Flame Failure Controls

Abstract

The invention has particular utility in connection with combustion control systems that employ lead sulfide (infrared sensitive) photoconductors in conjunction with band pass amplifiers. Flame relays in such systems would occasionally hold in after flame failure due to continued generation by the photoconductor of fluctuating signals. A flame failure transition responsive circuit is connected in series between the photoconductor and the flame relay and responds to an abrupt decrease in the flame signal to insure dropout of the flame relay.

Description

BACKGROUND OF THE INVENTION

This invention relates to electrical control circuitry and more particularly to control circuitry particularly adapted for use in combustion supervision systems.

In control systems of the type employed for the supervision of flame in a combustion chamber, the system arrangement must reliably and accurately indicate the presence or absence of flame, the absence of flame being promptly detected so that the fuel valve may be closed quickly, thus preventing an excessive amount of unburned fuel from accumulating in the combustion chamber. Known types of combustion supervision systems monitor the flame by sensing a fluctuating characteristic. The fluctuating characteristic of the flame enables discrimination against steady state sources of similar radiation such as the incandescent refractory in the combustion chamber. Upon certain circumstances, however, the steady state radiation from such sources may be modulated, for example a "shimmering" effect due to flow of air in the combustion chamber, or spraying of unignited oil into the combustion chamber. In such cases, a fluctuating radiation signal may be sensed which causes the control circuit to erroneously indicate the presence of flame.

SUMMARY OF INVENTION

It is an object of this invention to provide novel and improved control circuitry useful for supervising fuel burning systems.

Another object of the invention is to provide novel and improved combustion supervision circuitry particularly useful with flame sensors responsive to infrared radiation.

Still another object of the invention is to provide novel and improved combustion supervision circuitry which provides improved discrimination between flame signals and spurious modulated radiation.

In accordance with the invention there is provided a condition responsive system comprising a condition sensor which produces an output signal in response to the sensed radiation that has a fluctuating characteristic, and signal processing circuitry responsive to the fluctuating characteristic of the output signal produced by the condition sensor for controlling an output device arranged to assume a first state indicative of the presence of the condition being sensed when the sensor senses that condition and a second state indicative of the absence of the condition when the sensor does not sense that condition. The system includes circuitry responsive to an abrupt change in the magnitude of the output signal produced by the condition sensor for overriding the signal processing circuitry and placing the output device in its second stage, notwithstanding production by the condition sensor of a fluctuating output signal to which the signal processing circuitry would respond.

In combustion supervision systems, the abrupt change responsive circuitry responds to a change from a flame present to a flame absent condition (e.g., flame failure), the circuitry not responding to a signal change from flame absence to flame presence such as occurs on startup of the burner system. In response to the detected flame failure transition, the output device (for example, the flame relay) typically closes the fuel valve and takes such other action as determined by the system parameters. The system thus responds to the flame failure transition and overrides the discriminator circuitry that is responsive to the fluctuating signal characteristic and provides prompt, reliable response to the flame failure condition.

In particular combustion supervision embodiments, the flame failure transition responsive circuitry is connected either in series or in shunt with the frequency sensitive (discriminator) circuitry, and includes a resistance-capacitance network that generates an override signal. An asymmetrically conductive device is connected to the network such that the override signal is produced on flame failure transitions but not on flame startup transitions.

This invention has particular utility in conjunction with combustion control systems that employ lead sulfide (infrared sensitive) photoconductors in conjunction with band pass amplifiers tuned to pass a band of fluctuating signals in the subaudio region. Flame relays in prior art systems of that type would occasionally hold in due to continued generation by the photoconductor of fluctuating signals in the pass band after the main flame had been extinguished. In a preferred embodiment, the flame failure transition responsive circuit is connected in series between the photoconductor and the pass band amplifier and includes a series capacitor, a shunt resistor and a shunt diode--the time constant of the circuit being selected to block the amplifier for an interval sufficient to insure drop out of the flame relay. This circuit is single component fail safe in that failure of any one of its three components will not cause the flame relay held in.

The invention thus provides improved condition responsive system and particularly improved flame supervision systems. Other objects, features and advantages of the invention will be seen as the following description of particular embodiments progresses, in conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of a system in accordance with the invention for supervising the existence of flame in a combustion chamber; and

FIG. 2 is a portion of a modified circuit arrangement.

DESCRIPTION OF PARTICULAR EMBODIMENTS

The circuit shown in FIG. 1 includes a flame sensor 10 in the form of a lead sulfide cell which is positioned to supervise flame from a fuel burner in a combustion chamber that is under supervision. Cell 10 is connected in a voltage divider network that includes resistor 12 between positive bus 14 and grounded bus 16. Bus 14 is connected through diode 18 to tap 20 of the secondary winding of transformer 22. The transformer primary winding (not shown) is connected by conventional means to a suitable AC voltage source.

Junction 24 of the voltage divider network is connected to a flame failure transition responsive network 25 that includes capacitor 26, resistor 28 and diode 30 in shunt with resistor 28. Capacitor 26 and resistor 28 provide a time delay function in response to abrupt increases in the voltage at junction 24 such as occur when the flame is extinguished, while diode 30 provides a low impedance shunt path in response to abrupt decreases in the voltage at junction 24 such as occur when flame is sensed.

The system includes a frequency selective discriminator circuitry in the form of a high input impedance electronic amplifier which includes a pair of amplifier stages 32, 34. Interstage coupling capacitors 36, 38 cooperate with feedback circuits that include capacitors 40, 42, 44, 46 to provide a band pass amplifier circuit which, in this embodiment, peaks at about 10 Hertz and has primary response to signals in the 5-25 Hertz frequency range.

The output from coupling capacitor 38 is applied through an integrator circuit which includes resistor 50 and capacitor 52 to a bistable vacuum tube circuit that includes triode stages 60, 62. Tap 70 of the transformer secondary winding is connected to ground bus 16; and a portion 72 of that winding supplies filament voltage. A third tap 74 is connected to the cathode of stage 62 and the anode of stage 60 is connected to tap 74 through resistor 76. The grid of stage 62 is connected directly to the anode of stage 60 and capacitor 78 is connected between the cathode and grid of stage 62. Connected to the anode of stage 62 is flame relay coil 80 which is shunted by capacitor 82.

In operation, with the system in standby condition, triode stage 62 is cut off and relay 80 is de-energized. When sensor 10 sees fluctuating infrared radiation from the supervised flame, its resistance drops and is maintained at a low but pulsating value so that the voltage at junction 24 is low and pulsating. In response to this voltage transition, diode 30 enables the charge on capacitor 26 to be dissipated rapidly, the pulsating voltage at junction 24 is then coupled by capacitor 26 to the band pass amplifier; and that amplifier applies a signal to the integrator circuit of resistor 50 and capacitor 52. When capacitor 52 is sufficiently charged, stage 60 ceases conduction, and causes stage 62 to conduct and energize relay 80 to provide an indication of the presence of the sensed flame condition. Should the fluctuating signal terminate, the charge on capacitor 52 will dissipate, allowing the bistable stage 60 to conduct and causing the stage 62 to cease conduction so that the flame relay 80 will drop out after a predetermined time delay determined principally by the time constant of the integrator circuit. Should the flame go out, the voltage at junction 24 will rise abruptly. However, a modulated signal may continue to be present, due to continuing fuel spray, for example, and a fluctuating voltage (of much lower magnitude than when flame is sensed, however) will continue to be present at junction 24. Network 25 responds to the abrupt transition in voltage level and capacitor 26 applies a positive bias to the grid of tube 32 effectively blocking that amplifier stage and so that no signal is applied to maintain the charge on integrator capacitor 52. The flame relay 80 drops out as it should, terminates the flow of fuel into the combustion chamber by closing the fuel valve, and provides a proper indication of the absence of flame. The blocking condition on tube 32 is maintained by network 25 for a time interval greater than that of the drop out time of flame relay 80.

This network is single component fail safe in the system and blocks the spurious modulated signal from the input to the discriminator circuitry for an interval longer than the output relay drop out time.

In a second embodiment illustrated in FIG. 2, flame signal transition monitoring circuitry is connected in circuit between the photosensor 10 shown in FIG. 1 and transformer tap 74 (the junction between resistor 76 and capacitor 78). The signal from the photosensor, in addition to being coupled by capacitor 26' to the first stage 32 of band pass amplifier, is coupled by capacitor 100 to a time delay network 102 that includes resistor 104, capacitor 106 and diode 108. The output of the time delay network is applied to a programmable unijunction transistor 110 (e.g., type 2N6028) whose control electrode 112 is connected to a voltage divider network that includes resistors 114, 116 That voltage divider network is connected to diode rectifier 118 to tap 74 of the transformer 22 and junction 120 is maintained at about 4 volts. In response to an abrupt increase in voltage at junction 24, capacitor 106 is charged and triggers put 110 into conduction after a time delay determined by network 102. The resulting signal is applied through resistor 122 to trigger silicon control switch 124 (e.g., type 3N85) and that switch bypasses amplifier stage 60 and places stage 62 in non-conducting condition so that flame relay 80 is de-energized and drops out. It will be noted that this time delay network 102 responds to an abrupt increase in the magnitude of signal applied to capacitor 100. In this circuit arrangement the time delay of the override circuitry is independent of the normal drop out time of the flame relay, and may be set to cause drop out of the flame relay in an interval shorter than the normal drop out time provided by the main signal processing circuit between stages 32 and 60.

While particular embodiments of the invention have been shown and described, various modifications thereof will be apparent to those skilled in the art. For example, the invention is also applicable to solid state versions of combustion control systems. It is not intended that the invention be limited to the disclosed embodiments or to details thereof and departures may be made therefrom within the spirit and scope of the invention as defined in the claims.

Claims

What is claimed is:

1. A condition responsive system comprising a radiation sensitive condition sensor for producing an output signal in response to the sensing of a condition to be detected,

an output device arranged to assume a first state indicative of the condition being sensed when said sensor senses said condition and a second state indicative of the absence of said condition to be sensed when said sensor does not sense said condition,

discriminator circuitry connecting said sensor and said output device for placing said output device in said first state in response to the production of a fluctuating output signal by said condition sensor, and

circuitry responsive to an abrupt decrease in the condition sensed by said sensor for placing said output device in said second state independent of fluctuations of the signal produced by said sensor.

2. The system as claimed in claim 1 wherein said abrupt decrease responsive circuitry includes time delay circuitry.

3. The system as claimed in claim 1 wherein said abrupt decrease responsive circuitry is connected between said sensor and said discriminator circuitry.

4. The system as claimed in claim 1 wherein said abrupt decrease responsive circuitry is connected between said sensor and said output device in shunt with said discriminator circuitry.

5. The system as claimed in claim 1 wherein said sensor is a photoconductor.

6. The system as claimed in claim 1 wherein said condition to be detected is flame in a combustion chamber and said output device is the coil of a flame relay.

7. The system as claimed in claim 1 wherein said discriminator circuitry includes a band pass amplifier tuned to pass signals in the audio and subaudio range, and further including coupling means for applying output signals produced by said condition sensor to said band pass amplifier.

8. A combustion supervision system comprising a flame sensor for producing an output signal in response to the sensing of flame in a supervised combustion chamber,

an output device arranged to assume a first state indicative of flame when said sensor senses flame and a second state indicative of the absence of flame when said sensor does not sense flame,

discriminator circuitry connecting said sensor and said output device for placing said output device in said first state in response to the production of a fluctuating output signal by said flame sensor, and

flame failure transition responsive circuitry connected to said flame sensor for placing said output device in said second state in response to an abrupt decrease in flame and independently of fluctuations of the signal produced by said flame sensor.

9. A combustion supervision system comprising a flame sensor for producing an output signal in response to the sensing of flame in a supervised combustion chamber,

an output device arranged to assume a first state indicative of flame when said sensor senses flame and a second state indicative of the absence of flame when said sensor does not sense flame,

discriminator circuitry connecting said sensor and sai output device for placing said output device in said first state in response to the production of a fluctuating output signal by said flame sensor, and

flame failure transition responsive circuitry connected to said flame sensor for placing said output device in said second state in response to an abrupt decrease in flame and independently of fluctuations of the signal produced by said flame sensor, said flame failure transition responsive circuitry including time delay circuitry comprising a series capacitor connected in series between said flame sensor and said output device and a circuit of asymmetric resistance characteristics connected in shunt with said flame sensor.

10. The system as claimed in claim 9 wherein said flame failure transition responsive circuitry is connected between said flame sensor and said discriminator circuitry.

11. The system as claimed in claim 9 wherein said flame failure transition responsive circuitry is connected between said flame sensor and said output device in shunt with said discriminator.

12. The system as claimed in claim 8 wherein said flame sensor is a lead sulfide photocell, said discriminator circuitry includes a band pass amplifier tuned to pass signals in the 5-25 Hertz range, and said output device is the coil of a flame relay, and further including coupling means for applying output signals produced by said photocell to said band pass amplifier.