GB2201276A

GB2201276A - Burner combustion method and system

Info

Publication number: GB2201276A
Application number: GB08703605A
Authority: GB
Inventors: Andrew John Martin
Original assignee: British Gas PLC
Current assignee: British Gas PLC
Priority date: 1987-02-17
Filing date: 1987-02-17
Publication date: 1988-08-24
Also published as: GB8703605D0

Abstract

In the system a pre-mix burner 2 is utilised, and carbon monoxide levels in a flue 6 downstream thereof are continuously monitored by an SnO2 resistance monitor means 7 having a built-in heater 8, the heater being continuously run, and the output of the sensor means being continuously sensed, by a sensor means monitor 9, which is itself controlled by an overall system monitor 10 which sets gas and air admission rates and spark control in order to fire the burner. If the level of carbon monoxide exceeds a predetermined value, the supply of fuel to the burner is terminated. <IMAGE>

Description

Combustion Method and System The invention relates to a combustion method and system, particularly for operating in a burner that is of a kind in which fuel, usually gas, and air are mixed in the required proportions for operation of the , burner prior to being fed thereto and ignited.

In such systems, if the ratio of gas to air deviates from a preset safe level or for other operating reasons, a dangerous build up of carbon monoxide gas can occur, or the burner can be extinguished with nevertheless continuous supply of fuel and air which can form an explosive mixture which can lead to damage and injury.

It is an object of this invention to seek to mitigate these disadvantages.

According to one aspect of the invention there is provided a method of controlling a burner, comprising the steps of providing a burner, providing a sensor means adapted to respond to pre-determined operating parameters of the burner, and continuously operating and monitoring output of the sensor means, whereby to control operation of the burner.

The step of providing a sensor means adapt-ed.-to respond to pre-determined operating parameters of the burner may comprise providing a carbon monoxide sensordownstream of the burner.

pr:. < r W The sensor means may include a heating means.

The method may include the step of providing a monitor means adapted to control the burner supply and to monitor electrical output from the sensor means.

The control and monitor means may control -fuel and air flow to the burner and/or ignition means thereof.

The control and monitor means may be adapted to terminate the supply of fuel to the burner if the level of carbon monoxide exceeds a predetermined value.

According to a second aspect of the invention, there is provided a system for providing combustion, comprising a burner, a sensor means associated with the burner and adapted to respond to pre-determined operating parameters of the burner, and means continuously to operate and monitor the sensor means whereby to control operation of the burner.

The sensor means may comprise a carbon monoxide sensor means down-stream of the burner.

The sensor means may comprise a heater.

The sensor means may comprise a SnO2 semi-conductor material sensing means.

The system may include a monitor means adapted to control the burner supply and to monitor electrical output from the sensor means.

The monitor means may be connected with fuel and air supply to the burner and/or to ignition means thereof.

The monitor means may be adapted to terminate the supply of fuel to the burner if the level of carbon monoxide exceeds a predetermined value.

A method and system embodying the invention are diagrammatically illustrated, by way of example, with reference to the accompanying drawings.

Figure 1 shows schematically a pre-mix burner system, Figure 2 shows a block diagram of an operating sequence of a monitor and control means of the system of Figure 1.

Referring to the drawings, there is shown a burner system 1 (Figure 1) comprising a pre-mix burner 2 mounted in a combustion chamber 3 and arranged so that hot combustion gases pass over a heat exchanger 4 to heat fluid passing therethrough (as shown by arrows 5). Products of combustion pass from the combustion chamber 3 through a flue 6 in which is provided sensor means 7 for sensing carbon monoxide (CO) levels in the system 4. The sensor means 6 includes an integral heater 8 and is of SnO2 (stannic dioxide) semi-conductor material which provides a variable resistance dependent on the amount of carbon monoxide present.

The resistance and hence electrical output of the sensor means 7 is continuously monitored by a monitor 9 which also maintains the heater 8 in operation and is electrically connected with an overall system monitor means 10.

The burner 2 has ignition means in the form of a spark electrode 11, and a flame sense electrode 12 which are respectively connected with and controlled by the monitor means 10 via a spark generator 13 and flame sense circuitry 14.

Pre-mixed gas and air are fed to the burner by a fan 15 to which gas is fed through a gas supply pipe 16 including a multi-rate gas valve 17 controlled by the monitoring means 10 via control means 18 the fan 15 also being controlled by the monitor means 10 by its own speed control 19, the controls being operated according to pre-set gas and air firing rates 20.

As mentioned previously, the sensor monitor means 9 operates the sensor means 7 continuously by supplying a continuous voltage to the heater 8, for example a 5-volt supply.

The supply is maintained even when the rest of the system is off, which ensures long-term stability of the sensor means 7. Moreover, the sensor monitor 9 senses the sensor resistance and informs the system monitor means 10 if the sensor means 7 is running outside pre-set limits, whether those limits are at start-up or during running.

The spark generator 13 is to supply rapid series of sparks for ignition of gas.

The flame sense circuitry 14 is to detect presence of flame by flame rectification method.

The gas valve control 18 comprises electronics to control the gas valve.

The fan speed control 19 comprises electronics to control speed of fan (and therefore air rate), e.g. by phase angle control. Feed-back of fan speed (e.g. optically) may be necessary in some systems to ensure stable air rate.

Pre-set air and gas firing rates 20 comprise stored air/gas rates at which the system can operate. The rate selected is indicated by external parameters.

In operation of the system, the air and gas rates 20 are set independently according to stored data. These pre-set rates could range from a simple dual rate system (hi-lo) to a fully modulating system capable of firing at many rates. Also, since the air and gas rates are set independently, the aeration of the flame can be varied with firing rate, which is essential for some types of burner 2. The aeration (i.e. excess air level) of the flame is set such that under normal operation, CO production is within acceptable limits (i.e.

CO/CO2 < 0.02). If however a fault condition arises (e.g. gas valve jammed) that produces unacceptable levels of CO, the CO sensor means detects this and safely shuts the system off. It will be understood that any condition leading to unacceptable levels of CO will shut the system off.

The response of the sensor means is such that when it is heated to a suitable temperature (by the built-in heater 8), the resistance of the SnO2 varies with the concentration of CO present.

Therefore by measuring the resistance of the sensor means 7 the CO concentration can be determined. Thus the CO level at which the system should be shut down will correspond to a minimum allowable resistance of the sensor means 7 and this is detected electronically by the sensor monitor circuitry. An upper limit is also set, so that if the sensor means 7 fails and goes open-circuit this will also shut the system down. This is in addition to an initial test on the sensor means"7 before the system can be started.

The system monitor unit 10 operates as shown in Figure 2, to which reference is now made A. Before ignition the CO level in the flue 6 is zero, this will correspond to a certain resistance of the sensor means 7 within certain limits. This is checked by the sensor monitor 9, if the resistance is not within limits, it is assumed that the sensor has failed and the system "locks-out".

B. The air and gas are turned on to pre-set ignition rates 20, these are rates at which it has been found (for a particular system) that ignition is rapid and smooth.

C. The spark generator 13 is turned on and a timer set going. If the gas does not ignite within a pre-set time (up to 20 secs. say) then the system will shut down.

D. Upon ignition the spark generator 13 is switched off, and the air/gas rates 20 freed from ignition rates, so that the firing rates can be determined by external parameters. Once lit the system 1 is continuously monitored for (a) flame extinguished or flame lift, and (b) CO production.

E. If the flame extinguishes, the system reverts back to stage B and tries to re-light the gas. If it is not done within the set time the system shuts down.

F. If the system detects unacceptable levels of CO (ie. the sensor means resistance drops below the pre-set minimum) it does not immediately shut down. Instead the system waits 60 seconds and re-samples; if CO levels are still unacceptable only then will the system shut down. This 60 second delay is specified by BS 5258 part 1 and is to allow for brief CO production whilst the system stabilises, e.g. after ignition.

-Under normal operation, the flame is stable and CO levels minimal.

The system keeps running until shut off by external parameters. A post-purge by the fan may be necessary to clear out any residual CO from the flue, ready for re-start.

-The shut down procedure after a fault condition is: (i) turn gas off; (ii) turn fan to maximum (to clear any dangerous products); (iii) system lockout- requiring manual reset to re-start system; (iv) sound alarm if necessary.

The system 1 can be applied to any appliance which uses a pre-mix burner ranging from domestic devices to industrial devices. it is simple enough to be built using analogue electronics and thus avoids the complexity and expense of other systems (e.g. closed loop) which require microprocessors.

Claims

1. A method of controlling a burner, comprising the steps of providing a burner, providing a sensor means adapted to respond to pre-determined operating parameters of the burner, and continuously operating and monitoring output of the sensor means, whereby to control operation of the burner.

2. A method according to Claim 1, in which the step of providing a sensor means adapted to respond to pre-determined operating parameters of the burner comprises providing a carbon monoxide sensor downstream of the burner.

3. A method according to Claim 2, in which the sensor means includes a heating means.

4. A method according to Claim 2 or Claim 3, including the step of providing a monitor means adapted to control the burner supply and to monitor electrical output from the sensor means.

5. A method according to Claim 4, the control and monitor means controlling fuel and air flow to the burner and/or ignition means thereof.

6. A method as claimed in Claim 5 wherein the monitor means is adapted to terminate the supply of fuel to the burner if the level of carbon monoxide exceeds a predetermined value.

7. A method according to Claim 1, substantially as hereinbefore described.

8. A system for providing combustion, comprising a burner, a sensor means associated with the burner and adapted to respond to pre-determined operating parameters of the burner, and means continuously to operate and monitor the sensor means whereby to control operation of the burner.

9. A system according to Claim 8, the sensor means comprising a carbon monoxide sensor means downstream of the burner.

10. A system according to Claim 9, the sensor means comprising a heater.

11. A system according to Claim 9 or Claim 10, the sensor means comprising a SnO2 semi-conductor meaterial sensing means.

12. A system according to Claim 11, including a monitor means adapted to control the burner supply and to monitor electrical output from the sensor means.

13. A system according to Claim 12, the monitor means being connected with fuel and air supply to the burner and/or to ignition means thereof.

14. A system as claimed in Claim 13 in which the monitor means is adapted to terminate the supply of fuel to the burner if the level of carbon monoxide exceeds a predetermined value.

15. A system for providing combustion, substantially as hereinbefore described with reference to the accompanying drawings.