EP3135997B1

EP3135997B1 - Air/gas mixing apparatus for premix burners and premix burner provided with such an apparatus

Info

Publication number: EP3135997B1
Application number: EP16186697.5A
Authority: EP
Inventors: Emil Calzolari; Simona DI NINNI; Andrea Lovato; Vito Poletto
Original assignee: Riello SpA
Current assignee: Riello SpA
Priority date: 2015-08-31
Filing date: 2016-08-31
Publication date: 2019-04-03
Anticipated expiration: 2036-08-31
Also published as: ITUB20153299A1; PL3135997T3; EP3135997A1; TR201908121T4; ES2720031T3

Description

The present invention relates to an air/gas mixing apparatus for premix burners and to a premix burner provided with such an apparatus.
The invention relates also to a method for the preparation of an air/gas mixture in an air/gas mixing apparatus for premix burners.
In particular methane or LPG can be used as fuel gas.
In the field of combustion, burners are known traditionally called "premixers" (also known as "premix burners"), wherein the air/gas mixing occurs upstream of the combustion head in the strictest sense of the word; the mixing of the two components taking place in a suitable mixing apparatus. Among other things, these burners are characterised in that they are able to achieve a wide modulation of the heat power generated in the combustion head.
For instance, reference is made to a premix type apparatus described in the patent EP-B1-1 462 716 (of the same Applicant).
This document describes an air/gas mixing apparatus for a premix burner where the gas supply pipe (also known as "gas sleeve") is elbow-shaped at 90° and is interposed, inside the air supply cylinder (also called "air sleeve"), between two fixed metal helixes.
The air from a blower, passing through the first helix acquires a tangential component of speed, so it moves with a helical motion inside the air sleeve.
Such an air flow strikes the gas sleeve, at the end of which, through a series of cylindrical tubes arranged radially, the fuel gas escapes which is affected by a first coarse mixing.
Downstream of the gas sleeve the second helix gives this air/gas flow another tangential speed component, thus increasing the degree of mixing of the two components.
By means of the present invention it is possible to improve certain aspects of the apparatus described and illustrated in the patent EP-B1-1 462 716 , and in particular the following points:

the degree of air/gas mixing; by achieving a more even flow of the two components, thereby obtaining a better physical-chemical condition for the development of a flame having good thermal performance and limited nitrogen oxide emissions (NOx); and
an easier and cheaper construction of the metal components for performing the air/gas mixing.

US-B1-7 897 121 (HUGHES et al. ) describes an air/gas mixing apparatus according to the preamble portion of claim 1.
Consequently, the main purpose of the present invention is to make an air/gas mixing apparatus for premix burners with improved air/gas mixing compared to the apparatus of the same type on the market today.
For this reason, according to the present invention an air/gas mixing apparatus for premix burners as claimed in claim 1 is provided.
A further aim of the present invention is to provide a premix burner (premix) provided with such an air/gas mixing apparatus.
For a better understanding of the present invention, a preferred embodiment will now be described by way of a nonlimiting example and with reference to the appended drawings, wherein:

Figure 1 shows a perspective view (in longitudinal cross-section and partially exploded) of an air/gas mixing apparatus for premix burners, made according to the teaching of the present invention;
Figure 2 shows an enlargement of a fuel gas distribution device used in the air/gas mixing apparatus according to Figure 1;
Figure 3 shows a three-dimensional exploded view of the fuel gas distribution device shown in Figure 2;
Figure 4 shows a first group of elements assembled together; such a group being comprised in the fuel gas distribution device illustrated in figures 2 and 3; and
Figure 5 shows a second group of elements assembled together; such a group being comprised in the fuel gas distribution device illustrated in figures 2 and 3.

In Figure 1 reference numeral 10 globally denotes an air/gas mixing apparatus for premix burners according to the teachings of the present invention.
The apparatus 10 comprises a circular opening 12A for supplying combustion air to an air/gas mixing chamber 12. The circular opening 12A (made in the housing (CRT) of the mixing chamber 12) and the mixing chamber 12 have a common horizontal, longitudinal axis of symmetry (X).
The circular opening 12A is provided, advantageously but not necessarily, with a respective metal helix (EL) to give to the incoming air to the mixing chamber 12 a substantially helical and whirling motion (swirl).
Moreover, inside the mixing chamber 12, an intake and distribution device 14 of a combustible gas (methane, LPG, etc.) extends.
As shown in Figure 1, on the wall of the air/gas mixing chamber 12 opposite the wall on which the circular opening 12A is made, there is an opening 12B which in use is crossed by the air/gas mixture coming out of said mixing chamber 12 (see below).
In an embodiment not shown the opening 12B may be fitted with a propelling helix (or similar device) suitable to impart a substantially spiral swirling movement to the air/gas mixture before it enters the combustion head (not shown).
In actual use, on the wall which the opening 12B is made on, a combustion head of the known type and not shown is attached (by known means not shown).
As it is known, the assembly of the mixing apparatus 10 and the combustion head is known by the name of "premix burner".
According to the embodiment shown in Figure 1, the intake and distribution device 14 is laid, in actual use, on a bottom wall 12F of the air/gas mixing chamber 12, so that the circular base 14 of a central cylindrical main body 14C of the device 14 coincides with a circular opening 12C made on the aforesaid bottom wall 12F.
The device 14 is fixed to the bottom wall 12F by mechanical means of the known type and not shown.
It may be noted how, in order to increase the clarity of the design, the device 14 shown in Figure 1 has been raised from the bottom 14F.
The device 14, in the particular embodiment illustrated in Figures 1, 2 and 3, comprises, therefore:

(a) - a first vertical portion of a cylindrical tubular shape represented by the aforementioned central cylindrical main body 14C which extends symmetrically around a vertical axis of longitudinal symmetry (Y) substantially perpendicular to the aforementioned horizontal axis (X);
(b) - a second horizontal portion 14D (having an axis (X)) of tubular cylindrical shape spatially facing towards the circular opening 12A and the metal helix (EL); the second horizontal portion 14D being connected with the said main central cylindrical body 14C; said second horizontal portion 14D being suitable to introduce into the device 14 a first amount of fuel in counter-current with respect to the air flow so as to form a first air / gas mixture;
(c) - a third horizontal portion 14S (having the axis (X)) of a cylindrical tubular shape spatially facing the opening 12B and the combustion head (not shown); the third horizontal portion 14S is also connected with the main central cylindrical body 14C and with the second horizontal portion 14D; said third horizontal portion 14S being suitable to introduce, in co-current with the flow of said first gas / air mixture, into the device 14, a second quantity of fuel gas so as to form a second air / gas mixture.

The three portions 14C, 14D and 14F are in fluidic communication with each other and form a substantially asymmetrical T.
It may be noted that one portion may be considered the ideal continuation of the other along said horizontal axis (X) since the second horizontal portion 14D and the third horizontal portion 14S are both aligned along the same horizontal axis (X).
In other words, the vertical main central cylindrical body 14C ends (at its upper end) with the two horizontal portions 14D, 14S placed like a mirror image with respect to the vertical axis (Y) so as to form an asymmetric T (given the different dimensions of the two horizontal portions 14D, 14 S) .
It may be noted also that while the second horizontal portion 14D is directed toward the circular supply opening 12A of the combustion air, the third horizontal portion 14S is facing the opening 12D (and thus, ultimately, the combustion head) from which the air / gas mixture from the air / gas mixing chamber 12 comes out.
Observing Figures 1, 2, 3 it can be noticed that the third horizontal portion 14S differs, in terms of size, from the second horizontal portion 14D; in particular the third horizontal portion 14S is smaller than the second horizontal portion 14D, as it has been found experimentally that for optimum combustion (i.e. with a low content of NOx) it is necessary that the volumetric quantity of fuel gas facing the incoming air is greater than the volumetric quantity of fuel gas that is mixed in a second phase with the first air / gas mixture.
As shown in particular in figure 3, the exit opening 14D* of the first quantity of fuel gas from the device 14 is associated with a first gas distribution group 15.
Said first gas distribution group 15 comprises two circular plates 15A and 15B, parallel to each other, of equal diameter, in use, spaced by a plurality of cylindrical spacers 15C, each of which is crossed lengthwise by a corresponding axial through hole 15C*.
More particularly, while the circular plate 15B has the appearance of a circular disc, the circular plate 15A is in the shape of a circular crown (closed ring) having, therefore, a central through-hole 15A ** (figure 3).
The first gas distribution group 15 is completed by a sleeve 15D provided with a flange 15E, the latter, in use, facing towards the circular plate 15A.
Observing Figure 3 it can be noticed that the three elements 15A, 15B, 15D have respective through holes 15A*, 15B*, 15D*, in use aligned each other and with the respective through axial holes 15C* (along respective horizontal axes substantially parallel to the axis (X)).
For each assembly consisting of through holes 15A*, 15B, 15C*, 15D* it is possible to use a respective screw (not shown) to mechanically fix the first gas distribution group 15 to the second horizontal portion 14D of the device 14.
Therefore, in actual use, an operator, after having packed the elements 15A, 15B, 15C, 15D together in such a way as to form the first gas distribution group 15, can fix said first gas distribution group 15 to the device 14 using known mechanical means, such as, for example, a plurality of screws.
As will be seen better below where the general working of the apparatus 10 is explained, when the first portion of fuel gas flows from the sleeve 15D in the direction of the central through hole 15A** into the interspace 20 (created between the two plates 15A, 15B spaced from each other by the plurality of cylindrical spacers 15C) four radial flows of said gas are formed which mix with the primary air coming from the opening 12A (after being swirled by the metal helix (EL)) .
This first mixture is pushed forward towards the combustion head again by the action of the air blower.
As shown again in figure 3, at the exit opening 14S* of the second quantity of fuel gas from the device 14 a second gas distribution group 16 is associated, similar to but not the same as the first gas distribution group 15 described.
In fact, the second gas distribution group 16 comprises two circular plates 16A and 16B, parallel to each other, of equal diameter, in use, spaced by a plurality of cylindrical spacers 16C, each of which is crossed lengthwise by a corresponding axial through hole 16C*.
More particularly, while the circular plate 15B has the appearance of a circular disc, the circular plate 16A is in the shape of a circular crown (closed ring) having, therefore, a central through-hole 16A ** (figure 3).
In the second gas distribution group 16 however, the flanged sleeve is absent.
The two gas distribution groups 15, 16 can be made at least partially using suitable metals and/or plastics.
It is to be noted also, that the flanged sleeve 15D creates a kind of venturi effect, which ensures a proper division into two parts of the gas flow coming from the central supply sleeve, so that a part is always directed towards the gas distribution group 15, which is located underflow with respect to the air coming from the blower.
By observing Figure 3 it can also be noticed that the two elements 16A, 16B, have respective through holes 16A*, 16B*, in use aligned with each other and with the respective through axial holes 16C* (along respective horizontal axes substantially parallel to the axis (X)).
For each assembly consisting of through holes 16A*, 16B, 16C*, 16D* it is possible to use a respective screw (not shown) to mechanically fix the second gas distribution group 16 to the third horizontal portion 14S of the device 14.
Therefore, in use, an operator, after having packed the elements 16A, 16B, 16C together in such a way as to form the second gas distribution group 16, can fix said second gas distribution group 16 to the device 14 using known mechanical means, such as, for example, a plurality of screws.
As will be seen better below where the general working of the apparatus 10 is explained, when the second portion of fuel gas exits the third horizontal portion 14S, it meets the first mixture (which, as we have said previously was formed at the first distribution group 15) mixing in turn with it to form a second, and final, air / gas mixture flowing towards the combustion head (not shown) where it is ignited by a specially provided electrode (not shown).
In particular, in the interspace 30 (which is created between the two plates 16A, 16B spaced from each other by the plurality of cylindrical spacers 16C) in this case too, four radial flows of said fuel gas are formed which mix with the first air / gas mixture arriving from the first mixing zone, the area substantially corresponding to the internal space facing the second horizontal portion 14D.
The working of the air/gas mixing apparatus 10 for premix burners according to the present invention is as follows:

(s1) as shown in Figure 1, the primary combustion air enters the mixing chamber 12 from the circular opening 12A (arrow (F1)), advantageously, but not necessarily provided with a metal helix (EL); the primary air is blown by a blower (not shown);
(s2) the fuel gas enters the device 14 through the circular base 14A of the main central cylindrical body 14C (arrow (F2)); the fuel gas is also driven by a respective blower (not shown);
(s3) after travelling vertically upwards through the entire main central cylindrical body 14C the fuel gas is diverted to the second horizontal portion 14D (arrow (F3)) and to the third horizontal portion 14S (arrow (F4));
(s4) consequently, a first portion of fuel gas exits radially from the first gas distribution group 15 according to the directions indicated by the arrows (F5) and is mixed with the incipient primary air forming a primary air / gas mixture at the area surrounding the first gas distribution unit 15;
(s5) the first mixture is thus pushed towards the opening 12B, still inside the mixing chamber 12, where it meets a second portion of fuel gas in radial output from the second gas distribution group 16 (arrows F6) to form a second, and last, air/gas mixture at the area surrounding the second gas distribution group 16;
(s6) said second air / gas mixture then flows towards the opening 12B (arrow (F7)) to which the combustion head is attached (not shown).

The advantages of the present apparatus can be summarised in the following points:

the achievement of better mixing of air and fuel, to the benefit of the efficiency of the combustion head, and with a substantial lowering of the formation of harmful combustion products such as NOx; and
an easier and cheaper construction of the metal components for performing the air/gas mixing.

Claims

Air/gas mixing apparatus (10) for premix burners; apparatus (10) comprising:
- - means (12A) for feeding combustion air to a mixing chamber (12);

- - means (14) for feeding and distributing a fuel gas to said mixing chamber (12) so as to achieve the desired air/gas mixture;
wherein said means (14) for the intake and distribution of the fuel gas comprising:
(a) - a first vertical portion (14C), symmetrically extending around a longitudinal vertical symmetry axis (Y);

(b) - a second horizontal portion (14D), having a longitudinal horizontal symmetry axis (X) perpendicular to said axis (Y), spatially oriented towards an air inlet opening (12A) in said means (14) for the intake and distribution of a gas; said second horizontal portion (14D) being adapted to enter into said means (14) for the intake and distribution of a gas a first amount of fuel gas in counter-current with respect to the air flow, so as to form a first air/gas mixture; and

(c) - a third horizontal portion (14S), aligned with said second horizontal portion (14D) along said axis (X), spatially oriented towards an air/gas mixture outlet opening (12B); said third horizontal portion (14S) being able to enter, in co-current with respect to the flow of said first air/gas mixture, in said means (14) for the intake and distribution of a gas, a second amount of fuel gas so as to form a second air/gas mixture; and
wherein said second horizontal portion (14D) and said third horizontal portion (14S) are respectively provided with a first gas distribution assembly (15) and with a second gas distribution assembly (16);
mixing apparatus (10) characterized in that each of said first gas distribution assembly (15) and said second gas distribution assembly (16) comprises a pair of respectively parallel plates (15A, 15B) (16A, 16B); a plate of each pair being provided with an horizontal axial hole (15A**), respectively (16A**), to allow the passage of fuel gas fed by said intake and distribution means (14).
Apparatus (10), according to Claim 1, characterized in that radial distributions of the fuel gas within said mixing chamber (12) are implemented by means of said first gas distribution assembly (15) and of said second gas distribution assembly (16).
Apparatus (10), according to anyone of the preceding Claims, characterized in that said first gas distribution assembly (15) is further provided with a flanged sleeve (15D) .
Apparatus (10), according to anyone of the preceding Claims, characterized in that said second portion (14D) is dimensionally larger than said third portion (14S).
Apparatus (10), according to anyone of the preceding Claims, characterized in that said means (12A) for feeding combustion air into said mixing chamber (12) are provided with means (EL) to impart a helical and swirling motion to the inlet air.
Premix burner provided with at least an air/gas mixing apparatus according to Claims 1-5 and with at least one combustion head.