GB2132749A

GB2132749A - Ionisation sensor for controlling air demand in gas heated apparatus

Info

Publication number: GB2132749A
Application number: GB08333446A
Authority: GB
Inventors: Richard Hinrich Stoffels
Original assignee: FABER BV
Current assignee: FABER BV
Priority date: 1982-12-24
Filing date: 1983-12-15
Publication date: 1984-07-11
Also published as: ES8406699A1; GB8333446D0; BE898450A; FR2538506A1; ES528331A0; FR2538506B1; GB2132749B; CA1230816A; NL8204991A

Abstract

An auxiliary device for a gas central heating installation having a main flame 2 comprises a sensor 13 placed in the pilot flame 3. The sensor 13 delivers a signal to an electronic speed control 17 which controls a motor 27 driving a fan 28 so as to automatically control the throughflow rate of the air and/or flue gases in dependence on a measurement effected by the sensor 13. The measurement effected by the sensor 13 is based upon the determination of the ionisation level within the pilot flame 3 which is dependent on the CO% in the flue gases. <IMAGE>

Description

SPECIFICATION Electronic ionisation sensor for automatically controlling the air demand in gas heated apparatus The invention relates to an auxiliary device to be used in a heating installation heated by gas burners particularly with modulating main-flame, having a sensor placed in the pilot flame which sensor delivers a signal to an element that is to be operated so as to influence the supply of one of the components of the air-gas mixture to the gas burners.

Such a device is known from French Patent 2,169,451. Said device is particularly destined for central heating installations.

In gas heating apparatus for the heating of rooms, all is done to obtain the best possible efficiency. Too much air means that th'e excess air with the combustion gases evades through the chimney and carries heat away, outside, which is therefore excluded from heating the room. Too little air causes part of the gas to be not burned completely and the latent heat enclosed therein to not show to full advantage, and this is also a loss. The air supply must therefore be adjusted properly with respect to the gas quantity supplied. It should be kept in mind that when a stove. is well heated, a thermal draught in the outlet canal of the flue gases is developed, enhancing the discharge of the flue gases.Therefore in the reversed case, when the stove is still cold and so this thermal draught failing, it will have to be more helped artificially at a certain flame level than when the stove at the same flame level is heated (soaked) well.

The exigence of heating with favourable efficiency gives relatively little problems in heating installations with not-modulating flame; that are installations which only know two adjustments: "on" and "out", in comparison with installations in which the flame is quite certainly modulated. In apparatus with modulating flame, on the contrary, the problems to arrive at the purpose strived at are clearly greater. Whereas at high level of the gas flame adjustment on a reasonable efficiency is still possible (because there is a reasonable "average" of the modulating flame), this relatively favourable adjustment is lost completely especially at a lower level of the modulating gas flame. That means therefore that if one passes from high flame to low flame, the air supply must be reduced.

Accordingly the air supply should be adapted to the air demand of the gas flame, which demand varies with the size of the flame. In order to be able to ascertain whether the combustion occurs with reasonable efficiency, it is investigated whether a complete combustion has occurred. This is the case, when the combustible components of the gas mixture cannot be oxydised further; so when in the combustion gasses (flue gasses) evading through the chimney exclusively CO2, SO2, etc. occur and not some reduced form of it. This ideal situation is in practice unobtainable and hence, there will virtually always be a certain percentage of the flue gases being incompletly burned. Since, among all the gas mixture components of flue gases being incompletely burned, the CO is represented strongest, the CO percentage is determining for the degree of complete combustion.However, it would of course be possible to direct oneself to any other combustion gas. When there is henceforth question of CO, this flue gas is not meant specifically but every flue gas in general. Although a zero percentage is normally unachievable, for environmental-hygienic considerations, on the other hand a limit is imposed to the CO percentage being at maximum permissible. In practice it is strived at not exceeding this maximum permissible percentage. So, when the air supply is set such that the CO percentage in the flue gases remains just within the permissible maximum, the adjustment is "optimal". The maintenance, under all circumstances, of the optimal adjustment, requires a continuous controlling of the air demand. For, at high flame more air is needed than at low flame.

The supply of fresh air and the exhaust of flue gases to the exterior occurs through tubes communicating through an opening in a wall (or roof) with the external atmosphere. To this end use is made of an apparatus located within said tubes for promoting the throughflow-rate of the air and/or flue gases, such as a (small) pump or fan. Apart from other factors the speed is decisive for the quantity of intake-air, said fan being placed either on the input side for fresh air or on the output side for the combustion gases, or on both sides.

The proper quantity of air, necessary for complete combustion (optimal adjustment) is, among other things, dependent on the flame height (level) in the main-burner. Therefore the speed of the fan has to be controlled both at low and at high flame.

Controlling of the speed of the fan can be done by hand, that means more or less by feel or by using a normal, commercially available CO gauge which only measures the CO percentage after which the consumer controls the air supply and/or the discharge of the flue gases by means of the data so obtained. Such a controlling activity by hand in order to follow constantly the modulating flame in any variation of its magnitude, is in practice not feasible. Simply and more comfortably this activity ought to be done automatically. Then other factors can also be taken into account, which factors affect the optimal fan adjustment. These are-next to the modulation of the flame: the flame height; the resistance of the tubes through which the gas flows; the length of the tubes; the cross-area of the tube; atmospheric factors (storm and the like).The resistance as meant here, is great, because for esthetical considerations one wishes the tubes running through the space to be heated, as small as possible. For such controlling of a combustion with high efficiency being complicated by the various above mentioned factors, hand-control is impracticable and one is forced to have recourse to sensors which are able to indicate which quantity of air is desired at a certain flame level.

So far sensors have been applied in gas heated installations to measure the pressure of the gas and/of CO percentage; the latter to comply with the environmental safety regulations valid therefor. The controlling of the airdemand in order to come to a heating efficiency as high as possible, is quite a new terrain. The existing sensors being destined for measuring the pressure of the CO-percentage in the combustion gasses, are not suitable for the present (controlling) purpose and besides rather expensive, while their life is restricted. CO-measurement sensors have in general a relatively short life and are mostly equipped to stop heating installations being provided therewith.

The operation of the CO-sensor applied in the device according to the invention is, on the contrary, based on measuring the ionisation grade of the flame: the higher the flame, the higher the flame's temperature and the stronger the gas particles are ionised, and the more complete the combustion is, so that a low CO percentage in the combustion gases may be expected. The sensor proper consists of one or more electrodes, disposed in the heart (centre) of the flame. The ionisation is measured between the metal burner housing and the electrode. Hence, the sensor based on measuring the ionisation grade measures the CO percentage indirectly. The ionisation is dependent on: 1" the size of the flame; 2 the combustion grade of the air-gas mixture, so dependent on the degree in which the gas is burned completely.Is the combustion complete, then the CO percentage is minimum and the ionisation is maximum. Is the combustion incomplete, then the CO percentage is high and the ionisation is low.

On account of the above statements it will be clear that a modulating flame does not give a reliable indication of the ionisation grade. Hence, one is to have recourse to the pilot flame which has a constant flame size.

Also a measurement flame, under which is to be understood any other flame of constant shape, can be used. For convenience sake in the following there will be used the word "pilot flame" even though in a more general way a measurement flame is meant. The determination of the ionisation grade is brought back to a determination of the combustion grade.

Accordingly the invention is characterized in that the signal created by the sensor is conveyed to an apparatus, e.g. a fan, by means of which the through-flow-rate of the air and/or flue gases through the installation is automatically controlled, and that the measurement effected by the sensor is based upon an indirect determination of the CO percentage in the flue or combustion gasses.

This measure is favourable both for gas-stoves with modulating flame and for stoves with a central heating built in in which in the first case the advantages obtained with the invention are slightly more striking.

An additional advantage of the present system is, that the fan speed associated with a certain adjusted temperature is automatically adapted to kinds of gas with different caloric combustion values so that under those circumstances yet again an optimal efficiency has to be sought for.

The only disadvantage of the pilot flame is that a sensor when placed in the pilot flame, indicates an increase in CO2 often later than when it had been placed in the main flame, a phenomenon that demonstrates itself still stronger when the main-flame modulates at small flame level. This means that a sensor, placed in the pilot flame, shows a rather long response time. To this time the response time of the air supply should be added, which is defined by the time period which lapses between the moment that a command is given to a fan and the moment that thereby an accelerated or retarded air flow to the burners is initiated. For the purpose of permitting the fan(s) to operate in the ideal situation, they should be adjusted automatically on the correct speed for each flame level of the mainburner and this with a response time as short as possible.If one would be guided in these situations, however, by the slow response of the pilot flame, it could happen that the CO percentage in the flue gases of the main flame has already exceeded the maximum permissible limit, whereas this is not (yet) indicated by the sensor in the pilot flame. That is the reason whey care should be taken of "anticipating" or "advancing" the moment that the CO percentage in the combustion gases of the pilot flame attains the permissibility limit or is just not exceeding same; for, in that case there is still "nothing wrong" with the flue gases originating from the main flame and the CO percentage in those flue gases will remain a good deal below the permissibility limit, so that there will be no conflict with environmental regulations. One operates then absolutely on the safe side. Hence the fact that both the response of the sensor and that of the air supply initiated by the sensor actuated fan is accompanied by an inherent retardation (delay) time, renders it necessary to advance the pilot flame's reaction and this is achieved by the said "critical" adjustment of the pilot flame.

In connection herewith it is advantageous that the circumstances in and about the pilot flame are chosen such that the setting of the moment in which the CO percentage might become critical for the environmental burden, is automatically advanced.

For obtaining a heating efficiency as high as possible, it is sufficient to monitor the CO percentage in the flue gases of the pilot flame, that means to adjust this percentage critically and to maintain same under altering circumstances. The sensor with which the CO percentage in the flue gasses of the pilot flame is measured indirectly, is placed into the flame and creates a signal that is guided to the fan to regulate with this signal the speed of the fan via an electronic control means (regulator).

However, such a pilot flame is still becoming affected by the close vicinity of the modulating main-flame so that also the pilot flame is slightly modulated along. A solution for this problem is found in that the pilot flame is guarded from the main-flame by means of a metal gauze or foraminated metal, causing the pilot flame to be of a constant size. The ignition of the main burner by the pilot flame is established via one or more openings on the lower side of the screen. This is based on the same principle applied in the Davy miner's lamp. Starting point is here that a stable flame should be used which is not affected by the flame level of the near-by modulating main-burner.

This earlier premature warning relative to the CO percentage in the flue gases can be obtained according to the invention in that the air supply to the pilot flame, is throttled. This throttling is to be effected such that the CO percentage in the exhaust gases of the pilot flame is critical. This can be done in various manners. Since the air supply occurs mainly ( + 70%) via injection, the ratio of the components in the air-gas mixture to the pilot flame can be altered. Accordingly the device according to the invention is characterised by means which reduce or block the air-injection apertures in the gas supply pipe to the pilot flame.

In another embodiment the device is characterised by means which make part of the exhaust gases of the pilot flame to be returned to the gas supply pipe with air-injection apertures.

In a practical embodiment the device according to the invention is characterised by a bent pipe, having one end disposed above the pilot flame and its other end connected to one or more of the air-injection apertures in the gas supply pipe of the pilot flame. Due to the injection-action at said other end, part of the mixture of combustion gases above the pilot flame is sucked off via the free extremity of the bent pipe, so that the pilot flame is fed by an air-depleted air-gas mixture.

From said French patent is known a sensor with switch-circuit, which sensor, placed into the pilot flame, serves to ascertain whether the pilot flame is ON or OUT. Is the pilot flame ON, then a signal is created in the sensor, that after amplification, activates an electromagnetic circuit so that a valve in the flow of gas is opened. If the flame is OUT, or is going OUT, the signal created in the sensor stops immediately and the circuit is de-energized, so that the gas valve closes.This is a high degree of protection so that if the flow of gas fails by any external cause, whereby the pilot flame is shut off, the gas after restoring the flow of gas, cannot automatically flow out into the space where the boiler and burners are arranged, but that the valve automatically closes and remains closed, so that one is forced to push the valve open by hand to restore the pilot flame, after which the valve remains open again.

Such a protection is mostly obtained by a thermo-couple, a rather slow instrument. A similar protection is, of course, also present in the underlying installation but does not constitute itself part of the invention.

The adjustment of the desired temperature occurs in a conventional manner, viz. with a thermostat regulator via the gas block, by which the modulating flame obtains a certain magnitude (amplitude): the flame level. To this flame level the air demand of the heating installation is adapted by introducing more or less air and/or by acceleration or delaying the exhaust of the flue or combustion gases. This possible demand is ascertained by the sensor in the pilot flame which "informs" the electronic circuit thereabout, which on its turn controls the fan into the proper speed.

The invention will further be explained with reference to the figures of the enclosed drawing, in which Figure 1 shows schematically an elevational view of the device according to the invention; Figure 2 shows a plan view; and Figure 3 a side-elevational view thereof.

In Figs. 1-3 is shown a burner-installation of a gas heating device with modulating flame for heating a room or the like space. Herein can be observed a main-burner 1 with mainflame 2 and further a pilot flame 3. These flames are fed by an air-gas mixture. The gas for these flames is fed via pipes 4 and 6 resp., and their end connected to a gas burner 7 and 8 resp., and their other end to a gas control block 10. Connected to same is a gas supply tube 11, together with a modulating room-thermostat 1 2. The pipe 4 leads to the main gas burner 7 and the pipe 6 to the burner 8 of the pilot flame.

The device thus far described is conventional in the art of gas heating devices. To achieve the objective strived at by the invention, the conventional device has been altered in a number of respects in order to be able to measure the ionisation grade of the pilot flame 3, being indicative for the degree in which the gas fed to the pilot flame is being burned. In the pilot flame 3 is mounted a sensor 1 3 in the form of a hair pin shaped electrode (Fig. 3), although this form is not essential. By means of a wire 1 4 thermally insulated with refractory beads and line 16, the electrode 1 3 is connected to an electronic speed control means 1 7 being connected via a line 1 8 to the gas control block 1 0.

The pilot flame 3 should have a constant flame but due to the presence of the modulating main flame 2, the level of the pilot flame is affected. To avoid that the pilot "co"- modulates, a metal gauze or guard 24 is put between the main flame 2 and the pilot flame 3. The guard is also apt to mount onto it, via ceramic grommets 25, the sensor or electrode 1 3. This can, of course, be mounted on any other place.

The task of the sensor electrode 1 3 put in the pilot flame 3, is to allow for an electronic speed control. To this end the means 1 7 for electronic supply control via line 26, is connected to a motor 27 for driving a fan 28 mounted in a discharged pipe 29 for waste gases which can evade to the exterior through a wall 31.

The supply of air for the main-flame and pilot flame occurs via an air pipe 32, which starts in the wall 31 and terminates in the combustion room of the stove. The pilot flame 3 obtains its air from the immediate surroundings by injection, for which purpose openings 33 have been provided in the gas pipe 6.

The advancement of the critical CO-indication occurs here by means of a bent pipe 34 having its one end 36 placed above the pilot flame 3 and having its other end 37 placed above one or more openings 33 in the gas pipe 6. Due to injection-action in the openings 33 part of the flue gases of the pilot flame 3 is collected and next guided into the gas pipe 6. In this way the gas mixture contains less air. So doing the air gas mixture fed to the pilot flame is artificially altered and thus the ionisation grade in the pilot flame too. This modification of the ionisation grade produces a signal that through the component parts 1 3-1 7 controls the speed of the fan.

If thus by means of the room thermostat 1 2 another temperature is adjusted, then another ratio of the components of the air-gas mixture is associated therewith.

Claims

1. Auxiliary device to be used in a heating installation heated by gas burners, particularly with modulating main-flame, having a sensor placed in the pilot flame which sensor delivers a signal to an element that is to be operated so as to influence the supply of one of the components of the air-gas mixture to the gas burners, characterized in that the signal created by the sensor is conveyed to an apparatus, e.g. a fan, by means of which the through-flow-rate of the air and/or flue gases through the installation is automatically controlled, and that the measurement effected by the sensor is based upon an indirect determination of the CO percentage in the flue or combustion gases.

2. Auxiliary device according to claim 1, characterized in that the circumstances in and about the pilot flame are chosen such that the setting of the moment in which the CO percentage might become critical for the environmental burden is automatically advanced.

3. A device according to claim 1 or 2, characterized in that the air supply in the pilot flame is throttled.

4. A device according to any of claims 1-3, characterized by means which reduce or block the air-injection apertures in the gas supply pipe to the pilot flame.

5. A device according to any of claims 1-3, characterized by means which make part of the exhaust gases of the pilot flame return to the gas supply pipe with air-injection apertures.

6. A device according to any of claims 1-5, characterized by a bent pipe, having one end disposed above the pilot flame and its other end connected to one or more of the airinjection apertures in the gas supply pipe of the pilot flame.

7. A device according to any of the preceding claims, characterized in that the pilot flame is guarded from the main-flame by means of a metal gauze or foraminated metal, causing the pilot flame to be of a constant size.

8. An auxiliary device to be used in a heating installation heated by gas burners, the device being substantially as hereinbefore described with reference to the accompanying drawing.

9. A gas heating installation incorporating an auxiliary device according to any preceding claim.