EP0038193A1

EP0038193A1 - Burner control apparatus

Info

Publication number: EP0038193A1
Application number: EP81301587A
Authority: EP
Inventors: Yukuo Morohoshi; Ushio Date
Original assignee: Azbil Corp
Current assignee: Azbil Corp
Priority date: 1980-04-11
Filing date: 1981-04-10
Publication date: 1981-10-21
Also published as: JPS56144334A

Abstract

A burner for a furnace comprises an ignitor 42, a pilot valve 44, a main valve 43, an air blower 41, and a damper 37 controlled by a motor 47 to put the ais supply into high or low states. These elements are controlled by relays driven by electronic control circuitry, which comprises a combustion control circuit for controlling the start-up sequence (prepurge, ignition trial, pilot stabilization, main ignition trial), together with an air source control circuit for the air source sequence of high and low states. The combustion and air source control circuits are interlocked, and the circuitry is designed to be failsafe, with many features ensuring that the burner is shut down should a proper sequence of operations fail to take place with the proper timing.

Description

The present invention relates to an electronic burner control apparatus for use with a burner in which the air quantity for combustion may be controlled by means of a damper, and more particularly, to a burner control apparatus for fuel such as gases or oil in which a burner control circuit is failsafe.
A burner which is provided with a damper and having a relatively large combustion capacity (for example, more than 300,000 Kcal/h), normally requires, first, a firing sequence operation such as prepurge time, pilot ignition trial time, pilot stabilizing time and post purge time in order to achieve a complete combustion operation and secondly, a damper sequence operation associated with said first sequence operation at a given timing. However, in the past, it has been difficult to control the firing sequence and damper sequence electronically at the same time, and these sequences have been secured by a combination of numerous switches and cams driven by means of a synchronous motor.
Recently, however, it has been strongly desired from a viewpoint of safety control that such sequence operation is effected correctly and that sequence timing such as pilot ignition trial time is reduced. There has therefore been a tendency to replace control systems using cams and switches or using relay contacts employed in conventional burner control apparatuses by electronic burner control systems capable of providing more accurate sequence control.
As for example, devices for electronically controlling all of sequence timing are described in Japanese Patent Application Laid-open No. 51-117,335 or U.S. Patent No. 4,137,035. However, in these patents, only the combustion cycle sequence operation as described above is controlled by an electronic circuit, and none of them electronically controls those including the damper cycle sequence operation.
Accordingly the present invention provides burner control apparatus for energizing a fuel supply device and air supply deivce of a burner in response to a demand signal, characterized in that it comprises:

an electronic burner control circuit including a semi-conductor timer circuit for generating the combustion operation sequence in order of prepurge period, pilot ignition trial period, and combustion flame stabilizing period; and
an electronic air supply source control circuit associated with the semi-conductor timer circuit for controlling an air supply-quantity control device so as to place the quantity of air supplied to the furnace in a high or low state,
the electronic combustion air control circuit causing the air supply-quantity control device to effect an opening and closing operation sequence so that during the prepurge period, the air supply-quantity is placed in a high state whereas during said pilot ignition trial period and combustion flame stabilizing period, the air supply-quantity placed in a low state.

Thus the present invention provides a combustion control apparatus in which the damper cycle sequence operation is coupled to and associated with the combustion cycle sequence operation to form an overall sequence control system, thereby not only securing correct sequecne timing but providing extremely high reliability and failsafeness relative to trouble in the control apparatus itself.

INTRODUCTORY SUMMARY

The present system comprises a burner control apparatus which can achieve an operation sequence having a high safety by combination of a highly reliable and failsafe electronic burner control circuit for controlling normal combustion operation cycle sequences such as prepurge time, pilot ignition trial time, pilot stabilizing time and main ignition trial time, and an electronic control circuit for air source cycle sequence for sequentially controlling the drive of a burner air supply device such as a damper.
The electronic burner control circuit comprises an electronic timer circuit for securing prepurge time and a start check circuit connected to the electronic timer circuit. The start check circuit has an air pressure switch adapted to drive the electronic timer circuit after assurance that air is not being supplied into the combustion furnace. To monitor the state of the air supply source such as a damper, the start check circuit includes two air quantity state switches which indicate the state of the burner air supply device such as a damper, that is, high state (much air) of air supplied into the combustion furnace or low state (less air). Therefore, the electronic circuit is designed so as to start the timing operation only when the combustion air is not supplied and yet the air supply device is in the high state, at the time of start.
On the other hand, the air source cycle sequence electronic control circuit is connected to the electronic timer circuit so as to respond to the completion signal of prepurge time and completion signal of main ignition trial time from the electronic timer circuit. That is, the air source cycle sequence electronic control circuit has output switch means which, in response to the prepurge completion signal, again shifts the combustion air supply device to the low state, and in response to the main ignition trial completion signal, again shifts the combustion air supply device to the high state.
Further, in the present system, the electronic burner control circuit including the electronic timer circuit is not only formed into a failsafe circuit but the air source cycle sequence electronic control circuit connected to the electronic burner control circuit is also formed into a failsafe control circuit so that the air supply device such as a damper is suitably actuated at a given timing sequence time if trouble occurs in circuit. The air supply control device such as a damper may effect two-position control or proportional control during the normal combustion operation.
The present burner control apparatus further comprises a safety switch drive circuit, which is actuated when firing failure occurs during the ignition trial operation and when flame loss occurs during the normal combustion operation, said drive circuit being also formed into a failsafe circuit.
An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1A shows the burner and control contacts therefor;
Figure 1B to 1D show the electronic control ciruit; and
Figure 2 is a timing diagram for the system.

DETAILED DESCRIPTION - CONSTRUCTION

The present burner control apparatus is wired to the burner using gas or oil as fuel for controlling the combustion operation, and, in the drawings, various parts of the burner are shown as well as the control circuit itself. Small open circles generally indicate the boundary between the burner and the control circuit.
Figure 1A shows a load drive control circuit composed of a number of output switches or contacts of the present control apparatus, and an external burner (shown in symbolic form) connected to the load drive control circuit. The control system includes 6 relays Kl to K6, and their contacts are referenced as K1-1, Kl-2, etc., and shown in the normal (unenergized) positions. Change-over contacts may be regarded as pairs of contacts and are referenced as, e.g., K3-12. In the burner and control contacts, there is a blower electric motor 41 controlled by normally open contact Kl-4; an igniter 42 controlled by normally open contacts Kl-4 and K2-2 and contacts K3-12; a main (first). fuel valve 43 controlled by normally open contacts Kl-4 and K2-2 and contacts K5-12; and a pilot (second) fuel valve 44 controlled by contacts K6-34 from contacts K3-12 and K5-12. The two fuel valves 43 and 44 may be considered as first and second stage fuel supply devices as employed in a low-high change-over combustion system. Should the present burner control apparatus be used for the intermittent pilot system, the terminals ID and IT would be joined by a jumper wire 45. In the present embodiment, a description will be given of the case where no jumper wire 45 is present, that is where the apparatus is used for an interrupted pilot system. An alarm device 46 is controlled by a contact SS-2 to indicate occurrence of abnormal combustion operation. The normally open contact SS-2 of the safety cutoff switch is also connected to an indicator 33, which is in parallel with the alarm 46. The indicators (such as 33) consist of a light emitting diode in parallel with an oppositely poled normal diode, with a ballast resistor connected in series with either the two diodes or just the light emitting diode. Further, there is an indicator 34 for indicating whether or not the control apparatus is in operation and an indicator 35 indicating whether or not the second fuel supply device 44 is energized.
The load control circuit further includes a control circuit for controlling the operation of an air supply control device 47 adapted to control the quantity of air for combustion. The typical air supply control device 47 is normally provided with a damper device 37, but any device may be used as long as the total quantity of air supplied into the combustion furnace may be varied. The control device 47 is shown as comprising a well-known recycle motor and a low-high setting unit, being a known two-position or on-off control deivce. The air supply control device 47 may be operated as a well-known proportional control device, which will be described later. O (open) or H (high) indicatesthe high air state, C (closed) or L (low) the low air state.
Figures 1B to 1D show the electronic parts of the control circuitry, with power supplies not shown. The finer details of the construction will not be described at length, since they will become apparent from the circuit diagrams themselves taken in conjunction with the description of the operation later.
As shown the circuit comprises five main circuit sections: a first electronic timer circuit A, an air supply source control circuit B, an ignition source control circuit C, a second electronic timer circuit D, and a flame detection circuit E of well known type.
Power is supplied to most of the circuitry through a normally closed contact SS-1 of a safety cutoff switch SS described later and a start switch or thermostat switch 39. The first electronic timer circuit A is mainly composed of a safety cutoff switch drive circuit 53, a start check circuit 54, and a first semi-conductor timer circuit 55. The safety cutoff switch drive circuit 53 includes the safety cutoff switch heater SS, which may be a well known manual return type thermally responsive switch of the bimetal heating system, or any type of switch of the manual return on-delay type. The safety cutoff switch drive first series circuit 53 is connected to the first semi-conductor timer circuit 55 via a conductor 66, and the junction of the safety switch SS and a p gate thyristor 64 (hereinafter referred to as SCR) is connected to the start check circuit 54.
In the start check circuit 54, an air pressure switch 67 checks air supply into the furnace and is connected to a relay K2 for driving the second fuel supply source 44. Further, two monitoring switches 68 and 69 are provided to check the position of the combustion air supply control device 47, that is, the state of quantity of air supply. When the quantity of air supply is high, that is, when the damper is open, the high switch 68 is turned on, whereas when the quantity of air supply is low, that is, when the damper is closed, the low switch 69 is turned on. When the opening degree of the damper is in the intermediate position therebetween, both these high and low switches 68 and 69 remain off. These switches are connected via relay contacts Kl-12 to a trigger circuit 71 which controls an SCR 72. SCR 72 controls relay K1, and is shunted by a twin transistor 76 driven from a delay circuit including a capacitor 79 for securing a post purge period to thereby form a blower control circuit 75 for controlling the air supply blower 41.
The switch 67 and relay contacts Kl-12 are also connected to the first semi-conductor timer circuit 55. This semi-conductor timer circuit 55 utilizes two semi-conductor switch elements 85 and 97, which is disclosed in Japanese Patent No. 972,842 Specification. In the timer circuit 55, there is an n gate thyristor 85 (hereinafter referred to as PUT). The prepurge period termination signal or pilot ignition trial period signal is supplied from the PUT 85 via a capacitor 99 to the air supply source control circuit B, and also to ignition source control circuit C. The first electronic timer circuit A further includes a transistor 95 for the self-retaining or checking relay K2 and a normally open relay contact K2-1. This transistor 95 is provided to control conduction of another transistor 62 through the contact K2-1 and control heating of the safety switch heater SS.
Next, the electronics air supply source control circuit B will be considered. It will be noted that the air supply source control circuit is present also in the second electronic timer circuit described later. There are three series circuits across the power supply: a first control relay K4 for controlling the combustion air supply source and a transistor 102; a transistor 108 and its collector resistors; and the resistor chain controlling the bases of transistors 108 and 102. A transistor 111 forms, with transistor 108, a latch circuit so that when the pulse signal through capacitor 99 turns transistor 111 on, transistor 108 also turns on and both transistors 108 and 111 remain on thereafter.
The ignition source control circuit C is energized from the safety cutoff switch drive circuit 53 and the start check circuit 54. The circuit C includes relay K3, a twin transistor 124, and a second twin transistor 162.
The second electronic timer circuit D is energized from the collector of transistor 95 and the collector of transistor 162. Thus, if transistor 162 is turned on, power is supplied to the second timer circuit (D). This second electronic timer circuit D comprises an air fuel control circuit 140 having a second control relay K6 for controlling both the combustion air supply control source 47 and second fuel supply device 44, and a second semi-conductor timer circuit 145 having a load relay K5 for controlling the first fuel supply device 43. The air fuel control circuit 140 forms an air supply control circuit together with the control circuit B as previously mentioned. Relay K6 is connected in series with transistor 135, and relay K5 is connected in series with a transistor 136. A transistor 141 is shunted across relay K6 and transistor 135, and controls the latter transistor via a transistor 144. The transistor 136 and 135 are in the form of a pair having the same function as that of the transistors 108 and 111 in the air supply source control circuit B as previously mentioned.
The second semi-conductor timer circuit 145 also has a circuit construction similar to that of the first semi-conductor timer circuit as previously mentioned. That is, the charging circuit for the capacitor 146 connected to the anode side of n gate thyristor (PUT) 150 is from one end of relay K6 through load relay K5. The cathode side of PUT 150 feeds line 161 via diode 154, the base of transistor 141, and the gate of p gate thyristor 157.
Finally, the flame detection circuit E comprises a detection circuit 166 and a signal responsive control circuit 167. The detection circuit 166 may be of any well known type, such as flame rod detection, ultraviolet ray detection or CdS detection. Circuit 166 feeds a transistor 168 through a Zener diode 172, and the collector thereof is connected to the base of the transistor 95. The base of transistor 168 is connected to the safety switch drive circuit 53 through a Zener diode 169. Therefore, the potential of the connection 1^-75*varies with the combustion sequence operation, whereby the responsive control circuit may have the following two functions. One function is that the flame response timing quickens or delays depending on the sequence period, and the other is the function of return at the time of momentary power stoppage.

OPERATION

The operation of the present combustion control apparatus will be described in connection with Figure 2 showing relays and operation sequence. First, the normal combustion operation of the apparatus will be described and thereafter the significant characteristics of the system will be described. The major periods shown in Figure 2 are: a prepurge interval T1 of 30 s; a pilot ignition trial interval T2 of 5 s; a pilot stabilization interval T₃ of 7.5 s; a main trial interval T4 of 7.5 s; a main burn interval, which may be either damper high-low or proportional control, and has its duration determined by the thermostat 39; and a post-purge interval T5 of 20 s.
First, the start switch 39 is closed. This turns on transistor 102 and hence K4. At the same time, this turns on transistor 62,energizes the trigger circuit 71 through the safety switch heater SS and air pressure switch 67, and SCR72 and relay K1 are turned on. Thus, the damper control device 47 is driven open through contacts K4-12 changing over, and the blower 41 is energized through contact Kl-4 to introduce air into the furnace. The relay K1 is energized after the safety switch heater SS and air pressure switch 67 (no air) have been assured to be normal, and therefore, the contacts Kl-12 and air pressure switch 67 are immediately inverted and therefore, the heater SS stops its heating but SCR72 continues conducting. In the meantime, the low state switch 69 is turned off by the damper control 47 and the high state switch 68 waits for its turn-on. The low state switch 69 is designed so that it opens later than the inversion of the air flow switch 67.
The damper control 47 may require a time t₁, normally from a few seconds to a few minutes, to shift the position from low to high. Now, when the damper 37 assumes its open position, that is, when the high state switch 68 is turned on, the first semi-conductor timer circuit 55 begins to charge the capacitor 84 through the safety switch heater SS, air pressure switch 67, high state switch 68, and contact K1_÷12 to measure out the prepurge time T₁. When the capacitor 84 is fully charged and the anode voltage of PUT85 reaches the gate voltage (after the damper 37 is fully opened and the opening is confirmed), PUT85 turns on and the capacitor 84 begins to discharge. This applies a signal indicative of completion of prepurge time through the capacitor 99 as a pulse signal to transistor 111in the air supply control circuit from the cathode of PUT85, and as a DC voltage to transistor 124 in the ignition source circuit to turn these transistors on. Since at this time the voltage on line 66 is higher than that of the cathode PUT85, the diode 93 remains off. Therefore, when the transistor 111 turns on, the transistor 108 also turns on, after which both transistors 108 and 111 are kept on. This turns transistor 102 off so that control relay K4 is turned off. The air supply control circuit thus causes the contact K4-12 to change over, and the control 47 begins to move to close the damper 37.
When the high state switch 68 is on, the transistor 124 cannot be turned on because its emitter is at a relatively high voltage applied through the conductor 66 and relay K2, and after the high state switch 68 has been turned off, there is no current path and transistor 124 is still held off. Therefore, the transistor 124 awaits until the damper 37 is closed by the control 47 and the low state switch 69 is again turned on. Even during the period t₂, the capacitor 84 continues to be discharged, butthe resistor 92 is 1 MQ to prevent rapid discharge in the period t₂. Upon assurance that the damper 37 is fully closed by the low state switch 69 turning on, the emitter voltage of transistor 124 falls, through the diode 125 and switch 69, and hence transistor 124 becomes momentarily conductive and the relay K3 is turned on. This also turns on the transistor 95. This turns on transistor 76 and SCR 64 and the two trigger circuits 75 and 97 are thus energized.
When the transistor 76 is turned on, the applied voltage to the SCR72 is lowered and turned off, after which the relay K1 is controlled by the transistor 76. On the other hand, when SCR64 turns on, the voltage of line 66 is rapidly lowered and the diode 93 becomes conductive, and the charge remained in the capacitor 84 rapidly flows into SCR64 through the diode, relay K2, and air pressure switch 67 to turn relay K2 on. Also, at this time, since the contact K2-1 is closed, the relay K2 is latched through the already conducting transistor 95, contact K2-1, line 66, relay K2, air pressure switch 67 and SCR64. Therefore, the transistor 62 is turned off to stop heating by the heater SS.
The above mentioned operations are effected almost simultaneously with the turn-on of the switch 69, and the igniter 42 is energized by the contacts K3-12 with the second fuel supply source or pilot valve 44 being energized by the contacts K2-2, thus starting the pilot ignition trial time T_2' Even during the period of the pilot ignition trial time T₂, the charge of the capacitor 84 continues to be discharged through the transistor 124, the diode 125 and the switch 69.
If at this time, the second fuel supply source 44 provides no flame, after discharge of the capacitor 84, the transistor 124 turns off, the relay K3 turning off, the ignition source 42 deenergizing and the transistor 95 also turning off. Accordingly, the relay K2 is turned off because the latching circuit through transistor 95 is released and no discharge current from the PUT85 exists. When the contact Kl-1 is turned off at the same time, the transistor 62 again turns on_/the safety switch heater SS begins to be heated by current flowing into SCR64, the normally closed contact SS-1 thereof is cut off after a few seconds, and the contact SS-2 is turned on to energize the alarm 46.
On the other hand, when the pilot valve 44 provides a flame, the output of the amplifier 166 in the flame detection circuit 167 assumes a low-voltage. However, at this time, the voltage of line 175 is high, and therefore the Zener diodes 169 and 172 momentarily become conductive, and the transistor 168 also monentarily becomes conductive if a flame signal is present. Thus, when the flame is detected during the trial period T₂₁ the flame signal is momentarily fed to the base of the transistor 95. For this reason, even if the capacitor is completely discharged, the pilot trial period T2 is terminated and the transistor 124 and relay K3 are turned off so the transistor 95 and contact K2-1 maintain their on position, the relay K2 continues to be held on and the pilot valve 44 maintains the flame to advance the production of further timing sequence.
Next, when the transistor 95 is turned on, the transistor 124 is turned off and the transistor 162 is turned on; that is, when the provision of pilot flame and termination of pilot trial T₂ have been assured, electric power is for the first time supplied to the second electronic timer circuit D through lines 131 and 132. Then, first, the capacitor 146 begins to be charged through relay K6, resistor 143, and relay K5. That is, the pilot only period or pilot stabilizing period T₃ is counted at time constact determined by the resistor 143 and capacitor 146 while checking burn-out of relays K5 and K6. But in this period T₃₀ the relay K6 is prevented from turning on because of the size of the resistor 143. When the given stabilizing period T₃ has passed and the anode voltage of PUT150 reaches the gate voltage, PUT150 turns on. First, the transistor 141 is turned on, and the transistor 144 is then turned on. When the transistor 141 is turned on, this turns on the transistor 126, but at this time, the cathode current of PUT150 causes also the trigger circuit 155 to be energized and turns on SCR157 almost immediately. Thus, the voltage at point 161 is lowered, and hence the diode 154 becomes conductive, and the charge on capacitor 146 flows as a larger current to energize the diode 154, relay K5 and SCR154, and only the relay K5 is energized. Conduction of PUT150 and turning-on of relay K5 are effected almost immediately. As the relay K5 is energized, the next main trial period T₄ commences to energize the first fuel supply source, that is, the main valve 43, and consequently the main fuel is supplied into the furnace; this fuel is fired by the pilot flame which is already alight. Now, when the capacitor 146 is fully discharged, the transistors 141 and 144 are turned off. Therefore, the transistor 135, which has been locked by the transistor 144, turns on, after which during the normal combustion, the transistors 135 and 136 are maintained on whereby the second control relay K6 is energized. That is, simultaneously with the termination of the main trial period T₄₁ the damper control 47 again causes the damper 37 to drive open by the relay K6 through the load drive control circuit 30, and the damper 37 is fully opened to provide a completely normal combustion operation within the furnace.
Further, when the start switch 69 is turned off as a result of ending of thermal demand, all of circuits (except the relay K1 and flame detection circuit 166, which are supplied with electric power without passing through the start switch 39) are de-energized. Since the relays K2 to K6 are turned off, the damper control 47 again starts to drive the damper 37 closed. At the same time, the relay K1 is energized until the transistor 76 turns off after discharge of the capacitor 79 in the trigger circuit 75, that is, the post purge period T₅, after which the relay 73 is turned off and the blower 41 is stopped for completion of a series of combustion cycle.
As described above, the present system provides a highly reliable or failsafe circuit in the form of a semi-conductor fuel control circuit which includes four start sequences, the prepurge period T₁, pilot ignition trial period T₂, pilot stabilizing period T₃ and main trial period T₄ for entry into normal combustion operation, and further includes a post purge period T₅ as necessary in the stop sequence. Further, the combustion operation sequence of the semi-conductor combustion control circuit incorporates therein the opening and closing operation sequence of the damper for controlling the quantity of combustion air supplied into the furnace, that is the air supply control devices 37 and 47, so that the semi-conductor air source control circuit for opening and closing the air supply control device in accordance with the signal from the semi-conductor combustion control circuit is combined into the aforementioned semi-conductor combustion control circuit, which leads to a significant characteristic of the present system.
That is, the present control apparatus 50 starts only when detection is made of non-supply of air into the furnace by means of the blower 41. Prior to commencement of the prepurge period T1, the damper 37 is fully opened to ensure that the high state interlock 68 is provided thus starting the prepurge period T₁. During the prepurge period T₁, the damper 37 is held in high state and after the lapse of the prepurge period T₁, the damper 37 is again placed in closed state to ensure that the low state interlock 69 is provided before entry into the pilot trial period T₂. For this reason, the semi-conductor timer circuit 55 for the prepurge period T₁ effects the time counting operation only when the damper 37 is in open state and the air pressure switch 67 is monitoring the supply of air.
It will be noted that should a contact welding trouble occur by which both high and low state interlock switches 68 and 69 are closed, the timer circuit 55 and relay K1 are never energized. Also, in the case of trouble whereby no-air state is again involved after detection of air supply by means of the blower 41, a circuit for resetting the timer circuit 55 and a lock-out circuit function. That is, it is designed so that the timer circuit 55 may not operate completely since current flows through the transistor 62, SS, air pressure switch 67, contact Kl-12, high state switch 68, relay K2 and thence PUT85, with the gate voltage of PUT85 held at a low level by the division of mainly resistors 86 and 87. That is, since PUT85 is turned on while the anode voltage remains low, the transistor 111 is turned on to close the damper 37. Thus, when the switch 68 is turned off and switch 69 on, the transistors 124 and 95 and SCR64 are turned on, but the relay K2 is not pulled in due to the small discharge current of the capacitor 84, nor is the relay K3 pulled in because current is not supplied from the transistor 95 and contact K2-1. Since the transistor 64 is on, the safety switch SS actuates and the circuit is locked out by the contact SS-1. When the contacts Kl-12 are fused in the opposite position to that shown, the SCR72 is not triggered, and when the contacts are fused in the position shown, the voltage on capacitor 84 does not rise, because of resistor 81 and the resistor in the trigger circuit 71, far enough to drive the timer circuit 55. The first semi-conductor timer circuit 55 is composed mainly of PUT85 and SCR64, and the failsafe property of this circuit is known per se.
Since the second semi-conductor timer circuit 145 has a construction similar to that of the first semi-conductor timer circuit 55, its failsafe property will not be further described. The second control relay circuit 140 and second semi-conductor timer circuit 55 are connected in a unique manner. That is, the sequence necessary for the present control device to have the main trial period T₄ can prevent occurrence of trouble in which the damper 37 is not first placed in open state as a result of reduction in the period T₄, or trouble whereby the time required to open the damper 37 is too long or infinite. It is so designed that if the condition under which the relay K6 cannot be pulled in occurs, the main valve or the relay
K5 cannot be pulled in or even if the relay K5 is turned on, both relays K5 and K6 should be de-energized after the lapse of the main trial period T_4' That is, since the transistor 135 is not turned on unless the transistor 136 is turned on due to the off trouble thereof, the relay K6 is not turned on. At this time, the relay K5 cannot be self-retained, and the discharge of the capacitor 146 is terminated rapidly and the relay K5 is merely turned on momentarily, and thus the relay K5 cannot actually be pulled in. If off trouble should occur in the PUT150, the transistor 136 cannot be turned on but off by the transistor 141 simultaneously with the turn-off of PUT150 at the end of main trial T₄₁ thus dropping out also the relay K5.
While in the present embodiment, the provision of both state switches 68 and 69 has been described, it should be appreciated that only one of these state switches, the low state switch or high state switch, may be used to readily achieve similar damper sequence. For example, where only the low state switch 69 is used to effect control, the connection between switches 68 and 69 may be removed and switch 68 replaced by a short-circuit.
As described above, the operation sequence of opening and closing the damper by way of the combustion air supply control circuit is associated in an electronic circuit manner with the combustion operation sequence by way of the failsafe combustion control circuit, and with this arrangement, it is possible to achieve all of combustion controls with extremely high reliability and failsafe property.

Claims

1. Burner control apparatus for energizing a fuel supply device and air supply device of a burner in response to a demand signal, characterized in thatit comprises: an electronic burner control circuit including a semi-conductor timer circuit (A) for generating the combustion operation sequence in order of prepurge period (T1), pilot ignition trial period (T2) and combustion flame stabilizing period (T3); and an electronic air supply source control circuit (B) associated with the semi-conductor timer circuit for controlling an air supply-quantity control device (37,47) so as to place the quantity of air supplied to the furnace in a high or low state, the electronic combustion air control circuit causing the air supply-quantity control device to effect an opening and closing operation sequence so that during the prepurge period, the air supply-quantity is placed in a high state whereas during said pilot ignition trial period and combustion flame stabilizing period, the air supply-quantity placed in a low state.

2. Burner control apparatus according to Claim 1, wherein the combustion operation sequence includes a main ignition trial period following the flame stabilizing period, characterized in that it includes a starting check circuit (54) having means (67,68,69) for detecting the air supply-quantity state and controlling operation of said semi-conductor timer circuit in response thereto.

3. Burner control apparatus according to Claim 2, characterized in that said detecting means of the starting check circuit.comprises a first detecting means (67) for detecting presence of air supplied to the furnace, and a second detecting means (69) for detecting when the quantity of air supplied to the furnace is in the low state.

4. Burner control apparatus according to Claim 3, characterized in that said second detecting meons has a switch means and when a trouble occurs in conduction of the switch means, the starting check circuit prevents the prepurge period from proceeding by means of said semi-conductor timer circuit.

5. Burner control apparatus according to Claim 4, characterized in that the electronic combustion air control circuit has circuit means for energizing a controller in which the air supply-quantity control device changes quantity of air supplied to the furnace to the high state in accordance with the generation of said combustion operation demand signal, and when a prepurge termination signal from said semi-conductor timer circuit is further received by said circuit means, said air supply-quantity control device changes the quantity of air supplied to the furnace back to the low state.

6. A combustion control apparatus according to Claim 5, characterized in that said semi-conductor timer circuit has stop circuit connected across it and said circuit means, said stop circuit stopping the timing operation of said semi-conductor timer circuit within a period from after transmission of said prepurge termination signal to the closed circuit state of said switch means.

7. A combustion control apparatus according to Claim 6, characterized in that said semi-conductor timer circuit comprises a first semi-conductor timer circuit for timing said prepurge period and said pilot ignition trial period, and a second semi-conductor timer circuit for timing said combustion flame stabilizing period and said main ignition trial period, and said second semi-conductor timer circuit controls said electronic combustion air control circuit such that said air supply-quantity control device changes the quantity of air supplied to the furnace back to the high state after the lapse of said main ignition trial period.

8. A combustion control apparatus according to Claim 3, characterized in that said starting check circuit controls said electronic combustion control circuit such that the combustion operation sequence is started only whena said first detecting means confirms such air quantity state signal to the effect that combustion air to the furnace has not been supplied, and thereafter when it is detected that combustion air has been supplied to said furnace, said semi-conductor timer circuit starts said prepurge period.

9. A combustion control apparatus according to Claim 8, characterized in that said semi-conductor timer circuit has circuit means for interrupting said combustion operation sequence by said electronic combustion control circuit in accordance with output of said first detecting means which indicates such fact that supply of said combustion air to the furnace has been again stopped after starting said prepurge period.