Note: Descriptions are shown in the official language in which they were submitted.
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BLUE FLA~ BURNER
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Background of the Invention
This invention relates to blue ~lame burners,
and more particularly, to such burners adapted to operate
with fluid heaters for burning liquid hydrocarbon fuel
to produce a stable blue flame.
Summary of the Prior Art
Those familiar with the art to which~the present
invention pertains, are aware of a construction wherein
burner means are positioned at the combustion end of a
combustion chamber, the burner means including a cylin-- ~ ~-~-~ - -
drical tube which may be flared at the breech endt a nozzle
in the tube, fuel supply means for supplying a fuel spray
to the combustion chamber, means for supplying air under
pressure through the tube toward the combustion chamber,
and means such as a baffle to recirculate a portion of the
gaseous products of combustion to the breech end of the
cylindrical tube.
While such known constructions have represented
a positive development in the art and have achieved a degree
of commercial acceptance, experience has demonstrated that
they are subject to certain disadvantages in that the burner ~ ~ ;
was sensitive to both oil chemistry and nozzle spray varia- ~ ;
tions resulting in two independent occurrencesO Firstl the
air metering plate shroud which surrounded the end of the
burner accumulates carbon deposits which eventually re- ;
sulted in shorting out of the electrodes and a safety shut-
down of the burner. Second, the flame was often quenched,
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especially on cold starts. This latter occurrence was
manifested by burner start, flame out during transition
from yellow to blue flame and reignition in a fuel rich
environment that frequently resulted in sudden and rapid
combustion and consequent physical damage to the unit.
The carbon deposits were principally due to the
recirculation of combustion gases containing high levels
of unburned hydrocarbon and the effect of low eddy cur-
rents drawing fuel back into the cylindrical tube. The
recirculation of unburned hydrocarbons was in turn due in
part to the incorrect trajectory of combustion air in the
primary flame zone resulting in a highly substoichiometric
flame envelope. In like manner, the high level of unburned
hydrocarbons in the recirculating gases was caused in part
by the opening between the inside of the baffle and the
tube which allowed recirculation from a highly substoichio-
metric region of the 1ame. ~oreover, the levels of un-
burned hydrocarbons greatly increased if fuel s~ray dis-
tribution degraded, as by nozzle degradation, air leaks
and the like, or as fuel aromatics increased.
Other known constructions of interest in respect
of the subject matter of the present application are found
in U.S. Patent Nos. 3,399,022, 3,545,902, 3,597,134 and
3,652,194, and especially No. 3,741,166.
Summary of the Invention
Accordingly, I have conceived the present in-
vention to provide a blue flame burner which overcomes the
foregoing difficulties and disadvantages. As a ~eature of
my invention, the blue flame burner is adapted to operate
with a combustion chamber associated with a fluid heater
for burning liquid hydrocarbon fuel to ~roduce a stable
blue flame in which there is provided diffuser means having
an upstream end defining a breech, downstream end defining
a mouth and a constricted region between the breech and
the mouth. The fuel nozzle of the burner forms a fuel -
spray in the direction of the diffuser mouth. Upstream
of the breech of the diffuser means combustion air is
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provided under pressure and directed toward the breech.
In operation the burner has the mouth of the diffuser means
directed into the combustion chamber and there is also pro-
vided means defining a recirculation path for conducting a
portion of the gaseous products of combustion formed in
the combustion chamber back to the breech for entrainment
with the combustion air moving towards the diffuser means.
More specifically, the mouth of the diffuser means
is positioned within the combustion chamber at the combus-
tion end thereof so that the region between the diffuser
and the portion of the combustion chamber surrounding the
diffuser comprises the recirculation path, while the nozzle
itself is positioned within the diffuser substantialiv
axially thereof and upstream a radial plane through the
area of greatest constriction thereof. Additionally, the
means for providing combustion air under pressure may com-
prise simply a tube extending into the combustion chamber
and having an end plate formed with a plurality of aper-
tures therein opposite the diffuser breech, and means for
delivering air under pressure to the tube.
In the burner, the ignition means includes a
pair of electrodes positioned in a low pressure region in
the diffuser adjacent the fuel nozzle and out of the spray
pattern formed by the nozzle.
In one embodiment of the invention, the combus-
tion chamber has a zone of reduced diameter at the combus-
tion end thereof and the burner is positioned with the
breech of the diffuser means in that zone and in axial
alignment therewith and with its mouth positioned and
directed into the combustion chamber. In this case, I
provide means including annular baffle means surrounding
the diffuser mouth and spaced from the combustion chamber
wall to define therewith the recirculation path for con-
ducting a portion of the gaseous products of combustion
back to the breech.
Accordiny to another emhodiment of the invention,
I position the entire diffuser means downstream of the zone
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of reduced diameter of the combustion chamber and in this
case, I have found that I can thus eliminate the baffle
means altogether, the recirculation path then comprising
the region between the diffuser means and the combustion
chamber wall.
The present construction produces a stable blue
flame in the combustion chamber downstream of the mouth of
the diffuser means. Thus, the present construction alters
the trajectory of the combustion air and effectively closes
off the path of "inside" combustion gas recirculation, and
increases the stoichiometric ratio along the flame enve-
lope which in turn reduces the unburned hydrocarbon level
at the source of recirculating combustion gases. Addition-
ally, the modified electrode settings reduce the likelihood
of the arc interfering with the trajectory of the oil drop- -
lets and tends to counteract the effect of eddy currents
at the diffuser mouth.
_ It is known that the ignition speed of gases is
maximized as the stoichiometric ratio approaches unity,
and the known construction directed most of the combustion
air into the spray core resulting in the highly sub-
stoichiometric flame envelope already mentioned. The pre-
sent diffuser increases the level of air delivery to the
flame envelope without delivering excessive levels that
could cause autoignition and thus improves the stability
range of the burner. This improved function is achieved
by reason of the fact that the diffuser causes a portion
of the combustion air to flow along its smooth inside sur-
face and more uniformly distributes the air throughout the
flame zone. The present diffuser also reduces the axial
velocity of the gases passing through it because of its
configuration and larger average cross-sectional area
relative to that of the known cylindrical tube. The re-
sult is an aerodynamically anchored flame front due to
the reduced axial gas velocity, and increased flame
speed due to improved combustion air distribution.
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The diffuser also increases the air ~o fuel shear
angle in the primary combustion zone providing mare rapid
fuel preparation, effectively shortening the droplet re-
sidency time in the primary zone and reducing the physical
size of that zone.
There has thus been outlined rather broadly the
more important features of the invention in order that the
detailed description thereof that follows may be better
understood and in order that the present contribution to
the art may be better appreciated. There are, of course,
additional features of the invention that will be described
hereinafter and which will form the subject of the claims
appended hereto. Those skilled in the art will appreciate
that the conception upon which this disclosure is based
may readily be utilized as a basis for the designing of
other structures for carrying out the several purposes
of the invention. It is important, therefore, that the
claims be regarded as including such equivalent construc-
tions as do not depart from the spirit and scope of the
invention.
Brief Descr_ption of the Drawings
Specific embodiments of the invention have been
chosen for purposes of illustration and description, and
are shown in the accompanying drawings, forming a part of
the specification wherein:
Fig. 1 is a cross-sectional view in elevation
along the longitudinal axis of a burner arrangement ac-
cording to the prior art;
Fig. 2 is a cross-sectional view in elevation
along the longitudinal axis of a burner arrangement ac-
cording to a first embodiment of the present invention;
Fig. 3 is a view similar to Fig. 2, but illus-
trating a second embodiment of the present invention;
Fig. 4 is a top plan view of a burner assembly
illustrating an electrode arrangement according to the
present invention'
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Fig. 5 is an elevational view of the structure
of claim 4.
Description of the Preferred Embodiment
Referring now to the drawings and initially to
Fig. 1 thereof, there is shown a known burner 10 supported
by a support plate 12 in a combustion chamber 14 defined
- by a cylindrical wall 16. More specifically, the burner
10 includes an air tube 18 mounted in the plate 12 which
merges with a cylindrical wall 19 of reduced diameter
j relative to wall 16 the end of which is joined to the
i end of the wall 19 by an annular end pla-te 20.
The burner 10 includes a fuel supply line 22
passing through the air tube 18 and projecting outwardly
` of an air metering plate 24 fixed across the end of the
tube 20. The supply line 22 terminates in a nozzle 26
for directing a spray of fine fuel droplets towards the
combustion chamber 14. A pair of electrode rods 26 extend
through the tube 18 and the plate 24 and converge near -- ----
the spray nozzle to ignite the fuel.
It will be noted that the fuel nozzle is posi-
tioned axially of the combustion chamber and is surrounded
by a diffuser 28 which is flared as at 30 at its upstream
end, and that the metering plate 24 is formed with a
plurality of openings 32 centered on a circle concentric
with the axis of the diffuser 28.
Finally, an annular or disc shaped baffle 34 is
mounted in axial alignment with the combustion chamber 14
and with its central opening spaced from and surrounding
the downstream end of the tube 28.
According to this known construction, a core of
fuel spray emitted from the nozzle 26 into the combustion
chamber while air jets created by the apert~res 32 in the
plate 24 entered the upstream end of the diffuser 2~ to
mix with the fuel spray to enable combustion of same in
the combustion end of the chamber 14. A portion of the
combustion gases were recirculated around the baffle for
return to the upstream end of the diffuser 28 for passage
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therethrough along with the combus-tion air from the aper~
tures 32.
As already stated, this described construction
has acquired a degree of commercial acceptance in the
field, but is subject to various problems which applicant
has attacked and successfully resolved.
Referring now to Fig. 2 there is shown a con-
struction according to a first embodiment of the present
invention wherein the combustion chamber 14 is formed
essentially in the same manner as that of Fig. 1 to the
extent that the walls 16 and 19, support plate 12 and end
plate 20 are structurally similar in each case.
In Fig. 2 I have illustrated a housing assembly
34 which is removably attached by bolts to the support
plate 12 so that the burner assembly can readily be in- - ;
stalled.
The housing 34 includes a lower section 36 formed
with suitable air inlet openings and containing a blower
38 driven by an electric motor (not shown) in known manner
and which may also drive a fuel pump for a purpose to be
described. The housing 34 also includes an upper, gene-
rally tubular section 40 formed with a hinged partition 42
carrying an ignition transformer 44 and a mounting flange
46 for securement to the heat exchanger flange 12. The
section 40 extends forwardly through the flange 12 in
similar fashion as tube 18 of Fig. 1 and terminates in
a metering plate 41 formed with openings 43 centered on
a circle concentric with the axis section 40.
The fuel supply is controlled by conventional
equipment as disclosed in U.S. Patent No. 3,741,166 and
delivers fuel to the supply line 48 in the air tube
section ~0, which line extends forward to the nozzle 50.
The electrodes 60 extend forward through
ceramic insulating sleeves 62 fixed in a pair of suitably
located openings in the plate 41 and the electrode buss
bars contact respective high voltage terminals 68 of
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transformer 44 when the hinged partition 42 is closed.
A diffuser 70 having an upstream end defining
a breech 72, a downstream end defining a mou-th 74 and a
constricted region 76 between the breech and the mouth,
defines opposed arcuate configurations in cross section,
as shown, having a radius of curvature of the order of 4.5
inches, while the diameter of the breech and mouth is of
the order of 3 inches. The diffuser is mounted by means
not shown, such as thin spaced tabs extending between
the breech end of the diffuser and the tubular section 40.
An annular baffle 78 is supported by suitable
means, not shown, to surround the di~fuser mouth and is
spaced from the combustion chamber wall 16 to define
therebetween a recirculation path for conducting a portion
of the gaseous products of combustion formed in the com- -
bustion chamber hack to the diffuser breech for entrain-
ment with the combustion air from the apertures 43 in
plate 41 and movement through the diffuser.
In operation, fuel is pumped to the nozzle 50
to spray a cone shaped core of fine fuel droplets into
the combustion chamber while air under pressure is de-
livered by the blower 38 through the housing section 40
and the apertures 43 in the form of jets to the breech
of the diffuser for mixing with the fuel droplets. Fol-
lowing ignition by the electrode rods 60, a portion of the
combustion gases are recirculated, as described, to the
diffuser breech. The advantage derived by such recir-
culation is known to those skilled in the art and is set
forth in U.S. Patent No. 3,741,166. However, in known
constructions, as in Fig. 1, the tube 28 was spaced from
the end of the tube 18 to provide a breech width of about
3/8 inches, whereas I have found that by employing the
diffuser as here shown and described and by widening the
breech to the order of 5/8 inches, I obtain higher recir-
culation rates while showing the axial velocity of the
products passing through the diffuser and raising the
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primary flame zone temperature, both occurrences serving
to broaden the stability range of the burner. The dif-
fuser also serves to increase the air-to-fuel shear angle
in the primary combustion region resulting in more rapid
fuel preparation and effectively shortening the residency
time required of the fuel droplets in the primary zone
and reducing the physical size of the primary zone.
Turning now to Fig. 3, it will be seen that the
combustion chamber 14, the air, fuel and ignition means
are similar to that of Fig. 2 so that no detailed discus-
sion thereof is deemed necessary at this time. It will
be noted, however, that the tubular housing section 40
extends further into the combustion chamher than in the
previous embodiment so that the metering plate is almost
flush with the annular end plate 20. Moreover, the con-
tour of the diffuser is formed on a radius of the order
of 1.5 inches, the breech and mouth diameters are of the
order of 4 inches and the baffle 78 is omitted.
It should be noted that this metering plate 41
is designed with reduced swirl angles, i.e., angles based
upon the angle of the air metering plate jet from a plane
passing through the nozzle orifice and the centerline of
the jet with a minimum jet L/D ratio of .60. Proper
swirl angles serve to enhance burner stability by im-
proving mass transport rates in the combustion process.
Optimized swirl angles are necessary since increased
swirl tends to reduce recirculation rates which will have
detrimental effects on the process. I have been ahle to
reduce the swirl angle from about 20 to about 5 due to
improved distribution and mixing of combustion gases in
the combustion chamber. rloreover, because of venturi
geometry, there is no need for a convergence angle in
the air metering plate jets. Convergence of course re-
fers to the angle ~hat the air metering plate iet makes
with a plane normal to a plane passing through the nozzle
orifice and the centerline of the jet.
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In the present embodiment, I am able to realize
the advantages described in connection with that of Fig. 2
while eliminating the baffle because the larger diffuser
alters the combustion gas patterns to permit recirculation
of gases that are cool enough to permit advantageous re-
circulation rates and to prevent autoignition of the fuel
droplets.
In both embodiments I have also repositioned the
electrodes 60, as shown in Figs. 4 and 5, so that while
j the gap between the rod tips shown in Fig. 4 maintain the
conventional 1/8 inch spacing, these tips are actually
located 1/8 inch from the nozzle end, thus drawing them
back from the fuel spray pattern and into a low pressure
region in the diffuser.
I believe that the construction and operation
of my novel blue flame burner will now be understood and
that the advantages thereof will be fully appreciated by
those persons skilled in the art. _
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