US2817395A - Burner control system - Google Patents

Description

C. H. LUTZ ET AL BURNER CONTROL SYSTEM Dec. 24, 1957 Filed oct. 19, 1955 k ww nited Stats BURNER CONTROL SYSTEM Charles H. Lutz, Cleveland, William R. Durrett, Berea, Stephen'l. Posta, Parma, and Frank M. Susel, Mentor, Ohio, assignors, by mesne assignments, to White- Rodgers Company, `a corporation of Delaware Application October 19, 1955, Serial No. 541,467

2 Claims. (Cl. 158-28.)

This invention relates generallyto fluid fuel burning control systems and particularly to safety `provisions therein which insure against accumulations of unburned fuel in the furnace combustion chamber.

Conventionally, oil burner control systems include a lockout, safety time switch, which upon demand for heat uprovides a limited trail period during which fuel is suppliedto the burner with the igniter device in operation `to effect successful combustion of the fuel, and a flame .detection device, which responds to the presence of cornbustion flame when it occurs to shunt out the time switch so that the fuel supply may continue during the period .of heat demand so long as the combustion flame is present.

Apermit the use of a shorter and safer trial period without `the risk of unnecessary and annoying shut-downs which otherwise occur in shortened trial periods when the flame detecting device fails to respond quickly enough to avoid a lockout by the safety time switch.

A number of rapidlyresponding .flame detection devices have been'proposed in which the thermal ionization incident to the presence of combustion flame has been utilized `as a conduction path for the flow of signal energy to indicate the presence of flame. Many of these have required expensive or complicated amplifying means or at yleast electron discharge tubes having relatively limited operational life, others .have required the provision of a power source of high potential, and still others were confronted 'withiiame erosion problems and carbon build-up on the spaced electrodes of the detection circuit which were placed in the combustion name.

The `present invention has for Van object the provision of a generally new and improved name detection circuit for a burner control system which overcomes the problems and objectionable features of the earlier devices, is highly reliable, responds rapidly, and is of simple and inexpensive construction.

A further object of the invention is the provision .of a safety control circuit for a burner having a constantly operating igniter, which control circuit functions rapidly in response to a llame failure or ignition failure `to cut olf the fuel supply.

A further object is to provide a generally new and improved llame detection means for a burner, wherein the area of thermal ionization `incident to the presence `of combustion flame is utilized as a conductive path for signal energy with which to maintain operation of the fuel supply means so long `as combustion flame continues,

and in which the circuit electrodes bridged by the ionized 2,817,395 Patented Dec. 24, 195.7

path are constructed and arranged so as to be free from any llame erosion or carbon build-up which would affect the operation of the liame detection device.

A further object is to provide a novel flame detection circuit for a burner having an electric spark ignition device wherein ionization incident to the operation of the igniter device and ionization incident to successful combustion of fuel ignited by the igniter device are utilized in ,series relationship to jointly complete a conductive path for the ilow of sufficient energy with which to maintain operation of the fuel supply means so long as combustion flame is present and operation of the igniter contiuues.

More specifically, it is an object to provide a ame detection circuit for a fluid fuel burning control system having an electric spark igniter wherein use is made of one of the spark electrodes as an electron emission source, and wherein the thermal ionization incident to the combustion of the fuel ignited by the igniter is `utilized as aconductive path through which a portion of the energ of .the .iguiter dcviceisconducted and utilized to maintain operation of the fuel supply means so long as operation of the igniter and combustion of the fuel continues.

A further object is to provide a flame detection circuit for a pressure-type .oil burner having an electric spark igniter which includes a pair of spaced spark electrodes `'adjacent the fuel spray nozzle, but disposed so as not to be impinged by combustion flame.and Vmeans providing a forced draft `of `combustion air which attenuates the `ignition arc outwardly from the fuel nozzle in the direction ofthe combustion flame, wherein use is made of one of the spark electrodes as a source of electron .emissionand wherein the thermal `ionization incident to combustion of the fuel is `utilized as a conductive path and the attenuated ignition arc and its surrounding plasma are utilized to bridge the gap between the emitting electrode and the flame, thereby to provide a continuous conductive path for a portion of the energy ofthe ignition device which is utilized to maintain operation of the fuel supply device.

These and further objects and advantages of the invention will appear from the following description lwhen read in connection with the accompanying drawing.

In the drawing:

Fig. l is `a schematic view of a form of the invention adapted asa control system for a conventional pressuretype oil burner; and

`Figs. 2 and `3 are fragmentary sectional views of `the end ofthe burner tube of a conventional pressure-type oil burner showing the usual disposition of the-,fuel `nozzle and spark ignition electrodes.

Referring to Figs. l, 2, and 3, a furnace is generally indicated at 10, .having a combustion chamber 11 `and a pressure-type oil burner12 mountedtherein. The burner includes a blower.15, a fuel pumping 4device (not` indicated), `and ,a `burner tube13 `in which is longitudinally arranged `a liquid fuel spray nozzle 14. In operation, liquid fuel is supplied to the spray nozzle 14 by the fuel pumpandcombustion air is ,supplied tothe burner tube i3 bythe ,blower 15. Both the blower vand `fuelpump are driven by a-motor 16 so that, when motor 16 isenergized, fuel is sprayedfrom .nozzle 14iand air is supplied :by ythe blower which `pt-lsses'through the `burner tube `13 and around `the spray inozzle `i4 to supply :combustion `air and to further atomize the fuel.

The burnerensemble further includes a :pair of ignition spark electrodes 17and 18 which yare conventionally disposed yabove the spray nozzle and adjacent but `not :in the fuel spray cone, Vapproximately as shown. Infany event, the ignition electrodes `are `arranged rearward gof the fuelspraytso `as not :to beimpinged byiiarne. The

3 fact that the combustion air draft supplied by the blower blows the ignition arc outward from the ends of the electrodes, as indicated at 19 in Fig. 3, permits the electrodes to be disposed inwardly behind the spray cone somewhat so as to protect them from direct tlame irnpingement and yet insure ignition of the fuel.

A pair of leads 2t) and 21 for connection across a comy mercial source of A. C. power supply is provided, and the circuit for the energization of the burner motor may be l' traced from the power supply as follows; lead 20, a lead 22, a lead 23, the motor 16, a lead 24, a lead 2S, the normally open, upper switch contacts 26 and 27 of a relay RY28 which close when the relay is energized, a lead 29,

and through a normally closed, furnace temperature limiting switch 30 to the lead 21. The circuit for the ignition device comprises the secondary winding 31 of an ignition transformer 32, the lead 33, the electrode 17, the

spark gap, the electrode 18, and a lead 34 to the other side of secondary 31. kThe primary winding 35 of the ignition transformer is connected in parallel with the burner motor 16 by leads 36 and 37, and is energized simultaneously with motor 16.

The winding of relay RY28 is in a low voltage circuity under the control of a space thermostat 38.- This low voltage circuit may be traced as follows; a lead 39 from one side of the secondary winding 40 of a step-down transformer 41, the space thermostat 38, a lead 42, the relay winding 43, a lead 44, the electrothermal time switch 45,v

a lead 46, the Contact 47, and switch blade 48 of an electrostrictive switching device 49, a lead 50, and a lead 51 back to the other side of secondary winding 40. The primary winding 52 of the step-down transformer 41 is connected across the power supply leads and 21 by the leads 53 and 29 through limit switch 30 and by a lead S4.

The electrothermal time switch 45 consists of a bimetallic contact arm 55, a stationary contact 56, and a series connected resistance heater 57 for heating the bimetallic contact arm so that, when this circuit for energization of relay RY28 is closed for a predetermined v interval, the resistance heater 57 causes bimetallic arm 55 to warp leftward and to break the circuit at that point. This time switch is usually constructed so as to automatically lock in an open position and require manual resetting before relay RY28 can be re-energized and operation of the burner resumed.

The electrostrictive switch 49 comprises a crystal ele ment 58 which deforms or warps when a predetermined D. C. voltage is impressed across its terminals to effect actuation of the switch arm 48 to move it from its solid line position to its dotted line position. This actuator element 58 appears in its circuit as a capacitor with a very small leakage and is capable of storing energy pulses to effect its operation, whereby continuing unidirectional energy pulses which may be individually insufficient to effect operation of the switch are accumulated until a suicient charge is built up to eifect operation.

The switch actuator element 58 is connected between the high tension lead 34 of the ignition device and ground by a lead 59, resistors 60 and 61, a lead 62, and leads 63 and 64. With the igniter in operation and a combustion llame present, when the lower end of ignition secondary 31, the lead 34, and spark electrode 18 are plus, a positive pulse appears on the plus side of actuator element 58 due to conduction from the now negative spark electrode 17 to ground through an ionized path to be described and from ground to the negative side of element 58 through leads 64 and 63. A portion of the energy of the igniter device is thus conducted and utilized to actuate switch 49.

' The extreme tip of electrode 17, being intensely heated due to arcing, provides a hot cathode emitter from which electrons flow through the attenuated ignition arc 19 to the base of the combustion ame and through the combustion llame to ground at 65. The path from the :arne

i yto ground will be partially through the furnace and burner l believe that due to the high temperature existing at the extreme tip of the spark electrode substantial thermionic emission takes place and increases the electron flow.

It is to be further understood that, while the illustrated embodiment includes conventional means for providing a forced air draft which blows the arc forward toward the flame and thereby permits positioning of the spark electrodes rearward, in other arrangements wherein such air draft is absent and the spark electrodes are moved forward somewhat to insure ignition, the importance of the arc in providing a conductive path between the tip of an electrode at the base of the flame and the more intensely ionized forward areas of the combustion llame between ignition arc is not diminished.

A voltage regulating and filtering network is provided for the switch actuator element 58. This network includes a capacitor 67 connected between the high tension lead 34 and ground, which functions to by-pass any R. F. transient pulse occurring at the instant of break-down across the spark electrodes and as a filtering capacitor which discharges during the negative half-cycle to smooth out the D. C. signal applied to element 58. The network further includes the resistor 60 and the glow tubes 68 which are series connected across the capacitor 67 and regulate and limit the voltage applied to element 58. The resistor 61 further reduces the voltage applied to element 58. Connected across the leads 62 and 63 is a calibrated leakage resistor 69 which provides a predetermined discharge time for the element 58 and capacitor 67 when the signal fails due to failure of combusion flame or ignition arc.

Due to the fact that one side of secondary winding 31 is grounded through capacitor 67, the voltage swing of the ignition circuit is between ground potential and peak voltage so that, during that half of the cycle when spark electrode 18 is positive, electrode 17 is substantially at ground potential and the conductive path of the sensing circuit extends from the electrode 17 through the intensely ionized area created by the arc, through the combusion flame to the furnace and burner tube to ground, and from ground through leads `64 and 63 to the minus side of element 58. Capacitor 67 is, of course, charging during this half of the cycle. During the other half of the cycle when electrode 17 is plus, electrode 18 and lead 34 will be substantially at ground potential and there will be no flow of electrons from 18 to ground. Furthermore, there will be no appreciable flow of electrons from the negative side of element 58 or from capacitor 67 to electrode 17 via ground and the combustion flame when 17 is positive because of the well-known rectifying action of the llame, and because of the difference in the areas of the electrode 17 and the collector electrode (the furnace).

When combustion llame and ignition arc are present, the switch 49 is actuated and switch arm 48 breaks with contact 47 and closes with a contact 70. This breaks the circuit for the safety switch heater 57 and completes a shunt around the heater so as to maintain energization of relay RY28. This shunt is traced as follows; from lead 50 through switch blade 48 in its dotted line position, a contact 70, a lead 71, a lead 72, resistor 73, a lead 74, the lower contact arm and contact 76 of relay RY28, and a lead 77 to switch arm 55. This shunting of the safety switch heater maintains energization of the relay RY28 and, therefore, operation of the burner so long as a sufficient signal is applied to switch actuator element 58, and, of course, so long as the space thermostat is closed demanding heat and the maximum furnace temperature has not been exceeded.

The purpose of the resistor 73 is to maintain a substantially constant voltage drop in the thermostat circuit when the resistance heater 57 is removed so that the critical heat output of a thermostat anticipating heater 81 may be maintained constant.

In order to maintain energization of the relay RYZtS between the breaking of switch arm 48 with contact 47 and its closing with contact 70, a shunt around switch 49 is provided. This shunt, extending from a point 79 to the safety switch contact arm 55, comprises the resistor 78, lead 80, lead 72, resistor 73, lead 74, contact arm 75, contact 76, and lead 77 to contact arm 55. This shunt is, of course, in operation at all times when switch contacts 47 and 48 are closed, but the total resistance of the two resistors 78 and 73, which are series connected when switch arm 48 is closed with contact 47, is suicient to cause a current to llow through the parallel connected resistance heater 57 and effect a warping of safety switch contact arm 55 in the predetermined time. This shunt does, however, pass sucient current to hold the relay RYZS in as the contact arm 48 is swinging over from 47 to 70.

In operation, when the space theremostat closes, a timed starting circuit for relay RYZS is completed through contact arm 48 and contact 47 of switch 49, and through resistance heater 57 and safety switch contacts 55-56. When the relay is energized, its upper contacts 26 and 27 close, completing a circuit for the burner motor 16 and ignition transformer 32. Normally, combustion will occur within a very short period after the burner and ignition device are energized. Having thus established operation of the igniter and a combustion flame, a portion of the energy of the igniter circuit will be applied to the switch actuator 58, in the manner described, which will effect actuation of the switch 49 to its dotted line position thereby shunting out safety switch heater 57. Thereafter, so long as ignition arc and combustion flame exist, the relay RY28, and therefore the ignition circuit and fuel supply means, will remain energized until sufficient heat has been supplied by the furnace and the relay circuit is broken by the opening of the thermostat.

If, however, for any reason a satisfactory ignition arc and combustion flame do not occur within the trial period, the circuit for relay RYZS will be broken at safety switch 45, and as before stated, switch 45 is usually constructed so as to automatically lock open once it is opened so as to require manual resetting. If, on the other hand, either the ignition arc or combustion llame fail at any time after having been both established, or if the electrical power supply source fails momentarily, the ignition and flame sensing circuit will be broken and actuator element 58 will discharge to ground through resistor 69, causing switch arm 48 of switch 49 to return to its full line position, which again places the safety switch heater 57 in the circuit of relay RYZS. Under these conditions, the circuit will then attempt to re-establish ignition and combustion within the limited trial period. If this is successful, operation of the burner then continues until the end of the heat demand period, otherwise the circuit is locked open at the end of the trial period.

By way of example, a response time of two seconds for actuation of switch 49 in either direction was obtained in experiments using a standard commercial-type furnace and a standard pressure-type oil burner of the size and capacity used in heating medium-sized homes, and with an ignition transformer having the usual potential build up of 10,000 volts with a 60 cycle, 110 volt power supply, but having its secondary winding grounded at one side thereof instead of at its center as is conventional. The crystal element 58 of the electrostrictive switch which was used had a capacitance of .05 microfarad and a leakage approximately equal to that of a megohm resistor.

The values of filtering and voltage regulating network components were as follows:

Resistors 60 and 61 each 4.7 megohms. Glow tubes 68 NE2--6l Volt neon bulbs. Capacitor 67 .1 microfarad. Calibrated leak-olf resistor 69 20 megohrns.

Many variations of the foregoing embodiment which lie within the spirit of the invention will occur to those skilled in the art, and it is, therefore, the intent that the foregoing description be illustrative and not limiting, the scope of the invention being set forth in the appended claims.

We claim:

l. In a burner control system, a burner, a switching device for controlling the supply of fuel to the burner, a spark ignition circuit including an alternating power source and a pair of spaced spark electrodes arranged so that an arc bridging the electrodes will ignite the burner, means forming a sensing circuit for detecting the presence of ignition arc and burner flame when these two phenomena exist, said sensing circuit including electrode means forming with one of said spark electrodes a gap arranged to be bridged by a portion of the ignition arc and the combustion llame in series relationship, and means in said sensing circuit responsive to the flow of energy therethrough to actuate said switching device when both ignition arc and burner flame are present.

2. In a burner control system, a burner, a switching device for controlling the supply of fuel to the burner, a spark ignition circuit including an alternating power source and a pair of spaced spark electrodes arranged so that an arc bridging the electrodes will ignite the burner, means forming a sensing circuit for detecting the presence of ignition arc and burner llame when these two phenomena exist, said sensing circuit including electrode means forming with one of said spark electrodes a gap arranged to be bridged by a portion of the ignition arc and the combustion llame in series relationship, and an electrostrictive actuator in said sensing circuit deformable to actuate said switching device upon the accumulation thereon of a predetermined electrical charge.

References Cited in the le of this patent UNITED STATES PATENTS 2,003,624 Bower June 4, 1935 2,074,637 Ballentine Mar. 23, 1937 2,238,892 Fanger Apr. 22, 1941 2,386,648 Aubert Oct. 9, 1945 2,539,208 Schultz et al Ian. 23, 1951 2,581,188 Hibbard Jan. 1, 1952

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