EP1462716B1

EP1462716B1 - Air/gaseous fuel mixer for premix burners, and combustion system featuring such a mixer

Info

Publication number: EP1462716B1
Application number: EP03425185A
Authority: EP
Inventors: Andrea Lonardi; Andrea Lovato; Silvano Pavan; Giuseppe Toniato
Original assignee: Riello SpA
Current assignee: Riello SpA
Priority date: 2003-03-24
Filing date: 2003-03-24
Publication date: 2010-07-07
Anticipated expiration: 2023-03-24
Also published as: DE60333252D1; EP1462716A1; ATE473398T1

Abstract

An air/gaseous fuel mixer (10) for premix burners, having: a feed conduit (11) for feeding combustion air to a mixing chamber (12); a device (13) for feeding gaseous fuel to the mixing chamber (12) to form the desired air/gaseous fuel mixture; and a device (16; 160) for creating swirl in the air/gaseous fuel mixture issuing from the mixing chamber (12). <??>The mixer (10) also has at least a further device (17; 170) for creating swirl solely in the air from the feed conduit (11). <IMAGE>

Description

The present invention relates to an air/gaseous fuel mixer for premix burners, and to a combustion system featuring such a mixer.
The invention also relates to a process for preparing an air/gaseous fuel mixture in an air/gaseous fuel mixer for premix burners.
The invention therefore employs gaseous fuel, which may be methane or LPG.
In combustion engineering, so-called "premix" burners are known, in which air and fuel are mixed in a mixer upstream from the actual combustion head, and which, among other things, are characterized by permitting ample regulation of the heat developed in the combustion head.
In a first marketed solution, the mixture issuing from the mixing chamber is fed through the blades of a swirl device to improve the air/gaseous fuel mixture before it is fed to the combustion head of the actual burner.
For low-power premix burners, a second solution has also been devised, in which the air/gaseous fuel mixture is fed directly to a blower where the two components are mixed thoroughly.
This, however, as stated, is purely a low-power solution, owing to the difficulty, over 100 kW, of producing airtight fans suitable for air/gas mixtures.
The first solution, however, i.e. in which swirl is created in the mixture only, has also proved unsatisfactory, by failing to provide for thorough mixing of all air/gaseous fuel ratios.
From the document WO 00/25065 it is known to feed the mixture through the blades of a swirl device to improve the air/gaseous fuel mixture before it is fed to the combustion head of the burner. However, WO 00/25065 does not provide any hint to use swirl means for creating further swirl solely in the air too. Moreover, WO 98/51966 describes a premix burner wherein the combustion air solely is swirled before creating the air/gaseous fuel mixture.
On the other hand, also fitting the combustion air feed conduit with a swirl device has always been ruled out on the grounds of generating flow resistance.
Surprisingly, however, also fitting the air feed conduit with a swirl device has been found to greatly improve the quality of the mixture flowing from the mixing chamber to the combustion head.
It is therefore an object of the present invention to provide an air/gaseous fuel mixer for premix burners, designed to eliminate the aforementioned drawbacks, and which therefore also features a swirl device in the air feed conduit to the air/gaseous fuel mixing chamber.
According to the present invention, there is provided an air/gaseous fuel mixer for premix burners, as claimed in Claim 1.
It is a further object of the present invention to provide a combustion system featuring such an air/gaseous fuel mixer for premix burners.
Yet a further object of the invention is to provide a process for preparing an air/gaseous fuel mixture in an air/gaseous fuel mixer for premix burners.
A non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which:

Figure 1 shows a first embodiment of an air/gaseous fuel mixer for premix burners in accordance with the present invention;
Figure 2 shows a second embodiment of an air/gaseous fuel mixer for premix burners in accordance with the teachings of the invention;
Figure 3 shows a third embodiment of an air/gaseous fuel mixer for premix burners in accordance with the invention;
Figure 4 shows an enlarged front view of a first swirl device employed in the Figure 1 and 2 mixers;
Figure 5 shows an enlarged front view of a second swirl device employed in the Figure 3 mixer.

Number 10 in Figure 1 indicates a first embodiment of an air/gaseous fuel mixer for premix burners in accordance with the invention.
Mixer 10 comprises a tubular conduit 11 for feeding combustion air to a preferably, though not necessarily, circular-section mixing chamber 12. Conduit 11 and mixing chamber 12 are arranged in series with a common longitudinal axis of symmetry (a); and mixing chamber 12 comprises a cylindrical wall 12c.
Part of a gaseous fuel feed device 13 extends inside the mixing chamber 12.
In the Figure 1 embodiment, device 13 comprises a cylindrical tubular first portion 13a extending symmetrically about an axis (b) substantially perpendicular to axis (a); first portion 13a merges with a second portion 13b closed at one end; and second portion 13b is also substantially tubular, is aligned with axis (a), and is therefore substantially perpendicular to first portion 13a.
In other words, portions 13a, 13b of device 13 substantially form an L, and are a continuation of each other; and first portion 13a is fixed, by known means not shown, to wall 12c of mixing chamber 12.
A number of pipes 14, each having an axis (c) perpendicular to axis (a), are fixed to and radiate from second portion 13b.
As shown in Figure 1, gaseous fuel is fed into device 13 through an opening 13c, flows along portion 13a in the direction of arrow F1, turns into portion 13b in the direction of arrow F2, and is then fed by pipes 14 into mixing chamber 12 in the directions shown by arrows F3.
Combustion air is blown, by a blower not shown in Figure 1, along conduit 11 in the direction of arrow F4, and mixes with the gaseous fuel fed, as stated, into mixing chamber 12 by pipes 14.
In an embodiment not shown, pipes 14 are replaced with straightforward through holes.
The force of the air and that supplied by the blower (not shown) feed the air/gaseous fuel mixture so formed to a combustion head 15 (shown partly in Figure 1).
A first swirl device 16, shown in more detail in Figure 4, is interposed between mixing chamber 12 and combustion head 15.
Swirl device 16 comprises a plate 16a having holes 16b for assembly (by means of screws not shown) to a first end 12a of mixing chamber 12 (Figure 1).
As shown in Figure 4, plate 16a also comprises a central hole 16c, from which extend radially a number of openings 16d, each having an inclined plate 16e.
As a result of known phenomena, swirl is created in the air/gaseous fuel mixture flowing through device 16 towards head 15, substantially on account of plates 16e, which improve the mixture before it is fed to combustion head 15.
A second swirl device 17, identical with device 16 (Figure 4), is innovatively installed at a second end 12b of mixing chamber 12.
Device 17 creates a certain amount of turbulence solely in the air mass, before it is mixed with the gaseous fuel injected by device 13 into mixing chamber 12.
Theoretical calculations, confirmed by tests conducted for the purpose, show that injecting mixing chamber 12 with air which in itself is already turbulent provides for more thorough mixing with the gaseous fuel, and for improved combustion in head 15.
In the second embodiment of the present invention shown in Figure 2, the parts already described with reference to the first embodiment in Figure 1 are indicated using the same reference numbers, with the exception of fuel feed device 13, which is replaced by a similar device 130.
Device 130 comprises a cylindrical tubular portion 130a extending symmetrically about an axis (b) substantially perpendicular to axis (a); and portion 130a merges with a substantially toroidal conduit 130b resting on wall 12c of mixing chamber 12.
More specifically, conduit 130b comprises a curved wall 131, in which are formed a number of through holes 132, through which fuel is fed into a central region 133 of chamber 12, where it encounters and mixes with the combustion air from conduit 11.
More specifically, gaseous fuel is fed into device 130 through an opening 130c, flows along portion 130a in the direction of arrow F1, flows into conduit 130b in the direction of arrow F5, and is injected into central region 133 through holes 132 in the direction of respective arrows F6.
In the third embodiment shown in Figure 3, swirl devices 16 and 17 are replaced by respective propellers 160 and 170 located at first and second ends 12a and 12b of mixing chamber 12 respectively.
As shown in more detail in Figures 3 and 5, each propeller 160, 170 comprises a number of blades 161 fixed at one end to a hub 162 free to rotate about a respective shaft 163; and a bearing (not shown in Figure 3) is interposed between each hub 162 and respective shaft 163.
Both shafts 163 are fixed to second portion 13b of device 13 and aligned along axis (a).
Propeller 170 is rotated by the stream of pressurized air from conduit 11, and propeller 160 by the air/gaseous fuel mixture flowing from mixing chamber 12 towards combustion head 15.
In this case, too, propellers 160 and 170 provide for improved mixing of any air/fuel ratio, to achieve optimum combustion in combustion head 15.
In a further embodiment not shown, the two propellers are substituted for devices 16 and 17 in the Figure 2 embodiment.
In other embodiments, propellers 160, 170 are combined with respective devices 16, 17.
That is, in the Figure 1 and 2 embodiments, devices 16, 17 are provided together with respective propellers 160, 170 of the third embodiment of Figure 3.
The advantages of the mixer according to the present invention can be summed up in providing for a better air/fuel mixture, thus improving the efficiency of combustion head 15, and greatly reducing the formation of harmful combustion products, such as Nox.
The solutions proposed in the present invention are particularly useful for over 100 kW systems.
The present invention also relates to a combustion system featuring at least one mixer 10.

Claims

An air/gaseous fuel mixer (10) for premix burners, comprising:
- combustion air feed means (11) for feeding combustion air to a mixing chamber (12);

- gaseous fuel feed means (13; 130) for feeding gaseous fuel to said mixing chamber (12) to form the desired air/gaseous fuel mixture; and

- swirl means (16; 160) for creating swirl in the air/gaseous fuel mixture issuing from said mixing chamber (12);
the mixer (10) being characterized by also comprising further swirl means (17; 170) for creating further swirl solely in the air from said combustion air feed means (11).
A mixer (10) as claimed in Claim 1, characterized in that said gaseous fuel feed means (13) comprise a cylindrical tubular first portion (13a), which merges with a substantially cylindrical tubular second portion (13b) perpendicular to said first portion (13a).
A mixer (10) as claimed in Claim 2, characterized in that pipes (14) radiate from said second portion (13b) to feed gaseous fuel to said mixing chamber (12).
A mixer (10) as claimed in Claim 2, characterized in that said second portion (13b) has a number of through holes for feeding gaseous fuel to said mixing chamber (12).
A mixer (10) as claimed in Claim 1, characterized in that said gaseous fuel feed means (130) comprise a cylindrical tubular portion (130a), which merges with a substantially toroidal conduit (130b).
A mixer (10) as claimed in Claim 5, characterized in that said conduit (130b) comprises a curved wall (131), in which are formed a number of through holes (132) for feeding gaseous fuel into a central region (133) of said mixing chamber (12).
A mixer (10) as claimed in any one of the foregoing Claims, characterized in that said swirl means (16) and said further swirl means (17) each comprise a plate (16a) having a central hole (16c), from which extend radially a number of openings (16d), each having an inclined plate (16e).
A mixer (10) as claimed in any one of the foregoing Claims, characterized by comprising propellers (160, 170) located respectively at a first (12a) and a second (12b) end of said mixing chamber (12).
A mixer (10) as claimed in Claim 8, characterized in that each propeller (160, 170) comprises a number of blades (161) fixed at one end to a hub (162) free to rotate about a respective shaft (163).
A combustion system, characterized by comprising at least one mixer (10) as claimed in any one of the foregoing Claims.
A process for preparing an air/gaseous fuel mixture in an air/gaseous fuel mixer for premix burners; the process being characterized by the steps of:
- feeding air, in whose mass swirl has been created, into a mixing chamber;

- feeding gaseous fuel into said mixing chamber;

- mixing the air and gaseous fuel in said mixing chamber to obtain an air/gaseous fuel mixture;

- creating further swirl in the mass of said mixture; and

- feeding the turbulent said mixture to a combustion head.