Note: Descriptions are shown in the official language in which they were submitted.
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LOW NO P~IMARY ZONE RADIANT SCREEN DEVICE
Back round of the Invention
The present invention relates to combustion systems, and more
particularly, to a device and method ~or inhibi~îng formation
of oxides of nitrogen (NOX) in a two-zone diffusion flame of
a combustion system of a gas-fired furnace.
As a result of the combustion process, combustion systems
normally &enerate gaseous combustion products which include
oxides of nitrogen (NOX) which are vented to the atmosphere
as flue gases. It is desirable to limit these NOX emissions
since NO is considered a pollutant and combustion systems
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for gas-fired furnaces sold in certain geographical areas
must meet strict NOX emission standards.
~ It has been observed that the two most important parameters
- in the formation of NOX are peak combustion temperature and
residence time at peak temperatures. One technique for
limiting NOX emissions by controlling peak combustion flame
temperatures is to provide a supplemental air flow for
cooling the combustion flame in a combustion system. Another
technique is to alter the combustion system to minimize t~e
formation of NOX. However, these techniques, while limiting
NOX formation, may adversely affect the combustion process of
the combus~ion system by causing incomplete combustion and/or
by adversely affecting the combustion process in other ways.
Also, these techniques may require a major redesign of
Y.- certain components of the combustion system, such as a
redesign of burners and combustion chambers or heat
exchangers of the combustion system~ thereby rendering these
~;~ techniques undesirable for retrofitting existing combustion
systems. Another technique used with two-zone mono-port
inshot burners, which generally emit a lower level of NOX
- 35 emissions than multi-port burners, is to position a piece of
~; material at the periphery of the secondary flame of the
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two-zone diffusion flame to temper the combustion flame by
absorbing thermal energy from the combustion flame. In this
technique, the device absorbs ~hermal energy from the
combustion flame at a rate which inhibits formation of N0x
while allowing substantial complete combustion of the fuel
supplied to th~ burner. These devices are generally
cylindrical screens juxtaposed to the periphery of the
cooler, secondary combustion zone. Also, these devices
aerodynamically smooth at least a portion of the periphery of
the combustion flame, thereby inhibiting formation of eddies
by near-stoichiometric mixtures of combustion substances at
the periphery of the two-zone diffusion flame which are
capable of forming relatively large amounts of NOX.
.
Summary of the Invention
The present invention is directed to a method and apparatus
for reducing NOX emission levels from gas-fired burners
: having a two-zone diffusion flame.
-~ 20 It is an object of the present invention to limit N0x
.~ emissions from a combustion system to a desired level without
significantly reducing the combustion efficiency of the
combustion system or otherwise adversely affecting the
combustion process of the combustion system,
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A further object of the present invention is to provide an
easily installed and simple means for altering combustion
~` systems to limit their N0x emissions to a desired level
without significantly reducing the combustion efficiency or
otherwise adversely affecting the ~ombustion process of the
- combustion system.
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~- A still further object of the present invention is ~o reduce
the N0 production below a level of 40 nanograms/joule at a
~; 35 specific minimum California seasonal efficiency level of 71Z.
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Another object of the present invention is to limit the N0x
emissions from any burner, i.e. an inshot burner, a slotted
burner, or a ribbon burner having a two~zone diffusion
flame.
These and other objects of the present invention are attained
by providing a gas-fired combustion system having a two-zone
diffusion flame with a device positioned at the burning
interface of the primary air blue cone and secondary
diffusion layer of each flam~ to inhibit the formation of NOX
by the combustion system. The device comprises a piece of
material positioned relative to the primary zone or hottest
portion of the combustion flame produced by the burner to
temper the combustion flame by absorbing thermal energy from
the hottest portion, i.e. the blue primary cone, of the
conbustion flame. The device is sized, positioned, and
~configured relative to the primary zone of the two-zone
- diffusion ~lame to absorb thermal energy from the combustion
flame at a rate which limi~s peak flame temperatures and
residence times at these peak flame temperatures to levels
which inhibit formation of NOX while allowing substantially
~ complete combustion of the fuel supplied to the burner.
;~Preferably, the device is made of a material, such as
stainless steel J which is resistant to oxidation at the
:25 relatively high primary zone combustion flame temperatures,
and which radiates thermal energy which it absorbs from the
combustion flame, to its surroundings.
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~The various features of novelty which characterize the
-i~30 invention are pointed out with particularity in the claims
,annexed to and forming a part of this specification. For a
better understanding of the invention, its operating
ad~antages and specific objects attained by its use,
reference should be had to the accompanying drawings and
descriptive matter in which there is illustrated and
described a preferred embodiment of the invention.
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Brief Description of the Drawings
In the accompanying dr~wings, forming a part of the
specification, and in which reference numerals shown in the
drawings designate like or corresponding parts throughout the
same,
Figure 1 is a partially exploded and partly broken away
isometric view of a'gas-fired furnace having a ribbon type
burner;
- Figure 2 is a plan view of the ribbon type burner illustrated
in Figure 1 incorporating the,primary zone radiant screen
device made in accordance with.the present invention;
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Figure 3 is a side elevation view of the ribbon type burner
of Figure 2; and
Figure 4 i5 an elevation view of a two-zone diffusion flame
~- emanating from the ribbon type burner of Figures 2 and 3 ~-~
. 20 incorporating the primary zone radiant screen device of the
::~' present invention.
''' Descri tion of the Preferred Embodiment
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. Referring ~o Figure 1, a partly exploded and par~ly broken
:~` 25 away view is shown of a gas-fired furnace 10 having a natural
. draft or multi-port slotted type burner 12 and a primary zone
radiator structure 11 which is a preferre'd embodiment of a
' device fox inhibiting ~x formation by this type of
. combustion system in accordance with the principles of the
.-.' 30 present invention. As shown in Figure 1, the flame radiator
,'.~ structure 11 is an inverted V-shaped perforated structure
",;. having a generally triangular cross-section which will be
. described in more detail hereinafter. In addition to the
flame radiator structure 11 and the burner 12~ the gas-fired
, 35 furnace 10 includes h~at exchangers 14, a furnace cabinet 15,
','" and a flue pipe 16. Each of the heat exchangers 14 is a
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natural draft heat exchanger cell having an inlet opening 17
through which the ribbon type burner 12 projects. Also, each
of the hea~ exchangers 14 has an outlet opening 18 through
which flue gases are discharged from the heat exchanger into
a flue gas collection chamber 19 from which the flue gases
pass into the flue pipe 16 ~o be discharged from the furnace
1 0 .
For ease of illustration, only three heat exchangers 14 are
; 10 shown in Figure 1. However, it is to be understood that the
furnace 10 may have any num~er of heat exchangers 14 each
` with its own burner 12 and flame radiator structure ll.
.
When assembled in the gas-fired furnace 10, the multi-port
lS burner 12 with its attached inverted V-shaped radiator
structure 11 is generally located through the heat exchanger
inlet opening 17 and is held in this position by any suitable
means. Preferably, as shown in Figure 1, the ribbon type
burner 12 has a burner crossover tube 13 which is secured to
each burner which in turn is secured, for example by by a
~`~ bracket screwed to the area 21 surrounding the inlet opening
17. Alternatively, the crossover tube can be formed as part
of the burner.
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~'!' 25 The multi-por~ burner 12 includes a regulator and spud ~not
; shown) for supplying fuel to the burner 12. Fuel is supplied
through the regulator and ignited on the burner by way of the
-: crossover tube 13 by an ignition system (not shown). The
burner, when positioned inside the heat Pxchanger, produces a
two-zone diffusion flame resulting in off-stoichiometric
~- combustion in the primary zone. The combustion flame is
; actually divided into two distinct parts, a blue primary cone
33 and a secondary diffusion layer 31 as shown in Figure 4.
The air/fuel mixture entering the burner is fuel rich, that
is, the primary air is some value less than stoichiometric.
Primary combustion takes place near the burner producing a
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~riangular shaped, blue primary cone 33, and then at some
subsequent point further away from the burner secondary air
is added in order to produce a secondary diffusion layer 31.
The normal average operating temperatures of the blue primary
cone are about 3000F (1648C) while the average secondary
zone or diffusion layer operates about 1800F (1105C).
According to the present invention, undesirable formation of
NOX by the furnace lO is inhibited by the presence of the
inverted V-shaped flame radiator structure 11 which tempers
the prîmary combustion flame produced by the burner 12 by
absorbing thermal energy from the primary combustion flame.
The flame radiator structure ll limits peak flame
temperatures and residence times at these peak flame
temperatures, at the primary zone of the two-zone diffusion
flame, to levels which inhibit formation of N0x while
allowing substantially complete combustion of the fuel
supplied to the burner 12.
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Of course, different regions vf the diffusion flame have
different peak flame temperatures and different residence
times at these peak flame temperatures depending on their
~- location in the combustion flame. Normally, the temperature
of a given region of the combustion flame will vary within a
-~-` 25 certain temperature range as a function of time and will
`~; remain at the peak flame temperature within this tempera~ure
;~ range for a certain amount of time ~residence time) during
the time period of operation of the burner 12. Throughout
this patent application the terms "peak flame temperature'l
and "residence time at a peak flame temperature'l are used in
~! reference to a given region of the combustion flame and the
; plurals of these terms are used to collectively refer to
several of these regions.
It has been found that a desirable shape for the flame
radiator structure 11 of a ribbon type burner, as shown in
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Figures 2 and 39 iS an inverted V-shaped structure located at
the periphery of the primary zone where the radiator
structure 11 is able to efficiently and effectively reduce
peak flame temperatures and residence times at these peak
flame temperatures to desired ]evels. Wi~h an insho~ burner,
a desirable shape for the radiator structure would be a cone
structure loeated at the interface between the primary air
cone and the secondary diffusion layer. The periphery of the
primary cone of the combustion flame is the most desirable
location for the flame radiator structure 11 because 9 as
discussed above, the blue primary flame is the hot~est
portion of the two-zone combustion f3.ame. In fact, it has
been observed that if the flame radiator structure 11 is
moved to the periphery of the secondary flame, the structure
11 is substantially less effective in inhibiting N0x
formation compared to when the structure 11 is located at the
periphery of the primary cone.
Also, as best shown in Figure 4 the flame radiator structure
11 is a triangular shaped perforated sheet metal positioned
at the periphery 35 of the primary zone 33 of the two-zone
combustion flame produced by the burner 12 so that the flame
radiator structure 11 is adjacent to and in contact with the
outer surface or periphery of the primary zone of the
~- 25 combus~ion flame, i.e. the interface of the primary flame
blue cone. Of course, normally, the periphery of the primary
combustion flame will randomly fluctuate in location
throughout any time period of operation of the burner 12.
Therefore, for a primary combustion flame projected from any
~- 30 particular burner 12, it is desirable to determine the
:- average location of ~he periphery of the primary zone of the
combustion flame by observing the combustion flame during
; operation of the burner 12, and to position the triangular
shaped flame radiator structure 11 relative to this average
location of the periphery of the primary combustion flame.
Thus, in this patent application, when it is stated that the
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flame radiator structure 11 is position "at" the periphery of
the primary combustion flame, this means that the flame
radiator structure 11 is positioned relative to the average
location of the periphery of the primary combustion flame so
that when the burner 12 is operating the periphery 35 of ~he
primary combustion flame 33 randomly 1uctuates about the
position of the triangular shaped flame radiator structure
11 .
; 10 Also, it should be noted that the flame radiator structure 11
is made of a material having physical properties, such as
coefficient of thermal conductivity and radiation
characteristics (that is characteristics such as the rate at
which the material will radiate heat energy to its
surroundings at certain elevated temperatures), so that the
structure ll tempers the combustion flame pr~duced by the
burner 12 by absorbing thermal energy from the primary zone
of the combustion flame at a selected rate. Also, as will be
readily apparent to one of ordinary skill in the ~rt to which
the present invention pertains, the physical properties of
the material from which the structure 11 is made ~re very
important relative to the ability of a structure to absorb
and radiate thermal energy. Some materials are able to
absorb and radiate thermal energy faster and more efficiently
` 25 than other materials. In certain applications, such as
~ gas-fired furnace application~s, it is desirable to utilize
'~!, the available thermal energy in the most efficient manner
possible. Therefore, it is preferable in such a furnace
~- application to use a structure ll having a material which
3~ will allow the structure 11 to radia~e absorbed thermal
- energy to its surroundings, namely to the walls of a heat
exchanger surrounding the ribbon type burner 12 and structure
11, so tha~ the available thermal energy may be efficiently
utilized by the furnace. Further, the material from which
the flame radiator structure is made must be capable of being
~- cycled many times from normal room temperatures to relatively
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high combustion flame temperatures without being severely
damaged by oxidizing, corroding, breaking, bending, cracking,
or being damaged in other ways due to this thermal cycling.
In these applications, it has been found that metallic
materials, such as stainless steel, and other steel alloys
which are resistant to oxidation at relatively high
combustion flame temperatures are particularly suitable
materials from which to make the flame radiator structure 11.
Type 310, 314 and 330 stainless steels appear especially
desirable as a material from a reliability viewpoint.
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- As shown in Figures 1-3, the flame radiator structure 11 of
the present invention comprises a perforated sheet having a
generally inverted V-shaped cross section. This sheet
- 15 generally extends along the edges of the burner and covers
the ports 28 of the burner and extends from one end of the
burner to the other. Thus, this perforated inverted V-shaped
flame radiator structure 11 has the desired feature of not
interfering with the flow of products of combustion away from
the combustion 1ame produced by the burner 12. The
perforations 26 in the radiator structure are sized so that
~he products of combustion may flow freely through the
- structure 11 away irom the combustion flame while sufficient
material is present in the structure 11 to achieve the
~;: 25 desired tempering of the combustion flame.
Of course, the foregoing description is directéd to a
- preferred embodiment of the present invention and various
~. modifications and other embodiments of the prPsent invention
-~ 30 will be readily apparent to one of ordinary skill in the art
to which the present invention pertains. Thereore, while
the present invention has been described in conjunction with
- a particular embodiment it is to be understood that various
modifications and other embodiments of the present invention
may be made without departing from the scope of the invention
as described and as claimed in the appended claims.
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