represented as though viewed from the bottom and are designated I to 8, respectively. Ordinarily contact I is not used except when the tube is provided with an integral shield, in which event the shield is connected to the prong that fits into contact I, which is in turn grounded. However, in the present instance the electron tubes which are to be employed in the apparatus are not shielded, or if they are shielded the shields are connected to ground in some manner other than through the No. I contact in the tube socket. The filament 23 is'connected to the prongs that are received in contacts 2 and I, across which the secondary of the filament transformer 81 is connected. The contacts numbered I, and 8 are respectively adapted to receive the prongs that are connected, to the anode 22, cont rol grid 28 and screen grid 65 of the tube I8.

For the purpose which will appear presently, we prefer to provide each tube I8 with a special Jumper 66 which electrically interconnects the prongs of the tube I8 that are'adapted to be received in the contacts land 4 of the tube socket. If convenient, this jumper may be incorporated in the tube base in the course of manufacture thereof so that the two prongs to which the ends of the jumper are connected will be electrically in permanent connection with each other. The contact 4 of the tube socket is connected by a conductor 61 to the conductor 62 leading to the line terminal 60 of the control unit, while contact I is connected by a conductor 68 to a tertube as I 8 shown in Fig. 3. This arrangement insures that the heater of the thermal switch II will not be energized unless a tube particularly adapted for use in the control unit MU is employed. It should also be noted that the line wire LI, which is not grounded, normally has a direct electrical connection with the contact 4 of the tube I8 and thence through the jumper 66 to contact I, but if the jumper 66 is missing there is no electrical connection of 'the line wire LI with the contact I. Under the latter conditions the contact I is at substantially the same potential as the line wire L2, which is grounded as indicated at 86. (It will be understood, of course, that the wire L2 itself need not be grounded if the pole of the source of electrical current to which-this wire is connected by the switch S is grounded.) Hence, if an operator should carelessly insert into the tube socket a tube having a shield electrically connected to the prong which fitsinto the No. 1 contact, but not being provided with a jumper 86, there is no danger that he will receive a shock, since the potential of the ungrounded or hot line wire LI is not applied to the shield of the tube.

The purpose of the thermal switch I3 is to afford a preliminary warming-up period for the filament 23 of the tube It before any anode voltage is supplied to the tube, this being a precautionary measure toprevent damage to the tube. The heater II of the switch I! starts to warm up concurrently with the filament 23, and after .ample time has elapsed for the filament 23 to attain operating temperature, the bimetallic strip 80 ,ofthe switch It flexes and closes the switch to establish an electrical connection between the a aforesaid conductor I2 and another conductor ll minal 89 on the control unit MU. In the present I instance, where only'a single control unit is being employed, the terminal 69 is connected by a Jumper 10 to a similar terminal II from which a conductor I2 leads to one contact of a thermal switch generally designated I3. A branch conductor I4 electrically connects the conductor 12 to a movable contact blade 15 of a relay I6 included in the control unit MU. When the relay I8 is in a deenergized condition the blade I5 engages a stationary contact 15a from which a conductor 11 leads to one end of the winding I8 of-a heating element embodied in the thermal switch I3, the other end of this heater winding being connected by a conductor I9 tothe conductor 63 which leads to the terminal BI on the control unit MU.

When the line wires LI and L2 are potentialized a predetermined time following closure of the switch S, a circuit is completed from wire LI through the conductors 62 and 61, jumper 86 in the base of the tube l8, conductor 68, jumper III, conductors 12 and H, relay contact 15a and conductor 11 to the heater winding I8 and thence by conductors I9 and 63 back to the terminal 6| which is connected to the line wire L2. It is to be noted, however, that this circuit could not be established if a tube which was not equipped with a jumper as 66 had been used in lieu of a that leads to one end of the winding of the relay 18,-the other end of this relay winding being connected by the conductor 82 to the conductor I9 leading to the line wire L2. Closure of the switchI3 therefore causes relay It to energize, and-in so doing the relay moves its contact blade I5 away from the contact 15a and Into engagement with a contact 15b. The heater I! of the switch" is thereupon deenergized and the bimetallic strip III thereafter cools off and separates the contacts of the switch II. The contact 15b of the relay I6 is connected to the conductor 8| which leads to the winding of this relay, so that immediately upon energizatlon of relay 18 a holding circuit is established fromthe conductor I2 through conductor Hand contact 15b to the relay winding; hence opening of the switch II does not deenergize the relay I6, which remainsiocked up through the just described holding circuit. Thus the thermal switch I3 merely performs the function of initially energizing the relay I6 and is not called upon to operate continuously for holding the relay energized.

The relay I6 has another contact which is normally open when this relay is in a deenergized condition, but when the relay I8 is energized in the manner just described the contact 84 closes and electrically connects a conductor 83 with another conductor 85 that is tapped onto the cathode return wire 32. The conductor 83 is electrically connected by the conductors 61. and

' medium of the closed relay contact 84.

When the switch S is first closed, a circuit is completed through the wire L0 to the terminal the condenser 31 50, thence through a conductor 90 to a stationary contact 96a, of the relay I6, thence through a contact blade 86. which is normally engaged with the contact 80a, and a conductor 81 to a terminal 83 on the control unit MU. An alarm 99, which may be of either the visible or audible type, is connected on one side to the terminal 08 and on the other side to the line wire-L2, and hence when the switch S is first closed the alarm 89 is energized from the wire L to thereby indicate that the preliminary scavenging operations referred to hereinabove have commenced. When the relay I6 is energized in the manner just described, however, there is a momentary interruption in the operation of the alarm 89 while the contact blade 90 of the relay moves out of engagement with the contact 09a and into engagement with the contact 09b. A conductor 9I connects the contact 96b to a contact 92a of another relay 93. A contact blade 92 is normally engaged with the contact 92a when the relay 93 is deenerglzed, and this blade 92 is connected by a conductor 95 to the cathode return wire 22 leading to the cathode l9 of the tube I9. Inasmuch as the wire 32 is potentialized by the line wire LI upon energization oi the relay 19, as just explained. the alarm 09 is maintained energized from this source until such time as the relay 93 becomes energized in a manner to be described.

The control grid 28 01 the tube It is connected through a protective re'sistor29 and a conductor I00 to a terminal "II on the master unit MU, to which terminal the conductor 30 leading from the flame-sensing electrode I1 is connected exteriorly of the control unit. The sheath 34 enclosing the conductor 30 is electrically connected by a conductor II to a terminal I02 on the control unit MU, while the resistor 3| and condenser 91 (which perform functions analogous to those of the likenumbered parts in Fig. 1) are connected in parallel between the terminals I02 and IN. The terminal I02 is connected by the filament of a lamp 2! to a. terminal I03, and in the present instance where there is but a single burner to be controlled. a Jumper I09 interconnects the terminal I03 with another terminal I05 to which the cathode return wire 32 is connected. The pilot burner fuel pipe 52 is grounded as indicated at I09 so that an electrical connection between the grounded line wire L2 and the electrode I1 is aflorded through the medium of a flame from the pilot burner 90 whenever such a flame is present. I

During the preliminary operations which have been described thus far, the fuel valves 94 and 59 remain closed and no flame exists in either the pilot burner 50 or the main burner 5i; hence no current flow can take place through the flamesensing circuit including the electrode I1 and conductor 30. Inasmuch under these conditions, the control grid 28 of the tube I9 is at substantially the same potential as the cathode I9. Anode voltage is supplied to the tube I9 through a path comprising a conductor I09 which leads from the or anode contact in the socket of the tube I0 to one side of the winding of a relay I09, this winding being connected on its other side by conductors H0 and III to the conductor 93 leading to the terminal 0| to which the line wire L2 connected. It has been previously explained that the cathode I9 and its return wire 32 are connected to the line wire LI when the relay 16 is energized; hence energization of this relay causes a voltage to be impressed across the anode lay these blades 22 and cathode I9 and thus produces a flow of current through the tube I9. This current flows only during forward half-cycles of the alternating current, that is half-cycles in which the line wire L2 is positive with respect to the line wire LI, so that a pulsating direct current is passed by the tube I8. The relay I09 is a slow-to-release relay which energizes when such a pulsating current is sent through its winding and assumes a continuously operative condition so long as such current flow continues.

The relay I09 has a pair of movable contact blades H2 and H3 which are normally engaged with contacts IIZa and I'l3a when the relay is deenergized, but upon energization of the rebecome separated from the contacts I I2a and I I3a and engage the contacts I I2!) and II3b. The blade H3 is connected by. a conductor Ill to the cathode return wire 32 and the contact II3b is connected by a conductor H5 to a terminal IIS on the control unit MU. For single-burner operation the terminal H9 is connected by a jumper ill to a terminal II! on the control unit MU, which latter terminal is connected by a conductor II9 toanother conductor I20 interiorly of the unit MU. The conductor I20 leads'to one of a pair of spaced contacts I2I which are adapted to be bridged by the plunger of a push-button switch I22, the other of this pair of contacts I2 I being connected to a conductor I23. Normally the plunger of the switch I22 is retracted into engagement with a pair of spaced contacts I24, one of which is connected to the conductor I23 and the other to a conductor I25, the conductors I23 and I25 being connected to terminals I29 and I21, respectively, on the control unit MU. I

Assuming that the preliminary scavenging and like operations have been duly performed and that the tube I9 has become conductive to energize the relay I09, the operator manually depresses the push button I22 to thereby electrically connect the spaced contacts I2I. Thereupon a circuit is completed from the cathode return wire 5 32 (which is connected to the line wire LI) through conductor I, contact I I36 of relay I09, conductor II5, jumper II'I, conductor H9, contacts I2! and conductor I23 to the winding 01' relay 93, thence through this relay winding and conductors I28 and II I to the line wire L2. Re-

lay 93 is thus energized and closes a normally open contact I29 thereof, at the same time disengaging the contact blade 92 from the contact 92a and engaging this blade with the contact 92b 5 that is connected to the aforesaid conductor I20.

No. 2 05 r) Disengagernent of the blade 92 and contact 92a breaks the circuitto the alarm 09 and causes this alarm to cease operating.

Engagement of the blade 92 of the relay 93 as no charge xist on o with the contact 92b thereof closes a circuit from the cathode return wire 32 through conductor 95, contacts 92 and 92b, conductor I20, contacts HI 0! the switch I22 (provided the plunger of this switch is maintained depressed by i the operaand conductor I23 to terminal I29 and thence exteriorly of the control unit MU through a conductor I30 to the winding of the solenoid 56 which operates the pilot fuel valve 54, this solenoid being connected on its other side to the line wire L2 to complete the circuit. A parallel circuit is likewise established under control of th switch I22, this circuit extending from the conductor I23 through a conductor I3I connected therewith, contact blade H2 and contact II2b of relay I09 (this relay still being in an energized condition) r 17 and a conductor I92 to a terminal I99 on the control unit MU. and thence the circuit continues exteriorly of the unit MU through a conductor III to the primary of an ignition transformer I95 and. from this primary winding back to the line wire L2. Thus the pilot fuel valve solenoid 99 and theignition transformer I95v are concurrently energized when the push button I22 is de- Pressed. The pilot burner 50 is equipped with a spark plug I96, the terminal of which is connected by a conductor I31 'to one end of the secondary winding of the transformer III, the other end of this secondary, being grounded. High potential is thus applied to the spark plug as the pilot fuel valve opens so that the fuel issuing from the 5 pilot b""urner*l0'ma'"ybe ignited.

and II2b and reengage the contacts H21; and

I Ila, respectively, one of the results of this being that the circuit to the primary of the ignition 'Upon the establishment of a pilot flame in the burner "electrical current commences to flow through the flame-sensingcircuit', but due to the rectifying property of the pilot flame such current will flowbetween the electrodes I1 and the burner 99 only when the electrode I1 is positive with respect to ground potential. In other words,.

current flow through the pilot flame takes place only during those half-cycles when the line wire LI is positive with respect to the line wire'L2. The flame-sensing circuit is traced from the line wire LI to the cathode return wire 92 in the man- I ner described hereinabove, thence, through the jumper I04, lamp 92, resistor 9| and, condenser,

91 in parallel (as well as through the dielectric insulating medium between the sheath I9 and conductor 90), flame-sensing electrode I1, and thence through the pilot name to the pilot burner 90. which is electrically at the same potential as the line wire L2. However, during the half-cycle in which such current flow may take place through the flame-sensing circuit the tube I9 is e inoperative inasmuch as the cathode I9 which is connected to the wire 92 is then positive with respect to the anode 22, which is connected through the winding of the relay I09 to the conductor III leading to the line wire L2. That is to say, the current flow through the flame takes place only during the inverse, half-cycles of the alternating current, and therefore a rectifled current flows through the condenser 91 as well as through the capacitance between the sheath 34 and conductor 90, the direction of this current flow being such as to cause the terminal I92 of the control unit MU to assume a potential which is positive with respect to that of the terminal IN. the lamp 99, Jumper I04 and wire 92 to the cathode I9 of the tube I9, while the terminal IOI is connected through the conductor I00 and protective resistor 29 to the control grid 29 of this tube; therefore, the grid 29 becomes negative with respect to the cathode I9 due to the unidirectional charge which accumulates on the condenser 31. v

The charge on the condenser 91 (which will hereinafter be assumed to include the capacity of the insulation between the conductor 90 and sheath 94) may continue to build up throughout a plurality of inverse half-cycles until a substantially steady-state condition is attained so long as a flame is present in the burner 90; During forward half-cycles of the alternating cur-.

rent, when the relativepolarities of the line wires LI and L2 are such that the anode 22 of the tube I9 is positive with respect to "the cathode Terminal I02 is connected through I transformer I95 is broken and the spark plug I99 is deenergized. Also, the electrical connection between the conductors H4 and H5 is broken so that the contact I2I of the switch I22' whichis connected to the conductor I20 is no longer connected to the cathode return wire 92 through the path including the conductors -I It, II! and Ill. However, when the relay 99 was energized in the manner hereinabove explained, a circuit was closed from the wire 92 through conductor 95, contacts 92 and 92b of the relay 99, and conductor I20 to the lust-described contact I2I of the switch I22. Hence, so long as the relay 93 remains energized the energizing circuit to the pilot fuel valve solenoid .06 continues, providing the push button I22 is held depressed.

The relay 99 may be maintained energized through two possible holding circuits, one of these being traced from the cathode return wire 92 through the aforesaid contacts 92 and 92b of the relay 99, conductor I20, contacts I2I of the switch I22 and conductor I29 .to the winding of the relay 93. The other holding circuit is established when the contact blade H9 of the relay I09 engages the contact Illa upon deenergization of this relay, this circuit being traced from the cathode return wire 92 through the conductor I4, con'acts III and la, a conductor 0, contact I29 of the relay 93 (this contact being closed so long as the relay 9! is energized, conductor I29. winding of relay 99 and conductors I29 and III to the line wire L2. The relay 9! remains locked up through this latter holding circuit so long as relay I09 remains deenergized. During the interim while the blade III of the relay I09 is traversing the gap, between the spacedv contacts H911 and Ilia upon deenergization of the relay I09, the relay 93 is held ener-' gized solely through the flrst-mentioned holding circuit including the contacts 92 and 92b of the relay 99 and the contacts I2I of the push-button switch I22, and hence there is no interruption ofthe circuit through which the pilot fuel valve solenoid -59 is held energized. The same result couldbe obtained without the use of this manually controlled holding circuit by arranging the contacts 9:1 and H911 of the relay I09 in a r'nake-before-break manner so that the blade II! engages the contact II9a prior to disengaging the contact I I9b. However, in the present instance it is preferred not to employ make-before-break relay contacts but instead to utilize the manually controlled holding circuit for the aforesaid purpose.

To summarize the foregoing operations, the establishment of a pilot flame in the burner causes the tube I 9 to cease conducting current and brings about deenergization of the relay I09, as an incident to which a holding circuit is established for maintaining the relay 93 energized so long as relay I09 remains deenergized. As can HI and the contact M to the conductor |4| leading to a lamp I42 and thence through the filament of this lamp and conductors I43 and I II to the line wire L2. The lamp I42 thereupon lights to indicate that a holding circuit has been established by the relay I09 for the relay 93 and the pilot fuel valve solenoid 56. Upon observing this indication the operator releases the push button I22, and the plunger of the switch is thereupon restored to break the electrical connection between the contacts |2I and reestablish the connection between the contacts I24. This results in automatically closing a circuit from the conductor I23 through the contacts I24 and conductor I25 to the terminal I21 and thence through the conductor I45 and the winding of the solenoid 51, which control the main fuel valve 55, back to the line wire L2. The solenoid 51 energizes and opens the valve 55 to admit fuel to the main burner I, and as the fuel issues from the burner 5| it is ignited by the pilot flame from the burner 50 and thus the operation of the heater is initiated. As an incident to this a lamp I46, which is connected across the conductors I25 and III, lights to thereby indicate that the main burner 5| is in operation.

If for any reason there should be a failure of the pilot flame, or if this flame becomes too low or is otherwise rendered ineffective to ignite the fuel discharged from the nozzle of the main burner 5|, so that there is an interruption of the electrical connection through the flame between the electrode I1 and the pilot burner 50, rectified current ceases to flow in the flame-sensing circuit and the condenser 31 thereupon loses its charge. The potential of the control grid 28 of the tube I8 is thus brought to approximately the same level as that of the cathode I9, and the tube IB is thereupon rendered effective to pass current during the forward half-cycles. This results in energization of the relay I09, as an incident to which the lamp I42 is extinguished and the holding circuit for the relay 93 is broken, causing this relay to release and also bringing about the deenergization of the pilot valve solenoid 55, main valve solenoid 51 and lamp I46. As the relay 83 is deenergized, the blade 92 thereof engages the contact 92a and thereby closes circuit to the alarm 89, whereupon the alarm operates to give warning of what has happened.

The valves 54 and 55 close when the solenoids 56 and 51 are deenergized to thereby interrupt the admission of fuel to the burners 50 and 5|. In order to start the burners in operation again, it is necessary to repeat the above described steps commencing with actuation of the starting switch I22 and culminating in the opening of the main fuel valve 55. If combustion conditions in the heater are favorable so that the fuel issuing from the burners 50 and 5| is properly ignited, the system operates automatically to continue the supply of fuel to these burners unless and until another abnormal condition arises.

It is to be noted from the foregoing that in the enabling the relay 93 and pilot valve solenoid 56 as well a the ignition transformer I25 to become energized when the push button I22 is depressed, after which the tube |8 becomes nonconductive upon the establishment of a pilot flame and deenergizes the relay 1 I09 to enable the main valve solenoid 51 to become energized when the push button I22 is released. Thus, if there is an internal failure of the tube l8 which causes this tube to be permanently nonconductive or substantially so, the pilot fuel valve 54 is prevented from opening. On the other hand if the tube I3 fails in such a manner that it is permanently conductive, it will be possible to open the pilot fuel valve 54 but the main fuel valve will be prevented from opening because of the fact that the relay I09 cannot be deenergized. In either event the operator is duly apprised of the situation from an observation of either the burners themselves or the indicating lamps I42 and I46, and he may then shut down the system entirely by opening the switch S to enable a. check to be made.

As in the case of the previously described embodiments of the invention, it is desirable to check the system for abnormal conditions other than failure of combustion which may be a source of danger, and particularly to automatically detect any defective conditions in the flame-sensing circuit. Thus, for example, the electrode I1 may become deformed so that it engages the burner 50 or 5| and is thus connected directly to ground, that is to say, to the line wire L2. This causes the grid 26 to be positive with respect to the cathode I 8 during forward half-cycles, and hence the tube passes current to energize the relay I09 and thereby interrupt operation of the burners 50 and 5| in the manner hereinabove explained. Similarly, if a non-rectifying or bilateral leakage resistance is established between the electrode l1 or conductor 30 and ground, such as may be caused by carbonization of the electrode I1 or failure of the insulation around the conductor 30, this nullifies the current-rectifying action of the flame and causes an alternating current to pass through the condenser 31. On alternating current the condenser 31 acts more or less as a shunt and is not likely to build up any appreciably steady charge; or in other words, any charge which is imparted to this condenser during an inverse half-cycle may be lost during the succeeding forward half-cycle. This results in rendering the system inoperative in the same manprocess of initiating operation of the main burner 5| the tube I8 must initially be in a conductive condition for energizing the relay I09 and thereby ner as though there had been a flame failure or ground-out condition, the fuel supply either being interrupted immediately or being prepared for interruption in the event of a flame failure, depending upon the value of the leakage resistance. Likewise, failure of the insulation between the conductor 30 and the sheath 34 so as to establish a low-resistance leakage path in shunt with the condenser 31 renders this condenser ineifective to bias the control grid 28, with the above described results.

If more than one set of pilot and main burners is included in the burner system it is advantageous to have the fuel supply to all of the burn-- as controlled by a single master unit MU, the operation of which is in turn partially controlled by one or more secondary units SU, Fig. 4, the master unit MU being allocated to one of the main burners 5| and its associated pilot burner 50 and each secondary unit SU being likewise allocated to a respective main burner 5| and pilot burner 50. The secondary units SU are of identical construction with respect to each other but are greatly simplified as compared with the master unit MU. Thus, referring to Fig. 4, each secondary unit SU includes a tube I8 which is identical with the tube I8 of the master unit MU, Fig. 3, and which functions in a corresponding manner. Each unit SU may include an individual filament transformer 64', the primary of which is connected across the line wires LI and L2, but it is to be understood that a single transformer 64, Fig. 3, may be utilized to supply current for heating all of the tube filaments. Each tube I8 is provided with a jumper 66' which is electrically connected to the prongs that are received in the tube socket contacts I and 4 as in the case of the tube I 8, Fig. 3.

When the master unit MU is to be operated in conjunction with one or more secondary units SU the jumper I between the terminals 69 and II, Fig. 3, is omittedand a conductor I50, Fig. 4, leads from the terminal II of the master unit 'MU to the terminal 68' on the last secondary unit SU in the series. A conductor I5I connects the terminal 68 of the master unit MU to the terminal II of the first secondary unit SU in the series. Interiorly of this secondary unit a conductor I52 leads to the No. 4 tube socket contact, and if a tube I8 properly equipped with a jumper 66' is received in the tube socket the circuit continues from the conductor I52 and the No. 4 contact through this jumper 66 to the No. I contact and thence through a conductor I53 to the terminal 69 on the secondary unit SU. A conductor I54 then leads from this terminal 69 to the terminal I I on the next secondary unit SU in the series or if there is only one other secondary unit SU, this conductor I54 is connected to the terminal II of such unit. If there are more than two secondary units SU the terminal 69' of each such unit is connected by a conductor as I54 to the terminal II on the next unit, except for the last unit SU in the series,

the terminal 69' of which is connected to the conductor I50 leading back to the terminal II, Figs. 3 and 4, .of the master unit MU. As a result of these connections, if the proper typ of tube I8 has been installed in each secondary unit SU, a series circuit is established through the jumpers 66 and 66 of the tubes I8 and I8 in the master and secondary units, as can be seen by reference to the simplified circuit diagram in Fig. 5. This series circuit connects the line wire LI with the conductor I2 leading to the time delay device generally designated I55 in Fig. 5,

this device I55 comprising the thermal switch I3 and relay I6, Fig. 3, which are wired up in the manner shown in the latter view. The circuit then continues through the heater element and I relay winding of the device I55 and through a conductor I9 back to the line wire L2.

When the conductors LI and L2 are potentialized, assuming that the aforesaid series circuit has been established, the filaments of the tubes I8 and I8 immediately commence to heat up during the time delay interval which is afforded by the action of the device I55, Fig. 5. At the expiration of this interval the device I55 closes its contact 84, thereby connecting the cathode return wire 32 to the conductor 61 which is connected to the line wire LI, and thus applying the potential of the line wire LI to the cathode return wire 32. The device I55 likewise shifts its contact blade 86 into engagement with the contact 86b to thereby close a, circuit from the wire 32 through the alarm 89 to the line wire L2 for 22 maintaining the alarm 89 energized so long as the relay 93 remains deenergized.

Each secondary unit SU has a relay I58, the winding of which is connected on one side by a conductor I59 to the No. 3 or anode contact of the tube I8 in said unit, Fig. 4, and on its other side by a conductor I60 to the terminal BI which is electrically connected to the line wire L2, Thus, the relay I58 is included in the anode circuit of the tube I8 to be energized whenever the tube I8 is rendered conductive. Terminals I05 of all the secondary units SU are electrically connected to the cathode return wires 32' of their tubes I8 and are electrically connected exteriorly of the secondary units SU to a common conductor I which leads back to the terminal I05 of the master unit MU, Fig. 3. Thus, referring to Fig. 5, the cathodes of the tubes I8 and I8 are simultaneously potentialized when the relay device I55 connects the cathode return wire 32 of the master unit MU with the line wire LI The control grids 28 of the tube I8 in the various secondary control units SU, Fig. 4, are respectively connected to the flame-sensing electrodes I'I associated with the pilot burners 50 to which the units SU are respectively allocated. When a pilot flame is present in a particular burner 50 a circuit is established from the line wire L2, Fig. 5, through said burner 50 and the path afforded by the pilot flame to the associated electrode II, thence through the conductors 30 and I00 and the resistor3l, condenser 31 and lamp 33', Figs. 4 and 5, included in that particular control unit SU to the cathode return wire 32', from which point the circuit is completed through the common conductor I65 and wire 32 in the master unit MU to the line wire LI. Rectified current flowing through such circuit builds up a charge on the condenser 31' included therein so that the associated tube I8 is biased to the point where it is nonconductive. However, when a flame is absent in the particular pilot burner 50 the tube I8 is rendered conductive and passes current through its anode circuit to energize the relay I58.

Referring again to Figs. 3 and 4, the relay I09 in the anode circuit of the tube I8 in the master unit MU has a contact 31) which is connected to a terminal II6 on the master unit MU. For multiple burner operation the jumper I [1, Fig. 3, is omitted and a conductor I56, Fig. 4, is employed to connect this terminal I IE to a terminal IS! on the first of the secondary units SU in the series. The relay I58 in this secondary unit has a contact I6l which is normally open when the relay is deenergized but which closes when the relay is energized, in a manner presently to be explained, to electrically connect the terminal I51 on the secondary unit SU to a terminal I62 on said unit. The terminal IE2 is connected by a conductor I63 to the terminal as IE1 of the next secondary unit SU, and the terminal as I62 of this unit is in turn connected by another conductor I63 to the terminal as I51 of the next succeeding secondary unit as SU, and so on until the last of the secondary units SU in the series is reached. The terminal I62 of this last unit is connected by a conductor I66 to the terminal I I8 of the master unit MU, Figs. 3 and 4, Thus, referring to Fig. 5, all of the relays I 09 and I58 in the various control units must be energized in order to establish an electrical connection between the conductors H4 and H9, Figs. 3 and 5. Before operation of any of the burners can be initiated the tubes I8 and I8 must all be ren- Y 23 dered conductive, and if the tubes are all in working condition they will automatically become conductive as soon as the cathode return wires 82 and I2 are potentialized inasmuch as there is no substantial bias on the control grids of these tubes in the absence of flames in the associated burners. Assuming that all of the tubes are in good condition so that the relays I" and Ill in their anode circuits become energized, a series circuit is completed from the cathode return wire 82 of the master unit MU. Fi 3, through the conductor Ill and the contact Illb of the relay I" and thence through the contacts Iii of the relays I" to the conductor I", Figs. 4 and 5,1eadin: back to the conductor I It, Figs. 3 and 5, which is connected to the conductor I20 leading to a contact III of the switch m, The system is thereupon conditioned to become operative when the button of the switch I2! is pushed.

As has been described hereinabove, the switch III is manually actuated to energize the pilot fuel valve solenoid It and, the-ignition means. In the present instance, where a plurality of pilot burners are to be ignited, it is advantageous to employ a distributor Ill, Fig. 5, having a number of contact segments I'll, each of which is connected by a conductor ill to a spark plug I" on a pilot burner It or II. The distributor III has a wiper IIl insulatedly mounted on the shaft I" of a motor Ill. The motor I'll is connected in parallel with the primary of the ignition transformer I", while the high potential terminal of the transformer secondary is electrically connected by a conductor I10 to the wiper I'll of the distributor Ill, The motor "I is energized and commences rotating the wiper I'll simultaneously with the energisation of the ignition transformer I" and the opening of the pilot fuel valve N. As the wiper I'll successively enages the contacts "I, the spark plugs I" are energized in rapid succession for igniting the fuel in the various pilot burners I and 50', By employing such a distributor arrangement, it is possible to utilize a single ignition transformer which need only be capable of energizing a single spark plug at a time, and on the whole a more satisfactory and economical operation is insured than would be the case if a separate ignition transformer were provided for each burner or if a single transformer were arranged to supply potential to a number of spark plugs in parallel.

As each pilot burner 50 is ignited a rectified current flow is established in the flame-sensing circuit associated with such pilot burner, and in the manner explained hereinabove, this causes the corresponding tube I8 to become nonconductive, thereby causing the relay in in the anode circuit of such tube to deenergize. In the present instance, however, it is desirable that the relay I08, Figs. 3 and 5, in the anode circuit of the tube II of the master unit MU remain energized at least until all of the tubes I8 in th secondary units BU have been rendered operative in response to the establishment of pilot flames in their corresponding burners 50', regardless of whether the flame in the pilot burner 50 associated with the master unit MU has been established, for so to do insures that the holding circuit for the relay ll and pilot valve solenoid 56 will not be estab lished, nor will the ignition transformer I35 and motor I" be deenergized, at least until flames are present in all of the pilot burners M.

For the purpose of delaying the response of the master unit MU to the establishment of a pilot flame in the burner 50 until the last of the Pilot flames in the burners 50' has come into existence, the jumper IIM between the terminals I03 and III! of the master unit MU, Fig. 3, is omitted and the terminal I03 is connected by a conductor I'II to a terminal Ill! in the first of the series of secondary units EU. The terminal ill on this unit BU is electrically connected by a contact I19 of the relay I58 to another terminal III on this unit whenever the relay I58 is in a deenergized condition, but when the relay is energized this connection is broken. The terminal I" of the secondary unit BU is connected by a conductor Ill to the terminal I18 on the next secondary unit EU in the series until the last of the secondary units SU is reached. The terminal I" of this last unit is connected by a Jumper I82 to the terminal ID! to which the common conductor III is connected.

Thus,referring to Fig. 5, an electrical connection between the terminals Ill and III! of the master unit MU, Fig. 3, is not established until all of the relays III in the secondary units BU, 8. 4, have been deenergized to close their'contacts Ill; that is tosay, the flame-sensing circuit of the master unit MU does not become effective until flames have been established in all of the pilot burners ISO to render all the tubes II in the secondary units 8U nonconductive. Thereupon the flame-sensing circuit of the master unit MU is rendered effective, and if a flame is present in the pilot burner 50 the tube ll becomes nonconductive to deenergize the relay I". The relay I" closes its contact Ilia and thereby establishes a holding circuit for the relay II and the pilot fuel valve solenoid 58, at the same time opening its contact I Ilb to disable the ignition means including the transformer I 35 and motor Ill. The lamp I42 flashes on as the relay I09 closes its contact II2a to indicate that the push button I22 may be released to initiate operation of the main burners BI and I in the manner previously explained.

If an abnormal condition should arise at any of the burners with which the secondary units 8U, Fig. 4, are associated, such as a failure of combustion in such burner, th -tube II in the secondary unit SU becomes conductive and energizes its relay I58. As this relay energizes it opens its contact I19 and thereby breaks the flame-sensing circuit for the master unit MU. This causes the tube I! in the master unit MU to pass current and thereby energize the relay I08, causing the relay contact Illa to open and break the holding circuit for the relay 93, which in turn results in deenergizing the fuel valve solenoids 58 and 51, thus interrupting operation of all of the burners in this system. If the abnormal condition should arise at the burner with which the master unit MU is associated, then the tube I8 is immediately rendered conductive with the effect just described. It is manifest that the apparatus will function in a safe manner for any of the abnormal conditions referred to hereinabove, such as ground-out or leakage effects, as well as for a failure of combustion. It should also be noted that if any of the cable sheaths 34 or 34 becomes grounded, the associated warning lamp 33 or 33' lights to indicate the location of the fault.

While the master unit MU and secondary units SU have been described hereinabove as constituting separate control units which may be mounted in individual cabinets that are wired together by external connectiom and independently located with respect to each other, it is to be understood that these various control units may be embodied mmmmhxgipall ghp geries circuit exthkiipg through the jumpers.

trically connected to the cathode return wire 32 i by way or a series circuit extendin through the contacts I19 of the relays I58 in the secondary units SU, whereas the condensers 91 and resistors II of the secondary units SU are con-'' prises a relay unit BU which containscertain of the relays and associated devices for controlling operation or the fuel supply and ignition means.

Operation 01 this relay unit RU is] in turn governed by the flame-detecting units 0; olieoi'y which is operatively associatedwiflij each pilot" 1 burner 00 in the system. The controlLunits U an electron tube I92 and new; III-havingits winding connected in the anode circuit" or said tube. The filaments oi the tubes'ylll are pretalarm89 back to the line wire L2. Operation of the alarm 99 thus signifies that the necessary preliminary operations have been performed and that the heater system is ready for operatlon.

' As the line wire LI becomes potentialized the voltage existing between the wires LI and L2 is impressed across the cathodes I91 and anodes I99 of the various tubes I92. The control grids .205 of the tubes I92 are respectively connected byv conductors 209 to terminals 201 on the control units U.'irom which terminals the conductors 90 lead to therespective flame-sensing electrodes I1 associated with the pilot burners 50. The

sheaths 94 o! the conductors 30 are respectively.

connected toterminals 299 which are electrically connected through the iillments of lamps 209 to the terminals I99 on the units U. Each control unit U has a condenser 2I9 and a resistor 2I I which are electrically connected in parallel across the terminals 201 and 208. Since there is no J. current flow through the various name-sensing outfits-including the electrodes I1, conductors are 01 unii'orm construction and each includes 7 erably supplied with current by individual illament transformers I94 having their primaries,

connected across terminals I99 and. llllwhich are respec vely connected to the linewires L2 and L0. e cathodes I91 01 "the tubes I92 are connected to terminals I99 which are electrically connected to the line wire LI. The anode I99 of each tube I92 is connected by a conductor 200 to one end of the-winding 01 the relay I99, the other end of thia'..wlnding being connected by the conductor 29!" to the terminal I95. It will be understood that although the relay unit EU and flame-detecting units U are represented in Fig. 8 as constituting separate units, they may be conveniently incorporated in the same cabinet to afford a single control unit for the entire burner system. i

It will be recalled that the line wire LI ls'not immediately potentialized upon closure of the switch S but must await the establishment of 90,.conde'nsers 2I0andresistors 2 prior to the establishment orpuot 'flames in the burners 90.

the relay unit RU to initiate operation of the an electrical connection with the wire L0 through the medium of the preliminary control apparatus 99. Hence, while closure of the switch S immediately potentializes the wires L0 and. L2 for thereby energizing the filament transformers I94 and initiating heating of the cathodes I91, the line potential is not impressed upon the anode circuits of the tubes I92 until the scavenging and other preliminary operations have been performed under control of the apparatus. 59. Ordlnarily this afiordssufllcient time for the tubes I92 to be conditionedior operation, but if a longer delay period is desired this can be afforded by an arrangement such "as that comprising the thermal switch 19 and relay 19, Fig. 3. It is likewise apparent that the series circuit arrangement oi Jumpers 99 an 69', Figs. 4 and 5, may be employed to insure that the control apparatus is not conditioned for operation unless tubes of the proper type are installed in all of the active control units U. At the completion of the aforesaid scavenging cycle the line wire LI assumes the same potential as the line wire L0, whereupon a circuit is established from wire LI through a conductor 202, a contact blade 203 of a relay 204 included in the relay unit RU, a contact 203a with which the blade 203 cooperates when the relay 2 is deenergized, and thence thro g an pilot burners 90. Each relay contact 2I2 is arranged-to'bridge a pair of terminals 2I3 and 2 on its controlunit U when the relay I93 is energized. The terminal 2 of the last control unit U in the series is connected to the line wire LI, while the terminal 2 oi each of the other units U is connected by a conductor M5 to the terminal 2I3 of the next succeeding unit U in the series. A conductor 2; leads from the terminal 2|! 0! the first unit U to a contact 2031) 01 the relay 204 in the unit RU, Irom which relay contact the blade 203 is normally disengaged when the relay 204 is deenergized. A push button 2I1 is arranged to bridge a pair of spaced contacts 2I9, one of which is connected to the conductor 2 and the other to a conductor 2I9 leading to the winding of the pilot fuel valve solenoid 59. 'Assuming that all of the tubes I92 areconducting current so that all of the relays I99 are energized to maintain their contacts 2I2 closed, an electrical path is established from the line wire LI through the relay contacts 2I2 in series to the conductor 2 I6. However, if any of the tubes I92 is defective so that its relay I93 remains deenergized, the conductor 2IB remains disconnected from the line wir LI.

Assuming that all of the tubes I92 are conducting current, operation of the pilot burners 50 is initiated by manually closing the switch 2I1

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