GB2078398A

GB2078398A - Electric control circuit for a fuel-fired burner

Info

Publication number: GB2078398A
Application number: GB8118673A
Authority: GB
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 1980-06-18
Filing date: 1981-06-17
Publication date: 1982-01-06
Also published as: SE8103860L; DE3022635C2; DK259381A; DE3022635A1; FR2485165A1; CA1176730A; JPS5728921A; IT8167835A0; GB2078398B

Abstract

An electric control circuit for a fuel-fired burner comprises a preparatory control means (TG,T, one fuel release means (VI,VII), a switch (P1) actuable to render operative the fuel release means, a fuel ignition means (ZT), a flame monitoring means (FW,F) and a safety timer (SG,S). The safety timer is successively operable in two sequences during the course of a firing programme, the control means being operable during the first sequence so to condition the circuit that the fuel release means is rendered operative when the switch is actuated, actuation of the switch to render operative the fuel release means at the same time initiating the second sequence and the control means being operable to render inoperative the fuel release means later in that sequence if the flame monitoring means fails to respond. <IMAGE>

Description

SPECIFICATION Electric control circuit for a fuel-fired burner This invention relates to a control circuit for a firing installation comprising at least one fuel release element switched on by a switching-on element, ignition means, a flame monitor with a flame relay and an electronic safety timer which, in the course of the programme, successively operates in two sequences, during the first sequence at the end of the safety period actuates a preparatory apparatus with the aid of which the actuation of the fuel release element is prepared, commences its second sequence with switching on of the fuel release element and, with the aid of switching off apparatus, effects switching off of the fuel release element at the end of the safety period if the flame relay has failed to respond.

As a safety period, it is standard to take the longest permissible period during which the control circuit releases the supply of fuel without a flame being notified. It therefore commences with release of the fuel and must terminate with the signal for blocking the supply of fuel if no flame has been formed.

The safety periods are fixed. They decrease with an increase in the burner load and, particularly in the case of large burners, only amount to a few seconds, for example two to three seconds. in addition, there is the requirement that there must not be an excessive departure from these values even with an undervoltage of 15%, an overvoltage of 10% and a temperature change between 0 and 60"C. Such requirements cannot be met with thermal safety timers, for example those operating with bimetal.

A control circuit of the aforementioned kind is known (DE-AS 1 9 65 790), wherein an electronic safety timer is used in which a condenser is charged by way of a resistor and discharged by way of a unijunction transistor and an actuating relay when the threshold value of this transistor has been reached.

During a first sequence, a period is determined which is used as pre-ignition or preventilating period. The actuating relay thereupon transformingly delivers a switching pulse to an electronic switching element which switches on a burner relay. At the same time the armature of the actuating relay is brought to a preparatory position so that it may open a switching element at the input of the control circuit when the actuating relay receives a further pulse during the second sequence. The input of the timer is connected by way of a first diode to a point between the burner relay and the associated electronic switching element and by way of a second diode to a point between the flame relay and a transistor for switching this relay.Accordingly, the associated condenser is charged as long as the electronic switching element and/or the lastmentioned transistor are in the blocked condition.

In this way, one can check during the first sequence whether the electronic timer is in order. However, one cannot be certain that the fuel releasing element is actually switched off at the end of the safety period during the second sequence if, for example, the charging path leading through the second diode is interrupted or if the armature of the actuating relay fails to attract. In addition, the known circuit cannot be used for larger burners because in that case the safety periods are considerably longer than the corresonding preventilating periods.

The invention is based on the problem of providing a control circuit of the aforementioned kind in which it is ensured that the fuel releasing element is switched off if no flame occurs at the end of the saety period.

This problem is solved according to the invention in that, simultaneously with the fuel release element, the switching-on element applies a voltage to the input of the safety timer and that the preparatory apparatus serves as a switching-off apparatus during the second sequence.

In this circuit, it is not only the operability of the timer that is checked during the first sequence. Instead, one also ensures that the safety timer has voltage applied to it whenever the fuel releasing element is applied to a voltage. Should there be a fault in the supply to the safety timer, this would also prevent response of the fuel release element. By reason of the fact that one and the same device serves for preparation (preliminary ventilation or pre-ignition period) during the first sequence and for switching off during the second sequence, one ensures that at the end of the safety period there is actual switching off during the second sequence because the switching function during the first sequence was checked and there would have been no switching on of the fuel release element if there had been a fault.One thereby obtains a safety timer which is of extraordinary reliability.

Desirably, the safety timer comprises a relay which responds with a delay and remains energised as long as there is a voltage at the input. This not only defines the instant of switching on but also the switching-off point of the associated switching elements. This simplifies the construction of the combined preparatory and switching-off apparatus in comparison with an actuating relay which responds only during a momentary pulse.

There are numerous possibilities for constructing the circuit according to the invention. In a preferred example, it is ensured that the fuel release element can have voltage applied to it on the one hand by way of a first switching element which is in series with the switching-on element and which, by means of the preparatory and switching-off apparatus, is switched on at the end of each safety period during the first sequence and switched off during the second sequence and on the other hand by way of a second switching element which is in series with a first flame relay switching element in the operative position thereof and is switched on by the safety timer at the end of the safety period. The first switching element which has already been checked during the first sequence switches back during the second sequence basically at the end of the safety period.Whether the operation of the burner can be continued is tested with the aid of the second switching element which takes over the function of the first switching element if the first flame relay switching element has responded.

The first switching element may be a switch-over element by way of which the input of the safety timer can have voltage applied to it in the rest position thereof. This provides a simple possibility for introducing the first sequence of the safety timer independently of the switching-on element.

Further, it is favourable if the second switching element is a switch-over element by way of which the ignition means can have voltage applied to them in the rest position thereof. Since the flame should have existed at the end of the safety period during the second sequence, the ignition can be switched off by the second switching element.

In a further embodiment of the invention, it is ensured that the preparatory and switchingoff apparatus comprises a third switching element actuated by the safety timer at the end of the safety period and an auxiliary relay which is equipped with self-holding means, controls a switching element in the supply to the fuel releasing element, atracts during the first sequence in response to actuation of the third switching element and drops off during the second sequence. The use of the auxiliary relay provides a simple means of employing the same switching behaviour for the third switching element during the first sequence for switching on the first switching element and during the second sequence for switching off said switching element.

More particularly, the auxiliary relay may have voltage applied to it on the one hand by way of the series circuit of the operative position of the third switching element and the rest position of a second flame relay switching element and on the other hand by way of the series circuit of the rest position of the third switching element and the operative position of a self-holding switching element.

The auxiliary relay may be part of a preventilating timer. On the one hand, this provides a clear separation between the safety period and the preventilating period. On the other hand, it is possible for the auxiliary relay to be a constructional component which is necessary for other reasons, which simplifies the construction of the circuit.

In a preferred embodiment, the switchingon element is a first programme switch element of a programme timer which, after response of the auxiliary relay, can have voltage applied to it by way of the rest position of a third flame relay switching element. This programme timer ensures that a defined time interval exists between the end of the first sequence and the start of the second sequence during which the safety timer can reassume its initial condition.

Further, the programme timer may comprise a second programme switching element which is disposed at the input to the control circuit and opens after a predetermined delay period for the purpose of fault cut-off. If the third flame relay switching element fails to respond, the fault cut-off then occurs after the delay period.

Further the programme timer may comprise a third switching element which opens before the first programme switching element closes and is disposed in the supply to a second fuel feeding element. This second feeding element is therefore closed during ignition. However, if the voltage supply is removed from the programme timer by actuating the third flame relay switching element, the third programme switching element can subsequently operate the second fuel feeding element.

It is also favourable if the flame monitor can be switched to greater sensitivity prior to switching on of the fuel releasing element and the programme timer can during this time have voltage applied to it by way of an operative position of a fourth flame relay switching element. Should the flame monitor detect extraneous light, the programme timer is set into operation and there will be a fault cut-off after a certain time.

In a further embodiment of the invention, there is an operating relay which is energised .

depending on response of the auxiliary relay and comprises a self-holding switching element and an operating switching element in series with the switching-on element in the operative position thereof. With the aid of this operating relay, one ensures that the programme timer can be operated for the purpose of fault cut-off even before the instant at which the fuel release element is to be switched on, without the resultant operation of the switching-on element thereby giving rise to the release of fuel.

In particular, the operating relay may be connected to the self-holding input of the auxiliary relay by way of a delay element. The delay ensures that the associated operating switching element following the switching-on element has established communication to the fuel release element.

Further, the operating switching element may be a switch-over element preceding the first and fourth flame relay switching element in the rest position thereof. This simplifies the construction of the circuit.

It is also favourable if the self-holding switching element of the operating relay follows the operative position of the operating switching element and precedes the second and third flame relay. This duel function of the self-holding switching element likewise simplifies the circuit construction.

The same applies to the case where the second and third flame relay switching elements are combined to form a switching-over element or the first and fourth flame relay switching elements are combined to form a switching-over element.

The present invention also provides an electric control circuit for a fuel-fired burner, the circuit comprising a preparatory control means, at least one fuel release means, a switch actuable to render operative the fuel release means, a fuel ignition means, a flame monitoring means and a safety timer, wherein the safety timer is successively operable in two sequences during the course of a firing programme, the control means being operable during the first sequence so to condition the circuit that the fuel release means is rendered operative when the switch is actuated, actuation of the switch to render operative the fuel release means at the same time initiating the second sequence and the control means being operable to render inoperative the fuel release means later in that sequence if the flame monitoring means fails to respond.

An electric control circuit for a fuel-fired burner, the circuit being in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawing, wherein: Figure 1 is a circuit diagram of the control circuit, and Figure 2 is a timing diagram used for explaining the normal operation of the control circuit illustrated in Fig. 1.

Referring to the accompanying drawing, the circuit is energised by one phase of an alternating voltage applied between the terminals 1 and 2. The terminal 1 communicates with an input line 3 and the terminal 2 with a line 4 which is at earth potential. The following components are diagramatically illustrated next to each other in the lowermost row from left to right. A lamp 5 is lit up in the case of a fault. A relay BM serves to switch on a fan motor and has a self-holding contact bml. A preventilation timer TG comprises an auxiliary relay T which responds after a preventilating time (during which the combustion chamber of the burner is ventilated) of for example 30 seconds, a response input a and a holding input b and controls a self-holding switching element t1 as well as a first switching-over element t2 which is associated with the fuel release.An operating relay M comprises a response input cwhich is connected by way of a delay element 6 in the form of an NTC resistor to the holding input b of the prevent lation timer TG, and a holding input dand controls a switching-over element ml and a self-holding switching element m2. A programmer P operates with a thermal time element, closes a switching-on element p1 after a predetermined time, operates a switchingover element p2 in the input line 3 after a predetermined delay period and opens a third switching element P3 shortly before the switching element p1 is closed.A safety timer SG comprises a relay S which, with the aid of an electronic timer operates, after a safety period which may be short and amount to only, say, two seconds, a switching-over element s1 and a further switching-over element s2 which remain switched over for as long as the voltage is applied to the safety timer SG.

A flame monitor FW comprises a flame relay F to which a voltage can be fed by way of a line 7 and a signal for increasing the sensitivity by way of a line 8 and which, when a flame sensor 9 detects light, actuates a switchingover element f1 and a further switching-over element f2. This is followed by an ignition transformer ZT, a first fuel release element VI and a second fuel release element VII. Finally, the circuit comprises a temperature-dependently controlled switch KT, for example a boiler thermostat. A switch T is actuated in dependence on the flow of air from the fan.

During normal operation, the following sequence of functions will take place as described with partial reference to Fig. 2.

When heat energy is required and the switch KT therefore closes, voltage is applied to the fan motor relay BM by way of the switching elements p2, KT alld T. The selfholding switching element brn1 closes and the fan motor runs up to full speed.

As soon as the air flow from the fan motor is detected, the switching-over element T operates so that the voltage is now also applied to the line 10. This instant is designated ta in the diagram of Fig. 2. The safety timer SG thereby receives voltage by way of the switching-over element t2. At the end of the safety period, i.e. at the instant tb, the relay S switches the two switching-over elements s1 and s2 to their operative position.

The exciter input a of the preventilation timer TG thereby receives voltages by way of the switching elements ml, f1 and s2 so that, after a predetermined ventilating period, i.e.

at the instant tc, the auxiliary relay T attracts, the self-holding switching element t1 closes and the switching-over element t2 moves to the operative position. This disconnects the safety timer SG from the voltage. Its two switching elements s1 and s2 return to the rest position. The first sequence SI of the safety timer has been concluded. At the same time, switching over of t2 constitutes a preparatory step for effecting release of the fuel by energisation of the release element VI through the line 11.

From the instant tc, the operating relay M has also voltage applied to it by way of the delay element 6. This .therefore responds at the instant td, whereby the switching elements ml and m2 are actuated. The operating relay M is now held by the self-holding switching elementm2.

-rom the instant td, the programme timer P therefore receives voltage by :ay of the switching elements .1, m2 and f2. After a first period, namely at te, the swtching element P3 opens and a short time thereafter, namely at the instant t the switching-on element p1 closes. On the one hand, the result of this is that the safety timer SG has a voltage applied to it and a second sequence Sll is started.At the same time, voltage is applied to the fuel release element I by way of the switching elements ml, p1, t2 and the line 11 and to the ignition transformer by way of the switching element 1.

Consequently, the flame is ignited shortly thereafter, namely at the instant tg. The flame relay F responds and switches the two switching elements f1 and 2 to their operative position. As a resuit, the programme timer P is disconnected from the voltage so that, after some time, namely at the instant ti, the switching-on element p1 opens and the switching element p3 closes, whereupon the second fuel release element Vll is set in operation.

After the safety period started at the instant tf has expired for the second time, namely at the instant th, the relay S again switches the switching elements s1 and s2 to their operative position. As a result, the ignition transformer ZT is switched off and the fuel release element VI (and later the second release ele ment VII) receive voltage by way of the switching elements ml, m2, f2 and s2. As a result of the switching over of s2, the auxiliary relay T which had recently received voltage by way of the switching elements s1 and t1, is switched off so that the switching element t1 also returns to the rest position. This is insig nificant for the energisation of the fuel release elements because the line 11 is supplied with voltage over a different path.Simultaneously, the safety timer SG continues to receive volt age by way of the switching element t2, even if the switching-on element p1 opens.

The entire installation is now in operation and retains its conditions until the switching element KT opens.

Errors with regard to the safety period are avoided because all the elements necessary for the safety period during the second se quence Sll are checked in connection with the first sequence SI. This is because there is a preparatory and switching-off apparatus 1 2 which switches the auxiliary relay T and its switching element t2 on during the first sequence and off during the second sequence by simultaneous actuation of the switching element s2 during the first and second sequence. Failure during the second sequence is therefore inconceivable since not only the function of the electronic timer but also the function of the associated relay S with the functionally following sritching elements had been checked during the first sequence.

Should there be a fault (interruption, shortcircuit) in the supply to the safety timer that is responsible for the safety period during the second sequence, no fuel can be delivered and therefore no damage can be done.

If no flame is created during the safety period in the second sequence, the flame relay switching-over element f2 retains the illustrated position. At the end of the safety period, the switching element t2 moves to its illustrated rest position. This switches the fuel release element VI off and damage is avoided.

At the same time, however, the programme timer P continues to be heated by way of the switching elements ml, m2 and f2 so that, after a predetermined delay period, the switching element p2 is switched over whereby the entire installation is switched off and the signal lamp 5 is lit up.

Should the flame disappear during operation, the flame relay switching element f2 moves to the illustrated rest position whereby the fuel release elements are again switched off and voltage is supplied to the programme timer P until the actuating switching element p2 responds.

If extraneous light is detected before response of the operating relay M (because of phosphorescent walls, an open door, by reason of a defective flame sensor), the flame monitor FW responds because it receives a signal for increasing its sensitivity by way of the line 8. The flame relay switching-over element fl switches over so that the programme timer P receives voltage by way of the switching elements ml and f1 until the actuating switching element p2 responds.

Since the extraneous light is generally detected even before the realy S of the safety timer has responded, one also avoids switch ing on of the preventilation timer TG.

The illustrated circuit is designed for a gas burner firing installation with air blower and having a capacity of over 350 kW. In this case the safety period amounts to 2 secs. and the preventilation period is longer than 30 secs. Such control circuits can, however, also be used for oil burners and for those firing installations where the safety period is somewhat longer. Electromagneticaily controlled valves are primarily considered for use as the fuel release elements but it is also possible to employ an oil pump.

Claims

1. An electric control circuit for a fuel-fired burner, the circuit comprising a preparatory control means, at least one fuel release means, a switch actuable to render operative the fuel release means,a fuel ignition means, a flame monitoring means and safety timer, wherein the safety timer is successively operable in two sequences during the course of a firing programme, the control means being operable during the first sequence so to condition the circuit that the fuel release means is rendered operative when the switch is actuated, actuation of the switch to render operative the fuel release means at the same time initiating the second sequence and the control means being operable to render inoperative the fuel release means later in that sequence if the flame monitoring means fails to respond.

2. A control circuit as claimed in claim 1, wherein the safety timer comprises a relay which resonds with a delay and remains energised as long as a voltage is applied across it.

3. A control circuit as claimed in claim 1 or claim 2, wherein the fuel release means can have a voltage applied across it via a first switching element, which is in series with the said switch and which, by means of the control means is switched on at the end of each safety period during the first sequence and switched off during the second sequence, and via a second switching element which is in series with a first flame relay switching element in its operative position and which is switched on by the safety timer at the end of the safety period.

4. A control circuit as claimed in claim 3, wherein the first switching element is a switch-over element by which the safety timer can have a voltage applied it in its rest position.

5. A control circuit as claimed in claim 3 or claim 4, wherein the second switching element is a switch-over element by which the ignition means can have voltage applied across it in its rest position.

6. A control circuit as claimed in any one of claims 1 to 5, wherein the control means comprises a third switching element actuated by the safety timer at the end of the safety period and an auxiliary relay which is equipped with self-holding means, controls a switching element in the supply to the fuel releasing means, attracts during the first sequence in response to actuation of the third switching element and drops off during the second sequence.

7. A control circuit as claimed in claim 6, wherein the auxiliary relay can have voltage applied across it by the series circuit of the operative position of the third switching element and the rest position of a second flame relay switching element and by the series circuit of the rest position of the third switching element and the operative position of a self-holding switching element.

8. A control circuit as claimed in claim 6 or claim 7, wherein the auxiliary relay is part of a preventilating timer.

9. A control circuit as claimed in any one of claims 3 to 8, wherein the switching-on element is a first programme switch element of a programme timer which, after response of the auxiliary relay, can have voltage applied to it by way of the rest position of a third flame relay switching element.

10. A control circuit as claimed in claim 9, wherein the programme timer comprises a second programme switching element which is disposed at the input to the control circuit and opens after a predetermined delay period for the purpose of fault cut-off.

11. A control circuit as claimed in claim 9 or claim 10, wherein the programme timer comprises a third switching element which opens before the first programme switching element closes and is disposed in the supply to a second fuel releasing means.

1 2. A control circuit as claimed in any one of claims 9 to 11, wherein the flame monitor can be switched to greater sensitivity prior to switching on of the fuel releasing element and the programme timer can during this time have voltage applied across it by way of an operative position of a fault flame relay switching element.

1 3. A control circuit as claimed in any one of claims 3 to 12, wherein an operating relay is provided which is energisable depending on response of the auxiliary relay and comprises a self-holding switching element and an operating switching element in series with the switching-on element in its operative position.

1 4. A control circuit as claimed in claim 13, wherein the operating relay is connected to the self-holding input of the auxiliary relay via a delay element.

1 5. A control circuit as claimed in any one of claims 1 2 to 14, wherein the operating switching element is a switch over element preceding the first and fourth flame relay switching elements in their rest positions.

16. A control circuit as claimed in any one of claims 1 2 to 15, wherein the self-holding switching element of the operating relay follows the operative position of the operating switching element and precedes the second and third flame relay.

1 7. A control circuit as claimed in any one of claims 9 to 16, wherein the second and third flame relay switching elements are combined to form a switching-over element.

18. A control circuit as claimed in any one of claims 1 2 to 17, wherein the first and fourth flame relay switching elements are combined to form a switching-over element.

1 9. A control circuit for a firing installation comprising at least one fuel release element switched on by a switching-on element, ignition means, a flame monitor with a flame relay and an electronic safety timer which, in the course of the programme, successively operates in two sequences, during the first sequence at the end of the safety period actuates a preparatory apparatus with the aid of which the actuation of the fuel release element is prepared,commences its second sequence with switching on of the fuel release element and, with the aid of switching-off apparatus, effects switching off of the fuel release element at the end of the safety period if the flame relay has failed to respond, characterised in that, simultaneously with the fuel release element, the switching-on element applies a voltage to the input of the safety timer and that the preparatory apparatus serves as a switching-off apparatus during the second sequence.

20. An electric control circuit for a fuelfired burner, the circuit being substantially as hereinbefore described with reference to, and as illustrated by the accompanying drawing.