GB2387900A

GB2387900A - Flame failure device

Info

Publication number: GB2387900A
Application number: GB0208788A
Authority: GB
Inventors: Randolph Lofthouse
Original assignee: Diamond H Controls Ltd
Current assignee: Diamond H Controls Ltd
Priority date: 2002-04-17
Filing date: 2002-04-17
Publication date: 2003-10-29
Also published as: GB0208788D0

Abstract

A flame failure device comprises a detector circuit and a sensor 'S' which includes at least two conductors (3,4 fig 1), separated by a ceramics material (6 fig 1) having an open-circuit resistance at a no-flame temperature condition and a closed or short-circuit resistance at a temperature when a flame is present. The detector circuit may include the sensor and an electrical load 'L'. The sensor may be arranged in parallel or series configuration within an energising circuit, and comprise a switch within the detector circuit. The load may be in series with a normally open switch 'SWR' activated by a coil within the energising circuit. The sensor may be coupled to a control device or energy regulator utilising temperature-dependent deflection of a bimetal element (25 fig 4) used to actuate a switch mechanism to open and close electrical contacts for the supply of power to a load. The bimetal element is heated via a mounted heating element (31 fig 4) which provides heating dependent on flame conditions, and may prevent heating in the absence of a flame. The sensor may be in parallel with the heating element in order to prevent heating circuit energisation when the sensor has a closed circuit. The sensor may be coupled to the control device or energy regulator and series connected to the load, therefore the bimetal element and heating element are unnecessary. The detector circuit may allow the sensor to interface with remote sensing apparatus. The load may be an electric operated gas valve.

Description

FLAME FAILURE DEVICE

Field of the Invention

The present invention relates to a flame failure device for use in domestic and commercial apparatus in which the presence of a pilot flame or full flame is a pre 5 requisite of operation. The invention relates particularly to the fail-safe detection of "flame absence" so that said detection may be acted upon in an appropriate manner. Background to the Invention

In domestic and commercial gas cooking appliances and heating boilers it is 10 essential to have an operational pilot light so that when a gas valve is opened, the inflowing gas will be ignited. Alternatively, when the gas valve is opened an ignition circuit is fired until a flame is detected. In either case, in the absence of a flame, it is necessary for the apparatus to "fail safe", that is, the apparatus must fail in such a way as to default or revert to its safest operational mode or to shut 1 5 down.

There are many known techniques for detecting the presence or absence of a high temperature source, generating a suitable signal and feeding that signal to a control device or control system.

When detecting the presence or absence of a flame, however, the high 20 temperatures and aggressive environment within a flame removes the most common (and therefore cheapest) high temperature detection schemes from consideration.

-2 The use of resistance wire in such an environment is not particularly suitable and the resultant sensor requires electronic conditioning of the relatively small resistance changes in the wire. The resistance changes must be amplified to produce a useful voltage change.

5 Similarly, a thermocouple requires expensive electronic circuitry to amplify small voltage changes to produce a useful output voltage change. Furthermore, thermocouple sensors are in themselves relatively costly sensors.

One alternative is the use of the diodic properties associated with a low cost probe which utilises the diode property of a flame to produce an electric signal.

10 Unfortunately, the electronic circuitry associated with the detector/sensor raises the overall cost.

Finally, hydraulic expansion of a fluid within a probe is often utilised as a medium cost means for producing a mechanical action. A relatively simple, medium cost mechanical system is then required to convert the fluid expansion 15 (which is small) to an electrical signal. Most often the electric signal is realised by a switch.

Each of the known systems have disadvantages associated with reliability and cost which are immediately apparent to those skilled in the art.

It is an object of the present invention to provide a flame failure device which is 20 robust, reliable and inexpensive.

Summary of the Invention

The present invention provides a flame failure device comprising a sensor and a detector circuit, the sensor comprising at least two terminals separated by a ceramics material, the ceramics material having a first resistance value at 25 temperatures consistent with the absence of a flame and a second resistance value in presence of a flame, whereby in absence of a flame the detector circuit is in open circuit and in presence of a flame the detector circuit is in closed circuit.

This arrangement provides a sensor which acts effectively as a switch within a detector circuit so that in absence of a flame, the high resistance of the sensor is electrically equivalent to an open circuit.

Preferably, the detector circuit comprises an electrical load and most preferably, 5 the detector circuit is the electrical load.

It will be appreciated by the skilled addressee that a flame failure device which results in an open circuit for any fault condition, including absence of flame, is "fail-safe". The present invention also provides a control device of the type including an 10 energising circuit, the energising circuit including a parallel connected flame failure sensor so that in absence of a flame the energising circuit is open and the control device cannot be energised and in the presence of a flame, the circuit is made allowing the control device to be energised.

Optionally, or in accordance with the particular use of the control device, the 15 flame failure sensor is connected in series.

Conveniently, the energising circuit includes a switch activating coil which when energised closes a normally open switch. The switch is connected in series with the load which in turn is energised.

The above arrangement may be used in configuration with a standard energy 20 regulator of the type utilising temperature-dependent deflection of a bimetal element which is used to actuate a switch mechanism to open and close electrical contacts for the supply of power to a load. The bimetal element is heated via a heating element mounted to the active leg thereof. In one embodiment, a flame failure sensor is connected in series with the energising circuit for the heating 25 element so that in absence of a flame at the sensor site, the heater is inactive and deflection of the bimetal element is prevented. Optionally, the sensor is connected in parallel across the bimetal heating element so that when the sensor is in closed-circuit, power is shunted away from the heater preventing energisation

-4 of the heater circuit. In an alternative arrangement, the mechanical activation of the bimetal switch is obviated by the inclusion of a series connected flame failure sensor in the load circuit.

In an alternative arrangement the sensor is connected in series with the energising 5 coil of a relay, so that when the sensor is cool, that is, in the absence of flame, the relay normally-open contacts remain open and when the sensor is hot, that is, in intimate proximity or within a flame, the coil is energised and the relay contacts are closed.

Again, this resultant system or sub-system is inherently fail-safe, as in the absence 10 of a flame, the relay coil cannot be energised.

It will be seen by the skilled reader that a remote sensing apparatus may be implemented utilising the flame failure sensor with associated standard circuitry.

Additionally, a simple low component count circuit interfaces the flame failure sensor with most control devices.

15 The terms "glass" and "ceramics material" as used herein are directed to a vitrified compound having the requisite characteristic to effect the required parameter change between a first temperature range and at least a second temperature range. Examples include doped glass including borosilicate glass, overglaze and enamel, however, it is to be understood that no limitation is to be 20 implied by the exemplary materials referred to above.

Brief Description of the Drawings

The invention will now be described more particularly with reference to the accompanying drawings which show, by way of example only, several arrangements comprising two embodiments of flame failure device and a number 25 of associated applications in accordance with the invention. In the drawings: Figures la to to are elevations of three arrangements of a first embodiment of ceramic encapsulated sensor;

Figures 2a and 2b are elevations of two arrangements of a second embodiment of sensor comprising ceramic sensor probes; Figure 3 is a graph representing the resistance of the ceramic of the sensors and probes against temperature; 5 Figure 4 is a schematic representation of a flame failure sensor in circuit with an energy regulator; and Figures 5a to 5c are schematic representations of a flame failure sensor in circuit.

Detailed Description of the Drawings

Referring to the drawings and initially to Figures 1 a to lo, a flame failure sensor 1 10 comprises a pair of conductor terminals 3,4 which are separated by a gap 5 filled (and in this case encapsulated) by ceramics material 6. The conductors are coupled to a circuit via wires 7 or readout tape according to the desired application. In Figure 2a, a sensor is formed as a probe 10 and comprises a first centrally disposed conductor 13 and a second coaxial conductor 14 separated 15 therefrom by a gap 15, the conductor 14 being secured to a body of ceramics material 16 which substantially encapsulates the central conductor 13 to form the probe shape. The coaxial conductor 14 may be positioned at a predetermined distance along the length of the ceramics body to vary the temperature at which resistivity between the conductors begins to break down. In the probe 10 of 20 Figure 2b, the coaxial conductor 14 is formed as a collar which can be moved along the body of the probe.

Figure 3 is a graphical representation of the characteristic of a ceramics material such as glass, including doped glasses and enamel glaze, for example. At a temperature TOFF where many traditional probes and sensor arrangements suffer 25 reliability problems, the ceramic 6, 16 of the flame failure sensor 1, 10 changes state to allow current to flow against a lower resistance RON- The temperature range TOFF - TON over which the sensor changes from an open circuit ROFF to a low resistance RON or effectively to a closed circuit is relatively broad, for

-6- example, over 100C. The mid-point of this range P can be adjusted by sensor construction, that is, altering the proximity of the sensor conductors, and by appropriate material selection. It is envisaged that the mid-point transition temperature Tp can be easily adjusted between 300C and 600C.

5 The steep, non-linear characteristic curve of the sensor is advantageous for flame detection as the sensor resistance will not reside for long periods in the transition zone intermediate the effective open circuit ROFF and closed circuit RON conditions. Thus, the sensor acts as a switch, operating in a binary fashion, unlike most of the substantially linear systems acknowledged hereinabove. These 10 systems require an analogue threshold circuit with hysteresis to prevent cycling and to convert the signal from an analogue to a binary form.

The skilled addressee will appreciate that the temperature of a flame has no set or definitive value but will normally lie within the range of 500C to 1000C. Thus, by selecting a sensor which has a mid-point transition temperature of 400C, the 15 sensor will be ideally suited for flame detection. If the temperature in a "no flame" condition is the local ambient temperature TOFF, which should be no greater than 250C in a domestic oven or central heating boiler, then the resistance between the conductors will be high, resulting effectively in an open circuit ROFF.

Conversely, when a flame is present, the temperature of the sensor will rise above 20 450C at which temperature TON the resistance between the conductors has fallen away and is effectively a closed circuit RON.

On low flow rate or by-pass modes, a flame temperature in the region of 300C may be encountered. Thus, a mid-point transition temperature of 250C would be required. Where the transition interval (TON - TOFF) is no greater than 25 approximately 50C, a mid-point transition temperature of 275C may be chosen.

The major advantages provided by such a sensor are that it is inexpensive, robust, reliable, easily interfaced with existing control devices and systems and will fail-

safe.

Figure 4 is a schematic diagram of an energy regulator 20 comprising a base 23 in which the constituent parts of the regulator are housed. The main part of the regulator comprises a bimetal leg 25 to which a substrate thick film heater assembly 31 is secured by means of an eyelet 27 and a surrounding coil spring 29.

5 The further detail of the construction of the regulator may be found in the exemplifying disclosure of EP O 682 352 which is incorporated herein by

reference. In the control circuit of the regulator there is provided a basic single circuit connection through a heater assembly, secured to the active leg of a bimetallic 10 element, and a flame failure sensor. In this arrangement, a load which may comprise, for example, an electric gas valve is coupled to mains electrical supply through a switch actuated by the bimetallic element. A heating element is mounted to an active leg of the bimetallic element so that at a predetermined temperature of the heater the bimetallic element deflects to actuate the switch.

1 S With reference to Figures Sa to 5c, a number of possible circuit arrangements will be described beyond that of the simple arrangement whereby the load is connected in series with the flame failure switch. Figure Sa is a schematic circuit diagram of a flame failure sensors in series with the energising coil C of a relay R. The relay contacts form a switch SWR which may be connected to any one of a number of 20 devices or controllers and are normally associated with the actuation or energising of a load L. In the presence of a flame F. the normally-open (N.O.) sensor S short circuits energising the relay coil C. This in turn magnetically attracts the blade of the switch SWR to make the circuit powering the load L. Figure 5b is a schematic circuit diagram substantially similar to that illustrated in 25 Figure 5a, in this case however the sensor S is isolated from the mains voltage through a transformer T. One side of the transformer (or another series connected coil winding) acts as a relay coil to make the normally-open circuit powering the load L when a flame F causes the sensor S into the low resistance RON portion of its characteristic and facilitating the flow of induced current in the secondary 30 winding of the transformer T. When current flows, the necessary magnetic field

to attract the switch SWT blade is established.

-8 Finally, with reference to Figure 5c, a schematic circuit diagram of a circuit having a power supply for additional instrumentation and control is illustrated.

The mains voltage is stepped-down across a transformer T where it is rectified and regulated for the instrumentation and controls system feed. The sensor is 5 connected across the collector and base of a switching transistor ST so that when a flame F is present, the base current is increased, switching on the transistor and in turn providing base or gate current to a semiconductor power switch PT in the circuit containing the load L. In absence of a flame F. the sensor temperature drops below that corresponding to the high resistance ROFF part of the ceramic 10 sensor characteristic. Below this temperature TOFF, the sensor is in effect in an open circuit mode. This cuts off the switching transistor ST base current and the transistor turns OFF. Thus, the base or gate current to the power switch PT is cut off and the load is isolated from the mains supply.

The skilled reader will appreciate that the semiconductor power switch PT may be 15 a triac or any other suitable bi-directional device. In an alternative construction the power switch PT may be optionally isolated from the low voltage side of the circuit. In use, the flame failure device comprises a sensor which is positioned within the ordinary path of a flame. The sensor may then be used to detect the presence or 20 absence of a pilot flame, for example.

Selection of the correct ceramics material, for example, the doping concentration in a lithium glass or a boro-silicate glass will determine the suitability of a sensor for in flame use. It will be appreciated that certain applications of the invention will not be suited to in flame positioning of the sensor and that radiated heat or 25 ambient temperature within an enclosure may provide a better indication of correct operation of an appliance. Furthermore, certain sensor constructions are mountable on the surface of an enclosure and flame detection may be realised by measuring the temperature, for example, on the outer surface of an enclosure wall or an observation window.

The circuit arrangements shown herein are given by way of example and it will be appreciated that a sensor may be interfaced to a detection circuit, an energising circuit or a load in series or in parallel according to the intended use or switching function. Similarly, combinations of series and parallel connections may be used 5 to realise separate functions within an appliance. Logic circuit interfaces are facilitated by combinations of sensors to implement, for example, an OR or EXCLUSIVE-OR circuit.

It will of course be understood that the invention is not limited to the specific details described herein, which are given by way of example only, and that 10 various modifications and alterations are possible within the scope of the invention.

Claims

-10 CLAIMS:

1. A flame failure device comprising a sensor and a detector circuit, the sensor including at least two conductors separated by a ceramics material, the ceramics material having a first resistance value at temperatures consistent with the absence 5 of a flame and a second resistance value in presence of a flmne, whereby in absence of a flame the detector circuit is in the electrical equivalent of an open circuit and in presence of a flame the detector circuit is in the electrical equivalent of a short or closed circuit.

2. A flame failure device as claimed in claim 1, in which the sensor comprises a 10 switch within the detector circuit so that in absence of a flame, the resistance between the conductors of the sensor is electrically equivalent to an open circuit.

3. A flame failure device as claimed in claim 1 or claim 2, in which the detector circuit comprises an electrical load.

4. A flame failure device as claimed in any one of claims 1 to 3, in which the 15 sensor is operably coupled to a control device of the type including an energising circuit, the flame failure sensor being connected in series with the energising circuit so that, in absence of a flame, the energising circuit is in the electrical equivalent of an open circuit and the control device cannot be energised and, in the presence of a flame, the circuit is "made" or closed, allowing the control 20 device to be energised.

5. A flame failure device as claimed in any one of claims 1 to 3, in which the sensor is operably coupled to a control device of the type including an energising circuit, the flame failure sensor being connected in parallel with the energising circuit so that, in absence of a flame, the control device to be energised and, in the 25 presence of a flame, the energising circuit output is shunted through the sensor and the control device cannot be energised.

6. A flame failure device as claimed in claim 4 or claim 5, in which the energising circuit includes a switch activating coil which when energised closes a

-11 normally open switch, the switch being connected in series with a load which in turn is energised.

7. A flame failure device as claimed in any one of the preceding claims, in which the sensor is operably coupled to a control device or energy regulator of the type 5 utilising temperature-dependent deflection of a bimetal element which is used to actuate a switch mechanism to open and close electrical contacts for the supply of power to a load, the bimetal element being heated via a heating element mounted to the active leg thereof, whereby energising of the heating element is dependent on the presence or absence of a flame at the sensor.

10

8. A flame failure device as claimed in claim 7, in which the sensor is connected in series with the energising circuit for the heating element so that in absence of a flame at the sensor site, the heating element is inactive and deflection of the bimetal element is prevented.

9. A flame failure device as claimed in claim 7, in which the sensor is connected 15 in parallel across the bimetal heating element so that when the sensor is "made" or in closed circuit, power is shunted away from the heating element preventing energisation of the heater circuit.

10. A flame failure device as claumed in &Dy one of claims 1 to 6, in which the sensor is operably coupled to a control device or energy regulator so that the 20 sensor is connected in series to the load to be controlled when the control device or energy regulator is in uses thereby obviating the bimetal switch and the heated activation thereof

11. A flame failure device as claimed in any one of claims 1 to 6, in which the sensor is operably coupled to a control device or energy regulator so that the 25 sensor is connected in series with the energising coil of a relay, so that when the sensor is cool, that is, in the absence of flame, the normally-open contacts of the relay remain open and when the sensor is hot, that is, where the sensor is in intimate proximity or within a flame, the coil is energised and the relay contacts are closed.

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12 12. A flarne failure device as claimed in any one of the preceding claims, in which the detector circuit comprises an interface circuit facilitating the connection of the sensor with remote sensing apparatus.

13. A flame failure device as claimed in claim 12, in which the interface circuit 5 comprises a simple low component count circuit facilitating interfacing the flame failure sensor with most common control devices and/or instrumentation.

14. A flame failure device substantially as herein described, with reference to and as shown in the accompanying drawings.

15. A flame failure sensor substantially as herein described, with reference to and l O as shown in Figures 1 a to 2b of the accompanying drawings.

16. A control device or energy regulator substantially as herein described, with reference to and as shown in Figure 3 of the accompanying drawings.

17. An interface circuit or an energising circuit of a control device or energy regulator substantially as herein described, with reference to and as shown in 15 Figures 5a to 5c of the accompanying drawings.