Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
21 ~2741
Bo 21.12.1995 95/204
TITLE OF THE INVENTION
Premixing burner
BACKGROUND OF THE INVENTION
Field of the invention
The present invention relates to a premixing
burner according to the preamble of claim 1.
Discussion of backqround
EP-Bl-0,321,809 discloses a premixing burner
which, designed as a swirl burner, is composed of at
least two conical part shells which are fitted one in
the other in the direction of flow and the respective
axes of which are offset relative to a burner axis and
are parallel to one another, in such a way that the
adjacent walls of the conical part shells form, in
their longitudinal extent, tangential ducts or air
inlet slits for the throughflow of a combustion air
stream into the interior of the premixing burner. On
the head side, the premixing burner has a nozzle which
is arranged essentially on the burner axis and which is
preferably operated by means of a liquid fuel. The
combustion air flowing tangentially into the interior
of the premixing burner takes up the conical fuel spray
released by the nozzle, whereupon a fuel/air mixture is
formed. A flame front having a stabilized backflow zone
or backflow bubble forms in the region of the outlet of
the premixing burner. Further nozzles are arranged in
the region of the tangential air inlet slits in their
longitudinal extent and are preferably operated by
means of a gaseous fuel. To supply this fuel, a fuel
line extends alongs each of the tangential air inlet
slits and is preferably widened by means of said
nozzles. This fuel line is welded, preferably in
sectors, in the axle dir~ction to the conical part
shell. It therefore forms the outer delimitation of the
respective tangential air inlet slit; the inner
delimitation is formed by the other conical part shell
arranged offset there. The different thermal expansions
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between the conical part shell and the welded-on fuel
line which occur during operation cause distortions
along the connecting plane of the two elements, and
these distortions lead to the formation of gaps between
the two. The result of this is that the combustion
airflow runs in an uncontrolled manner within this
difficult region, that is to say some combustion air
flows through these gaps onto the outer face of the
conical part shell, with all the attendant flow-related
and heat-related disadvantages. Furthermore, it has
become clear that the fabrication of the fuel nozzles,
whether by drilling open, punching, laser, etc. could
not be carried out satisfactorily, in particular after
the welding of the fuel line to the conical part shell,
on account of the very small diameters and the
unfavorable machining conditions. The difficulties in
connection with the uniformity of the hole routing,
which is eminently important for fuel management, are
of prime concern here.
SUMMARY OF THE INVENTION
Accordingly, the object of the invention is to
remedy this. The invention, as defined in the claims,
is based on the object of providing, in a premixing
burner of the type initially mentioned, the formation
of the fuel line in such a way that no uncontrolled
flow conditions occur in this region during operation.
The object is achieved, according to the
invention, in that the conical part shell and the fuel
line are composed of a single piece. In this case, the
fuel line is formed by making a bend in the sheet metal
around the conical part shell, the end then being
welded to the outer surface of the co.nical part shell.
The essential advantage of the invention is to
be seen in that there is no welded joini~ between two
different bodies in the region of the tangential air
inlet slits at the point where the inflow of combustion
air takes place. Since the shell, together with the
fuel line, is produced from one piece, the inherent
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risk of an uncontrolled flow through the gaps which
form in the region of the weld no longer exists.
A further advantage of the invention is to be
seen in that the fabrication of the holes which form
the fuel nozzles can be carried out prior to the
formation of the fuel line under optimum machining
conditions and using optimum machining methods.
Advantageous and expedient developments of the
solution according to the invention are defined in the
further dependent claims.
Exemplary embodiments of the invention are
explained in more detail below by means of the
drawings. All elements not necessary for immediate
understanding of the invention are omitted. Like
elements are provided with the same reference symbols
in the various Figures. The direction of flow of media
is indicated by arrows.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention
and many of the attendant advantages thereof will be
readily obtained as the same becomes better understood
by reference to the following detailed description,
when considered in connection with the accompanying
drawings, wherein:
Figure 1 shows a perspective representation of a
premixing burner, appropriately cut away;
Figure 2 shows a view through the sectional plane
II.-II, with one configuration of the fuel
lines, and
Figure 3 shows a further view through the same
sectional plane as in Figure 2, with a
further configuration of the fuel lines.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, wherein like
reference numerals designate identical or corresponding
parts throughout the several views, in order to
understand the design of the burner 100 better it is
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advantageous to refer to Figures 2 and 3 at the same
time as Figure 1. In the description of Figure 1 below,
reference is made to the remaining Figures as required.
The burner 100 according to Figure 1 is a
premixing burner and is composed of two conical part
shells 101, 102 which are fitted one in the other so as
to be offset relative to one another. The offset of the
respective mid-axis or longitudinal axes of symmetry
lOlb, 102b of the conical part shells 101, 102 in
relation to one another leaves free on each of the two
sides, in a mirror-symmetrical arrangement, a
tangential air inlet slit 119, 120 (Figures 2 and 3)
through which combustion air 115 flows into the
interior of the premixing burner 100, that is to say
into the conical cavity 114. The conical shape of the
illustrated part shells 101, 102 in the direction of
flow has a specific fixed angle. Of course, depending
on operational use, the conical part shells 101, 102
can have an increasing or decreasing cone taper in the
direction of flow, in a similar way to a diffuser or
confuser. The two last-mentioned shapes are not
included in the drawing, since the average person
skilled in the art can readily understand them. The two
conical part shells 101, 102 each have a cylindrical
initial part lOla, 102a, the said initial parts
likewise extending so as to be offset relative to one
another in a similar way to the conical part shells
101, 102, so that the tangential air inlet slits 119,
120 are present over the entire length of the premixing
burner 100. Accommodated in the region of the
cylindrical initial part is a nozzle 103, the injection
point 104 of which coincides approximately with the
narrowest cross section of the conical cavity 114
formed by the conical part shells 101, 102. The
injection capacity and the nature of this nozzle 103
depend on the predetermined parameters of the relevant
premixing burner 100. Of course, the premixing burner
can be composed of part shells which extend purely
conically, that is to say without a cylindrical initial
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part, as is evident from Figure 1. Furthermore, the
conical part shells 101, 102 each have a fuel line 108,
109, said fuel lines running along the tangential air
inlet slits 119, 120 and being provided with injection
orifices 117, through which preferably a gaseous fuel
113 is injected into the combustion air 115 flowing
through there, as the arrows 116 are intended to
symbolize. These fuel lines 108, 109 are to be arranged
in the region of the tangential air inlet slits in such
a way that an optimum air/fuel mixture can be achieved.
The more exact types of design of these fuel lines 108,
109 are discussed in more detail further below. On the
combustion space side 122, the outlet orifice of the
premixing burner 100 merges into a front wall 110, in
which there are a number of bores 110a. The last-
mentioned come into operation as required and ensure
that diluent air or cooling air 110b is supplied to the
front part of the combustion space 122. Furthermore,
this air supply ensures flame stabilization at the
outlet of the premixing burner 100. This flame
stabilization becomes important when a principal
concern is to support the compactness of the flame by
radial flattening. The fuel supplied to the nozzle 103
is a liquid fuel 112 which, if need be, can be enriched
with a recycled waste gas. This fuel 112 is injected
into the conical cavity 114 at an acute angle. A
conical fuel profile 105 therefore forms out of the
nozzle 103 and is surrounded by the rotating combustion
air 115 flowing in tangentially. In the axial
direction, the concentration of the fuel 112 is reduced
continuously by the inflowing combustion air 115 to the
point of optimum mixing. The position of the fuel
nozzle 103 on the burner axis can be displaced a
specific distance upstream in relation to the narrowest
cross sect~ n of the conical part shells 101, 102. This
depends on the compactness of the fuel spray 105 which
- the inflowing combustion air 115 already has to
penetrate on the head side of the premixing burner 100
in order to make it possible to produce an optimum
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mixture. If the premixing burner 100 is operated with a
gaseous fuel 113, the latter is preferably introduced
via orifice nozzles 117, the formation of this fuel/air
mixture taking place directly at the end of the air
inlet slits 119, 120. During the injection of the fuel
112 via the nozzle 103, the optimum homogeneous fuel
concentration over the cross section is achieved in the
region of the vortex breakdown, that is to say in the
region of the backflow zone 106 at the end of the
premixing burner 100. Ignition takes place at the tip
of the backflow zone 106. Only at this point can a
stable flame front 107 be obtained. There is no fear,
in this case, of a flashback of the ~lame into the
interior of the premixing burner 100, as is potentially
the case in known premixing stages, these resorting to
complicated flame holders to remedy this. If the
combustion air 115 is additionally preheated or
enriched with a recycled waste gas, this assists the
evaporation of the liquid fuel 112 in a sustained
manner, before the combustion zone is reached. The same
considerations also apply when liquid fuels are
supplied via the fuel lines 108, 109 instead of gaseous
fuels. Narrow limits on the cone angle and the width of
the tangential air inlet slits 119, 120 must be adhered
to in the design of the conical part bodies 101, 102,
so that the desired flow field of the combustion air
115 can be set by means of the flow zone 106 at the
outlet of the premixing burner. It may be said, in
general, that a reduction in size of the tangential air
inlet slits 119, 120 displaces the backflow zone 106
further upstream, although the mixture then ignites
earlier as result. It should be noted, nevertheless,
that the backflow zone 106, once fixed, is positionally
stable per se, since the swirl factor increases in the
direction of flow in the r gion of the conical shape of
the premlxing burner 100. The axial velocity within the
premixing burner 100 can be changed by means of a
corresponding supply (not shown) of an axial combustion
air stream. Furthermore, the design of the premixing
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burner 100 is preeminently suitable for changing the
size of the tangential air inlet slits 119, 120, with
the result that a relatively large operating bandwidth
can be covered without any change in the overall length
of the premixing burner 100. The individual conical
part shells 101, 102, can also be fitted spirally one
in the other.
The geometrical configuration of the fuel lines
108, 109 now emerges from Figure 2. These are formed in
one piece from the corresponding part shells 101, 102,
in that their ends are bent round continuously and are
welded on the outer surface of the respective part
shell.
The fuel nozzles 117 are located directly in
the region of the tangential air inlet slits 119, 120,
through which the combustion air 115 flows into the
conical cavity 114. In this region, the inflow of
combustion air 115 remains free of interferences
harmful to flow, because the fuel lines 108, 109 do not
undergo any heat-related distortions in relation to the
part shells. The homogeneity in the inflow of
combustion air 115 into the conical cavity 114 brings
about optimum mixture formation by means of the fuel
injection 116 taking place there. In such a design, the
fuel lines can also be drawn upstream over the
tangential air inlet slits 119, 120, as shown in Figure
2 by the body lO9a represented by broken lines. In such
a configuration, the fuel nozzles 117a can, if
required, also be arranged above the tangential air
inlet slits 119, 120. These can have oblique fuel
injection 116a, such that the fuel jet flows closely
past the edge of the opposite part shell 101, thus
triggering at least a cooling effect. Furthermore, an
advantage of this design is the possibility of
lengthening the mixing stage, thus res~lting in an
improvement in intermixing, along with an accompanying
,
reduction in the pollutant emissions.
Figure 3 is essentially a modification in the
design of the fuel lines 108, 109, the largest
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throughflow cross section of which is not located
directly in the region of the tangential air inlet
slits 119, 120, as in Figure 2, but largely in the
plane of the conical cavity 114. Accordingly, fuel
injection 116 also takes place further downstream of
the tangential air inlet slits 119, 120. Here too, the
inflow of combustion air 115 remains free of possible
bypass gaps caused by heat-related distortions.
Obviously, numerous modifications and
variations of the present invention are possible in
- light of the above teachings. It is therefore to be
understood that, within the scope of the appended
claims, the invention may be practiced otherwise than
as specifically described herein.
