AU427452B2 - Improvements relating to ignition and control devices for fuel burners - Google Patents

Description

39.254/72 In French Patent No. 1.598.786 dated December 27,

1968, the applicants have described an ignition and safety

device for burner consuming hydrocarbons or other combustible

fluids in which the ignition and the detecting of the pre-

sence or absence of a flame are effected using a single

electrode, by a spark repeated at a very low frequency, below

1Hz. The ignition is obtained'by'dischiarging a'contiensor in-

the primary of the ignition transformer.

The principle on which the detection is based is

the difference, in con-ditions of ionisation and non-ionisat-

ion, in the value of the damping coefficient, in a transipnt

mode, of a very small third coil which, with a condenser con-

nected in parallel, forms a damped oscillating circuit.

However, in this device, the repetition of the

sparks used for ignition at the very low frequency, below 1Hz,

has certain disadvantages with certain types of gas, and with

imperfect gas mixtures and also when large flow burners are

employed. In particular, sparks of this kind are unsuitable

for the proper ignition of heavy hydrocarbons.

A serious disadvantage of other devices which use

a single electrode for ignition and detection and which pro-

duce a permanent supply of sparks at the frequency of the

supply circuit is their noisy operation. They also generate

interference which affects radio and television reception to

such an extent that it could lead to their being prohibited.

In a device embodying the present invention, a

pulse generating means is operable to deliver to the single

electrode a succession of spark-producing pulse trains, the

pulse trains being separated from one another by intervals

which are longer than the duration of each train, a detector

2 427452

39254/72 means detects the presence or absence of a flame when a

spark appears across the said gap, and there are means where-

by when ignition takes place the pulse generating means changes

over automatically to cause the production of sparks at a fre-

quency substantially lower than the frequency of the pulses in

the trains. Thus, the frequency of the pulses in the pulse

trains may be that of the mains supply and the frequency of

the sparks produced for detection purposes, once ignition

has taken place, may be not greater than 1 Hz. The interval

between successive pulse trains when ignition is attempted

allows the unburnt mixture to disperse in the atmosphere, to

reduce risk of explosion.

In the preferred form of apparatus, if the flame

is.extinguished re-ignition at the low frequency is attempted

and then, if this is unsuccessful, several successive trains

of pulses at the higher frequency are supplied before the

device finally resets itself to a safety condition such that

manual intervention is required before further attempts can

be made.

The invention is also applicable to temperature

regulation, for example in heating systems, and it can also

be applied in safety systems, in particular those which

operate:-

if the flame is accidentally extinguished (for

example if it blows out) for a short period, to attempt re-

ignition, first at low frequency, 'and then by successive

trains of sparks;

if the flame remains extinguished, to close and

lock electrically, the shut-off member for the combustible

fluid so that a manual operation is required to restore the

3 427452 supply; 3 2 472 in the case of a general failure of the gas or electricity supply, to provide the possibility of .automatic re-ignition of the burner as soon as the supply is restored, this being effected by means of an additional contact con- trolled by a pressure-responsive device; in the case of an ignition failure, tolock the shut-off member electrically, so that manual reactivation is required, as in case in the case of a short circuit of the ignition electrode, to take action as described for case in the case of an abnormally high burner temperat- ure, to take action as described for case

In order that the invention may be better under-

stood, an example of apparatus embodying the invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 illustrates diagrammatically.the operation

of currently employed.devices;

Figure 2 illustrates diagrammatically the operation

of the device described in the abovementioned patent;

Figure 3 illustrates diagrammatically the operation

of a device embodying the present invention;

Figure 4 shows a circuit similar to that shown in the abovementioned patent;

Figures 5 to 12 show oscillograms illustrating the

method of functioning of this device;

Figure 13 is the overall circuit of a device embody-

ing the present invention;

Figure 14 is a circuit for a variant intended par-

39 254/72 ticularly for igniting heavy hydrocarbons;

Figure 15 shows the synchronisation signal used in

the latter;

Figure 16 shows the pilot signal; and Figure 17 shows the interruptions of the inductive

current.

From the earlier French Patent No. 1.598.786, it

will be remembered that the circuit shown in figure 4 funct- ions as follows. The damped oscillatory discharge in the

principal discharge circuit constituted by a condenser C4 and the primary winding N1 of an ignition coil, supplies across

the secondary winding N 2 the sparks required for ignition and for detection. At each detection spark, the oscillating cir-

cuit N 3 Co is excited, and produces a damped oscillation at approximately 200 KHz (see figure The negative part of this oscillation is detected by a condenser C 5 and by the base-emitter junction of a transistor T, (see Figure A negative potential is therefore applied to the base of T1 during the entire oscillation period, which blocks it and produces on its collector a rectangular signal whose duration is proportional to the oscillation period t i (see Figure 7). This signal allows the condenser C to be charged through two 7

resistances R 6 and R 8 according to the time constant

(R6 R8 C7 If there is no flame, and consequently t 1 is greater than this time constant, the voltage across the ter- minals of the condenser C07 reaches the threshold of a junct- ion transistor T 5 triggering this transistor (see Figure 8),

and producing a positive pulse across the terminals of a resistance R1 0

In contrast to this, Figures 9 to 12, which corres-

3- 254/72 pond respectively to Figures 5 to 8, show that if the flame

is burning, the damping is very small, the detected signal

is alsi' weak, and consequently, the rectangular signal is

also weak and its duration t2 becomes less than t Thus, the threshold of the unijunction transistor T is not reached, 5

and consequently, no pulse appears across the resistance R.1 0 As represented in the part A of Figure 13, the

frequency of the series of sparks, which is the supply fre-

quency, is attained by means of a diac whose response thres-

hold is reached when the two condensers C and C3 which are 2

connected in series, are charged up to a voltage correspond-

ing to this threshold. The condenser C4 is charged by a rectifying diode D and discharged by a thyristor T 2

The switching at the detection frequency is

obtained by short circuiting the condenser C which is very 3

small compared to the condenser C2 by means of an NPN trans- istor T 3 This transistor T 3 is saturated when the condenser

C1 reaches the saturation threshold. This period is deter-

mined by the time constant R 2 C1 It is necessary to pro-

vide a voltage divider consisting of resistances R and R 3 2

such that the voltage across the condenser C is prevented 1

from reaching excessively high values.

The inductive train of sparks used for igniting

heavy fuel products is obtained, as shown in Figure 14, by

deriving a comparatively heavy current with the help of a

resistance R 0 This current is passed, by means of a power

transistor T in the primary coil N 1 of the ignition trans-

former. As shown in Figure 15, the synchronisation signal of this transistor is derived by means of a zener diode Zn

and applied to the monostable integrated circuit CM. This

6 427452

39.254/7 circuit acts through a transistor Ta to cause the power

transistor Tp to become conductive at the beginning of a

half sinewave of the supply voltage (this is the pilot sig-

nal shown in Figure 16) and to block the power transistor

T when the inductive current reaches its maximum (see

Figure 17). The integrated circuit CM is locked.when the condenser C 2 is charged and the transistor T is saturated. In this device, the detection sparks can be obtained at low frequency over the circuit R2 C1 diac. The pulses used for attempted re-ignition can also be

obtained at the terminals of the circuit N C 3 0 There will now be described the means used for detection, for safety locking and for integrating the pulses

produced in the absence of a flame, shown in part B of Figure

13).

During the ignition phase, which can be long or

short in duration, the detection circuit naturally indicates

absence of flame. It is therefore necessary to lock the safety system during the ignition period. This is obtained by short circuiting the condenser C7 by means of a PNP trans- istor T4, as soon as the voltage is applied and by causing the

transistor T 4 to be unblocked progressively before the end of

the ignition period. To obtain this,.a diode D2 is interposed in series with the base of the transistor T the anode- cathode path of the diode D 2 being conductive as soon as volt-

age is applied. As soon as the potential across the conden- ser C 6 is sufficient to block the diode D 2 this blocks the transistor T4, allowing the condenser C 7 to charge up.

In order to close the fuel supply member, (for

example an electrovalve EV) and to cause the repeated

39f 25 4/72 attempts to re-ignite, in the case where the flame goes out either for a short period or finally, the Pulses indicating absence of flame, derived from the terminals of the resist- ance R0are integrated through the diode D 5 in a condenser 016. When this condenser 016 is charged, the transistor T1 saturates opening a relay RL (in part D of Figure 10) having two contacts C EL and C RL. This intdrrupts the control 2 sparks and the condenser C 1 6 discharges slowly, until the transistor T15 becomes blocked again, whereupon the relay RL closes, producing a fresh sequence of sparks at the ignition electrode Al. If the fuel fails to Ignite or is absent, and assuming that the detection system'(part B of figure 13) is unlocked, pulses appear over the terminals of the resistance R1o* These pulses, integrated again in the condenser C1 6 reopen the relay RL. The process Is repeated until a flame appears.

To allow the system to be put into a state of safety at the end of a certain number of attempts to reignite, the number of attempts is counted by the circuit represented In the part E of figure 13. This circuit will now be described.

The counting circuit comprises a non-locked detect- ion circuit consisting of a transistor T 6and a uni-junction transistor T1 7 During the ignition phase the circuit pro- duces a sequence of pulses across a resistance R3 3 The pulses are integrated in a condenser 019, by way of a diode D3 A transistor T 8.coninected in series with a resistance R beccmes saturated, charging the condenser C2 Each time the relay RL closes and a fresh sequence of sparks occurs, the condenser 020 charges up step by step until the

39 25472 threshold of the uni-junction transistor T 1 9 is reached. As soon as this threshold is reached, the transis- tor T 1 9 is triggered. A pulse appears on the terminals of the resistance R 3 The thyristor T 1 6 3 is triggered, short circuiting the relay RL and so ensuring that it remains open. Consequently, the device can be returned to zero only by an intervention by the operator. If the fuel ignites at the first attempt, or at the second attempt, the control spark occurs in an ionised medium, and consequently no pulse appears across the resistances R and R and the process of reignition does not take place.

There will now be described the safety device which comes into operation when an electrode short circuits, or when there is an ignition failure (part C of figure 1j). The detection signal derived from the terminals of the cir- cuit N 3 C 0 is also applied, over a condensor C9 to a fur- ther detection circuit which is essentially similar to the circuit shown in part B of figure 13, except that the means for locking during the ignition phase are omitted. In this circuit, the time constant is chosen such that, in the case of an ionised medium, the uni-junction transistor T 8 delivers a pulse to the terminals of a resistance R3 7 As soon as the relay RL is closed, the cordensor C1 1 charges up over the resistance R 9 A transistor T99 1 connected in parallel, partially discharges this condensor at each detection spark. When anelectrode short circuits, the damping is very weak and consequently no pulse occurs across the resistance R The partial discharges of the 3 7

condenser C 1 1 are therefore interrupted and this allows the response threshold of the uni-junction transistor T1 0 to be

-9

39 254/ 7 reached. Consequently, this transistor conducts and a posit-

ive pulse appears across the resistance R2 3 This pulse

triggers the thyristor T1 3 finally closing the shut-off

member. In this circuit, there can, if desired, be provided

a positive-temperature-coefficient resistance CTP, connected

in parallel with the resistance R1 6 and selected on the basis

of the highest temperature to be detected. When the temper-

ature rises abnormally the value of the positive-temperature-

coefficient resistance increases and consequently, the value

of the combination of this resistance with R 1 6 This in-

creases the time constant and consequently, no further pulses

appear across resistance R3 7 This brings the system into a

condition corresponding to a short circuited electrode, or to

an ignition failure.

The temperature control which will now be described

(see part D of figure 13) is carried out by means of a relax-

ation oscillator comprising a uni-junction transistor T 11

The high frequency pulses appearing across a resistance R27 are integrated in the condensor C1 5 by way of a diode D5

When the voltage on this condensor reaches the threshold of

a zener diode Z 2 the transistor T 1 2 becomes saturated, clos-

ing the main relay RL and opening the electrovalve EV for the fuel. When the temperature rises, after being previously set

by means of a potentiometer R 3 5 a negative temperature co- efficient resistance CTN decreases in value until the satur-

ation current of the transistor T1 1 is attained, whereupon the

circuit ceases, to oscillate and consequently, the transistor

T12 becomes blocked, opening the relay RL.

Figure 13 shows a pressure controlled contact Pr'

which interrupts the general feed to the system as soon as

10

39 254/ the gas pressure drops. lWhen the pressure returns to normal, the entire system is restarted automatically by the pressure controlled contacts.

The low tension required by the system is provided

by a circuit (the part F of Figure 13) which has no input transformer, the isolation from the supply circuit being pro- vided by the ignition transformer. The circuit has a condensor C12 connected in series with a diode bridge D 4 The filtering is done by a condensor C1 A resistance R 1 3 3 limits the short circuit current when the system is in the safety condition.

427 45 2

Claims (1)

1. 39 254/72 The claims defining the invention are as follows:-

   1. An ignition and safety device for burners consuming

   hydrocarbon fuels or other combustible fluid fuels, comprising:

   a single electrode for igniting the fuel when placed to form

   a spark gap between itself and the burner; pulse generating means operable to deliver to the electrode a succession of

   spark-producing pulse trains, the pulse trains.being separ-

   ated from one another by intervals which are longer than the

   duration of each train; detector means for detecting the presence or absence of a flame when a spark occurs across

   the said gap; and means whereby when ignition takes place the pulse-generating means changes over automatically to

   cause the production of sparks at a frequency substantially

   lower than the frequency of the pulses in the said trains.

   2. A device in accordance with claim 1, in which the

   frequency of the pulses in the said pulse trains is that .of

   .the mains supply, and in which the frequency of the sparks

   produced once ignition has taken place is not greater than 1 Hz.

   3. A device in accordance with claim 1 or 2, in .which the detector means includes means responsive to the extinction

   of the flame to cause a number of attempts to re-ignite the

   flame by sparks at the said lower frequency and, if this is

   unsuccessful, to cause the application to the:electrode of a

   number of the said high-frequency pulse trains.

   4. A device in accordance with any.one of the preceding

   -12 427452

   39 254/72 claims, in which the detector means includes means responsive

   to a predetermined number of the said pulse trains without

   ignition taking place to close and block a fuel valve in such

   a manner that manual intervention is required to re-open it.

   A device in accordance with any one of the preceding

   claims for igniting light hydrocarbons, in which the ignition

   means are of the capacitive discharge type.

   6. A device in accordance with any one of the preceding

   claims for igniting heavy hydrocarbons, in which the ignition

   means are of the inductive type.

   7. A device in accordance ,with any one of the preceding

   claims, including a pressure-controlled device responsive to a failure of the fuel supply to cut the electrical supply to the circuit and operative to re-establish the electrical supply automatically when the fuel supply is re-established.

   8. A device in accordance with any one of the preceding

   claims, in which the detector means includes means responsive

   to a short-circuit condition between the electrode and the burner to close a fuel valve in such a manner that manual

   intervention is required to re-open it.

   9. A device in accordance with any one of the preceding

   claims, in which the detector means includes means responsive to an abnormally high burner temperature to close a fuel

   valve in such a manner that manual intervention is required to open it.

   13 427452

   30 254/7 A device in accordance with any one of the preceding claims, for controlling temperature in heat exchanges, in which the temperature control is maintained by successive ignition and extinction periods, the device including a relaxation oscillator controlled by variations in the resist- ance of a thermistor.

   11. A device in accordance with any one of the preceding

   claims, in which the pulses of the said higher frequency are interrupted to form pulse trains by a relay operated by pulses produced in the absence of a flame.

   12. A device in accordance with any one of the preceding claims, in which the individual sparks in the said trains of sparks are generated at the frequency of the alternating current supply by means of a circuit including a threshold diode connected in series with the gate of a discharge thyristor.

   13. A device in accordance with any one of the preceding

   claims, in which the pulses for producing the sparks at the said lower frequency are obtained by means of two series- connected capacitors connected in a charging circuit and a transistor arranged to short-circuit one of the said capacit- ors when it is driven into conduction by the periodic charging of a further capacitor.

   14 A device in accordance with claim 6, including a power transistor connected to interrupt the current flowing through the primary coil of an ignition transformer and

   14- 427452

   39 254/72 thereby to produce an inductive spark having a duration of at

   least 1,500 microseconds, the power transistor being biased

   by a monostable circuit.

   A device in accordance with claim 4, including, to

   count the attempts to re-ignite the fuel, an integrating

   capacitor which is charged, one step at a time, with each

   train of pulses, and a transistor operative when this

   capacitor is charged to its threshold level to energise a

   relay which closes a fuel valve.

   16. A device in accordance with claim 7 or 8, including

   a circuit having a capacitor which is charged over a resistor,

   the capacitor being partially discharged with each low-

   frequency detection spark when the burner is functioning

   normally, and a transistor responsive to the charge on the

   said capacitor and operable when the detection spark fails

   to take place or when there is a.short circuit in the elect-

   rode circuit, to close the said fuel valve.

   17. A device in accordance with claim 10, including,

   for detecting an abnormal increase in the burner temperature,

   a resistance having a positive temperature coefficient, in a

   resistance-capacitance circuit such that in the case of an

   abnormal increase in temperature, the voltage across a capac-

   itance fails to reach the threshold of a transistor to the

   control electrode of which the capacitor is connected.

   18. A device in accordance with claim 1, substantially

   as herein described with reference to Figure 13 of the

   15 427452 accoripanYing drawin g3. 30T 254/72

   19. A device in 'accordance with claim 1, substantialiy as herein described with reference to Figure 13, as modified by Figure 14j,.of the accomPanying drawin~p.

   DATED this 17th day Of FEBRUARY, 1972

   ATELIERSDE LA MOTOBECANE,

   EDWD. WATERS SONS, Patent Attorneys, Russell Street, MLBOURNE,

   16 6 -427452

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