GB2082752A

GB2082752A - Burner Igniter/Detector

Info

Publication number: GB2082752A
Application number: GB8027176A
Authority: GB
Original assignee: British Gas Corp
Current assignee: British Gas Corp
Priority date: 1980-08-21
Filing date: 1980-08-21
Publication date: 1982-03-10
Also published as: GB2082752B

Abstract

A simple adaptor is disclosed which enables a burner control system employing rectification flame detection to be modified so that a single electrode (5) Fig. 1 (not shown) may be used for both flame ignition and detection. The adaptor comprises an encapsulated assembly 1 of two spark gaps S1, S2 and a resistor A which can readily be connected into the high tension lead 13 from the ignition transformer (T) to the single electrode. <IMAGE>

Description

SPECIFICATION Single Probe Ignition/Detection Systems and Devices Therefore This invention relates to fuel gas burners and in particular to flame ignition and detection systems for such burners.

In the industrial field, the ignition of gas burners is usually accomplished using sparks generated from a high voltage ignition transformer circuit arrangement. These sparks are applied in most cases between an electrode positioned close to the burner and the body of the burner itself, which is earthed.

To detect the presence of a flame, two methods are commonly used. One is to detect ultra-violet radiation from the flame, the other is to place an electrode in the flame and use the rectification property of a flame to generate a signal indicating that the flame is present.

Ultra-violet detection can be difficult to apply as the sensors of the system must not overheat and, in an application with more than one burner, the detector may fail to discriminate between flames from adjacent burners. As the sensors age they become more sensitive until an indication of flame presence is received continuously; this means that regular checking must be carried out on the detector.

In contrast, rectification detection, as it uses an electrode probe in the flame to be detected, does not suffer such discrimination problems. The probe will stand much higher ambient temperatures than an ultra-violet cell and, as it erodes gradually in the flame, it will eventually fail safe. For these reasons, as well as the comparative cheapness of rectification against ultra-violet sensors, rectification probes are extensively used, especially on small burners.

On some types of burner, however, where the advantages of rectification detection would make it the best choice, it is difficult to fit the necessary two electrodes for flame ignition and detection because of space restrictions, or it may be that fitting the electrodes is not economically desirable. Systems are known, for example, a single probe ignition/detection system, in which a single electrode is used for both flame ignition and detection. They use relays in the circuit arrangements to switch from flame ignition to flame detection and are designed for use with their own special control system rather than normal control units. Greater complexity in the control system, however, adding to manufacturing costs, is not always acceptable.

It is an object of the present invention to enable a single electrode at a burner head to be used for both spark ignition and flame detection in a simple and inexpensive manner.

Accordingly, the invention provides a device for converting existing burner control units to single probe burner ignition and flame detection operation, which device consists of at least two spark-gaps and a resistor connected so as to form part of an ignition/detection electric circuit arrangement of said control unit, the said components being encapsulated in a block of electrically insulating material with external terminal connections to and from said part circuit arrangement, whereby in use to provide a simple adaptor for electrical connection into the remaining part of said ignition/detection circuit arrangement.

A single probe ignition/detection control system must have the capability both to detect a rectified direct current across the flame of a few microamperes with a voltage of about 250 volts a.c. applied to the probe, and to prevent damage to the flame detection amplifier of the control system during the ignition period when the ignition spark is generated by an applied voltage of several kilovolts. The function of the invention is to introduce the requisite protection of the flame detection amplifier into the control circuitry normally used with two electrodes, enabling one electrode to be dispensed with.

The invention will now be described, by way of example, with reference to the accompanying drawing in which: Figure 1, is a simple circuit diagram of a single probe ignition/detection system and, Figure 2 is a perspective view of the adaptor for use in the system.

Referring to the drawing, the encapsulated components of the adaptor 1 are spark gaps S1 and S2 and resistor R. Typically, the breakdown voltage of the spark gaps is 600 volts. When the voltage applied to a spark gap is less than 600 volts it does not conduct but when a greater voltage is applied and spark is formed then it does conduct. The voltage drop across a gan when it is conducting is about 80 volts and it continues to conduct until the applied voltage falls below this value. A typical value of the resistance of resistor R is 2.2 megohms.

In use of the invention, while the burner 2 is lit and a flame 3 is being detected, an a.c. voltage of about 250 volts is applied at the flame detector terminal 4. This voltage is not sufficient to break down the spark gaps and the only difference from normal operation of the control system is the presence of resistor R in series with the probe 5.

As the value of 2.2 megohms of the resistor R is much less than the flame impedance, this has a negligible effect on the flame signal and flame detection is not impaired. When the ignition transformer T is energised, the voltage is high enough to break down the spark gap S1 and so the spark appears at the burner as usual. The ignition transformer voltage, less the 80 volts or so across spark gap S1, is also applied across resistor R and spark gap S2. This causes spark gap S2 to break down as weil and so the voltage which appears at the flame detector terminal 4 never rises above 600 volts and for the major part of each mains half-cycle is only 80 volts.The resistance R limits the current into the flame amplifier (not shown) and ensures that most of the transformer current passes in the spark between the electrode probe 5 and burner 2 to maintain satisfactory ignition. Because both the voltage and the current applied to the flame detector during ignition are limited, no damage results to the flame amplifier circuitry.

Referring in particular to Figure 2, the spark gaps S1 and S2, resistor R and suitable connectors 7, 8 and leads 9, 10 are connected on printed circuit boards 11, 12, which assembly is encapsulated in a solid resin block 6, conveniently of cylindrical form some 50 mm in length and 30 mm in diameter. The connectors 7, 8 ensure that the adaptor 1 can be easily and safely inserted into the high tension cable 1 3 from the ignition transformer to the probe. The flying lead 9 is used to connect the device to a flame detector terminal on the control unit (not shown) and the lead 10 is connected to an earthing point on the burner body 2.

Claims

1. A device for converting existing burner control units to single probe burner ignition and flame detection operation, which device consists of at least two spark-gaps and a resistor connected so as to form part of an ignition/detection electric circuit arrangement of said control unit, the said components being encapsulated in a block of electrically insulating material with external terminal connections to and from said part circuit arrangement, whereby in use to provide a simple adaptor for electrical connection into the remaining part of said ignition/detection circuit arrangement.

2. A device according to Claim 1 , wherein said components are carried by and electrically connected by one or more printed circuit boards.

3. A single probe burner ignition and flame detection electric circuit arrangement incorporating a device according to Claim 1 or Claim 2.

4. A circuit arrangement according to Claim 3 arranged to operate substantially as shown in and as hereinbefore described with reference to Figure 1 of the accompanying drawing.

5. A device, for use in a circuit arrangement according to Claim 4, substantially as shown in and as hereinbefore described with reference to Figure 2 of the accompanying drawing.