BACKGROUND

The present invention relates to the mixing of fuel with a source of oxygen (for example, air) and the burning of that mixture. Particularly, but not exclusively, the invention relates to the mixing and burning of hydrogen with a source of oxygen in a boiler.

In prior art heating systems, it is known for boilers to mix fuel gas with a source of gaseous oxidiser, such as air, upstream of a burner, where the fuel and oxygen is subsequently burnt. This is known as a pre-mixed burning, as opposed to diffusion burning where the process of mixing fuel and oxygen together occurs simultaneously with the process of burning.

A problem with pre-mixed burning in boilers is that the fuel and oxygen mixture present upstream of the burner is susceptible to being ignited and burnt prematurely. This can occur in a flashback condition, when a normally stable flame downstream of a burner surface causes ignition of the supply of unburnt fuel and oxygen mixture present on the upstream side of the burner. Features provided in a boiler to promote and improve the mixing of fuel and oxygen typically do so by introducing swirl or turbulence into the fuel/oxygen fluid flow, or by providing a sufficient flow distance for mixing processes to progress, and this would often accelerate a flashback flame and exacerbate its severity, resulting in damage to components of appliances.

The likelihood of flashback is increased when the fuel is hydrogen, rather than the more common natural gas used in appliances such as boilers. This is because hydrogen gas has a much greater flame speed than natural gas and produces greater overpressures in transient ignition events. Flame accelerations can also generate flame speeds approaching or exceeding the speed of sound, which can produce damaging shockwaves. It is an object of the present disclosure to obviate or reduce problems associated with flashback.

BRIEF DESCRIPTION OF THE DRAWINGS

Some non-limiting examples of the present disclosure will be described in the following with reference to the appended drawings in which:

Figure 1 is a schematic side view of a first boiler in accordance with aspects of the present disclosure;

Figure 1a is an end view showing an arrangement of venturi outlets in accordance with aspects of the present disclosure;

Figure 1 b is an end view showing an arrangement of venturi outlets in accordance with aspects of the present disclosure; Figure 2 is a schematic cross- sectional end view of a second boiler in accordance with aspects of the present disclosure;

Figure 3 is a schematic view of a fuel/air flow from a single venturi compared with a fuel/air flow from a plurality of venturis;

Figure 4 is a schematic cross-sectional side view of a venturi;

Figure 5 is a schematic view of a fuel/air flow from one of a plurality of venturis; Figure 6a-k provide schematic views of eleven different arrangements for a plurality of venturis;

Figure 7a is an exploded perspective view of a burner assembly of a third boiler according to the present disclosure;

Figure 7b is an exploded perspective view of a venturi array assembly of the burner assembly shown in Figure 7a; and

Figure 8 is a cross-sectional side view of some of the plurality of venturis in the burner assembly of Figure 7a.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific examples in which the disclosure may be practiced. It is to be understood that other examples may be utilized and structural or logical changes may be made without departing from the scope of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims. It is to be understood that features of the various examples described herein may be combined, in part or whole, with each other, unless specifically noted otherwise.

Schematic examples of the present invention are illustrated in Figures 1 and 2 of the accompanying drawings. With reference to Figure 1 , an appliance 2 is shown having a mixer 4 to mix together fuel and oxidiser, and a burner 6 to burn a mixture of fuel and oxidiser from the mixer 4. The mixer 4 has a plurality of venturis 8,10 and a mixing flow pathway 12 extending from the plurality of venturis 8,10. The mixing flow pathway 12 is configured to provide fluid communication between the plurality of venturis 8,10 and the burner 6.

Each venturi 8,10 comprises a mixing chamber 14, a fuel inlet 16 to provide fluid communication between the mixing chamber 14 and a source of fuel 18, an oxidiser inlet 20 to provide fluid communication between the mixing chamber 14 and a source of oxidiser 22, and a venturi outlet 24 to provide fluid communication between the mixing chamber 14 and the mixing flow pathway 12. The venturi outlets 24 of the plurality of venturis 8,10 open into the mixing flow pathway 12. In Figure 1 , the venturi outlets 24 are shown lying in the same plane denoted by imaginary line 23.

All of the oxidiser for fuel burnt at the burner is provided through the plurality of venturis.

In the example of Figure 1 , the oxidiser is air and, accordingly, the oxidiser inlet 20 is an air inlet and the source of oxidiser 22 is a source of air.

The fluid flow distance (denoted by arrow 25 in Figure 1 ) from the venturi outlets multiple of the detonation cell size (DCS) of a stoichiometric mixture of the fuel and the oxidiser (for example, air) at operating conditions for the mix of fuel and oxidiser during use of the appliance. The predetermined multiple is no more than 6. In an example, the predetermined multiple is between 2 and 6. In the example schematically shown in Figure 1 , the predetermined multiple is 3.

The fluid flow distance (denoted by arrow 27 in Figure 1 ) from the mixing chambers 14 of the plurality of venturis 8,10 to the burner 6 is no more than a given multiple of the detonation cell size (DCS) of a stoichiometric mixture of the fuel and the oxidiser (for example, air) at operating conditions for the mix of fuel and oxidiser during use of the appliance. The given multiple is no more than 12. In an example, the given multiple is between 4 and 12. In the example schematically shown in Figure 1 , the given multiple is 6.

The detonation cell size is dependent upon the type of fuel used in the mixture with the oxidiser (such as air), and on other factors, which include the temperature and pressure of the mixture. As pressure of the fuel and oxidiser mixture increases, the detonation cell size decreases. As temperature of the mixture increases, the detonation cell size decreases.

The temperature and pressure assumed when selecting the detonation cell size is that at operating conditions for the mix of fuel and air during use of the appliance. In this regard, the operating conditions are the conditions in the mixing flow pathway. The operating conditions include the temperature and pressure of the fuel and oxidiser mixture inside the appliance when the fuel is being combusted and the temperature has increased within the appliance (at least) as a result. In an example, when operating conditions vary (for example, as a consequence of how the appliance is used), the operating conditions used for selecting detonation cell size are those which, in the range of possible operating conditions, result in the smallest detonation cell size.

The operating conditions for the mix of fuel and air (i.e. in the mixing flow pathway) during use of a boiler will typically be up to 40 degrees Celcius (specifically between zero and 40 degrees Celcius) and up to 1200 millibars (specifically between 800 and 1200 millibars). In the example of Figure 1 , the operating conditions are Normal Temperature and Pressure (20 degrees Celcius and 1 atmosphere). Also, the operating conditions are dry (i.e. zero humidity). Accordingly, the detonation cell size for a stoichiometric mixture of hydrogen and air for these operating conditions is 15mm. Accordingly, in the appliance of Figure 1 , which is provided to function in the operating conditions mentioned, the first fluid flow distance 25 is no more than 45mm (i.e. three times the DCS) and the second fluid flow distance 27 is no more than 90mm (i.e. six times the DCS). Specifically, in the appliance 2, the first fluid flow distance 25 is 40mm (i.e. no more three times the DCS) and the second fluid flow distance 27 is 55mm (i.e. no more than six times the DCS).

In a further example, the predetermined multiple in relation to the first fluid flow distance 25 is no more than 6 while the given multiple in relation to the second fluid flow distance 27 is no more than 12.

The first fluid flow distance 25 has a maximum, as mentioned above, and a distance between the burner 6 and the plurality of venturis 8,10 which is no greater than this maximum will ensure the severity of flashback passing upstream of the burner 6 in the event of deflagration remains at an acceptable level. If the distance was greater than the maximum, then, in the event of deflagration, this would allow flame to “run-up” and its speed to increase beyond

an acceptable level where the severity of flashback passing upstream of the burner would become unacceptably high.

The second fluid flow distance 27 has a maximum, as mentioned above, and again this ensures the severity of deflagration passing upstream of the burner 6 in the event of flashback remains at an acceptable level.

The severity of flashback can be maintained at an acceptable level by not exceeding the maximum for either of the first or second distances fluid flow 25,27.

A short fluid flow distance 25,27 reduces the opportunity for mixing of the fuel and air. The mixture of fuel and air must be sufficiently mixed to support combustion of the fuel by the time the mixture reaches the burner. This influences the minimum length of the mixing flow pathway 12 (i.e. the minimum value of the first fluid flow distance 25). However, the number of venturis in the plurality of venturis 8,10 provides mixing of the fuel and air to a sufficient extent to support combustion of the fuel at the burner. By increasing the number of venturis, and as a consequence reducing their size, the distance required for sufficient mixing reduces (see below in relation to Figure 3). So, the number of venturis, and their associated size, is selected to ensure sufficient mixing is achieved in a first fluid flow distance 25 equal to or less than the maximum permitted. In the appliance 2 shown schematically in Figure 1 , the number of venturis required to ensure sufficient mixing is only two.

In the appliance 2, the mixing of the fuel and air provides a stoichiometric mix of fuel and air. Also, the mixing also provides a homogenous mixture (or, alternatively, a mixture homogenous to a sufficient degree for adequate combustion). It will be understood that complete homogeneity throughout the mix at the burner is not required to allow for combustion of the fuel at the burner. Also, complete combustion of all fuel at the burner is not achieved or required in many appliances generally (for example, boilers). Reference herein to homogenous only requires that a mixture is suitable for adequately stable and complete combustion.

The appliance 2 is a boiler. The appliance has an arrangement to transfer heat to a supply of water, external and separate to the appliance, which passes through the appliance during.

The source of air 22 may be ambient air.

The mixing flow pathway 12 extending from the plurality of venturis 8,10 is a single mixing flow pathway common to the plurality of venturis 8,10. In an alternative example, a plurality of mixing flow pathways is provided wherein a mixing flow pathway extends from each venturi or from a subset of venturis.

The venturi outlets 24 are uniformly distributed in the mixing flow pathway 12. As shown in Figure 1 , the venturi outlets 24 are uniformly distributed across the entire cross-sectional flow area of the mixing flow pathway 12. The venturi outlets 24 are uniformly distributed in that they are evenly spaced across the flow area of the mixing flow pathway 12 to introduce fuel and air from the venturi outlets 24 evenly within the mixing flow pathway 12. This promotes a rapid formation of a homogenous mixture within the mixing flow pathway 12.

In the appliance 2, the venturi outlets 24 are uniformly distributed and located in a single plane. In other examples, the venturi outlets 24 are uniformly distributed and located in one of (i) a part cylindrical shape, and (ii) a part spherical shape.

In another example, the venturi outlets are distributed and located in one of (i) a triangular pitch, (ii) a square pitch, (iii) a rotated square pitch, and (iv) a staggered array. The longitudinal pitch is either equal to the transverse pitch or not equal to the transverse pitch.

Further arrangements of venturi outlets are shown in Figures 1a and 1 b. In Figure 1a, sixteen venturi outlets are arranged with a first venturi outlet 102 located in a central position relative to the other venturi outlets, with the other venturi outlets being located in two concentric circles centred on the first venturi outlet. The inner concentric circle has five venturi outlets 104, and the outer concentric circle has ten venturi outlets 106. The venturi outlets in each concentric circle are equi-spaced from each other i.e. the spacing between neighbouring venturi outlets is the same along the circumference of the circle. The venturi outlets lie in the same plane.

In Figure 1 b, thirteen venturi outlets are arranged with three venturi outlets 108 located in a triangular pitch centred on an imaginary point 110, with the remaining ten venturi outlets 112 being located in a concentric circle which is also centred on an imaginary point 110. The venturi outlets in the concentric circle are equi- spaced from each other i.e. the spacing between neighbouring venturi outlets is the same along the circumference of the circle. The venturi outlets lie in the same plane.

Any of appliances disclosed herein as having a plurality of venturis can have the venturi outlets arranged as shown in Figures 1a and 1 b.

Another example is shown schematically in Figure 2. Common features between the boilers of Figures 1 and 2 are provided with like reference numerals, for example, venturi outlets 24’ in Figure 2. Figure 2 is a cross-sectional view taken through a plane in which the venturi outlets 24’ are located (the plane being equivalent to the plane denoted by the imaginary line 23 in Figure 1). The boiler 2’ of Figure 2 has the same arrangement as the boiler 2 of Figure 1 except for the number of venturis provided. As shown Figure 2, the venturi outlets 24’ are uniformly distributed in a plurality of rows 26,28, neighbouring rows 26,28 being equispaced from one another. In the example of Figure 2, the plurality of venturis has four venturis. The venturi outlets 24’ are arranged in a square pitch.

In the example of Figure 2, neighbouring venturi outlets 24’ are equispaced from one another. In both Figures 1 and 2, the venturi outlets 24,24’ are equidistant from the burner i.e. the venturi outlets are all spaced from the burner by the same amount (the same fluid flow distance).

In a further example (not shown), a first subset of venturi outlets is spaced from the burner by a first distance and a second subset of venturi outlets is spaced from the burner by a second distance, the first and second distances being different to one another. Figure 6d shows an arrangement of a plurality of venturis which can be used in such an example.

The mixing flow pathway 12 is configured to present unobstructed and uncongested passage for fluid flowing between the plurality of venturis 8,10 and the burner 6. In this respect, the fluid has a clear passage through the mixing flow pathway without flowing over or around features (for example, protuberances) in the mixing flow pathway 12 which tend to promote swirl, eddy currents and/or turbulence.

The mixing flow pathway 12 is straight for fluid flowing between the plurality of venturis 8,12 and the burner 6. However, in a further example, the mixing flow pathway comprises one or more bends for fluid flowing between the plurality of venturis 8,12 and the burner 6. Figure 6b shows an arrangement of a plurality of venturis which can be used in such an example.

The burner 6 is planar and nominally flat. However, in a further example, the burner is curved in one or more axis. In yet another example, the burner is cylindrical.

The use of a plurality of venturis allows a volume of fuel gas and a volume of gaseous oxidiser (such as air) to be separated into sub-volumes of fuel gas and gaseous oxidiser, wherein each sub-volume is mixed in a different venturi. The mixed sub-volumes of fuel gas and gaseous oxidiser leave each venturi and further mix downstream in the mixing flow pathway. The sub-volumes from different venturis also mix with one another in the mixing flow pathway where they form a single volume of gas, which becomes a homogenous flow.

The division of the fuel gas and gaseous oxidiser (such as air) into sub-volumes mixed together separately, allows a homogenous mixing of the total volume of gasses to be completed more quickly, and thus over a shorter distance in the direction of flow, than if the whole volume of fuel gas (as a single body of fuel gas) and the whole volume of gaseous oxidiser (as a single body of gaseous oxidiser) were mixed together. So, the use of a plurality of venturis allows for a homogenous mixing of fuel and oxidiser to be completed more quickly and over a shorter distance in the direction of flow than with the use of a single venturi, for a given volume flow rate. This is shown schematically in Figure 3.

On the left hand side of Figure 3, flows of fuel 30 and air 32 from a single venturi (not shown) are shown mixing in a mixing flow pathway (not shown) downstream of a plane 34 in which a venturi outlet is located. The mixing distance 36 is the distance the fuel 30 and air 32 flows before a homogenous condition is achieved.

On the right hand side of Figure 3, multiple flows of fuel 31 and air 33 from a plurality of venturis (not shown) are shown mixing in a mixing flow pathway (not shown) downstream of a plane 34 in which a plurality of venturi outlets is located. The venturis on the right hand side of Figure 3 are smaller than the single venturi on the left hand side of Figure 3. The mixing distance 37 is the distance the fuel 31 and air 33 flows before a homogenous condition is achieved. It is seen that the mixing distance 37, when fuel gas and air is divided into sub-volumes and mixed, is less than the mixing distance 36, when the same volumes of fuel gas and air are mixed as a whole.

The distance between the venturi outlets 24 and the burner 6 is such that, in at least one use condition of the boiler 2, fluid flowing in the mixing flow pathway 12 becomes homogenously mixed in the mixing flow pathway 12 at or adjacent the location of the burner 6. In other words, for at least one flow condition of the boiler 2, the position of the burner 6 relative to the venturi outlets 24 is such that the burner position and the position within the mixing flow pathway 12 at which the fuel and air become homogenously mixed (or, alternatively, homogenous to a sufficient degree for combustion) are coincident or close to being coincident. This positioning of the burner 6 ensures that a homogenous mix of fuel and air (or a mix suitable for combustion) is burnt early or at the earliest opportunity. Consequently, the length of the mixing flow pathway is not unnecessarily or overly long. This minimises the likelihood of turbulence developing and precludes a significant run-up distance upstream of the burner 6 and, as a result, reduces the severity of a flashback occurring in the mixing flow pathway 12. The volume of pre-mixed fuel upstream of the burner is also thereby reduced, to a minimum or close to a minimum. Then, in the event of a flashback, damage to components resulting from the burn of the pre-mixed fuel upstream of the burner will be less than would have been the case with a larger volume of pre-mixed fuel.

The flow conditions in the mixing flow pathway 12 will vary during use of the boiler 2, depending on how the boiler is being used and its operational settings. For example, the velocity of fluid flow in the mixing flow pathway 12 will vary during use of the boiler 2. Variations in flow conditions will vary the position within the mixing flow pathway 12 at which the fuel and air become homogenously mixed (or mixed to a required degree for combustion). So, for some use conditions of the boiler 2, the position within the mixing flow pathway 12 at which the fuel and air become homogenously mixed will not be coincident with the position of the burner 6.

In examples, the distance between the venturi outlets 24 and the burner 6 is such that, in all use conditions of the boiler 2, fluid flowing in the mixing flow pathway 12 is homogenously mixed in the mixing flow pathway 12 at the location of the burner 6. In this example, fluid flowing in the mixing flow pathway 12 becomes homogenously mixed in the mixing flow pathway 12 at the location of the burner 6 for at least one use condition of the boiler. The boiler 2 has a temperature sensor 40 (or a thermostat) configured to sense the temperature of the plurality of venturis 8,10. In this way, the presence of a stabilised flame on or in the plurality of venturis 8,10 can be determined. A controller (not shown) receives measurement data from the temperature sensor 40 and determines whether or not a temperature threshold (indicative of a stabilised flame at the plurality of venturis 8,10) is exceeded. If the controller determines a stabilised flame is present, then the control provides a warning to the user and/or takes corrective action to reduce the severity of damage to the boiler, for example, isolating the boiler from the source of fuel.

The boiler 2 has a single fuel chamber 42 common to the fuel inlets 16 of the plurality of venturis 8,10 wherein said fuel inlets 16 open into the fuel chamber 42. Furthermore, the boiler 2 has an oxidiser chamber 44 common to the oxidiser inlets 20 of the plurality of venturis 8,10 wherein said oxidiser inlets 20 open into the oxidiser chamber 44. The oxidiser is air, and so the oxidiser chamber 44 is an air chamber.

Figure 4 shows a detailed view of a venturi 8 for use in a plurality of venturis of the present disclosure. Figure 5 shows the flow of fuel 31 and air 33 from the venturi 8, wherein the fuel 31 and air 33 combine in the venturi 8 and mix together along the mixing distance 37 before becoming a homogenous mix 39 of fuel and air.

Each venturi has a circular cross-sectional shape. Flowever, in other examples, each venturi has a non-circular cross-sectional shape, such as an oval or rectangular shape, or some geometric form with multiple degrees of symmetry.

The boiler 2 is a hydrogen fuel boiler.

The present disclosure also provides a heating system having a boiler with a plurality of venturis, wherein the fuel inlet is in fluid communication with a source of hydrogen fuel. The present disclosure provides a method of burning fuel (hydrogen) having the steps of combining a flow having fuel (hydrogen) and a flow having oxidiser in a plurality of venturis, forming a mixture having fuel (hydrogen) and oxidiser by combining outlet flows from the plurality of venturis in a single mixing flow pathway common to and extending from the plurality of venturis, and burning the mixture having fuel (hydrogen) and oxidiser at a burner. In an example, the oxidiser is air.

The step of forming a mixture of fuel (hydrogen) and oxidiser (such as air) flows hydrogen and oxidiser along a fluid flow distance extending from the venturi outlets of the plurality of venturis to the burner wherein said distance is no more than a predetermined multiple of the detonation cell size (DCS) of a stoichiometric mixture of the fuel (hydrogen) and the oxidiser (for example, air) for the conditions of the mix of fuel (hydrogen) and oxidiser. The predetermined multiple is no more than 6. In an example, the predetermined multiple is between 2 and 6. In an example, the predetermined multiple is 3.

The step of forming a mixture of fuel (hydrogen) and oxidiser flows fuel and oxidiser from a fuel (hydrogen) and oxidiser mixing chamber of each venturi to the burner wherein the fluid flow distance between the mixing chamber of each venturi and the burner is no more than a given multiple of the detonation cell size (DCS) of a stoichiometric mixture of the fuel (hydrogen) and the oxidiser (for example, air) for the conditions of the mix of fuel and oxidiser. The given multiple is no more than 12. In an example, the given multiple is between 4 and 12. In an example, the given multiple is 6.

Furthermore, the present disclosure also provides a method of burning fuel, such as hydrogen, the method having the steps of combining flows of fuel (hydrogen) and oxidiser in a plurality of venturis, forming a homogenous mixture of fuel (hydrogen) and oxidiser by combining outlet flows from the plurality of venturis in a mixing flow pathway extending from the plurality of venturis, and burning the homogenous mixture of fuel (hydrogen) and oxidiser at a burner. The homogenous mixture of fuel (hydrogen) and oxygen is burnt at the burner prior to the establishment of developed turbulent flow. The homogenous mixture of fuel (hydrogen) and oxidiser is burnt at the burner at the point where the homogenous mixture is formed.

Figures 6a-k provide schematic views of eleven different arrangements for a plurality of venturis. These schematic views shown the general arrangement of the plurality of venturis but are not to scale and any flow distances to a burner are not to scale. These arrangements are described briefly below.

Figure 6a shows a schematic view of a plurality of venturis arranged with venturi outlets 24 opening adjacent a turbulator 45 so as to promote mixing.

Figure 6b shows a schematic view of a plurality of venturis arranged along three sides of a square/rectangle with the venturi outlets 24 facing inwards towards the interior of the square/rectangle. The venturi outlets 24 open into a mixing flow pathway 12 and at various locations along the mixing flow pathway 12, which itself extends along the three sides of the mentioned square/rectangle before extending towards a burner.

Figure 6c shows a schematic view of a plurality of venturis arranged on a cylindrical surface of a cylinder with the venturi outlets 24 facing outwards away from the interior of cylinder into which air is directed.

Figure 6d shows a schematic view of a plurality of venturis oriented in parallel with one another and arranged in parallel rows, wherein every other row is positioned offset (in the direction of flow through the venturis) relative the remaining rows.

Figure 6e shows a schematic view of a plurality of venturis oriented in parallel with one another and arranged in parallel rows, wherein every venturis is located in the same plane. Figure 6f shows a schematic view of a plurality of venturis arranged on a spherical surface of a sphere with the venturi outlets 24 facing outwards away from the interior of sphere into which air is directed.

Figure 6g shows a schematic view of a plurality of venturis oriented in parallel with one another and arranged in a single line/row.

Figure 6h shows a schematic view of a plurality of venturis arranged in a circular and oriented on radials of the circle wherein the venturi outlets 24 face outwards away from the centre of the circular, from which air is directed through the venturis.

Figure 6i shows a schematic view of a plurality of venturis arranged on a part spherical surface of half of a sphere with the venturi outlets 24 facing outwards away from the concave interior surface of half-sphere on to which air is directed.

Figure 6j shows a schematic view of a plurality of venturis arranged on a half circular surface forming an arch wherein the venturi outlets 24 face outwards away from the centre of the a half circular, from which air is directed through the venturis.

Figure 6k shows a schematic view of a plurality of venturis arranged on a conical surface with the venturi outlets 24 facing outwards away from the interior of the cone from which air is directed.

Figures 7 and 8 show a burner assembly 50 (or a part thereof) of a third boiler. Features of the burner assembly 50 common with the examples described elsewhere herein are provided with common reference numerals.

Figure 7a shows an exploded perspective view of the burner assembly 50, which has a number of components arranged in a stack and fastened together with appropriate fasteners (for example, bolts or screws) or integrated as combined components. The components include a heat exchanger 52, on top of which is integrated a combustion chamber assembly 54, on top of which and within a receiving recess of the combustion chamber assembly 54 is mounted a burner 6, on top of which is mounted a venturi array assembly 56 (which includes a plurality of venturis), on top of which is mounted an air intake cover 58. A first ring seal 60 is located between the air intake cover 58 and the venturi array assembly 56. A second ring seal 62 is located between the burner 6 and combustion chamber 54 on the one hand, and the venturi array assembly 56 on the other hand. The fasteners extend through the venturi array assembly 56 and into a housing of the combustion chamber assembly 54 and the heat exchanger 52. In this way, all components of the burner assembly 50 are retained together. All of the oxidiser for fuel burnt at the burner is provided through the plurality of venturis.

Figure 7b shows an exploded perspective view of the venturi array assembly 56. The venturi array assembly 56 has a first row of seven venturis 70 and a second row of eight venturis 80. The venturis 70,80 are formed from upper and lower mating planar members 90,92 which have a ring seal 94 located therebetween and are secured together by fasteners 96 (see Figure 8). Specifically, in the appliance 50, the first fluid flow distance 25 is 40mm (i.e. no more three times the DCS) and the second fluid flow distance 27 is 50mm (i.e. no more than six times the DCS)

In another example, a boiler is provided as described above in relation to the arrangement of Figure 1 or 2 but where the predetermined multiple in relation to the first fluid flow distance 25 is more than 6 and the given multiple in relation to the second fluid flow distance 27 is no more than 12.

Although specific examples have been illustrated and described herein, a variety of alternate and/or equivalent implementations may be substituted for the specific examples shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific examples discussed herein. Therefore, it is intended that this disclosure be limited by the claims and the equivalents thereof.