GB2107050A

GB2107050A - Monitoring oil and gas flames

Info

Publication number: GB2107050A
Application number: GB08226531A
Authority: GB
Inventors: Karl-Friedrich Haupenthal
Original assignee: Landis and Gyr AG; LGZ Landis and Gyr Zug AG
Current assignee: Siemens Building Technologies AG; Landis and Gyr AG
Priority date: 1981-09-30
Filing date: 1982-09-17
Publication date: 1983-04-20
Also published as: DE3142987C2; DE3142987A1; FR2513743A1

Abstract

An automatic firing device (1) for the selective combustion of oil or gaseous fuels includes a programmer (2) and a single flame monitoring means (4) which is controlled by two series-connected flame sensing means, namely a UV-cell (5) and a photoelectric resistor (6). The photoelectric resistor (6) is bridged by a switch (12) in a gas flame monitoring mode while the switch (12) remains open in an oil flame monitoring mode so that undesired shutdown does not occur due to the UV-cell (5) responding to the ignition spark. <IMAGE>

Description

SPECIFICATION Methods of and apparatus for monitoring oil and gas flames This invention relates to methods of and apparatus for monitoring oil and gas flames.

Previous methods of monitoring burners differ in regard to the nature of a flame sensing means and an associated flame monitoring means or an amplifier thereof. Photoelectric resistors in particular are used for oil flames which give off light, because photoelectric resistors are highly suitable by virtue of their spectra! sensitivity and can be readily manufactured. In contrast, the low level of brightness of gas flames is generally insufficient for them to be monitored by means of photoelectric resistors, and ionisation probes have instead been utilised for that purpose. In principle, ionisation probes can also be used for oil flames, but they are not suitable for long-term use because of the carbonisation which occurs in oil flames.

Sensors which are responsive to ultra-violet (UV) radiation, herein referred to as UV-cells, are equally suitable for both kinds of flame, but they suffer from the disadvantage that they also respond to the arc of an ignition spark and, in the case of oil burners which are operated with preignition, would result in undesired fault shut-down phenomena.

Due to the severe increase in the price of heating oil and the price relationship between oil and gas, which is changing rapidly nowadays, there is a requirement for burner constructions which are equally suitable for both fuels. The operator of an installation can then always use the fuel which is cheaper at the time, depending on the market situation in the energy field.

The above-described kinds of flame sensors, namely photoelectric resistors for oil flames and UV-cells for gas flames, hitherto also required two different flame monitoring means.

The present invention provides a method of monitoring oil and gas flames with an automatic firing device for the selective combustion of oil or gaseous fuels, wherein a UV-cell and a photoelectric resistor are connected as flame sensing means in series in a sensor circuit which controls a single flame monitoring means, and both such flame sensing means are jointly exposed to the same flame radiation, the photoelectric resistor being electrically bridged in a gas flame monitoring mode.

The present invention also provides apparatus for monitoring oil and gas flames in conjunction with an automatic firing device for the selective combustion of oil or gaseous fuels, the apparatus comprising a UV-cell and a photoelectric resistor connected as flame sensing means in series in a sensor circuit which controls a single flame monitoring means, both such flame sensing means being arranged to be jointly exposed to the same flame radiation, and means for electrically bridging the photoelectric resistor when a gas flame is to be monitored.

Instead of the two flame monitoring means which are required at the present time for the purpose of selectively monitoring an oil flame or a gas flame, an embodiment of the invention described hereinbelow uses a single flame monitoring means, which hitherto was suitable solely for UV-cells, for the purposes of distinguishing between UV-radiation and visible radiation, so that it can also be used for oil burners with a long pre-ignition period.

The invention will now be further described, by way of illustrative and non-limiting xample, with reference to the accompanying drawing, the sole figure of which is a circuit diagram of an automatic firing device in conjunction with apparatus for monitoring oil and gas flames.

In the drawing, reference numeral 1 denotes an automatic firing device which controls a burner (not shown) by way of elements which are also not shown, such as motors for an oil pump and/or an air blower, a fuel valve, and the like. The automatic firing device 1 includes a programmer 2 which supplies a sensor circuit 3. The sensor circuit 3 controls a single flame monitoring means 4. Two flame sensing means, namely a UV-cell 5 and a photoelectric resistor 6, are connected in series and are both jointly exposed to the same flame radiation, the photoelectric resistor 6 being electrically bridged or shunted in the gas flame monitoring mode. Also disposed in the sensor circuit 3 is a resistor 7 which is connected in series with the two flame sensing means 5 and 6, the voltage drop across the resistor 7 controlling an amplifier 8 in the flame monitoring means 4.

For that purpose, inputs 9 and 10 of the amplifier 8 are connected in parailel with the resistor 7.

When the amplifier 8 is controlled from the sensor circuit 3, a flame relay 11 of the flame monitoring means 4 is actuated and signals the presence of a flame to the programmer 2.

The photoelectric resistor 6 can be bridged by means of a switch 12. In the gas flame monitoring mode, the switch 1 2 is always closed, that is to say, the monitoring action is then effected exclusively by means of the UV-cell 5 and the photoelectric resistor 6 is no longer active.

In contrast, in the oil flame monitoring mode, the switch 12 is open and the UV-cell 5 and the photoelectric resistor 6 are then connected in series. As the UV-cell 5 responds to ignition sparks, the cell 5, without the photoelectric resistor 6, would signal a flame just from the presence of an ignition spark. Therefore, the photoelectric resistor must be active at least during the ignition period, for in that period the purpose of the photoelectric resistor is to prevent actuation of the flame relay 11 because of the UVcell 5 responding to the ignition spark.

In a simple embodiment, the switch 12 is in the form of a manual switch which is advantageously incorporated in the flame monitoring means 4 and which always remains in the open condition as long as oil is being burnt.

In present-day burners, when an oil flame is to be started, the ignition system is switched on right at the beginning of a pre-scavenging operation, while in the gas fired mode the ignition system is switched on only directly before or at the same time as the supply of gas is switched on.

Therefore, when there is a change in the firing mode of the burner, a change in programme is also required. This means that it may be desirable for the switch 12 to be moved into the programmer 2 and for the switch 12 to be disposed therein, from the design point of view, in such a way that it is necessarily actuated when changing over the pre-ignition times, which are different as between the gas-fired mode and the oil fired mode. The association of the switch 12 with the programmer 2, in that situation, is indicated in the drawing by a broken line 1 3.

In flame monitoring means with a test darkness system for continuous self-checking of the flame sensor, for example in accordance with Swiss Patent Specification No. 604 086, it is sufficient for only the UV-cell 5 to be operated in the test darkness mode. In the situation involving monitoring of a gas flame, the photoelectric resistor 6 is inoperative in any case, while in the oil flame monitoring mode, the required degree of inherent safety or fail safety is also provided, in respect of the photoelectric resistor 6, by virtue of the series connection of the flame sensing means 5 and 6. If the photoelectric resistor 6 suffers a short-circuit in operation, then the flame is still monitored by the UV-cell 5 which is operated with test darkening.In that case, before the burner is next set in operation, the short circuit results in a fault shut-down procedure because of extraneous light during the pre-ignition period, because the UV-cell 5 detects the ignition spark as light. A defect in the photoelectric resistor 6 in a sense tending to cause a circuit interruption produces fault tripping after the safety period has expired, more particularly both when setting the burner in operation and also during operation of the burner.

In the case of the UV-cell 5 which is operated with a test darkening mode, in accordance with the foregoing description, it is also advantageous to have a circuit which allows the photoelectric resistor 6 to become active only during the ignition period, with the photoelectric resistor being bridged for the remainder of the time. That can be effected by means of a relay contact or by a switch contact of the programmer 2.

If on the other hand the flame monitoring operation is effected without the above mentioned test darkening, then the photoelectric resistor 6 may never be bridged in the oil-fired mode. As long as the photoelectric resistor 6 is operating correctly, a UV-cell 5 which has a tendency to uncontrolled actiation response is not detected, but that is of no significance in the present arrangement because the photoelectric resistor 6 takes over the flame detection operation. If, however, a photoelectric resistor 6 which for example becomes of low resistance signals extraneous light in a pre-ventilation period while the UV-cell 5 is responding in any case as a result of the ignition spark, then a fault shut-down procedure is carried out. Interruptions are detected in any case.

The technique described above permits the flame monitoring system to be switched over to one fuel or the other by means of a single switch contact, thereby substantially simplifying the wiring of the installation and enhancing operational reliability.

For a type of burner which can be operated both with oil and with gas, with the same preignition and pre-ventilation times, actuation of the switch 1 2 advantageously includes switching over the entire automatic firing device to one kind of fuel or the other.

Claims

1. A method of monitoring oil and gas flames with an automatic firing device for the selective combustion of oil or gaseous fuels, wherein a UVcell and a photoelectric resistor are connected as flame sensing means in series in a sensor circuit which controls a single flame monitoring means, and both such flame sensing means are jointly exposed to the same flame radiation, the photoelectric resistor being electrically bridged in a gas flame monitoring mode.

2. A method according to claim 1, wherein the photoelectric resistor is bridged by means of a switch.

3. A method according to claim 2, wherein the switch is installed in a programmer of the automatic firing device and is there positively actuated by switching over of the pre-ignition periods, which are different as between a gas-fired mode and an oil-fired mode.

4. A method according to claim 1, claim 2 or claim 3, wherein, when in an oil flame monitoring mode, the photoelectric resistor is active at least during ignition.

5. A method according to any one of claims 1 to 4, wherein the flame monitoring means has a test darkening mode for continuous self-checking of the flame sensing means, only the UV-cell being operated during test darkening.

6. A method according to claim 1, substantially as herein described with reference to the accompanying drawing.

7. Apparatus for monitoring oil and gas flames in conjunction with an automatic firing device for the selective combustion of oil or gaseous fuels, the apparatus comprising a UV-cell and a photoelectric resistor connected as flame sensing means in series in a sensor circuit which controls a single flame monitoring means, both such flame sensing means being arranged to be jointly exposed to the same flame radiation, and means for electrically bridging the photoelectric resistor when a gas flame is to be monitored.

8. Apparatus according to claim 7, wherein the means for electrically bridging the photoelectric resistor comprises a switch.

9. Apparatus according to claim 8, wherein the switch is installed in a programmer of the automatic firing device and is there arranged to be positively actuated by switching over of the preignition periods which are different as between a gas-fired mode and an oil-fired mode.

1 0. Apparatus according to claim 7, claim 8 or claim 9, so arranged that, when in an oil flame monitoring mode, the photoelectric resistor is active at least during ignition.

11. A method according to any one of claims 7 to 10, wherein the flame monitoring means has a test darkening mode for continuous self-checking of the flame sensing means, the arrangement being such that only the UV-cell is operated during test darkening.

12. Apparatus for monitoring oil and gas flames in conjunction with an automatic firing device for the selective combustion of oil or gaseous fuels, the apparatus being substantially as herein described with reference to the accompanying drawing.