The present invention relates to a filtering-bed burner
of the type comprising a pre-mixing chamber to be
associated with feeding means to supply an air-combustible
gas mixture, and a filtering bed formed of
granular elements disposed to define a delimitation wall
for the pre-mixing chamber, said filtering bed being
arranged so as to be passed through by the air-gas
mixture that is fired close to an outer surface provided
on the filtering bed itself on the opposite side relative
to the pre-mixing chamber.
The invention also relates to a method of combustion of
a gas carried out by said burner, said method comprising
the steps of: pre-mixing the gas with a predetermined
amount of air to form an air-gas mixture; feeding said
air-gas mixture through a filtering bed defined by
granular elements to cause an intimate mixing between the
air and the gas forming said mixture, firing the air-gas
mixture close to an outlet surface of said filtering bed.
It is known that burners currently used in boilers and
water heaters or similar apparatuses for household and/or
industrial use, are essentially comprised of a mixing
chamber in which a combustive air-combustible gas mixture
is formed. This mixture is then ejected from the burner
through a plurality of holes or shaped openings
conveniently distributed on a burner wall, intended to
delimit the mixing chamber.
At the burner outlet, the air-gas mixture is fired,
thereby forming a plurality of small flames
correspondingly distributed at the location of said
through holes or openings provided in the burner wall.
The above described perforated-wall burners have the
special feature of being of relatively cheap
construction, so that use of same is widespread in
different application fields.
However these burners have some drawbacks of technical
and operational character that are essentially due to the
fact that they do not enable an intimate mixing between
the air and gas before ejection and firing of the mixture
formed by them. This fact can give rise to a defective
burning of the mixture which results in production of
polluting discharges, and CO and NOx emissions.
This situation has a tendency to become worse when the
air-gas mixture flow rate is wished to be modulated in
order to adjust the power of the flame produced by the
burner within a desired value range. This modulation, on
the other hand, can take place only within a relatively
limited value range. Actually, when the mixture flow rate
is increased beyond certain values, depending on the
burner construction features, undesired phenomena may
arise that involve detachment or separation of the flame
from the perforated burner wall. On the contrary, when
the mixture flow rate tends to be reduced beyond given
values, the flame can move too close to the perforated
burner wall thereby causing overheating of said mixture.
Due to thermal expansions, a deformation of the through
openings or holes may occur that will lead to a
consequent loss of control in the outgoing mixture flow,
as well as to the risk of dangerous backfires.
Presently, one of the possible alternatives to burners
provided with a perforated wall of the type described
above involves employment of the so-called "filtering-bed"
burners. In this type of burners, the air-gas
mixture conveyed to the inside of the burner is forced to
pass through a filtering bed formed of granular elements
disposed in several superposed layers, to define a
labyrinth formed with hollow spaces promoting an intimate
mixing between the air and gas forming the mixture, said
mixture being fired close to an outlet surface of the
filtering bed.
For example, US Patent 3,322,179 describes a burner
having a filtering bed formed of a plurality of spherules
or granular elements of different shape made of glass,
quartz, silicon carbide or ceramic material, mutually
joined at the points of mutual tangency by a sintering
process and distributed to form two different superposed
layers.
A first layer, turned towards the pre-mixing chamber, is
formed of spherules or granular elements of different
shape having a particle size included between about 0.15
and 0.3 mm. A second layer, defining the outlet surface
of the filtering bed, is formed of spherules or granular
elements of a particle size included between 0.5 and 1
mm.
Another filtering-bed burner is described in US Patent
3,947,233. In the burner disclosed in this document the
filtering bed is essentially defined by a plate-like
element of sintered metal particles, defined by spherules
or irregular bodies. In this case too, particles are
distributed to form two distinct layers. A first layer,
of a thickness in the order of 3 mm, is formed of
spherules the size of which is such selected that the
interspaces defined between them have a size in the order
of 0.1 mm. The spherules forming the second layer, of a
thickness in the order of 1 mm and defining the outlet
surface of the filtering bed, have such sizes that the
interspaces defined between them have a diameter in the
order of 0.02 mm.
US Patent 5,591,095, in the name of the same Applicant,
discloses a burner the filtering element of which is
formed of spherules of ceramic material of predetermined
sizes, included by way of example between 1 and 10 mm,
optionally welded together by a sintering process. The
mutual divergence of the spherule surfaces at the burner
outlet causes the outgoing flow speed to slow down
immediately downstream of the interspaces defined between
the spherules themselves, even at high flow rates of the
air-gas mixture conveyed through the filtering bed,
thereby generating a continuous flame front confined
between the spherule surfaces.
In other words, the flame stays always adherent to the
spherule surfaces, so that a localized heating produced
on said surfaces may cause incandescence thereof and
consequent heat emission by radiation.
In accordance with the present invention it has been
found that by making the filtering bed of granular
elements of irregular shape having a particle size
included between 2.5 and 7 mm, preferably between 3 and
5 mm and randomly laid down upon each other so as to
form an overall thickness included between 10 and 20 mm,
the possibility of making the burner operate in an
optimal manner, also modulating the flow rate value of
the air-gas mixture within exceptionally wide value
ranges, is advantageously achieved.
Such a burner can be in addition produced at very reduced
costs, comparable to those of conventional burners
provided with a perforated plate, even in the case in
which the burner is of cylindrical form.
In particular, the invention relates to a filtering-bed
burner characterized in that said filtering bed of a
thickness included between 10 mm and 20 mm, is made up of
granular elements of irregular shape, having a particle
size included between 2.5 and 7 mm, randomly disposed
upon each other.
Still in accordance with the present invention, a
preferential solution of a burner of cylindrical form is
obtained according to a process which is characterized in
that it comprises the following steps: forming an inlet
header having a base wall in the form of an annulus, as
well as an outer perimetric wall and an inner perimetric
wall substantially extending at right angles from
respectively opposite perimetric edges of the base wall;
associating first and second concentrically-disposed
holding nets with said outer perimetric wall and said
inner perimetric wall respectively; filling a space
defined between said holding nets at least partly with
granular elements disposed randomly against each other so
as to define a filtering bed; forming a closing element
having a connecting portion of shape and sizes
substantially identical with those of the inlet header,
and a closing wall; causing engagement of an inner
perimetric wall and an outer perimetric wall of said
connecting portion respectively with the first and second
holding nets on an opposite side relative to the inlet
header.
Further features and advantages will become more apparent
from the detailed description of a preferred but non-exclusive
embodiment of a filtering-bed burner and a
process carried out by it in accordance with the present
invention. This description will be taken hereinafter
with reference to the accompanying drawings, given by way
of non-limiting example, in which:
- Fig. 1 is a diametrical sectional view of a filtering-bed
burner in accordance with a first embodiment of the
invention; - Fig. 2 shows a different embodiment of the burner in
question.
With reference to the drawings, a filtering-bed burner in
accordance with the present invention has been generally
identified by reference numeral 1.
Burner 1 comprises a pre-mixing chamber 2 to be
operatively associated with feeding means for supply of
an air-gas mixture, said means being only
diagrammatically shown in that it can be made in a manner
known per se and conventional.
Burner 1 further comprises at least one filtering bed 3
arranged to define, by its inlet surface 3a, at least
one wall delimiting the pre-mixing chamber 2. In more
detail, in the embodiment shown the pre-mixing chamber 2
is directly defined within the filtering bed 3 and
circumscribed by the inlet wall 3a of the filtering bed,
the latter being made of a cylindrical tubular form,
preferably of circular section.
In a manner known per se, the filtering bed 3 is adapted
to be passed through by the air-gas mixture introduced
into the pre-mixing chamber 2 which is fired at the
outside of the burner, close to an outlet surface 3b
provided by the filtering bed on the opposite side
relative to the pre-mixing chamber 2.
The pre-mixing chamber 2 is connected to the feeding
means 1a by an inlet header 4 associated with one end 3c
of the filtering bed 3. The inlet header 4 is preferably
made of a die-cut and drawn metal sheet and has a base
portion 4a in the form of an annulus which is such
disposed that the first end 3c of the filtering bed 3 is
delimited thereby. Header 4 further has an outer
perimetric portion 4b of cylindrical conformation
extending in a substantially perpendicular direction from
an outer perimetric edge of the base portion 4a.
Furthermore, extending from an inner perimetric edge of
the base portion 4a, preferably in the opposite direction
relative to the outer perimetric portion 4b, there is an
inner perimetric portion 4c which is also extended in a
direction substantially perpendicular to the base portion
4a and has a substantially cylindrical conformation.
Burner 1 further has a closing element 5 associated with
a second end 3d of the filtering bed 3, on the opposite
side relative to the inlet header 4. Advantageously, this
closing element 5 has a connecting portion 6 the shape
and sizes of which are substantially identical with those
of the inlet header 4, and a closing wall 7 arranged to
close the end of the pre-mixing chamber 2. In more
detail, the connecting portion 6 of the closing element
5 has a respective base wall 6a in the form of an
annulus, so as to delimit the second end 3d of the
filtering bed 3, as well as an outer perimetric wall 6b
and an inner perimetric wall 6c extending at right angles
and in opposite directions from an outer perimetric edge
and an inner perimetric edge respectively of the base
wall 6a. The closing wall 7 extends in a diametrical
plane from an end edge of the inner perimetric wall 6c.
It is to note that advantageously the closing element 5,
of a conformation and structure substantially identical
with those of the inlet header 4, is adapted to be made
with the aid of the same machines and equipment as
employed for making the header itself. In particular,
since both elements are obtained by carrying out die-cutting
and drawing of a metal sheet, it will be
possible to produce these two components with one and the
same mould by arranging a movable punch in said mould,
which is adapted to selectively cause either forming of
the closing wall 7 so as to obtain the closing element 6,
or removal of the metal sheet portion corresponding to
such a wall so as to form the inlet header 4.
As can be easily recognized by comparing the accompanying
drawings, header 4 can have the same orientation as the
connecting portion 6 of the closing element 5, as shown
in Fig. 1, or it may be disposed in mirror image
relationship relative to this connecting portion 6, as
shown in Fig. 2.
The filtering bed 3 is substantially formed of a
homogeneous mixture of granular elements 8 of irregular
shape, preferably of crystallized silicon dioxide, having
differentiated particle sizes of a value included between
2.5 and 7 mm, and preferably included between 3 and 5 mm.
The embodiment involving granular elements of different
size homogeneously mixed with each other is preferred in
that it has been found that it enables a more efficient
mixing between the air and gas forming the mixture
admitted to the pre-mixing chamber 2. However, employment
of granular elements of same size, the particle size of
which is included within the above specified value
ranges, may be also taken into account.
The granular elements 8 are freely and randomly disposed
against each other to fill a space defined between a
first holding net 9 extending at the inlet surface 3a of
the filtering bed 3 and a second holding net 10 extending
at the outlet surface 3b. In the embodiment shown the
first and second nets 9 and 10 have a cylindrical
configuration and are concentrically disposed with
respect to each other.
Each of the holding nets 9, 10 can be made either of a
metal wire for example, or of a die-cut and optionally
stretched metal sheet so as to form the net meshes by
spreading apart cuts produced by the die-cutting
operation.
Preferably the holding nets have the same mesh width as
the particle size of the smallest granular elements or a
slightly smaller width.
In this manner, the holding nets are adapted to carry out
an efficient holding action of the granular elements 8
without putting up too much resistance to the air-gas
mixture passage.
It is to note that, since the granular elements 8 are of
random and irregular shape, the interspaces defined
between them and passed through by the air-gas mixture
have lower sizes than the particle size values of the
granular elements themselves.
In order to eliminate any risk of backfire, without
hindering the air-gas mixture passage through the
filtering bed 3 too much, thickness of said bed is
provided to be included between 10 and 20 mm.
In the embodiment shown in Fig. 1, the first holding net
9 is fastened by its opposite end edges 9a, 9b to the
inner perimetric walls 4c, 6c provided by the inlet
header 4 and the connecting portion 6 of the closing
element 5, respectively. The outer holding net 10, in
turn, has its opposite end edges 10a, 10b fastened to the
outer perimetric walls 4b, 6b belonging to the inlet
header 4 and the closing element 5, respectively.
In more detail, in the embodiment shown in Fig. 1 the
first holding net 9 is internally in engagement both with
the inner perimetric wall 6c of the closing element 5 and
the inner perimetric wall 4c of header 4. The second
holding net 10, in turn, is engaged externally of the
outer perimetric wall 6b of the closing element 5 and
externally of the outer perimetric wall 4b of header 4.
In the embodiment shown in Fig. 2, on the contrary, both
holding nets 9, 10 are engaged internally of the
corresponding perimetric walls 4b, 4c, 6b, 6c belonging
to the header 4 and closing element 5.
Connection between nets 9, 10 and header 4, as well as
the closing element 5, can be carried out by welding, for
example.
At least one insulating element 11, preferably made of an
elastically-yielding refractory material, a ceramic fibre
for example, may be interposed between at least one of
the filtering bed 3 ends and the corresponding base wall
6a, 4a of the closing element 5 and/or header 4.
In accordance with a further preferential feature of the
present invention, burner 1 further comprises at least
one shielding element 12 extending from the closing
element 5 close to the inlet surface 3a of the filtering
bed 3, to the inside of the pre-mixing chamber 2. In more
detail, this shielding element 12 extends axially over
at least 1/10 of the axial length of the filtering bed
past the corresponding end 3d of the filtering bed 3
itself. Owing to the presence of the shielding element
12, distribution of the air-gas mixture through the
filtering bed 3 is made still more homogeneous, thereby
eliminating the undesired effect of overpressures that
are likely to be created close to the corresponding end
3d of the filtering bed, due to the presence of the
closing wall 7 that suddenly stops mixture advancing
along the pre-mixing chamber 2.
A second shielding element 13 may be also provided and it
extends from the inlet header 4 close to the inlet
surface 3a of the filtering bed 3 to the inside of the
pre-mixing chamber 2. This second shielding element 13
can be either defined by the inner perimetric wall 4c of
header 4, as shown in Fig. 1, or made as an extension of
said wall. Alternatively, as provided in Fig. 2, the
second shielding element 13 can be coaxially inserted in
the inner perimetric wall of header 4.
The burner in accordance with the invention can be
advantageously obtained by a process first providing
fastening, by welding for example, of the first and
second holding nets 9, 10 to the inlet header 4.
Subsequently the interspace defined between nets 9, 10 is
filled with the granular elements 8 forming the filtering
bed 3. When filling has been completed, the closing
element 5 is fitted to the holding nets 9, 10 and
fastened to the first holding net 9 by welding, for
example. Concurrently with, or before engagement of the
closing element 5, insertion of the optional shielding
element or elements 12 is carried out.
Obviously, assembling can be also executed with a
reverse sequence with reference to the above described
one, that is the holding nets 9, 10 are first engaged in
the closing element 5, whereas mounting of header 4 is
executed once the filtering bed 3 has been formed by
introduction of the granular elements 8.
If arrangement of the insulating elements 11 is provided,
important tolerances in the axial size of the filtering
bed 3 are enabled. Actually, the elastically-yielding
insulating element or elements 11 act on the filtering
bed ends causing compacting of said bed on mounting of
the closing element 5 and/or header 4. In addition, the
insulating elements 11 prevent the flame produced by
combustion from being propagated internally of the outer
perimetric wall 6b, 6c of the closing element 5 and/or
header 4, thereby giving rise to an undesired
overheating of said element 5 and header 4.
In addition, the insulating elements enable compensation
for thermal expansions undergone by the material during
operation of the burner.
In accordance with a further feature of the present
invention, the feeding means 1a is advantageously
arranged to feed the air-gas mixture at a specific flame
power included between a minimum value of about 25 W/cm2
and a maximum value of about 350 W/cm2.
To the ends of the present description, by specific flame
power it is intended a power delivered by the flame for
each cm2 of the outlet surface 3b of the filtering bed 3.
Advantageously, the feeding means 1a may be of a type
capable of modulating the air-gas mixture flow rate so as
to adjust the specific power flame at each moment
depending on requirements within a value range included
between said minimum and maximum values.
It is also preferably provided for the feeding means to
be of the type capable of supplying a greatly
hyperstoichiometric air-gas mixture, with an oxygen
excess in the combustion fumes included between 3.5% and
9%.
Use of plenty of air in excess enables the flame
temperature to he lowered to advantage of the combustion
quality and also the flow rate and consequently the flow
speed of the mixture passing through the filtering bed 3
to be increased.
It is to note that feeding of excess air would tend to
make achievement of a perfectly homogeneous mixing of gas
in air quite difficult. However, due to the typology,
size features and arrangement of the granular elements 8
forming the filtering bed 3 in accordance with the
present invention, homogeneity of gas-mixing in air is in
any case excellent, at any flow rate value and also when
the excess air is brought to the maximum values.
Therefore an excellent quality of combustion is ensured,
with a very low production of CO and NOx, under any
operating conditions. The optimal mixing quality also
eliminates any risks of flame detachment even under the
highest running conditions, as well as any risks of
backfire under the lowest running conditions. During
operation at low running conditions, that is at 25 W/cm2,
the outgoing flow rate can be in any case maintained, by
increasing the air percentage in the air-gas mixture for
example, to such a level that the flame stays slightly
detached from the outlet surface of the filtering bed, so
that undesired overheating of the second holding net 10
and the granular elements 8 located close to the outlet
surface 3b is not caused.
By using granular elements 8 of ceramic material as well
as known materials adapted to the purpose in the
manufacture of the second holding net 10, it is in any
case possible to reduce the air excess and/or the mixture
flow rate so that the flame stays in contact with the
outlet surface of the filtering bed to bring the latter
to an incandescence condition and dissipate heat by
radiation.
Heat dissipation by radiation can be also achieved by
arranging an auxiliary radiating net-like element 14
(shown in chain line in Fig. 1) to a position
conveniently spaced apart from the outlet surface of the
filtering bed 3.
The present invention achieves the intended purposes.
The burner in reference enables operation to be carried
out in a greatly adjustable manner and ensures an
excellent combustion at any load level, while at the same
time having production costs comparable with those of
standard perforated-plate burners.
The characteristic parameters of an embodiment of a
burner made in accordance with the present invention are
set forth herebelow:
- diameter of the outlet surface 3b of the filtering bed
3:60 mm;
- material forming the granular elements 8: crystallized
silicon dioxide;
- particle size of the granular elements 8: 3 mm to 5 mm;
- thickness of the filtering bed: 14 mm;
- mesh width of the holding nets 9 and 10: 3 mm;
- possibility of modulation in operation: 30 W/cm2 to 300
W/cm2;
- feeding: air-methane gas mixture with a ratio λ equal
to 1.56 corresponding to an oxygen excess of 7.5%.
During operation of this burner at a specific power of 30
W/cm2, a CO emission lower than 10 ppm and an NOx
emission lower than 20 ppm is obtained.
During operation at 300 W/cm2, CO emission is lower than
10 ppm and NOx emission is lower than 20 ppm.