The present invention relates to a premix gas burner designed for operation with a premixing of a combustible air and gas mixture.
The invention relates in particular to the specific technical field of modulating gas burners, in which the burner power can be modulated within a predetermined range of modulation, according to the specific operating requirements.
Conventional burners can be classified according to the form of combustion, as either diffusion flame or premixed flame burners. The term "diffusion flame" denotes a combustion process in which the reagents are mixed by fluid-dynamic action downstream of the burner head. The term "premixed flame" denotes a process in which the mixture is ready for burning when it reaches the burner head.
In the case of diffusion flame combustion, there are two principal phenomena, namely a mixing phenomenon (characterized by a mixing time) and a combustion phenomenon (characterized by a combustion time). In premixed combustion, the mixing time is zero and only the combustion time remains; therefore flames of this type generally have a higher velocity. Moreover, a premixed flame takes up less space than a diffusion flame, for a given burner power, because the reaction is faster since it is independent of the mixing time.
As regards the combustion products, nitrogen oxide emissions are related to various aspects such as the mixing of the reagents, the time for which the combustion products remain in "hot spots", the general temperature within the flame, and the uniformity of the flame. The premixing of the reagents upstream of the burner head tends to eliminate the presence of "hot spots" in the combustion area and thus tends to make the temperature more uniform. This characteristic cannot be obtained with diffusion flames in which there is a lack of uniformity of the thermal field.
In the field of diffusion flames, however, combustion conditions have been developed which lead to a reduction in nitrogen oxide emissions by fluid dynamic methods, that is, by making the thermal field more uniform and reducing the mean temperature in the combustion chamber. Burners which have been developed, in the diffusion combustion field, to reduce nitrogen oxides by acting on the fluid dynamics of the combustion are called "swirl" burners or recirculation burners. These cause the combustion products to be recirculated in the combustion area and this recirculation tends to reduce the mean temperature in the combustion area, thus reducing the nitrogen oxides of thermal origin. These burners are usually operated with limited modulation ratios, for example, with a ratio of 1 to 2, because the recirculation phenomenon is a fluid dynamic phenomenon which is present only in specific reagent velocity conditions. Burners of this type are not widely used in the domestic field because of the limited range of modulation and because of the space taken up by the flame. In the domestic field, the use of premix burners is preferred in condensation boilers. This type of burner has a valve and a mixing system upstream of the burner. With conventional premix burners, it is possible to achieve modulation ratios of 1 to 5 whilst limiting nitrogen oxides because the thermal field in the flame is made uniform and the periods spent in the combustion area are reduced. The possibility of providing wider modulation ranges, that is, of reducing the power at the burner head, is dependent on the availability of a burner head with apertures that are designed to stabilize the flame above them and to eliminate the effects of flash-back or flame detachment. With known condensation boiler technology, one possible method of reducing nitrogen oxides is to increase the quantity of air whilst keeping the quantity of fuel constant. Thus, the maximum temperature which can be reached within the flame is reduced and the nitrogen oxides of thermal original are consequently also reduced. However, this method is subject to two drawbacks the first of which lies in the fact that the heat exchanger associated with the burner operates optimally only for certain air/fuel ratios and, consequently, the exchanger no longer works at maximum efficiency if these ratios are varied; the second drawback is due to the fact that excessively lean flames, that is, flames with a large excess of air, are unstable.
The problem underlying the present invention is that of providing a modulating burner of the above-mentioned type which is designed structurally and functionally to overcome the limitations discussed with reference to the prior art mentioned.
This problem is solved by the invention by means of a burner constructed in accordance with the appended claims.
Further characteristics and advantages of the invention will become clearer from the following detailed description of some preferred embodiments thereof which are described by way of example with reference to the appended drawings, in which:
With reference to the drawings mentioned, a first embodiment of a burner, in particular a modulating burner for combustible gas of the premixed type, formed in accordance with the present invention, is generally indicated 1. The burner 1 comprises a burner body 2 with a burner head 3 downstream of which the air and gas mixture that is supplied to the burner, for example, by means of a fan device, not shown, is burnt.
The burner is designed to be housed in a combustion chamber, not shown, of a heating apparatus, also not shown, which also houses a heat exchanger within which a working fluid circulates and is heated by the burner.
The burner body 2 comprises, at the end axially remote from the head 3, a connecting flange 4 for fixing the burner body to a shutter element which is provided for removably sealing the combustion chamber.
More specifically, starting from the flange 4, the burner body has a first, cylindrical portion 5 which is axially symmetrical with a principal axis X and is extended axially by a second, tapered portion 6.
The cylindrical shell portion 5 is connected, upstream of the tapered portion 6, to a front surface 7 of the head 3 extending transversely to the axis X. The tapered portion 6 has generatrices which converge towards the surface 7, as shown in
Upstream of the burner head 3, with respect to the axial direction of the flow, there is a device, shown only schematically and generally indicated 8, for premixing the air and gas flows that are supplied to the burner.
This device may have a conventional system for injecting the combustible gas into the air flow supplied to the burner and may also be associated with a fan unit the intake end of which is supplied with an air and gas mixture which is thus subjected to premixing.
The burner head 3 is also provided with a device for guiding the flow of the mixture, which device can impart rotational components to the velocity of the flow that is supplied through the burner head, as well as with a device for preventing flashback.
In particular, a plurality of distributor compartments 9 are provided in the surface 7 of the head; the compartments extend below the profile of the surface 7 towards the flange 4 and are spaced apart from each other. In a preferred configuration, the distributor compartments 9 are spaced at regular angular intervals and radiate from the centre of the circular surface 7 located on the axis X.
Each compartment 9 is delimited by a pair of walls 9a, 9b which are interconnnected along an edge 10 which extends radially and is connected to the surface 7. At the ends radially remote from the axis X, the walls 9a, 9b are connected to a further lateral wall 11 which defines the corresponding compartment 9 jointly with them.
In each compartment 9, the walls 9a, 9b are inclined to one another to define an obtuse angle inside the compartment, about the edge 10, as shown in the drawings.
The wall 9a of each compartment 9 also has a plurality of slotted and/or circular through apertures 12 which are arranged in a preselected configuration and through which the air and gas mixture to be burnt on the surface 7 of the head is supplied. The configuration of the apertures 12 is also selected so as to prevent flash-back when the burner is operating at low power, thus ensuring stable attachment of the flame to the burner. In each compartment, the wall 9b is a solid wall and its inclination to the contiguous wall 9a is such that the flow of the mixture supplied through the head is diverted and a recirculation phenomenon is created by means of a rotational component imparted to the flow of reagents, as explained further below.
The flow guide device which can cause the above-mentioned recirculation is thus formed by the plurality of walls 9b formed in the compartments 9 of the burner head 3.
The flash-back prevention device is formed by the plurality of through apertures 12 in a suitable quantity and of suitable size, which are created in the plurality of walls 9a of the distributor compartments 9.
In operation, the air and gas reagent mixture, premixed upstream of the burner head 3, is supplied through the apertures 12 of each distributor compartment 9 and is diverted by the inclined profile of the compartment. A low pressure region is thus created in the centre of the burner, in the area downstream of the head, as shown in the diagram of
Conversely, if the power, and consequently the reagent flow rate, is reduced, the low pressure region in the centre of the burner can no longer draw the flow towards it and the burner tends to operate as a conventional premix burner, without recirculation. In this operating phase, moreover, the burner is not affected by flash-back problems, by virtue of the operation of the flash-back prevention device provided in the burner, or by instability problems due to flame detachment, by virtue of the stabilizing function of the device.
Consequently, the burner can adapt to the combustion conditions as the burner power varies, in the range of modulation in which it is used.
During the transition from a recirculation burner to a conventional premix burner, a fluid-dynamically unstable transition area may be present, with fluctuations of the reagent flow which may lead to pulsations or extinction of the flame during this phase. To ensure the presence of a stabilized flame even in this transition area, the lateral wall (portion 6) of the burner body is tapered, as described above. The presence of this tapered wall creates a flame attachment area and thus makes the transition between the two combustion conditions stable.
With reference to
The burner 1' comprises a burner body 2' with a burner head 3' downstream of which the air and gas mixture that is supplied to the burner, for example, by means of a fan device, not shown, is burnt.
The burner is designed to be housed in a combustion chamber, not shown, of a heating apparatus, also not shown, which also houses a heat exchanger within which a working fluid circulates and is heated by the burner.
The burner body 2' comprises, at the end axially remote from the head 3', a connecting flange 4' for fixing the burner body to a shutter element (not shown) which is provided for removably sealing the combustion chamber. More specifically, starting from the flange 4', the burner body 1' has a cylindrical portion 5' which is axially symmetrical with a principal axis X' and is connected, at its opposite end, to the front portion of the head 3' which extends transversely to the axis X'.
Upstream of the burner head 3', with respect to the axial direction of the flow, there may also be a device, shown only schematically in the drawings, for premixing the air and gas flows that are supplied to the burner.
This device may have a conventional system for injecting the combustible gas into the air flow supplied to the burner and may also be associated with a fan unit the intake end of which is supplied with an air and gas mixture which is thus subjected to premixing.
The burner head 3' may also be provided with a device, generally indicated 20, for guiding the flow of the mixture, which device can impart rotational components to the velocity of the flow that is supplied through the burner head, as well as with a flash-back prevention device.
In particular, the above-mentioned devices are constructed so as to be structurally independent of one another and the flash-back prevention device is advantageously arranged downstream of the flow guide device 20. The flow guide device 20 comprises a plurality of radial deflector fins 21 spaced at regular angular intervals and radiating from a central support area 22 extending in the region of the axis X'.
The fins 21 are preferably inclined to the axial direction of the flow through the head so as to impart a rotational component to the flow that is conveyed through respective passageway sections 23 which are defined between each pair of adjacent fins 21.
More particularly, each fin 21 comprises a first surface portion 21a extending radially between the support 22 and the circumferential edge of the burner body and a second surface portion 21b which forms an extension of the first portion and is inclined to the axial direction X', as shown in the drawings.
The surface portion 21b of each fin also has a substantially flat surface which is inclined at 45° to a plane perpendicular to the axis X'. In each fin 21, the second surface portion 21b is also advantageously obtained by cutting and subsequent bending of a respective portion of a flat disk extending transversely to the axis X in the region of the burner head.
This disk may be produced integrally with the tubular burner body 2' or, alternatively, may be structurally independent of the body 2' and connectible to the body by suitable connection means, for example, by welding along the circumferential edge.
The flash-back prevention device comprises a plate-like element 24 provided with a plurality of slotted and/or circular through apertures 25 which extend through its thickness in a preselected configuration and through which the air and gas mixture to be burnt on the surface of the head is supplied. The configuration of the apertures 25 is also selected so as to prevent flash-back when the burner is operating at low power, thus ensuring stable attachment of the flame to the burner.
Preferably, the plate-like element 24 has a flat disk-like shape and is produced so as to be structurally independent of the burner body and to be fixable thereto, for example, by welding or clinching along the circumferential profile thereof.
Moreover, the element 24 may be made of blanked sheet-metal with apertures 25 produced by drilling or punching.
Alternatively, the flash-back prevention device may be formed with a mesh structure, for example, with a metal mesh suitably connected to the burner along the circumferential profile of the tubular body 2'.
As a further alternative, the flash-back prevention device may be made of metal fibre.
In a first variant of the embodiment just described, which is shown in
In operation, the air and gas reagent mixture premixed upstream of the burner head 3' is supplied through the apertures 25 and is diverted beforehand by the inclined profiles of the deflector fins 21. A low pressure region is created in the centre of the burner, in the area downstream of the head; in certain conditions, the low pressure region can draw the flow into itself, thus forming a toroidal recirculation bubble which serves to stabilize the flame and reduce the mean temperature of the flame front, thus reducing nitrogen oxide (NOx) emissions. In this phase, the burner operates in a manner typical of a recirculation burner with a predetermined number of "swirls" and this manner of operation is maintained, in the modulation range, as the power gradually increases up to the maximum power permitted for the burner.
Conversely, if the power, and consequently the reagent flow rate, is reduced, the low pressure region in the centre of the burner can no longer draw the flow towards it and the burner tends to operate in the manner of a conventional premix burner, without recirculation. In this operating phase, moreover, the burner is not affected by flash-back problems, by virtue of the operation of the flash-back prevention device provided in the burner, or by instability problems due to flame detachment, by virtue of the stabilizing function of the device.
Consequently, the burner can adapt to the combustion conditions as the burner power varies, in the modulation range in which it is used.
During the transition from recirculation burner to conventional premix burner, a fluid-dynamically unstable transition area may be present with fluctuations of the reagent flow which may lead to pulsations or extinction of the flame during this phase. To ensure the presence of a stabilized flame even in this transition area, the lateral wall (wall 26) of the burner body is tapered, as described above. The presence of this tapered wall creates a flame attachment area and thus makes the transition between the two combustion conditions stable.
In a second variant, shown in
The second device 27 is structurally and functionally equivalent to the guide device 20 described above to which reference should be made for a detailed description. The device 27 also comprises a plurality of deflector fins, all of which are indicated 21 for simplicity, and which are structurally identical to those provided in the device 20. In this case, the fins 21 are connected to the outer circumferential edge by a ring 28 with a cylindrical wall which is intended to be connected to the cylindrical burner body 2'. The second guide device 27 provides an additional contribution to the rotational component imparted to the flow of the mixture. Moreover, since it is disposed downstream of the flash-back prevention device, it performs its function in an area in which flame is already present, in contrast with the guide device 20 which is active in a mixing area of the burner, in the absence of flame.
A tapered lateral wall 26 is provided in the burner body 2' and extends in the region of the front surface of the head, coaxially therewith. More particularly, the tapered wall 26, which converges towards the burner head 3', is provided downstream of the second finned guide device 27, with respect to the axial direction X' of the flow supplied to the head 3', as is clearly shown in the respective drawings. The function of the tapered wall is substantially that already described above with reference to the embodiment of
The invention thus solves the problem posed, affording many advantages over known solutions.
The dual operating conditions incorporated in the modulating boiler according to the invention afford, in the first place, a greater power modulation range and greater efficiency with lower nitrogen oxide emissions.
It is also possible to produce a burner which is smaller than conventional burners for the same burning power. Moreover, it is possible to provide burners which are interchangeable with conventional burners both with respect to their dimensions and with respect to the connection flange. Furthermore, the nitrogen oxide emissions are reduced in the burner of the invention in comparison with a conventional boiler, owing to the recirculation of the combustion products in the flame area.
By virtue of the combustion conditions with maximum flame detachment, the burner head of the burner according to the invention is less prone to overheating than conventional burners.
It is also possible to reduce the cost of the burner in comparison with conventional systems of the same power by virtue of the smaller dimensions of the burner and the fact that it can be produced from a single component, as well as the fact that less expensive materials can be used by virtue of the less critical thermal operating conditions.
In comparison with the preceding solutions, the solution of the second embodiment described and of its variants also resolves the technological aspects of construction since the construction is much simpler, given the known sheet-metal processing techniques. This constructional simplicity also leads to reduced stressing of the various parts of the burner and hence to an increase in its life and strength.
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.