Patent Description:
From the state of the art, an air-gas mixing unit having an air inlet, a gas inlet, an air-gas outlet, and an air-gas mixing section for mixing air with gas to form a combustible air-gas mixture is known. More specifically, known air-gas mixing units such as venturi mixers take in separate gas (fuel) and air (oxidizer) streams and provide a pre-mixed (or aerated) air-gas mixture for the burning unit at a desired concentration or ratio.

A venturi mixer typically contains a converging channel to accelerate the air stream in order to create a decreased pressure in the throat of the mixing unit. This decreased pressure, which is sometimes also referred to as suction pressure, in the throat causes a suction effect on the gas stream, which causes the gas stream to flow into the air-gas mixing unit and mix with the air stream. The upstream side of the gas stream is usually controlled by a gas valve, which regulates the gas pressure relative to the air pressure measured at some location in the air-gas mixture burning appliance.

Current air-gas mixture burning appliances usually include a controller that controls the amount of gas and the amount of air that enter the air-gas mixing unit.

Moreover, the air-gas mixing units of such appliances are often designed to operate with a predetermined gas that has certain properties, which are determined by a combination of density, viscosity, heating value, heat input, and air-gas ratio. Furthermore, the air-gas mixing units are often very sensitive to changes in gas quality levels, including the level of impurities such as nitrogen or carbon dioxide in the gas.

Conventional air-gas mixing units that are designed for a certain air-gas ratio have a predetermined pressure drop in the gas channel that, when combined with the setting of the gas valve regulator, precisely results in the target air-gas ratio. However, if a different fuel gas is used, the pressure drop in the gas channel changes. This results in a different air-gas ratio for the venturi mixer. As a result, a venturi designed for one fuel gas may no longer be suited to operating on another type of fuel gas.

In the remainder of this description, the term "gas" refers to any fuel in gaseous form that, when mixed with air, forms a combustible air-gas mixture. Examples for such a gas include hydrogen, propane, butane, methane, liquefied petroleum gas, etc. Moreover, the term "air" refers to any suitable oxidizer that may be mixed with a fuel gas to form a combustible air-gas mixture.

The concentration of the combustible air-gas mixture, which is sometimes also referred to as the air-gas ratio or the air to gas ratio, is the mass of air per mass of gas in the air-gas mixture. A complete combustion takes place when all the gas of the combustible air-gas mixture is burned. In other words, the exhaust gas is free of unburned gas. The air-gas ratio of a complete combustion is referred to as the stoichiometric air-gas ratio, and the ideal gas-air ratio is called stoichiometric gas-air ratio.

Document <CIT> describes a method for regulating a mixing ratio of fuel gas and air for a heating device. Initially, an air mass flow is subdivided and mixed with the fuel gas mass flow to form a premix via a partial flow. Material properties of the premix are detected by a sensor and the mixing ratio of fuel gas and air is calculated therefrom.

Document <CIT> describes a burner and a tile for producing a flame in a furnace. The burner includes a pre-mixer with a venturi that mixes fuel with a first portion of combustion air. The mixture of the fuel and combustion air is passed through the tile. The tile also passes additional portions of combustion air there through. The tile may include a wall to form a staged fuel tile with flow channels splitting the mixture of fuel and combustion air so that some exits the tile inside of the wall, and some exits the tile outside of the wall. The tile may be two pieces. Also described is a process for improving a flame produced by such a burner with a tile.

Document <CIT> describes a ventilated hob comprising a motor-driven fan adapted to provide high pressure air to two or more burners thereof, each of said two or more burners at least comprising means for supplying and regulating the flow of fuel gas in input into each of said two or more burners, means for supplying the flow of combustion air in input into each of said two or more burners, able to mix with said fuel gas, shut-off valves of the flow of said combustion air, an electronic unit for controlling said air flow. The air flow varies as a function of said flow of fuel gas in input into each of said two or more burners so as to ensure a gas-air mixture with a stoichiometric or substantially stoichiometric ratio without the need of input of secondary air from the outside, said variation of air flow being obtained by varying the pressure of the combustion air in said sealed circuit inside said hob. Said pressure is intermediate to the single pressures that each active burner should have.

The present invention relates to a two-stage air-gas mixing unit for mixing of air with gas to form a combustible air-gas mixture for a burning unit of an air-gas mixture burning appliance. The two-stage air-gas mixing unit comprises an air inlet for providing a flow of the air, an air flow channel that is arranged downstream of the air inlet, a gas inlet for providing the gas, a gas valve that is arranged downstream of the gas inlet and comprises a reference pressure port, wherein the air flow channel is adapted for providing the flow of the air to the reference pressure port, and wherein the gas valve is adapted for regulating the flow of the gas based on the pressure of the flow of the air at the reference pressure port, a first stage air-gas mixer that is arranged downstream of the gas valve and adapted for mixing the gas from the gas valve with a first portion of the air to form a preliminary air-gas mixture, a second stage air-gas mixer that is arranged downstream of the first stage air-gas mixer and the air inlet, wherein the second stage air-gas mixer is adapted for mixing a second portion of the air from the air inlet with the preliminary air-gas mixture from the first stage air-gas mixer to form the combustible air-gas mixture, an air-gas outlet that is adapted for providing the combustible air-gas mixture to the burning unit, and an additional air flow channel that is arranged upstream or downstream of the air flow channel and adapted for providing the first portion of the air to the first stage air-gas mixer.

Accordingly, the inventive two-stage air-gas mixing unit may be able to control the pressure drop in the gas channel as a result of a changing density and/or viscosity of the gas, thereby making the air-gas mixing unit suitable for mixing different gas types, for reacting promptly to changes in gas quality, and for providing a uniform combustible air-gas mixture at the air-gas outlet.

Optionally, the two-stage air-gas mixing unit may include a restricting device that is arranged in the additional air flow channel and adapted for regulating the first portion of the air to the first stage air-gas mixer.

Thus, the amount of air that mixes with the gas in the first stage air-gas mixer may be restricted such that the preliminary air-gas mixture remains non-flammable.

According to one embodiment, the restricting device may be adapted for regulating the first portion of the air based at least on the type of the gas.

Thus, the two-stage air-gas mixing unit may be adjusted to allow for the use of different gas types and to react to different levels of impurities in the gas.

Optionally, the two-stage air-gas mixing unit may include a second additional air flow channel that is arranged downstream of the air inlet, and an additional first stage air-gas mixer that is arranged downstream of the gas valve and adapted for mixing the gas from the gas valve with a third portion of the air from the second additional air flow channel to form the preliminary air-gas mixture.

Accordingly, the two-stage air-gas mixing unit may be able to provide a combustible air-gas mixture for at least two different predetermined gas types.

Optionally, the two-stage air-gas mixing unit includes at least one of a first restricting device or a second restricting device, wherein the first restricting device is arranged in the additional air flow channel and adapted for regulating the first portion of the air to the first stage air-gas mixer, and wherein the second restricting device is arranged in the second additional air flow channel and adapted for regulating the third portion of the air to the additional first stage air-gas mixer. Thus, the amount of air that mixes with the gas in the first stage air-gas mixer may be restricted for at least one of the two different predetermined gas types such that the preliminary air-gas mixture remains non-flammable.

Preferably, the first restricting device is adapted for regulating the first portion of the air based at least on the type of the gas, and the second restricting device is adapted for regulating the third portion of the air based at least on the type of the gas.

Accordingly, the two-stage air-gas mixing unit may react to different levels of impurities in the gas of at least one of the two different predetermined gas types.

Preferably, the additional air flow channel and the second additional air flow channel are arranged hydraulically parallel to each other and adapted for providing different quantities of the flow of the air to the first stage air-gas mixer and to the additional first stage air-gas mixer, respectively.

Thus, the additional air flow channel may be adapted to provide air to the first stage air-gas mixer, and, in parallel, the second additional air flow channel may be adapted to provide air to the additional first stage air-gas mixer.

Preferably, the additional air flow channel and the second additional air flow channel are operable independent of each other.

Accordingly, the additional air flow channel and the second additional air flow channel may be adjustable independent of each other, for example to adjust for different levels of impurities in the gas.

Optionally, the two-stage air-gas mixing unit may include an additional air inlet that is adapted for providing the air directly to the first stage air-gas mixer.

Thus, air may be provided from outside the two-stage air-gas mixing unit directly to the first stage air-gas mixer.

Optionally, the two-stage air-gas mixing unit may include a fan that is arranged upstream of the air flow channel and adapted for propagating the flow of the air towards the first and second stage air-gas mixers.

Accordingly, the fan may propel air from the air inlet towards the first and second stage air-gas mixers.

Optionally, the two-stage air-gas mixing unit may include a fan that is arranged downstream of the second stage air-gas mixer and adapted for propagating the combustible air-gas mixture towards the air-gas outlet.

Accordingly, the fan may propel the combustible air-gas mixture from the second stage air-gas mixer towards the air-gas outlet.

Optionally, the two-stage air-gas mixing unit may include a gas sensor that is arranged upstream of the gas valve and adapted for detecting the type of the gas.

Thus, the two-stage air-gas mixing unit may advantageously detected the type of the gas upstream of the gas valve.

Furthermore, an air-gas mixture burning appliance includes an air inlet for providing air, a gas inlet for providing gas, a burning unit for burning a combustible air-gas mixture, and the two-stage air-gas mixing unit described above.

Accordingly, the inventive air-gas mixture burning appliance may be suitable for being operated with different gas types using the same air-gas mixing unit and for reacting promptly to changes in gas quality.

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>. By way of example, the air-gas mixture burning appliance <NUM> may be used in any application that requires the use of a heat exchanger. For example, the air-gas mixture burning appliance <NUM> may be used in a building heating system or a water heater.

As shown in <FIG>, the air-gas mixture burning appliance <NUM> may include an air inlet <NUM> for providing air <NUM>, a gas inlet <NUM> for providing gas <NUM>, and a flue outlet <NUM> for the evacuation of exhaust gas <NUM>. Illustratively, the air-gas mixture burning appliance <NUM> may further include an air-gas mixing unit <NUM>, a burning unit <NUM>, and a heat exchanger <NUM>.

The gas <NUM> may be any gas such as hydrogen, propane, butane, methane, liquefied petroleum gas, etc. Illustratively, the gas <NUM> may have a certain gas quality range. The gas quality range may be specified by the gas distributer. Thus, the gas <NUM> may contain other, usually non-flammable, species besides the pure fuel itself. These other species in the gas <NUM> are called impurities. Typical impurities are nitrogen or carbon dioxide.

The density and viscosity of the pure fuel are different from the density and viscosity of the typical impurities. When the gas quality changes, within limits as allowed by the gas quality standards, the concentration of impurities in the gas <NUM> will change. This results in a change in density and viscosity of the gas <NUM>. Consequently, the flow rate of gas <NUM> drawn in by the air-gas mixing unit <NUM> changes. As a result, the heat input and air-gas ratio of the air-gas mixture burning appliance will change. The larger the difference in density and viscosity between the gas <NUM> before the gas quality change and after the gas quality change, the larger the change in heat input and air-gas ratio of the air-gas mixture burning appliance at the moment of the gas quality change.

The air-gas mixing unit <NUM> is preferably adapted for mixing of air <NUM> and gas <NUM> to form a combustible air-gas mixture <NUM>. Preferentially, the combustible air-gas mixture <NUM> is a homogenous mixture of the air <NUM> and the gas <NUM>. To adapt to different gas types and/or to different gas quality ranges, the air-gas mixing unit <NUM> is preferably embodied as a two-stage air-gas mixing unit <NUM> as shown in <FIG>.

By way of example, the air-gas mixing unit <NUM> includes an air inlet <NUM> that receives air <NUM> from the air inlet <NUM> of the air-gas mixture burning appliance <NUM> and a gas inlet <NUM> that receives gas <NUM> from the gas inlet <NUM> of the air-gas mixture burning appliance <NUM>.

Illustratively, the combustible air-gas mixture <NUM> is formed in the two-stage air-gas mixing unit <NUM> from the air <NUM> received at the air inlet <NUM> and the gas <NUM> received at the gas inlet <NUM>. In the two-stage air-gas mixing unit <NUM>, a first stage air-gas mixer may mix a portion of the air <NUM> from the air inlet <NUM> with the gas <NUM> from the gas inlet <NUM> to produce a preliminary air-gas mixture. The preliminary air-gas mixture may have a predetermined air-gas ratio. For example, the preliminary air-gas mixture may have an air-gas ratio such that the preliminary air-gas mixture is non-flammable.

By way of example, a second stage air-gas mixer may mix the remaining air <NUM> from the air inlet <NUM> with the preliminary air-gas mixture to produce a combustible air-gas mixture <NUM>. The combustible air-gas mixture <NUM> is provided via the air-gas outlet <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 in the two-stage air-gas mixing unit <NUM> flows towards the burner surface <NUM>. If desired, a fan may drive the combustible air-gas mixture <NUM> 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>. The heat exchanger <NUM> may transfer the heat that is generated at the burner surface <NUM> to another medium. For example, the heat exchanger <NUM> may transfer the heat that is generated at the burner surface <NUM> to water in a water circuit. The flue outlet <NUM> may evacuate the exhaust gas <NUM> from the air-gas mixture burning appliance <NUM>.

<FIG> shows a schematic view of the air-gas mixture burning appliance <NUM> of <FIG> with a two-stage air-gas mixing unit <NUM>. The two-stage air-gas mixing unit <NUM> is suitable for mixing of air <NUM> with gas <NUM> to form a combustible air-gas mixture <NUM> for burning unit <NUM> of air-gas mixture burning appliance <NUM>.

As shown in <FIG>, the two-stage air-gas mixing unit <NUM> includes an air inlet <NUM> for providing a flow of the air <NUM>, a gas inlet <NUM> for providing the gas <NUM>, and an air-gas outlet <NUM> that is adapted for providing the combustible air-gas mixture <NUM> to the burning unit <NUM>. Preferably, the flow of the air <NUM> is unregulated. For example, the flow of the air <NUM> may have an ambient air pressure. Thus, the air inlet <NUM> may be adapted for providing an unregulated flow of the air <NUM>.

The two-stage air-gas mixing unit <NUM> further includes an air flow channel <NUM> that is arranged downstream of the air inlet <NUM> and a gas valve <NUM> that is arranged downstream of the gas inlet <NUM>. The gas valve <NUM> may include a reference pressure port <NUM>.

Preferably, the air flow channel <NUM> is adapted for providing the flow of the air <NUM> to the reference pressure port <NUM>, and the gas valve <NUM> is adapted for regulating the flow of the gas <NUM> based on the pressure of the flow of the air <NUM> at the reference pressure port <NUM>.

Illustratively, the two-stage air-gas mixing unit <NUM> may include a first stage air-gas mixer <NUM> that is arranged downstream of the gas valve <NUM>. The first stage air-gas mixer <NUM> may be adapted for mixing the gas <NUM> from the gas valve <NUM> with a first portion of the air <NUM> to form a preliminary air-gas mixture <NUM>.

By way of example, the two-stage air-gas mixing unit <NUM> may include a second stage air-gas mixer <NUM> that is arranged downstream of the first stage air-gas mixer <NUM> and the air inlet <NUM>. The second stage air-gas mixer <NUM> may be adapted for mixing a second portion of the air <NUM> from the air inlet <NUM> with the preliminary air-gas mixture <NUM> from the first stage air-gas mixer <NUM> to form the combustible air-gas mixture <NUM>.

As shown in <FIG>, the two-stage air-gas mixing unit <NUM> includes a fan <NUM>. The fan <NUM> may be arranged downstream of the second stage air-gas mixer <NUM>, if desired. The fan <NUM> may propagate the combustible air-gas mixture <NUM> towards the air-gas outlet <NUM>.

Preferably, the first portion of air <NUM> that mixes with the gas <NUM> from the gas valve <NUM> in the first-stage air-gas mixer <NUM> to form the preliminary air-gas mixture <NUM> is controlled such that the preliminary air-gas mixture <NUM> remains above the upper flammability limit of the gas <NUM>.

Illustratively, the two-stage air-gas mixing unit <NUM> may include an additional air flow channel <NUM>. The additional air flow channel <NUM> may provide the first portion of the air <NUM> to the first stage air-gas mixer <NUM>. By way of example, the additional air flow channel <NUM> may control the first portion of air <NUM> (e.g., through the dimensions of the additional air flow channel <NUM>) such that the preliminary air-gas mixture <NUM> remains non-flammable. If desired, the additional air flow channel <NUM> may be arranged downstream of the air flow channel <NUM>.

<FIG> shows a schematic view of the air-gas mixture burning appliance <NUM> of <FIG> with air inlet <NUM>, gas inlet <NUM>, burning unit <NUM>, heat exchanger <NUM>, flue outlet <NUM>, and a two-stage air-gas mixing unit <NUM>.

However, in contrast to the two-stage air-gas mixing unit <NUM> of <FIG>, the additional air flow channel <NUM> to the first stage air-gas mixer <NUM> in the two-stage air-gas mixing unit <NUM> of <FIG> is arranged upstream of the air flow channel <NUM> to the reference pressure port <NUM> of gas valve <NUM>. Moreover, the two-stage air-gas mixing unit <NUM> of <FIG> includes a gas sensor <NUM> and a restricting device <NUM>.

The gas sensor <NUM> may be adapted for detecting the type of the gas <NUM>. Illustratively, the gas sensor <NUM> may be arranged upstream of the gas valve <NUM>. If desired, the gas sensor <NUM> may be arranged between the gas valve <NUM> and the first stage air-gas mixer <NUM>.

The restricting device <NUM> may be adapted for regulating the first portion of the air <NUM> to the first stage air-gas mixer <NUM>. Preferably, the restricting device <NUM> may be arranged in the additional air flow channel <NUM>. If desired, the restricting device <NUM> may regulate the first portion of the air <NUM> based at least on the type of the gas <NUM>. Thus, the two-stage air-gas mixing unit <NUM> may adapt to the use of different gas types.

However, the two-stage air-gas mixing unit <NUM> of <FIG> differs from the two-stage air-gas mixing unit <NUM> of <FIG> in that the air flow channel <NUM> to the reference pressure port <NUM> of gas valve <NUM> is decoupled from the air inlet <NUM>. Instead, the reference pressure port <NUM> of gas valve <NUM> is subjected to ambient air pressure, for example from outside the air-gas mixture burning appliance <NUM>. If desired, the reference pressure port <NUM> of gas valve <NUM> may be subjected to ambient air pressure from inside the enclosure of the air-gas mixture burning appliance <NUM>.

However, in contrast to the two-stage air-gas mixing unit <NUM> of <FIG>, the additional air flow channel <NUM> to the first stage air-gas mixer <NUM> of the two-stage air-gas mixing unit <NUM> of <FIG> is decoupled from the air inlet <NUM>. Instead, the two-stage air-gas mixing unit <NUM> includes an additional air inlet <NUM> that is adapted for providing the air <NUM> directly to the first stage air-gas mixer <NUM>. In other words, the first stage air-gas mixer <NUM> receives the air <NUM> from the ambient air within the enclosure of the air-gas mixture burning appliance <NUM> at an ambient air pressure.

The two-stage air-gas mixing unit <NUM> of <FIG> differs from the two-stage air-gas mixing unit of <FIG> in that the fan <NUM> of <FIG> is removed, and, instead, a fan <NUM> is arranged upstream of the air flow channel <NUM>. Fan <NUM> is adapted for propagating the flow of the air <NUM> towards the first and second stage air-gas mixers <NUM>, <NUM>.

However, in contrast to the two-stage air-gas mixing unit <NUM> of <FIG>, the additional air flow channel <NUM> to the first stage air-gas mixer <NUM> in the two-stage air-gas mixing unit <NUM> of <FIG> is arranged upstream of the air flow channel <NUM> to the reference pressure port <NUM> of gas valve <NUM>. In other words, the additional air flow channel <NUM> connects an opening at the output of fan <NUM> upstream of an opening of air flow channel <NUM> with the first stage air-gas mixer <NUM> such that a first portion of air <NUM> from the fan output may arrive at the first stage air-gas mixer <NUM>.

<FIG> shows a schematic view of a two-stage air-gas mixing unit <NUM>. The two-stage air-gas mixing unit <NUM> is suitable for mixing of air <NUM> with gas <NUM> to form a combustible air-gas mixture <NUM> for a burning unit of an air-gas mixture burning appliance (e.g., burning unit <NUM> of air-gas mixture burning appliance <NUM> of <FIG>).

As shown in <FIG>, the two-stage air-gas mixing unit <NUM> includes an air inlet <NUM> for providing a flow of the air <NUM>, a gas inlet <NUM> for providing the gas <NUM>, and an air-gas outlet <NUM> that is adapted for providing the combustible air-gas mixture <NUM> to the burning unit. Preferably, the flow of the air <NUM> is unregulated. For example, the flow of the air <NUM> may have an ambient air pressure. Thus, the air inlet <NUM> may be adapted for providing an unregulated flow of the air <NUM>.

By way of example, the two-stage air-gas mixing unit <NUM> may include an additional air flow channel <NUM>. The additional air flow channel <NUM> may provide a first portion of the air <NUM> to the first stage air-gas mixer <NUM>. The additional air flow channel <NUM> may be dimensioned to control the first portion of air <NUM> such that the preliminary air-gas mixture <NUM> remains non-flammable. If desired, the additional air flow channel <NUM> may be arranged downstream of the air flow channel <NUM>.

Illustratively, the two-stage air-gas mixing unit <NUM> may include a first stage air-gas mixer <NUM> that is arranged downstream of the gas valve <NUM>. The first stage air-gas mixer <NUM> is adapted for mixing the gas <NUM> from the gas valve <NUM> with the first portion of the air <NUM> to form a preliminary air-gas mixture <NUM>.

As shown in <FIG>, the two-stage air-gas mixing unit <NUM> may include a second additional air flow channel <NUM> that is arranged downstream of the air inlet <NUM>. Illustratively, the additional air flow channel <NUM> and the second additional air flow channel <NUM> may be arranged hydraulically parallel to each other. If desired, the additional air flow channel <NUM> and the second additional air flow channel <NUM> may be adapted for conveying different quantities of the flow of the air <NUM>. Preferably, the additional air flow channel <NUM> and the second additional air flow channel <NUM> are operable independent of each other.

Illustratively, the two-stage air-gas mixing unit <NUM> may include an additional first stage air-gas mixer <NUM>. The additional first stage air-gas mixer <NUM> may be arranged downstream of the first stage air-gas mixer <NUM> and mix the preliminary air-gas mixture <NUM> from the first stage air-gas mixer <NUM> with a third portion of the air <NUM> from the second additional air flow channel <NUM> to form a second preliminary air-gas mixture <NUM>.

If desired, the additional first stage air-gas mixer <NUM> may be arranged downstream of the gas valve <NUM> and mix the gas <NUM> from the gas valve <NUM> with the third portion of the air <NUM> from the second additional air flow channel <NUM> to form the preliminary air-gas mixture <NUM>.

Preferably, the additional air flow channel <NUM> and the second additional air flow channel <NUM> may be adapted for providing different quantities of the flow of the air <NUM> to the first stage air-gas mixer <NUM> and to the additional first stage air-gas mixer <NUM>, respectively. As an example, the additional air flow channel <NUM> may be closed such that no air <NUM> is flowing from the air inlet <NUM> to the first stage air-gas mixer <NUM> of <FIG>. As another example, the second additional air flow channel <NUM> may be closed such that no air <NUM> is flowing from the air inlet <NUM> to the additional first stage air-gas mixer <NUM>.

Illustratively, a gas sensor <NUM> may be arranged upstream of the gas valve <NUM> or between the gas valve <NUM> and the first stage air-gas mixer <NUM>. The gas sensor <NUM> may exemplarily be adapted for detecting the type of the gas <NUM>. If desired, the gas sensor <NUM> may sense other properties than the type of the gas <NUM>. As an example, the gas sensor <NUM> may measure the pressure drop in the gas channel between the gas inlet <NUM> and the gas valve <NUM>. As another example, the gas sensor <NUM> may measure the viscosity of the gas <NUM>. As yet another example, the gas sensor <NUM> may measure the amount of impurities in the gas <NUM>.

By way of example, the two-stage air-gas mixing unit <NUM> may include at least one of a first restricting device <NUM> or a second restricting device <NUM>. The first restricting device <NUM> may be arranged in the additional air flow channel <NUM> and adapted for regulating the first portion of the air <NUM> to the first stage air-gas mixer <NUM>. The second restricting device <NUM> may be arranged in the second additional air flow channel <NUM> and adapted for regulating the third portion of the air <NUM> to the additional first stage air-gas mixer <NUM>.

As an example, the first restricting device <NUM> may be adapted for regulating the first portion of the air <NUM> based at least on the type of the gas <NUM>. As another example, the second restricting device <NUM> may be adapted for regulating the third portion of the air <NUM> based at least on the type of the gas <NUM>.

For example, a controller may receive information about the type of the gas <NUM> from the gas sensor <NUM>. The controller may be coupled to the first restricting device <NUM> and adjust the first portion of air <NUM> that flows through the additional air flow channel <NUM> from the air inlet <NUM> to the first stage air-gas mixer <NUM>. If desired, the controller may be coupled to the second restricting device <NUM> and adjust the third portion of air <NUM> that flows through the second additional air flow channel <NUM> from the air inlet <NUM> to the additional first stage air-gas mixer <NUM>.

Illustratively, the two-stage air-gas mixing unit <NUM> may use the additional air flow channel <NUM> and provide a first portion of air <NUM> to the first stage air-gas mixer <NUM> for one type of gas <NUM>, and the two-stage air-gas mixing unit <NUM> may use the second additional air flow channel <NUM> and provide a third portion of air <NUM> to the additional first stage air-gas mixer <NUM> for another type of gas <NUM>.

For example, the two-stage air-gas mixing unit <NUM> may close the second additional air flow channel <NUM> when the additional air flow channel <NUM> is providing the first portion of the air <NUM> to the first stage air-gas mixer <NUM>, and the two-stage air-gas mixing unit <NUM> may close the additional air flow channel <NUM> when the second additional air flow channel <NUM> is providing the third portion of the air <NUM> to the additional first stage air-gas mixer <NUM>. Thus, the two-stage air-gas mixing unit <NUM> may use a first subset of the air channels for one type of gas <NUM> and a second subset of the air channels for another type of gas <NUM>.

By way of example, the two-stage air-gas mixing unit <NUM> may include a second stage air-gas mixer <NUM> that is arranged downstream of the first stage air-gas mixer <NUM> and the air inlet <NUM>. The second stage air-gas mixer <NUM> is adapted for mixing a second portion of the air <NUM> from the air inlet <NUM> with the preliminary air-gas mixture <NUM> from the first stage air-gas mixer <NUM> and/or the additional first stage air-gas mixer <NUM> to form the combustible air-gas mixture <NUM>.

Preferably, the preliminary air-gas mixture <NUM> that is provided to the second stage air-gas mixer <NUM> is controlled such that the preliminary air-gas mixture <NUM> remains above the upper flammability limit of the gas <NUM>. In other words, the first portion of air <NUM> that mixes with the gas <NUM> from the gas valve <NUM> in the first-stage air-gas mixer <NUM> and/or the third portion of air <NUM> that mixes with the gas <NUM> or the preliminary air-gas mixture <NUM> from the first stage air-gas mixer <NUM> in the additional first stage air-gas mixer <NUM> to form the preliminary air-gas mixture <NUM> that is provided to the second stage air-gas mixer <NUM> is controlled such that the preliminary air-gas mixture <NUM> remains non-flammable.

If desired, the two-stage air-gas mixing unit <NUM> may include a fan. The fan may be arranged downstream of the second stage air-gas mixer <NUM> and propagate the combustible air-gas mixture <NUM> towards the air-gas outlet <NUM>. Alternatively, or in addition, the two-stage air-gas mixing unit <NUM> may include a fan that is arranged between the air inlet <NUM> and the air flow channel <NUM>.

Claim 1:
A two-stage air-gas mixing unit (<NUM>) for mixing of air (<NUM>) with gas (<NUM>) to form a combustible air-gas mixture (<NUM>) for a burning unit (<NUM>) of an air-gas mixture burning appliance (<NUM>), comprising:
an air inlet (<NUM>) for providing a flow of the air (<NUM>);
an air flow channel (<NUM>) that is arranged downstream of the air inlet (<NUM>);
a gas inlet (<NUM>) for providing the gas (<NUM>);
a gas valve (<NUM>) that is arranged downstream of the gas inlet (<NUM>) and comprises a reference pressure port (<NUM>), wherein the air flow channel (<NUM>) is adapted for providing the flow of the air (<NUM>) to the reference pressure port (<NUM>), and wherein the gas valve (<NUM>) is adapted for regulating the flow of the gas (<NUM>) based on the pressure of the flow of the air (<NUM>) at the reference pressure port (<NUM>);
a first stage air-gas mixer (<NUM>) that is arranged downstream of the gas valve (<NUM>) and adapted for mixing the gas (<NUM>) from the gas valve (<NUM>) with a first portion of the air (<NUM>) to form a preliminary air-gas mixture (<NUM>);
a second stage air-gas mixer (<NUM>) that is arranged downstream of the first stage air-gas mixer (<NUM>) and the air inlet (<NUM>), wherein the second stage air-gas mixer (<NUM>) is adapted for mixing a second portion of the air (<NUM>) from the air inlet (<NUM>) with the preliminary air-gas mixture (<NUM>) from the first stage air-gas mixer (<NUM>) to form the combustible air-gas mixture (<NUM>);
an air-gas outlet (<NUM>) that is adapted for providing the combustible air-gas mixture (<NUM>) to the burning unit (<NUM>); and
an additional air flow channel (<NUM>) that is arranged upstream or downstream of the air flow channel (<NUM>) and adapted for providing the first portion of the air (<NUM>) to the first stage air-gas mixer (<NUM>).