Patent Description:
From the state of the art an air-gas mixture burning appliance with an air-gas mixing unit, a burning unit, and a flame detector is known. In this air-gas mixture burning appliance, hydrogen may be used as gas and mixed with air to form a combustible air-gas mixture.

More specifically, such an air-gas mixture burning appliance usually performs "just in time" mixing of air and gas directly before the burning unit to minimise a respective volume of combustible air-gas mixture available in the event of a flashback. Such a "just in time" mixing may be performed by means of discrete multiple mixers provided at the air-gas mixing unit. Thus, a predefined point of mixing for the air and gas may be located near an associated combustion space directly downstream of the burning unit.

If a flashback occurs, it is therefore possible that a flame may stabilise at the predefined point of mixing close to a burner surface of the burning unit. Furthermore, if the air-gas mixing unit comprises discrete multiple mixers, a flame may stabilise in one or more of the discrete multiple mixers upon occurrence of a flashback, while one or more other discrete multiple mixers may continue to provide combustible air-gas mixture to the burning unit, thus, supporting a residual flame on the burner surface.

In order to prevent flame stabilisation at the predefined point of mixing, flame detection for sensing presence of a flame adjacent to the burner surface may be performed. Therefore, a respective sensing element of the flame detector of the air-gas mixture burning appliance is usually positioned near the burner surface in order to acquire signals from the combustion space directly downstream of the burning unit. The flame detector determines whether a flame is well established at the burner surface with reference to a predetermined flame detection threshold.

The predetermined flame detection threshold is generally fixed and independent of any time-variant functional variables. More specifically, in order to ensure reliable flame detection of a flame at the burner surface of the burning unit and to avoid false "no flame" indications, the predetermined flame detection threshold that is applied to a sensed flame signal must be significantly smaller than a maximum value of a detectable flame signal given off by a normal flame stabilised against the burner surface. However, in order to avoid overheating of the air-gas mixing unit and unwanted noise in case of a flashback, the predetermined flame detection threshold must be significantly larger than a maximum value of a detectable flame signal given off by either one or more flames incorrectly stabilised at the predefined point of mixing, or a small residual flame on the burner surface of the burning unit of the air-gas mixture burning appliance, which is supported by the remaining unignited discrete multiple mixers.

In general, for an air-gas mixture burning appliance, a respective total volume of gas available to burn and an underlying flame length determine the maximum value of a detectable flame signal. The respective total volume of gas available to burn and the underlying flame length are both dictated by heat input and air-gas ratio.

Document <CIT> describes a gas appliance that includes a burner, a gas valve, an igniter, a thermocouple, and a control device. The control device is adapted to execute a control method comprising the following steps: controlling the igniter to ignite and controlling the gas valve to open; receiving a detected voltage output from the thermocouple; controlling the igniter to stop igniting and keeping the gas valve open when the detected voltage reaches a first voltage value; receiving the detected voltage output from the thermocouple continuously, and controlling the gas valve to close the gas pipe when the detected voltage above a second voltage falls below the second voltage. The second voltage value is higher than the first voltage value. As such, an ignition procedure may be speeded up and the supply of the fuel gas may be interrupted more quickly when the flame is extinguished.

The present invention relates to a method of operating an air-gas mixture burning appliance that comprises an air-gas mixing unit for mixing of air and gas, in particular hydrogen, to form a combustible air-gas mixture, a burning unit with a burner surface that is arranged downstream of the air-gas mixing unit for burning the combustible air-gas mixture at the burner surface, and a flame detector for sensing presence of a flame at the burner surface on the basis of a predetermined flame detection threshold. The method comprises adjusting the predetermined flame detection threshold for at least two different operating points of the air-gas mixture burning appliance, wherein the predetermined flame detection threshold is configured to avoid indication of a stable flame upon occurrence of a flame in the air-gas mixing unit under a sustained flashback event and/or in the event of a flame stabilising undesirably upstream of the burner surface.

Advantageously, the inventive method allows an improved flame detection at the burner surface of the burning unit of the air-gas mixture burning appliance by means of an adjustment of the predetermined flame detection threshold for at least two different operating points of the air-gas mixture burning appliance. More specifically, by adjusting the predetermined flame detection threshold for at least two different operating points, e.g. a detectable flame signal from a normal flame on the burner surface at a low firing rate of the air-gas mixing unit may reliably be recognised and distinguished from a detectable flame signal resulting from a sustained flashback at a high firing rate of the air-gas mixing unit. In this case, a conventionally predetermined flame detection threshold which is fixed and independent of any time-variant functional variables would either fail to detect the normal flame at the low firing rate, or fail to detect the sustained flashback at the high firing rate.

Moreover, overheating of the air-gas mixing unit and unwanted noise in case of a flashback may securely be avoided.

Preferably, adjusting the predetermined flame detection threshold comprises assigning a predetermined discrete threshold value to each one of the at least two different operating points.

Accordingly, the predetermined flame detection threshold may reliably be adjusted between the predetermined discrete threshold values. Advantageously, the predetermined discrete threshold values may be selected to enable reliably detection of a normal flame stabilised against the burner surface whilst failing to allow the detection of a flame, in particular at the air-gas mixing unit, under a sustained flashback event.

According to one aspect, adjusting the predetermined flame detection threshold comprises determining at least one of an operating state of the burning unit or at least one operating parameter of the air-gas mixture burning appliance, wherein at least one of the at least two different operating points corresponds to the at least one of an operating state of the burning unit or at least one operating parameter of the air-gas mixture burning appliance.

Thus, the predetermined flame detection threshold may advantageously be adjusted on the basis of a suitable time-variant operating state/parameter.

Preferably, a first operating state of the burning unit corresponds to an ignition phase of the burning unit, wherein a second operating state of the burning unit corresponds to a normal operating range of the burning unit.

Accordingly, a distinction between the ignition phase and the normal operating range of the burning unit may be made for a reliable and secure flame detection.

Preferably, adjusting the predetermined flame detection threshold further comprises associating a first predetermined flame detection threshold with the first operating state of the burning unit, and associating a second predetermined flame detection threshold with the second operating state of the burning unit, wherein the first predetermined flame detection threshold is smaller than the second predetermined flame detection threshold.

Thus, flame detection during the ignition phase and the normal operating range may easily be improved and adapted during operation of the burning unit.

According to one aspect, a first operating parameter of the air-gas mixture burning appliance corresponds to a first firing rate or a first range of firing rates of the air-gas mixing unit, or a first fan speed or a first range of fan speeds of a fan of the air-gas mixing unit, and a second operating parameter of the air-gas mixture burning appliance corresponds to a second firing rate or a second range of firing rates of the air-gas mixing unit, or a second fan speed or a second range of fan speeds of the fan.

Accordingly, a distinction between different firing rates or ranges of firing rates of the air-gas mixing unit, or between different fan speeds or ranges of fan speeds of the fan of the air-gas mixing unit may be made for a reliable and secure flame detection.

Preferably, adjusting the predetermined flame detection threshold further comprises associating a first predetermined flame detection threshold with the first operating parameter of the air-gas mixture burning appliance, and associating a second predetermined flame detection threshold with the second operating parameter of the air-gas mixture burning appliance, wherein the first predetermined flame detection threshold is smaller than the second predetermined flame detection threshold.

Thus, flame detection may be made dependent on varying operating parameters and easily be improved and adapted during operation of the burning unit.

According to one aspect, the at least one operating parameter corresponds to at least one of a firing rate, a range of firing rates, a fan speed, a range of fan speeds, an air-gas ratio or a range of air-gas ratios.

Thus, various different operating parameters may be used for adjustment of the predetermined flame detection threshold.

Preferably, the predetermined flame detection threshold is associated with, and adjusted based on, the at least one operating parameter.

Accordingly, the predetermined flame detection threshold is not restricted to simply switching between two discrete flame detection thresholds, but will switch continuously depending on one of: the firing rate, range of firing rates, fan speed, range of fan speeds, air-gas ratio or range of air-gas ratios.

Exemplary embodiments of the present invention are described in detail hereinafter with reference to the attached drawings. In these attached drawings, identical or identically functioning components and elements are labelled with identical reference signs and they are generally only described once in the following description.

<FIG> shows an exemplary air-gas mixture burning appliance <NUM> with an air-gas mixing unit <NUM>, a burning unit <NUM>, and a flame detector <NUM>. By way of example, the air-gas mixture burning appliance <NUM> may be used in a boiler or, more generally, in a building heating system. Preferably, the gas used is hydrogen such that the air-gas mixture burning appliance <NUM> forms an air-hydrogen mixture burning appliance.

The air-gas mixing unit <NUM> is preferably adapted for mixing of air and gas to form a combustible air-gas mixture <NUM>. Preferentially, the combustible air-gas mixture <NUM> is a homogenous mixture of the air and the gas.

By way of example, the air-gas mixing unit <NUM> includes an air supply <NUM> and a gas supply <NUM>. Illustratively, the air supply <NUM> includes a fan <NUM> that may be operated with an adaptable fan speed and/or within predetermined ranges of fan speeds to draw air into the air-gas mixing unit <NUM>.

The air supply <NUM> and the gas supply <NUM> may be interconnected via a predetermined number of mixers <NUM> which form a corresponding predetermined number of discrete points of mixing <NUM>. Preferably, the combustible air-gas mixture <NUM> is formed at the predetermined number of discrete points of mixing <NUM> and guided via the predetermined number of mixers <NUM> to the burning unit <NUM>.

Illustratively, the burning unit <NUM> is provided with a burner surface <NUM> that is arranged downstream of the air-gas mixing unit <NUM> such that the combustible air-gas mixture <NUM> that is formed at the predetermined number of discrete points of mixing <NUM> flows towards the burner surface <NUM>. The combustible air-gas mixture <NUM> is burned by the burning unit <NUM> and, more specifically, at the burner surface <NUM>.

By way of example, the burner surface <NUM> is illustrated with a comparatively small flame <NUM> which occurs e.g. at a low firing rate of the air-gas mixing unit <NUM>, i.e. at a comparatively low rate at which feed of the combustible air-gas mixture <NUM> from the air-gas mixing unit <NUM> to the burning unit <NUM> occurs, in terms of volume, heat units, or weight per unit time. Such a low firing rate may e.g. be applied to the air-gas mixing unit <NUM> during an ignition phase of the air-gas mixture burning appliance <NUM>. The comparatively small flame <NUM> is illustratively stabilised against the burner surface <NUM> and detected by means of the flame detector <NUM>.

According to one aspect, the flame detector <NUM> is provided for sensing presence of the flame <NUM> at the burner surface <NUM> on the basis of a predetermined flame detection threshold. The predetermined flame detection threshold is preferably adjustable to vary for at least two different operating points of the air-gas mixture burning appliance <NUM>, as explained in detail at <FIG>.

By way of example, the flame detector <NUM> detects a flame signal <NUM> that is directed toward the flame detector <NUM> and suitable for determining whether the flame <NUM> is present on the burner surface <NUM>, or not. However, it should be noted that suitable flame detection techniques that may be used with the flame detector <NUM> are well-known to the person skilled in the art and are, therefore, not described in more detail, for brevity and conciseness. For instance, the flame detector <NUM> may use any suitable sensing element for sensing presence of the flame <NUM> at the burner surface <NUM>.

Illustratively, the flame detector <NUM> is connected to a controller <NUM>. Preferably, the controller <NUM> is adapted to control supply of gas to the air-gas mixing unit <NUM>, in particular to the gas supply <NUM>, on the basis of a detection signal <NUM> provided by the flame detector <NUM>. The detection signal <NUM> may be created by the flame detector <NUM>, or alternatively by the controller <NUM>, by comparing the detected flame signal <NUM> to the predetermined flame detection threshold. Thus, the controller <NUM> may create a control signal <NUM> on the basis of the detection signal <NUM> which can be used e.g. to interrupt supply of gas to the air-gas mixing unit <NUM>, in particular to the gas supply <NUM>, if the detection signal <NUM> indicates an abnormal flame state at the burner surface <NUM>. The controller <NUM> may be an integral part of the flame detector <NUM> or, as illustrated, a separate component of the air-gas mixture burning appliance <NUM>.

<FIG> shows the air-gas mixture burning appliance <NUM> of <FIG> with the air-gas mixing unit <NUM>, the burning unit <NUM>, the controller <NUM>, and the flame detector <NUM>. However, in contrast to <FIG> the air-gas mixing unit <NUM> is now exemplarily operated at a high firing rate, which is e.g. associated with a normal operating range of the burning unit <NUM> compared to the low firing rate that is associated with the ignition phase of the air-gas mixture burning appliance <NUM>, as described above at <FIG>.

The high firing rate leads, e.g. compared to the low firing rate, to a greater volume of the combustible air-gas mixture <NUM> that is available and burned at the burner surface <NUM> of the burning unit <NUM>. Therefore, the flame <NUM> is now illustratively greater than in <FIG> and emits toward the flame detector <NUM> a flame signal <NUM> having a higher intensity than the flame signal <NUM> emitted by the flame <NUM> of <FIG>.

<FIG> shows the air-gas mixture burning appliance <NUM> of <FIG> with the air-gas mixing unit <NUM>, the burning unit <NUM>, the controller <NUM>, and the flame detector <NUM>, wherein the air-gas mixing unit <NUM> is illustratively again operated at the high firing rate. However, in contrast to <FIG> the air-gas mixture burning appliance <NUM> is now shown in an undesirable operating scenario, wherein a flame <NUM> occurs in the air-gas mixing unit <NUM> under a sustained flashback event.

In other words, if a flashback occurs e.g. at the high firing rate in the normal operating range of the burning unit <NUM>, the flame <NUM> may stabilise at the point of mixing <NUM> of the air-gas mixing unit <NUM>. This, however, may lead to an unwanted overheating of the air-gas mixing unit <NUM>, as well as to unwanted noise. According to one aspect, such an unwanted overheating of the air-gas mixing unit <NUM>, as well as unwanted noise, may be avoided according to the present invention, as described below at <FIG>.

More specifically, the flame <NUM> illustratively emits a flame signal <NUM>, similar to the flame signal <NUM> emitted by the flame <NUM> of <FIG>, which is directed toward the flame detector <NUM>. If the flame signal <NUM> is detected by the flame detector <NUM> as a flame signal of a flame that is stabilised at the burner surface <NUM> of the burning unit <NUM>, i.e. if a normal flame state is detected, the unwanted overheating of the air-gas mixing unit <NUM>, as well as unwanted noise, may occur. Therefore, detection of the flame signal <NUM> by the flame detector <NUM> as a flame signal of a flame that is stabilised at the burner surface <NUM> of the burning unit <NUM> must be prevented and an abnormal flame state must instead be detected, as described below at <FIG>.

<FIG> shows the air-gas mixture burning appliance <NUM> of <FIG> with the air-gas mixing unit <NUM>, the burning unit <NUM>, the controller <NUM>, and the flame detector <NUM>. However, in contrast to <FIG> the air-gas mixture burning appliance <NUM> is now shown in another undesirable operating scenario, wherein the flame <NUM> also occurs in the air-gas mixing unit <NUM> under a sustained flashback event.

Nevertheless, the flame <NUM> illustratively only occurs at a part of the predetermined number of mixers <NUM> of the air-gas mixing unit <NUM>, while the combustible air-gas mixture <NUM> from the other mixers of the predetermined number of mixers <NUM> is burned at the burner surface <NUM> of the burning unit <NUM>, thereby creating a flame <NUM>, which illustratively corresponds to the small flame <NUM> of <FIG>. This other undesirable operating scenario may occur at a given firing rate that is somewhere between the low firing rate of <FIG> and the high firing rate of <FIG>.

The flame <NUM> illustratively emits a flame signal <NUM>, similar to the flame signal <NUM> emitted by the flame <NUM> of <FIG>, which is directed toward the flame detector <NUM>. If the flame signal <NUM> is detected by the flame detector <NUM> as a flame signal of a flame that is stabilised at the burner surface <NUM> of the burning unit <NUM>, i.e. if a normal flame state is detected, operation of the air-gas mixture burning appliance <NUM> is continued such that an unwanted overheating of the air-gas mixing unit <NUM>, as well as unwanted noise, may occur. Therefore, detection of the flame signal <NUM> by the flame detector <NUM> as a flame signal of a flame that is stabilised at the burner surface <NUM> of the burning unit <NUM> must be prevented and an abnormal flame state must instead be detected, as described below at <FIG>.

<FIG> shows a diagram <NUM> with an axis of abscissae <NUM> that may e.g. represent different values of firing rates, fan speeds or air-gas ratios of the air-gas mixture burning appliance <NUM> of <FIG>, as well as with an axis of ordinates <NUM> that illustrates intensities of different flame signals detected for the air-gas mixture burning appliance <NUM> of <FIG>. More specifically, three different flame signal intensities <NUM>, <NUM>, and <NUM> are shown. By way of example, the flame signal intensity <NUM> is associated with the flame signal <NUM> of <FIG>, the flame signal intensity <NUM> is associated with the flame signal <NUM> of <FIG>, and the flame signal intensity <NUM> is associated with the flame signal <NUM> of <FIG> as well as the flame signal <NUM> of <FIG>.

In order to avoid in the air-gas mixture burning appliance <NUM> of <FIG> an unwanted overheating of the air-gas mixing unit <NUM>, as well as unwanted noise, the flame signal intensity <NUM> must be recognised as being associated with an abnormal flame state at the burner surface <NUM> of the burning unit <NUM> of <FIG>, while the flame signal intensities <NUM>, <NUM> must be recognised as being associated with a normal flame state at the burner surface <NUM> of the burning unit <NUM> of <FIG>. Thus, the controller <NUM> of the air-gas mixture burning appliance <NUM> of <FIG> may control supply of gas to the air-gas mixing unit <NUM> correctly, as described above at <FIG>.

As described above at <FIG>, the flame detector <NUM> of the air-gas mixture burning appliance <NUM> of <FIG> is provided for sensing presence of a flame at the burner surface <NUM> on the basis of a predetermined flame detection threshold. If the predetermined flame detection threshold is merely fixed and independent of any time-variant functional variables, the undesirable operating scenarios of <FIG> may occur, as described hereinafter with respect to two different fixed constant flame detection thresholds <NUM>, <NUM>. If, however, the predetermined flame detection threshold is adjustable, as described below with respect to an exemplary adjustable flame detection threshold <NUM>, presence of the flame (<NUM> in <FIG>) in the air-gas mixing unit <NUM> under a sustained flashback event may be detected and suppressed. In other words, if the predetermined flame detection threshold <NUM> is adjustable, indication of a stable flame upon occurrence of a flame (<NUM> in <FIG>) in the air-gas mixing unit <NUM> under a sustained flashback event and/or in the event of a flame (<NUM> in <FIG>) stabilising undesirably upstream of the burner surface <NUM> may be avoided, as described in detail below.

By way of example, the fixed constant flame detection threshold <NUM> is selected such that the flame signal intensity <NUM> is below the fixed constant flame detection threshold <NUM> and may, thus, be recognised as being associated with an abnormal flame state. However, in this case the flame signal intensity <NUM> would also be recognised as being associated with an abnormal flame state, as it is also below the fixed constant flame detection threshold <NUM>. Thus, a suitable functioning of the air-gas mixture burning appliance of <FIG> may not be guaranteed.

Furthermore, the fixed constant flame detection threshold <NUM> is illustratively selected such that the flame signal intensity <NUM> is above the fixed constant flame detection threshold <NUM> and may, thus, be recognised as being associated with a normal flame state. However, in this case the flame signal intensity <NUM> would also be recognised as being associated with a normal flame state, as it is also above the fixed constant flame detection threshold <NUM>. Thus, a suitable functioning of the air-gas mixture burning appliance <NUM> of <FIG> may again not be guaranteed.

In contrast to the fixed constant flame detection thresholds <NUM>, <NUM>, the adjustable flame detection threshold <NUM> preferably comprises at least one predetermined discrete threshold value that is assigned to each one of at least two different operating points. At least one of the at least two different operating points may correspond to an operating state of the burning unit <NUM> of the air-gas mixture burning appliance <NUM> of <FIG>, or at least one operating parameter of the air-gas mixture burning appliance <NUM> of <FIG>.

By way of example, the first operating state of the burning unit <NUM> of the air-gas mixture burning appliance <NUM> of <FIG> corresponds to an ignition phase <NUM> of the burning unit <NUM>, as illustrated in <FIG>, and a second operating state of the burning unit <NUM> of the air-gas mixture burning appliance <NUM> of <FIG> corresponds to a normal operating range or operating phase <NUM> of the burning unit <NUM>, as illustrated in <FIG>. Duration of the ignition phase <NUM> and the normal operating phase <NUM> are exemplarily illustrated on an associated time-axis <NUM>.

Accordingly, a first predetermined flame detection threshold <NUM> is associated by way of example with the first operating state, and a second predetermined flame detection threshold <NUM> is associated with the second operating state, the first predetermined flame detection threshold <NUM> being smaller than the second predetermined flame detection threshold <NUM>. Illustratively, the first predetermined flame detection threshold <NUM> corresponds to the fixed constant flame detection threshold <NUM>, and the second predetermined flame detection threshold <NUM> corresponds to the fixed constant flame detection threshold <NUM>. The transition between the first and second predetermined flame detection thresholds <NUM>, <NUM> may be abrupt, as illustrated, or alternatively be implemented as a ramp (<NUM> in <FIG>) with one or more different slopes, as described below at <FIG>.

Illustratively, the first and second predetermined flame detection thresholds <NUM>, <NUM> are selected such that all flame signal entities above the first and second predetermined flame detection thresholds <NUM>, <NUM> are falling into a region <NUM> that is associated with a normal flame state. Similarly, the first and second predetermined flame detection thresholds <NUM>, <NUM> are selected such that all flame signal intensities below the first and second predetermined flame detection thresholds <NUM>, <NUM> are falling into a region <NUM> that is illustratively hatched and associated with an abnormal flame state.

More specifically, the first predetermined flame detection threshold <NUM> is illustratively selected such that the flame signal intensity <NUM> is above the first predetermined flame detection threshold <NUM> and may, thus, be recognised as being associated with a normal flame state. The second predetermined flame detection threshold <NUM> is illustratively selected such that the flame signal intensity <NUM> is above the second predetermined flame detection threshold <NUM> and may, thus, be recognised as being associated with a normal flame state, while the flame signal intensity <NUM> is below the second predetermined flame detection threshold <NUM> and may, thus, be recognised as being associated with an abnormal flame state. Accordingly, a suitable functioning of the air-gas mixture burning appliance <NUM> of <FIG> may be guaranteed.

It should be noted that the example described above only relates to possible operating states of the burning unit <NUM> of the air-gas mixture burning appliance <NUM> of <FIG>. However, as described above the at least two different operating points may not only correspond to the operating states of the burning unit <NUM> of the air-gas mixture burning appliance <NUM> of <FIG>, but also, or alternatively, to operating parameters of the air-gas mixture burning appliance <NUM> of <FIG>. By way of example, a first operating parameter of the air-gas mixture burning appliance <NUM> of <FIG> may correspond to a first firing rate or a first range of firing rates of the air-gas mixing unit <NUM> of the air-gas mixture burning appliance <NUM> of <FIG>, or a first fan speed or a first range of fan speeds of the fan <NUM> of the air-gas mixing unit <NUM>, and a second operating parameter of the air-gas mixture burning appliance <NUM> of <FIG> may correspond to a second firing rate or a second range of firing rates of the air-gas mixing unit <NUM> of the air-gas mixture burning appliance <NUM> of <FIG>, or a second fan speed or a second range of fan speeds of the fan <NUM> of the air-gas mixture burning appliance <NUM> of <FIG>. In this case, the first predetermined flame detection threshold <NUM> may be associated with the first operating parameter of the air-gas mixture burning appliance <NUM> of <FIG>, and the second predetermined flame detection threshold <NUM> may be associated with the second operating parameter of the air-gas mixture burning appliance <NUM> of <FIG>, the first predetermined flame detection threshold <NUM> being smaller than the second predetermined flame detection threshold <NUM>.

It should be noted that the firing rates or ranges of firing rates of the air-gas mixing unit <NUM> of the air-gas mixture burning appliance <NUM> of <FIG>, as well as the fan speeds or ranges of fan speeds of the fan <NUM> of the air-gas mixing unit <NUM> are only cited by way of example, and not for limiting the invention accordingly. Instead, other operating parameters are likewise contemplated, such as e.g. air-gas ratios or ranges of air-gas ratios of the air-gas mixture burning appliance <NUM> of <FIG>.

In an exemplary operation of the air-gas mixture burning appliance <NUM> of <FIG>, the predetermined flame detection threshold <NUM> is preferably adjusted at least for the two different operating points described above. For instance, if the first operating point is associated with an ignition phase <NUM> and the second operating point is associated with a normal operating range <NUM> of the air-gas mixture burning appliance <NUM> of <FIG>, then the predetermined flame detection threshold <NUM> may be adjusted from the first predetermined flame detection threshold <NUM> to the second predetermined flame detection threshold <NUM> upon transition from the ignition phase <NUM> to the normal operating range <NUM>.

<FIG> shows a diagram <NUM> with an axis of abscissae <NUM> that may e.g. represent different values of firing rates, fan speeds or air-gas ratios of the air-gas mixture burning appliance <NUM> of <FIG>, as well as with an axis of ordinates <NUM> that illustrates intensities of different flame signals detected for the air-gas mixture burning appliance <NUM> of <FIG>. Illustratively, the three different flame signal intensities <NUM>, <NUM>, and <NUM> of <FIG> are shown, as well as the adjustable flame detection threshold <NUM> with the first predetermined flame detection threshold <NUM> and the second predetermined flame detection threshold <NUM> of <FIG>.

More generally, the diagram <NUM> essentially corresponds to the diagram <NUM> of <FIG> and the time-axis <NUM> of <FIG> is also shown. However, in contrast to the diagram <NUM>, the transition of the adjustable flame detection threshold <NUM> between the first and second predetermined flame detection thresholds <NUM>, <NUM> is now no more abrupt, but implemented as a ramp <NUM>.

<FIG> shows a diagram <NUM> with an axis of abscissae <NUM> that may e.g. represent different values of firing rates, fan speeds or air-gas ratios of the air-gas mixture burning appliance <NUM> of <FIG>, as well as with an axis of ordinates <NUM> that illustrates intensities of different flame signals detected for the air-gas mixture burning appliance <NUM> of <FIG>. Illustratively, the three different flame signal intensities <NUM>, <NUM>, and <NUM> of <FIG> are shown, as well as the adjustable flame detection threshold <NUM> with the first predetermined flame detection threshold <NUM>, the second predetermined flame detection threshold <NUM>, and the ramp <NUM> of <FIG>.

More generally, the diagram <NUM> essentially corresponds to the diagram <NUM> of <FIG>, and the time-axis <NUM> of <FIG> is also shown. However, in contrast to the diagram <NUM>, the diagram <NUM> further comprises two additional flame signal intensities <NUM>, <NUM>, which are exemplarily occurring in the normal operating range <NUM>. For instance, the flame signal intensity <NUM> relates to an abnormal flame state and results e.g. from an increase of the firing rate of the air-gas mixture burning appliance <NUM> of <FIG> starting at the firing rate underlying the flame signal intensity <NUM>. In contrast, the flame signal intensity <NUM> exemplarily relates to a normal flame state. Accordingly, in order to guarantee a correct detection of the abnormal flame state associated with the flame signal intensity <NUM> and the normal flame state associated with the flame signal intensity <NUM>, the adjustable flame detection threshold <NUM> is adjusted from the second predetermined flame detection threshold <NUM> to a third predetermined flame detection threshold <NUM>, which illustratively corresponds to a fixed constant flame detection threshold <NUM>, by means of a transition <NUM> in form of a ramp.

By way of example, the firing rate is decreased after detection of the flame signal intensities <NUM>, <NUM>. Therefore, the adjustable flame detection threshold <NUM> is re-adjusted from the third predetermined flame detection threshold <NUM> back to the second predetermined flame detection threshold <NUM> by means of another transition <NUM> in form of another ramp.

Claim 1:
A method of operating an air-gas mixture burning appliance (<NUM>) that comprises an air-gas mixing unit (<NUM>) for mixing of air and gas, in particular hydrogen, to form a combustible air-gas mixture (<NUM>), a burning unit (<NUM>) with a burner surface (<NUM>) that is arranged downstream of the air-gas mixing unit (<NUM>) for burning the combustible air-gas mixture (<NUM>) at the burner surface (<NUM>), and a flame detector (<NUM>) for sensing presence of a flame (<NUM>) at the burner surface (<NUM>) on the basis of a predetermined flame detection threshold (<NUM>), the method comprising:
adjusting the predetermined flame detection threshold (<NUM>) for at least two different operating points of the air-gas mixture burning appliance (<NUM>), characterised in that the predetermined flame detection threshold (<NUM>) is configured to avoid indication of a stable flame upon occurrence of a flame (<NUM>) in the air-gas mixing unit (<NUM>) under a sustained flashback event and/or in the event of a flame (<NUM>) stabilising undesirably upstream of the burner surface (<NUM>).