US3449055A

US3449055A - Burner control apparatus with prepurge timing

Info

Publication number: US3449055A
Application number: US685193A
Authority: US
Inventors: James C Blackett
Original assignee: Honeywell Inc
Current assignee: Honeywell Inc
Priority date: 1967-11-22
Filing date: 1967-11-22
Publication date: 1969-06-10
Anticipated expiration: 1986-06-10
Also published as: GB1250655A; FR1592907A; DE1809942A1; NL6816670A; SE343671B

Description

June l0, 1969 J.`c. BLACKETT Filed Nov. 22, 1967 t Njomo United States Patent O U.S. Cl. 431-26 9 Claims ABSTRACT OF THE DISCLOSURE A burner control apparatus having a burner motor relay adapted to energize a burner motor to institute air flow to the burner, a unijunction prepurge timer having a capacitor and a unijunction transistor which are energized when an air flow switch responds to air flow, a main control relay which is energized by the discharge of the capacitor through the unijunction transistor and which is adapted to energize a pilot valve and ignition means, an electronic flame detector having transistors and a flame relay which is adapted to de-energize the ignition and energize a main valve upon the presence of pilot flame, and a bimetal operated safety lockout device which is de-energized upon the presence of pilot flame.

Background of the invention My invention relates to the art of burner control apparatus having a prepurge timer. Prior art burner control apparatus provide this prepurge function by the use of various timers, including unijunction timers. Unijunction timers have been used generally in many arts to delay the energization of a relay by connecting an R-C circuit to the emitter of the unijunction transistor and then discharging the capacitor through the unijunction emitter to base circuit and the relay winding. As is well known, the time of firing of the unijunction is determined by the relationship between the emitter voltage (capacitor charge) and the base-to-base voltage.

In the art of burner controls, the prepurge timer, whatever type it might be, is usually energized by circuit means including a normally closed switch of the flame relay, to insure that the flame detector is not falsely detecting flame.

Summary of the invention Having the above background in mind, my invention can be understood by considering an instance of unsafe failure of the ame detector during the prepurge period. Such a failure is evidenced by energization of the flame relay to falsely indicate that flame is present. The energization of the flame relay reduces the unijunction baseto-base voltage to zero and the capacitor of the R-C circuit then tends to discharge through the winding of the main control relay to energize the winding. Energization of the main control relay under these conditions is undesirable.

My invention utilizes an electronic switching element in the form of a transistor to cause the flame relay winding to be energized when the transistor is conductive. The relay armature does not immediately move to its energized state because of inertia. To insure that the capacitor is always discharged before the flame relay armature can react to energization, I connect the transistor to the capacitor to discharge the capacitor through the transistor when the transistor is rendered conductive. Thus, if an unsafe failure occurs during the prepurge period, at the time that the llame relay is energized and the unijunction base-to-base voltage is reduced to zero, the capacitor ice has been primarily discharged by the transistor and the main control relay winding is not energized.

Other features of my invention include, in combination with the above defined structure, a prepurge relay and a time delayed safety switch, interconnected by means of unique circuit means to provide a versatile and a reliable burner control apparatus.

Description of the drawing The single figure is a schematic showing of a burner control .apparatus embodying the preferred embodiment of my invention.

Description of the preferred embodiment Referring to the single figure, a fuel burner installation 1 0 includes a burner motor 11, pilot valve 12, ignition means 13, main valve 14, and flame rod 15. For purposes of simplicity, the burner installation has been shown in block diagram form, recognizing that such a burner installation may take many specific forms. Flame rod 15 is structurally associated with a pilot burner, not shown, to sense the presence of flame at the pilot burner. The pilot burner is structurally associated with the main burner, not shown, to ignite fuel flowing from the main burner, once a pilot flame has been established at the pilot burner.

Also associated with the fuel burner installation is a burner air flow switch 16. This switch is a normally open switch which is constructed and arranged to close in response to air flow initiated by energization of burner motor 11.

A controller 17, which likewise may take many forms, is a normally open switch constructed and arranged to close upon a need for operation of the fuel burner installation and to open at the end of a period of such a need.

The burner control apparatus of my invention provides prepurge timing by means of a conventional unijunction timer identified generally by means of reference numeral 18. This timer is connected to control the energization of winding 19 of a main control relay identified as relay 1R. Relay 1R includes normally open switches 1R1, 1K3, and 1R4, and a normally closed switch 1R2.

A conventional electronic flame detector 20, having flame rod 15 connected to the input of the ame detector, includes as an output, a llame relay identified as relay 2R. Relay 2R includes a winding 21, normally closed switches 2R1 and 2R2, and a normally open switch 2R3.

A conventional time delayed safety lockout means 22 may take the form of a self-latching bimetal operated safety switch including a heater 23 which when energized for a time period is effective to cause a bimetal (not shown) to warp to open a normally closed switch 24. For purposes of simplicity, I have shown the interaction of heater 23, the bimetal and switch 24 by means of broken line 25.

In order to provide energization of burner motor 11 during the prepurge period, I provide a burner motor relay identified as relay 3R. Relay 3R includes a winding 26 and normally open switches SR1 and SR2.

Operating voltage for the fuel burner installation and the burner control apparatus is provided from

conductors

27 and 28 which are adapted to be connected to a source of alternating voltage. A transformer 29 has its primary winding 30l connected to

conductors

27 and 28. Secondary winding 31 provides a low magnitude alternating voltage to bridge rectiiier 32 to charge capacitor 33 to the polarity indicated. Secondary winding 34 provides a high magnitude AC voltage, one terminal of which is connected to ground at 35 and the other terminal of which is connected through condenser 36 and conductor 37 to flame rod 15. As is well known, when a flame is present at the pilot burner of the fuel burner installation, rectification current fiows to the ground connection 38 through the llame to fiame rod 15 to charge capacitor 36 to the polarity indicated.

Capacitor 36 is charged only so long as flame is present at the pilot burner and the charge on capacitor 36 passes through a filter network 39 to render transistor 40 nonconductive in the presence of flame. Upon nonconduction of transistor 40,

transistors

41, 42 and 43 are rendered conductive.

Referring now to unijunction prepurge timer 18, this timer includes a unijunction transistor 44 having an emitter electrode 4S, a first base electrode 46 and a second base electrode 47. A capacitor 48 is connected in circuit with emitter 45 and base 47 of this transistor, through winding 19 of relay 1R.

Capacitor 48 and the base-to-base circuit of the unijunction transistor are connected to be energized from DC source 33 through a circuit which includes burner air flow switch 16, switch 2R1 of relay 2R and

resistors

49, 50 and 51.

While I have elected to disclose a unijunction transistor 44, it is within the teachings of my invention to use the electrical analogy of this structural element, for example, the two transistor analogy of a unijunction transistor. More broadly, unijunction transistor 44 can be considered to be an electronic switching element having an input and an output, and characterized as a switching element by means of which capacitor 48 will discharge through winding 19 rwhen the voltage on the capacitor exceeds a normally given value, which value-however reduces in magnitude as the voltage applied to the output of the switching element is reduced.

Considering now the operation of the apparatus disclosed in the drawing, let it be assumed that controller 17 closes to energize transformer 29. Bridge rectifier 32 charges capacitor 33 and winding 26 of relay 3R is energized through a circuit including the series elements switch 1R2 and heater 23. Thus, relay 3R is energized only if relay 1R is de-energized and only if heater 23 has circuit continuity.

Energization of relay 3R closes switch 3R1 to complete a holding circuit. Specifically switch 3R1 shorts the above mentioned series circuit and thereby insures that heater 23 is not energized during the purge period. The closing of switch 3R2 is effective to energize burner motor 11 to institute the flow of air at the fuel burner installation.

Air flow switch 16 responds to the ow of air to energize unijunction timer 18. Specifically, the charging of capacitor 48 begins and a base-to-base voltage is applied to unijunction transistor 44. It will be noted that this energizing circuit is through switch 2R1 of flame relay 2R.

At this point in the explanation, it is assumed that flame detector 20 is operating properly. Of course, if an unsafe failure had occurred, such that relay 2R was falsely energized to indicate the presence of flame when in fact a flame could not be present at the fuel burner installation, then switch 2R1 woud be open at this time and control of the fuel burner installation would be interrupted at this point with only burner motor 11 energized.

However, with the assumption that the iiame detector is operating properly, the prepurge period is instituted. During this time capacitor 48 charges, the upper plate of the capacitor becoming positive. Thus, the voltage level of the emitter is gradually increasing and, in accordance with the well-known characteristics of unijunction transistor 44, after a time delay a voltage will be reached where transistor 44 becomes conductive and capacitor 48 discharges through winding 19 of relay 1R to energize this winding.

The energization of winding 19 completes a first maintaining circuit for winding 19 which includes switch 1R3, resistor 53, diode 52, resistor 49 and switch 2R1 of the flame relay.

Energization ol winding 19 also closes switch 'IRI to complete an energizing circuit for heater 23. This euergizing circuit also includes resistor 49 and switch 2R1.

Relay 1R, by way of switch 1R4, is connected in overall control of fuel flow to the fuel burner installation. The closing of switch 1R4 is adapted to energize pilot valve 12 and ignition means 13 to thereby institute a trialfor-pilot-flame period.

During this trial-for-pilot-flame period, heater 23 is energized. It is therefore necessary that pilot flame be established and detected by flame detector 20 Within the timing period of safety lockout 22.

Normally, flame is established within this period, to render transistor 40 nonconductive and

transistors

41, 42 and 43 conductive.

Conduction of transistor 43 is effective to energize winding 21 of flame relay 2R. Energization of this relay causes switch 2R1 to open, thereby de-energizing heater 23 and opening the circuit to capacitor 48 and unijunction transistor 44. It will also be noted that conduction of transistor 42 is effective to short capacitor 48 through diode 54. However, capacitor 48 at this time has discharged through winding 19 to energize the winding and relay 1R is maintained energized through the above mentioned first maintaining circuit.

Conduction of transistor 43 is also effective to complete a second maintaining circuit for winding 19 of relay 1R. This second maintaining circuit extends through diode 55 and resistor 56 to switch 1R3 and then through winding 19.

Energization of relay 2R is also effective to open switch 2R2, de-energizing ignition means 13, and to close switch 2R3, energizing main valve 14.

The apparatus is now operating in the run period, with burner motor 11, pilot valve 12 and main valve 14 energized. Should a flame failure occur during the run period, transistor 40 is rendered conductive and

transistors

41, 42 and 43 are rendered nonconductive. As transistor 43 is rendered nonconductive, the windings of both relays 1R and 2R are deenergized to shut down the fuel burner installation. Relay 3R remains energized and the closing of switch 2R1 is effective to institute a recycle of the control apparatus.

As I have mentioned, the structure of my invention insures that winding 19 of relay 1R will not be energized should an unsafe failure of flame detector 20 occur during the purge period. An unsafe failure of the flame detector is evidenced by energization of relay 2R during the purge period. Energization of this relay during the purge period would open switch 2R1 and this would have the undesirable effect of removing the base-to-base voltage of transistor 44. Capacitor 48 would be charged to some voltage level at this time and with no base-to-base voltage on the unijunction capacitor 48 would discharge through the winding of relay 1R energizing the winding.

My invention involves the unique structural relationship of transistor 42 and capacitor 48. Transistor 42 and the manner in which this transistor shorts capacitor 48 through diode 54 insures that this unsafe operation cannot take place. With the construction of my invention, it is not possible for relay switch 2R1 to open before capacitor 48 is shorted. It will be remembered that the energization of winding of flame relay 2R is assumed to occur because of the unsafe failure of flame detector 20. Immediately upon such an unsafe failure, transistor 42 becomes conductive, rendering transistor 43 conductive and energizing winding 21. However, because of the initial properties of the relay, and the failure of the armature to respond immediately to energization of its winding, transistor 42 always becomes conductive before relay switch 2R1 can open.

I have elected to show a flame rod type flame detector and, as is Well known in the art, an unsafe failure can occurs with such a flame detector in the event that energization of burner motor 11 causes vibration of the iiame rod to periodically short the fiame rod to ground, this vibration simulating a rectification condition as through a pilot fiame. My invention likewise has application to structures utilizing other types of fiame detection, for example, an ultraviolet flame detector wherein an unsafe failure may occur due to a high background count of the Geiger tube detector utilized to sense ultraviolet radiation.

My invention is also directed to the structural combination of the use of transistor 42 to short capacitor 48 in combination with the above described structure including the two maintaining circuits for relay 1R, the energizing and holding circuits for relay 3R, and the circuits including heater 23, as will be apparent from the following claims.

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

1. Burner control apparatus, comprising;

main control means,

a flame detector including a first electronic switching element adapted to be rendered conductive upon the presence of flame and means connecting said rst switching element in controlling relation to a flame `detector control means having a time delay in actuation from a first to a second state upon conduction of said switching element,

a prepurge timer having an input including a capacitor connected to the input of a second electronic switching element, the output of said second electronic switching element including said main control means, said second electronic switching element being characterized as one in which the capacitor will discharge through said main control means when the voltage on said capacitor exceeds a given value, which value reduces as the voltage applied to the output of Said second electronic switching element reduces,

means controlled by said detector control means when in said first state to complete a circuit to energize the output of said second electronic switching element u and to charge said capacitor, to thereby energize said main control means after a time delay,

and means connecting said first electronic switching element to said capacitor to discharge said capacitor through said first switching element upon said first switching element being rendered conductive prior to the end of said time delay.

2. Burner control apparatus as defined in claim 1 wherein said main control means includes an actuator connected in circuit with the output of said second electronic switching element, and wherein said main control means is actuated from a first to a second state upon energization of said main control means actuator, and having a first maintaining circuit tomaintain energization of said maincontrol means actuator, said first maintaining circuit being controlled by said main control means when in said second state and said detector control means when in said first state.

3. Burner control apparatus as defined in claim 2 having a second maintaining circuit to maintain energization of said main control means actuator, said second maintaining circuit being controlled by said main control means when in said second state and by means connecting said first electronic switching element in controlling relation to said second maintaining circuit.

4. Burner control apparatus as defined in claim 3 having safety lockout means including a time delay actuator and a burner motor control means having an actuator, wherein said motor control means is actuated from a first to a second state upon energization of said motor control means actuator; including initial energizing circuit means to initially energize said motor control means actuator and including in a series circuit said lockout means actuator and said main control means in said first state, holding energizing circuit means for said motor control means actuator including said motor control means in said second state connected to short said series circuit, and energizing circuit means for said lockout means actuator controlled by said main control means in said second state and said detector control means in said first state.

5. Burner control apparatus as defined in claim 4 wherein said burner motor control means is adapted when in said second state to energize the fan of a burner installation, wherein said circuit to energize the output of said second electronic switching element and to charge said capacitor is additionally adapted to -be controlled by a burner air fiow switch, wherein said main control means is adapted when in said second state to energize the pilot valve and ignition means of the burner installation, and wherein said detector control means is adapted when in said second state to energize the main valve of the burner installation.

6. Burner control apparatus as defined in claim 1 wherein said main control means includes an actuator and wherein said second electronic switching element is a unijunction transistor with said capacitor connected to the emitter electrode thereof and with said main control means actuator connected to the base-to-base circuit of said unijunction transistor.

7. Burner control apparatus as defined in claim 6 wherein said main control means is a relay having a winding as the actuator and having switch means actuated from a first to a second state upon energization of said main control relay winding, and wherein said flame detector control means is a flame relay having a Winding which is energized by conduction of said first switching element, and having switch means which is actuated from a first to a second state upon conduction of said first switching element, thetime delay being provided by the inertia property of the relay.

8. Burner control apparatus as defined in claim 7, wherein said rst electronic switching element is a first transistor whose output electrodes are connected to said capacitor, wherein said means connecting said first transistor in controlling relation to the winding of said flame relay is a second transistor which is rendered conductive by conduction of said first transistor; and including a first maintaining circuit to maintain energization of said main control relay winding, said first maintaining circuit being completed by the switching means of said main control relay when in said second state and the switch means of said flame relay when in said first state; and a second maintaining circuit to maintain energization of said main control relay winding, said second maintaining circuit being completed by the switch means of said main control relay when in said second state and by said second transistor when conductive.

9. Burner control apparatus as dened in claim 8, having safety lockout means including a time delay actuator in the form of a heater, and having a burner motor relay having a winding and switch means which are actuated from a first to a second state upon energization of said winding, and including initial energizing circuit means to energize said motor relay winding and including in a series circuit said heater and the switch means of said main control relay inpsaid first state, holding energizing circuit means for said motor relay winding including the switch means of said motor relay in said second state to short said series circuit, and energizing circuit means for said heater controlled by the switch means of said main control relay in said second state and the switch means of said flame in said first state.

References Cited UNITED STATES PATENTS 2,839,132 6/1958 Blackett et al. 431-26 3,263,730 8/ 1966 Giuffrida 431-26 3,270,799 9/1966 Pinackers 431-25 3,376,099 4/ 1968 Giuffrida et al. 4311-26 3,393,037 7/ 1968 Giuffrida et al 431-24 FREDERICK L. MATTESON, JR., Primary Examiner. ROBERT A. DUA, Assistant Examiner.

U.S. Cl. X.R. 431-27, 31, 46