US3664803A

US3664803A - Combustion control system

Info

Publication number: US3664803A
Application number: US60230A
Authority: US
Inventors: Phillip J Cade
Original assignee: Electronics Corp of America
Current assignee: Electronics Corp of America
Priority date: 1970-08-03
Filing date: 1970-08-03
Publication date: 1972-05-23
Anticipated expiration: 1989-05-23
Also published as: CA948739A; GB1355376A; CH557505A; FR2104827A1; BE770632A; DE2138868A1

Abstract

A COMBUSTION CONTROL SYSTEM INCLUDES A FUEL CONTROL, A FLAME RELAY, AN IGNITER AND A FLAME SENSOR FOR SENSING THE PRESENCE OF FLAME IN THE COMBUSTION CHAMBER. THE CONTROL CIRCUITRY IS ARRANGED TO PERMIT FLOW OF FUEL INTO THE COMBUSTION CHAMBER IN RESPONSE TO A REQUEST FOR IGNITION AND A RESISTIVE CAPACITIVE TIMING CIRCUIT PERMITS THE FLOW OF FUEL INTO THE COMBUSTION CHAMBER FOR A PREDETERMINED TIME AND THEN TERMINATES THE FLOW OF UEL UNLESS THE FLAME SENSOR INDICATES THAT FLAME IS PRESNT BEFORE THE END OF THE PREDETERMINED TIME INTERVAL.

Description

May 23, 1972 P. J. CADE COMBUSTION CONTROL SYSTEM Filed Aug. :5, 1970 mm/ mm \QQN kQ m Q wmw w wnwnew W? N A o 8 /W kww mm mmkw amn in T k QQ mm mm .H. 3 kb@ United States Patent 3,664,803 COlVIBUSTION CONTROL SYSTEM Phillip J. Cade, Winchester, Mass., assignor to Electronics Corporation of America, Cambridge, Mass. Filed Aug. 3, 1970, Ser. No. 60,230 Int. Cl. F23n 5/20 US. Cl. 431-69 8 Claims ABSTRACT OF THE DISCLOSURE A combustion control system includes a fuel control, a flame relay, an igniter and a flame sensor for sensing the presence of flame in the combustion chamber. The control circuitry is arranged to permit flow of fuel into the combustion chamber in response to a request for ignition and a resistive capacitive timing circuit permits the flow of fuel into the combustion chamber for a predetermined time and then terminates the flow of fuel unless the flame sensor indicates that flame is present before the end of the predetermined time interval.

This invention relates to combustion control systems.

In combustion control systems, the existence of a flame in the combustion chamber is conventionally supervised by a sensor such as a flame rod or an infrared or an ultraviolet radiation detector. Prior to ignition, the sensor generates a signal indicating that no flame is present, after flame is established in the combustion chamber the sensor generates a signal indicating that flame is present and upon flame fail, a flame failure signal is generated. In typical ignition sequence, fuel is allowed to flow into the combustion chamber for ignition prior to establishment of flame. If flame is not established promptly, the fuel flow is terminated and a purge sequence is provided prior to restart of the ignition sequence to clear the unburned fuel from the combustion chamber. The trial for ignition" interval must be terminated after a predetermined time or unburned fuel will continue to flow into the combustion chamber, creating a hazardous condition. A variety of circuit arrangements for timing this trial for ignition interval have been suggested. In a conventional arrangement, a motor driven timer is employed which closes a switch to prevent the flow of fuel into the combustion chamber for a predetermined interval and terminates that flow of fuel if flame ignition is not established within that interval. Such a system can fail unsafely in that the trial for ignition period could be indefinitely long if the motor driving the cam should stall. Sometimes a thermal lockout switch is used to avoid this problem. The lockout switch heater is energized during the trial for ignition period so that the lockout switch operates if the timer motor stalls. Such lockout switches are not sufliciently accurate for short trial intervals, such as two to fifteen seconds, and also the timing is dependent on voltage.

It is an object of this invention to provide a novel and improved combustion control system.

A more specific object of the invention is to provide novel and improved circuit arrangements for limiting the trial for ignition interval.

Another object of the invention is to provide novel and improved combustion control circuit arrangements that provide a reliable trial for ignition interval in a simple and inexpensive manner.

Another object of the invention is to improve safe burner operation by providing improved means for limiting the interval during which fuel flows to a burner without being ignited. Further objects are to effect the limitation simply, inexpensively, and in a fail-safe manner.

In accordance with the invention there is provided a combustion control system having a main fuel valve for 3,664,803 Patented May 23, 1972 controlling the flow of fuel to a combustion chamber, a relay for controlling the main fuel valve, an igniter in the combustion chamber for igniting fuel in the combustion chamber to produce flame, a flame sensor for sensing the presence of flame in the combustion chamber, and control circuitry for opening the main fuel valve to permit flow of fuel into the combustion chamber in response to a request for ignition, and a timing circuit consisting solely of passive components for producing an output signal permitting flow of fuel into said combustion chamber for a predetermined time interval and terminating the flow of fuel to the combustion chamber in the absence of a flame present signal from the flame sensor before the end of the predetermined interval.

In a preferred embodiment the circuitry includes a capacitor and a switch responsive to a trial for ignition request. The capacitor is connected to a source of power prior to initiation of the ignition request and in response to the ignition request is disconnected from the source of power and connected to circuitry to operate the fuel control. In a particular embodiment, this switch is a cam operated single pole double throw switch that, in a first position, connects the capacitor to a source of power and in a second position connects the capacitor in a discharging circuit to control the operation of the fuel control. In this embodiment the circuit between the capacitor and the fuel control includes a flame relay that is connected to respond to the flame present signal from the flame sensor and the capacitor is connected in a timing circuit with at least one resistor so that the trial for ignition interval is determined by the time constant of the capacitor and the resistor and energizes the flame relay during the trial for ignition interval independent of the signal from the flame sensor.

Other objects, features, and advantages will appear from the following description of a preferred embodiment of the invention taken together with the attached drawing which shows a diagram of a burner control constructed in accordance with the invention.

Referring to the figure, terminals 12 and 14 provide connections for a suitable source of AC power supplied to transformer 16. Flame detector 1 8 employing ultraviolet sensitive tube 20, is connected in circuit with first secondary winding 22 of transformer 16 and primary winding 24 of readout transformer 26. Secondary winding 28 of transformer 26 is connected with

diodes

30, 32 to form a full wave rectifier circuit, the output of which is connected through lead 34 to the base electrode of transistor 36. A second secondary winding 38 of transfonner 16 is connected in a circuit with diode 40 and capacitor 42 to provide a B+ power supply. Electronic circuitry connected to this power supply includes

transistors

36 and 46 and controls the flow of current through relay coil 48. Relay 48 governs the valve 50 that controls the admission of fuel to combustion chamber 52. An igniter 54 is disposed in chamber 52. A motor drives cams which operate contacts 60-1, 60-2 and 60-3. Contacts 60-1 and 60-2 are arranged as a single pole, double throw switch 60 that has a first position in which capacitor 62 is connected via contacts 60-1 through resistor 64 to the B-I- supply and a second position in which capacitor 62 is connected via contacts 60-2 to a voltage divider consisting of

resistors

66 and 68, the center junction 70 of which is connected to the base of transistor 72 and its collector electrode is connected to the base electrode of transistor 46. Additional details of a combustion control system incorporating apparatus of this type may be had with reference to co-pending application Ser. No. 60,229, entitled Electrical Control Apparatus, filed concurrently herewith and assigned to the same assignee as this appli cation.

The operation of the apparatus is as follows:

In the pre-ignition period valve 50 is closed, contact 60-1 is closed, and contacts 60-2, and 60-3 are open. Flame sensor 20 sees no flame radiation and is therefore in a nonconductive state. No current flows through winding 24 of transformer 26 and there is therefore no output on lead 34. Transistor 36 is therefore in ofl? (non-conductive) condition. Transistor 72 is also in non-conductive state with its base electrode at approximately ground voltage. Approximately B-lvoltage is therefore applied to the base of electrode transistor 46 with the result that transistor 46 is also non-conductive and no current flows through relay coil 48. The fuel valve 50 controlled by relay 48 is thus closed. Capacitor 62 is maintained during this period in a charged condition by connection through switch contact 60-1 to the B+ power supply.

A trial for ignition mode of operation is initiated when cam operated switch 60 is switched thereby opening contact 60-1 and closing contacts 60-2 and 60-3. Capacitor 62 thereupon supplies a voltage through contact 60-2 and resistor 66 to the base of transistor 72, causing transistor 72 to become conductive and lowering the voltage on the base of transistor 46, which in turn becomes conductive allowing current to flow through relay coil 48. The fuel valve 50 is thereupon opened. If the fuel flowing into chamber 52 is ignited, ultraviolet sensor tube 20 in response to the flame becomes conductive, and current flows through winding 24 of transformer 26 with the result that a positive voltage is applied through lead 34 to the base of transistor 36. Transistor 36 thereupon becomes conductive and maintains a reduced voltage on the base of transistor 46 so that transistor 46 remains conductive, and current flow is maintained through relay coil 48. The fuel valve 50 thus continues in open condition. However, capacitor 62 is being discharged through resistors 64, '66 and '68. The time constant for this discharge is the product of the capacitance of capacitor 62 times the sum of the resistances of

resistors

64, 66 and 68. The voltage on the base of transistor 72 will decline and that transistor will go to non-conductive state. If ignition in chamber 52 is not achieved within a time interval that is a function of that time constant, the voltage on the base of transistor 46 will thereupon rise so that transistor 46 will become nonconductive with the result that the current through relay 48 and consquently fuel valve 50* is closed. The apparatus thus simply, reliably and in fail safe manner limits the interval during which fuel flows without ignition to approximately the time constant of capacitor 62 with

resistors

64, 66 and 68 in series.

While a particular embodiment of the invention has been shown and described, various modifications thereof will be obvious to those skilled in the art. For example, it will be obvious that principles of the invention are applicable to circuitry using other types of flame detectors. Also, other forms of switches and charging circuitry may be employed in the practice of various aspects of the invention. Therefore, it is not intended that the invention be limited to the disclosed embodiment or to details thereof, but departures may be made therefrom within the spirit and scope of the invention.

What is claimed is:

1. A combustion control system comprising a valve for controlling the flow of fuel to a combustion chamber, a flame sensor for sensing the presence of flame in said combustion chamber and producing a flame present signal in response thereto, a flame relay connected to respond to said flame present signal from said flame sensor for controlling said fuel valve, an igniter in said combustion chamher for igniting fuel in said combustion chamber to produce flame, control circuitry responsive to said flame sensor for energizing said relay to operate said fuel control and permit flow of fuel into said combustion chamber, and a timing circuit consisting solely of passive components for producing an output signal in response to a request for ignition that defines a trial for ignition interval, said output signal energizing said flame relay during said trial for ignition interval independent of the signal from said flame sensor and permitting flow of fuel into said combustion chamber for a predetermined time interval and terminating the flow of fuel to the combination chamber in the absence of a flame present signal from said flame sensor before the end of said predetermined interval.

2. The system as claimed in claim 1 wherein said timing circuit includes a capacitor and a switch responsive to a trial for ignition request, said capacitor being connected to a source of power prior to initiation of the ignition request and said switch, in response to the ignition request, disconnecting said capacitor from the source of power and connecting said capacitor to energize said flame relay.

3. The system as claimed in claim 2 wherein said switch is a cam operated single pole double throw switch which, in a first position, connects said capacitor to said source of power and, in a second position, connects said capacitor in a discharging circuit to control the operation of said fuel control.

4. The system as claimed in claim 1 wherein said timing circuit includes a capacitor and further including a circuit between said capacitor and said flame relay said capacitor being connected in said timing circuit so that the trial for ignition interval is determined by the time constant of said timing circuit and said flame relay is energized during said trial for ignition interval independent of the signal from said flame sensor.

5. In control apparatus for a burner including a flame detector, a relay adapted to effect withholding of fuel to the burner when said relay is in its normal state and to effect admission of fuel when in its energized state, circuitry adapted to receive a signal from said flame detector when a flame is detected and thereupon to effect the energizing of said relay, and a switch adapted to provide a trial-for-ignition mode of operation of said apparatus,

a capacitor adapted to be connected to a source of power prior to initiation of said trial-for-ignition mode and to be disconnected from said source during said mode, and means to connect said capacitor in circuit with said relay to provide for a limited interval subsequent to said initiation a voltage effecting the energizing of said relay.

6. The apparatus of claim 5 in which a single pole, double throw switch in a first position connects said capacitor to said source and in a second position connects said capacitor to second circuitry controlling the energizing of said relay.

7. The apparatus of claim 4 in which said single pole, double throw switch is actuated by a timing cam.

8. The apparatus of claim 5 in which said apparatus includes an igniter for igniting said fuel and further including means for energizing said igniter concurrently with the connection of said capacitor in circuit with said relay.

References Cited UNITED STATES PATENTS 3,270,800 9/ 1966 Deziel et al 431--69 3,488,131 1/1970 Myers et a1 43169 X 3,276,507 10/1966 Eldridge, Jr. et a1 431-69 3,350,581 10/1967 Stein, Jr 307-117 3,388,566 6/1968 Kaper 307-117 X EDWARD G. FAVORS, Primary Examiner