U.S. patent application number 16/504298 was filed with the patent office on 2021-01-07 for apparatus for firetube boiler and ultra low nox burner.
The applicant listed for this patent is Jianhui Hong. Invention is credited to Jianhui Hong.
Application Number | 20210003277 16/504298 |
Document ID | / |
Family ID | |
Filed Date | 2021-01-07 |
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United States Patent
Application |
20210003277 |
Kind Code |
A1 |
Hong; Jianhui |
January 7, 2021 |
Apparatus for Firetube Boiler and Ultra Low NOx Burner
Abstract
The current invention disclose a method and apparatus for
production of hot water or steam in a firetube boiler, said method
comprising the steps of producing a first flue gas using a first
stage of a burner in a first pass of a firetube boiler; passing at
least a portion of said first flue gas through a second pass of
said boiler, wherein said second pass comprises a plurality of
firetubes; routing said portion of said first flue gas to a second
stage of said burner to reduce NOx emissions from said second stage
of said burner; producing a second flue gas from said second stage
of said burner in a third pass of said boiler; passing said second
flue gas through a fourth pass of said boiler, wherein said fourth
pass comprises a plurality of firetubes.
Inventors: |
Hong; Jianhui; (Buffalo
Grove, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hong; Jianhui |
Buffalo Grove |
IL |
US |
|
|
Appl. No.: |
16/504298 |
Filed: |
July 7, 2019 |
Current U.S.
Class: |
1/1 |
International
Class: |
F22B 9/08 20060101
F22B009/08; F24H 1/20 20060101 F24H001/20 |
Claims
1. An apparatus for producing hot water or steam, said apparatus
comprising 1) a firetube boiler comprising 1a) a shell
substantially cylindrical in shape, having a front end and a back
end; 1b) a front tube sheet and at least one back tube sheet; 1c)
two furnace tubes and a plurality of firetubes positioned inside
the shell and substantially extending the length of the shell from
the front end to the back end, said furnace tubes form a first pass
and a third pass, said firetubes form a second pass and a fourth
pass in the boiler, wherein said first pass comprises a furnace
tube and allows a first flue gas to be produced and flow in the
direction from the front end to the back end; said second pass
comprises a plurality of firetubes and allows said first flue gas
to flow in the direction from the back end to the front end; said
third pass comprises a furnace tube and allows a second flue gas to
be produced and flow in the direction from the front end to the
back end; and said fourth pass comprises a plurality of firetubes
and allows said second flue gas to flow in the direction from the
back end to the front end. 2) a burner affixed to the front end of
said firetube boiler comprising 2a) a first stage that produces
said first flue gas; 2b) a second stage that produces said second
flue gas; wherein at least a portion of said first flue gas goes
through said second pass of said boiler, and is routed to the
second stage of the burner to reduce NOx emissions from said second
flue gas; said second flue gas goes through said fourth pass of
said boiler.
2. The apparatus as described in claim 1 wherein said firetube
boiler further comprises a fifth pass that comprises a plurality of
firetubes and allows flue gas to flow in the direction from the
front end to the back end.
3. The apparatus as described in claim 2 wherein said firetube
boiler further comprises a sixth pass that comprises a plurality of
firetubes and allows flue gas to flow in the direction from the
back end to the front end.
4. The apparatus as described in claim 1 wherein the first stage
and second stage of the burner are supplied with combustion air
from a single blower.
5. The apparatus as described in claim 1 wherein the first stage of
the burner use premix type combustion.
6. The apparatus as described in claim 1 wherein the firetube
boiler is a dry back design.
7. The apparatus as described in claim 1 wherein the firetube
boiler is a wet back design.
8. The apparatus as described in claim 1 wherein the first stage
accounts for 10%-33% of the heat input of the burner, and the
second stage accounts for the rest of the heat input of the
burner.
9. The apparatus as described in claim 8 wherein the first stage of
the burner is run at a higher oxygen level in the first flue gas
than the second flue gas from the second stage of the burner.
10. The apparatus as described in claim 9 wherein the first stage
of the burner is run at 5-11% oxygen level in the first flue gas,
and the second stage is run at 1-5% oxygen level in the second flue
gas.
11. The apparatus as described in claim 9 wherein the second stage
is run at 1-3% oxygen level in the second flue gas dry volume
based.
Description
[0001] This is a divisional application of application Ser. No.
15/347,900 (filed on Nov. 10, 2016), which is a continuation of
application Ser. No. 14/941,842 filed on Nov. 16, 2015.
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0002] This invention relates generally to a firetube boiler and a
burner system for the production of hot water or steam. More
particularly, this invention relates to the production of hot water
or steam using a firetube boiler and a burner system that are
designed to produce ultra low NOx emissions and higher efficiency
at the same time. The firetube boiler and the burner of this
invention are specifically designed to work with each other.
2. Description of the Related Art
[0003] Boilers are widely used for the generation of hot water and
steam. A conventional boiler (excluding Heat Recovery Steam
Generator or HRSG) comprises a furnace in which fuel is burned, and
surfaces typically in the form of steel tubes to transfer heat from
the flue gas to the water. A conventional boiler has a furnace that
burns a fossil fuel or, in some installations, waste fuels or
biomass derived fuels. According to the website of Britannica, the
first boiler with a safety valve was designed by Denis Papin of
France in 1679; boilers were made and used in England by the turn
of the 18th century. Most conventional boilers are classified as
either firetube boilers or watertube boilers. In a firetube boiler,
the water surrounds the steel tubes through which hot flue gases
from the furnace flow. In a watertube boiler, the water is inside
the tubes with the hot flue gases flowing outside the tubes. One
example of firetube boilers is Scotch Marine firetube boilers.
There have been relatively few innovations in the designs of
firetube boilers in the last few decades. Several incremental
improvements aimed to enhance the heat transfer efficiency of the
firetube boilers. The introduction of helical ribs inside the
firetubes (also known as "spiral tubes" or "gun barrel tubes") and
spiral turbulators inserted inside the tubes, are a couple of
examples of such incremental improvements. The recent innovations
have been focused on low NOx and ultra low NOx burner technologies.
However, innovations in the low emission burners have been hampered
by the lack of innovations in the firetube boilers. In order for
low emission burner technologies to make further progress, there
exists a need for a holistic approach, to treat the firetube boiler
and the burner as an integral system, rather than two separate and
loosely related sub-systems.
[0004] NOx is a recognized air pollutant. Regulations on NOx tend
to get more stringent in densely populated areas of the world. In
some areas, local regulations require low NOx or even ultra low NOx
emissions in the exhaust from the combustion processes of the
boilers. Various low NOx and ultra low NOx burners are available in
the market to meet these requirements. A review of typical NOx
reduction methods can be found in the article "NOx emissions:
Reduction Strategy" in "Today's Boiler" magazine Spring 2015 by
Jianhui Hong. FGR (Flue gas recirculation) is a commonly used
technique for NOx reduction. In one approach called "forced FGR",
FGR is added into the burner system downstream of the combustion
air blower with the help of a separate FGR blower. Flue gas is
pulled from the stack and pushed through some sort of manifold or
bustle ring into the flame. The forced FGR approach is more energy
efficient in terms of electrical consumption for the combustion air
blower. However it often requires a factory modified burner capable
of forced FGR. From a control standpoint, the forced FGR cannot be
substantially self-controlled like induced FGR. The addition of a
separate FGR blower demands accurate and reliable control of the
rate of FGR relative to the rate of combustion air, making the
control more complicated.
[0005] In another approach called "Induced FGR", flue gas is drawn
through a duct to the inlet of the air blower and mixed with the
combustion air by using the blower wheel as a mixing device. The
flue gas is typically at a higher temperature than the ambient air.
The introduction of flue gas into the blower can sometimes lead to
condensation, corrosion, and heat damage to some burner equipment.
For example, condensation on the spark ignition system could render
it inoperable due to electric short-circuit. Corrosion to the
internal parts of the blower and the burner head can occur due to
condensation. Heat and condensation from the flue gas can damage or
interfere with the flame scanner, which is a part of the burner
management system. The heat can also transmit through the shaft of
the electric motor and damage the motor if the shaft is not
properly cooled.
[0006] According to the Perry's Chemical Engineers' Handbook
(7.sup.th Edition) Section 10-46, the horsepower requirement for a
blower is determined by the multiplication of two factors, the
volumetric flow rate through the blower in cubic feet per minute,
and the blower operating pressure in inches water column. Induced
FGR increases both the volumetric flow rate through the blower and
the pressure drop through the burner and the boiler (hence
increasing the blower operating pressure required), and therefore
greatly increases the horsepower requirement for the blower motor.
In this sense, induced FGR penalizes the blower horsepower
requirement twice, once for the extra volumetric flow rate, and
another for the extra pressure drop through the boiler and burner
system.
[0007] U.S. Pat. No. 5,407,347A teaches an apparatus and method for
reducing NOx, CO and hydrocarbon emissions when burning gaseous
fuels. The advantage of this invention is that ultra low NOx
emission can be achieved at relatively low oxygen level (such as 3%
dry volume basis) in the flue gas. The shortcoming of this
technology is that a large amount of FGR (up to 40% of combustion
air by mass) is required to achieve <9 ppm NOx emissions. In
addition, the rapid mixing design requires large pressure drops
across the swirl vanes in the combustion air pathway near the
burner head. Since mixing rate slows down with flow velocity, this
design also has a limited turndown for ultra low NOx performance.
Due to the large amount of FGR and the high pressure drop the
air/FGR mixture has to overcome, a larger motor and a larger
combustion air blower are required compared to some other
alternative ultra low NOx burner technologies. The larger motor
means higher initial capital costs, higher electricity consumption
and higher noise during the burner's operation. In the state of
California in particular, operators of boilers often dislike use of
FGR, perhaps due to the concerns of earthquake and the additional
mandatory structural inspection related to the field installation
of the FGR pipe. U.S. Pat. No. 6,776,609 also discussed the motor
size penalty problem in details related to the use of Induced
FGR.
[0008] Another commonly used technique for ultra low NOx is called
"lean premixed combustion". U.S. Pat. No. 6,776,609 was intended to
teach a method for operating a burner with FGR, but it also
discussed the disadvantages of the lean premixed combustion method
based on fiber matrix. It disclosed that "Alzeta Corp. of Santa
Clara, Calif. sells a burner for use in food processing and other
industries that utilizes only excess combustion air (no FGR) to
achieve the flame dilution necessary for 9-ppm NOx emissions. A
dilution level of 60% on a mass basis is required". Using the same
dilution principle, Power Flame Corporation offers a product called
Nova Plus. It uses metal fiber matrix elements for the "fully
premixed surface stabilized combustion".
[0009] Another technique for ultra low NOx relies on lean premixed
combustion, but it does not use fiber matrix elements. Sellers
Manufacturing, a subsidiary of Green Boiler Technologies, sells an
S-Series boiler that generates 9 ppm NOx level using lean premixed
combustion based on injection nozzles instead of fiber matrix
elements. A fully premixed air/fuel mixture goes through multiple
injection nozzles at relatively high velocity to resist flashback
to areas upstream of these injection nozzles. These premix burners
have limited turndown due to flashback concerns.
[0010] The shortcomings of the "lean premixed combustion" technique
are well recognized in the combustion community: low thermal
efficiency due to the very high excess air level and the resultant
very high oxygen level in the flue gas (9% oxygen is typical), and
the extra electricity consumption due to the extra excess air for
the dilution and flame cooling. The large amount of excess air was
intended to reduce the peak flame temperature by dilution effects.
The extra dilution air carries additional heat into the atmosphere
(wasted heat) when the exhaust is vented out of the stack, and
causes a reduction of thermal efficiency.
[0011] In view of the foregoing, there exists a need for an
improved method and apparatus for production of hot water and steam
that can produce low NOx (including ultra low NOx) emissions, low
electricity consumption for the motor and high thermal efficiency
at the same time.
SUMMARY OF THE INVENTION
[0012] It is a general object of the present invention to provide a
method and apparatus for the production of hot water or steam in a
firetube boiler and a burner system that produces low NOx
emissions, low electricity consumption and high thermal
efficiency.
[0013] A more specific object of the present invention is to
provide a method and apparatus for the production of hot water or
steam in a firetube boiler and burner system that produces ultra
low NOx emissions in the flue gas, low oxygen level in the flue gas
which leads to higher thermal efficiency, low horsepower
requirement for the blower motor for the burner.
[0014] These objects are achieved by a method of producing hot
water or steam, comprising the steps of, producing a first flue gas
using a first stage of a burner in a first pass of a firetube
boiler;
[0015] passing at least a portion of said first flue gas through a
second pass of said boiler, wherein said second pass comprises a
plurality of firetubes; routing said portion of said first flue gas
to a second stage of said burner to reduce NOx emissions from said
second stage of said burner; producing a second flue gas from said
second stage of said burner in a third pass of said boiler; passing
said second flue gas through a fourth pass of said boiler, wherein
said fourth pass comprises a plurality of firetubes.
[0016] These objects are achieved by an apparatus for producing hot
water or steam, comprising a firetube boiler and a burner; said
firetube boiler comprising a shell substantially cylindrical in
shape, having a front end and a back end; a front tube sheet and at
least one back tube sheet; two furnace tubes and a plurality of
firetubes positioned inside the shell and substantially extending
the length of the shell from the front end to the back end, said
furnace tubes form a first pass and a third pass, said firetubes
form a second pass and a fourth pass in the boiler, wherein said
first pass comprises a furnace tube and allows a first flue gas to
be produced and flow in the direction from the front end to the
back end; said second pass comprises a plurality of firetubes and
allows said first flue gas to flow in the direction from the back
end to the front end; said third pass comprises a furnace tube and
allows a second flue gas to be produced and flow in the direction
from the front end to the back end; and said fourth pass comprises
a plurality of firetubes and allows said second flue gas to flow in
the direction from the back end to the front end; and said burner
comprising a burner affixed to the front end of said firetube
boiler comprising a first stage that produces said first flue gas;
a second stage that produces said second flue gas; wherein at least
a portion of said first flue gas goes through said second pass of
said boiler, and is routed to the second stage of the burner to
reduce NOx emissions from said second flue gas; said second flue
gas goes through said fourth pass of said boiler.
[0017] Additional objects and features of the invention will appear
from the following description from which the preferred embodiments
are set forth in detail in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a schematic view of an apparatus for producing
steam in accordance with the present invention.
[0019] FIG. 2 is a front view of an embodiment of the firetube
boiler with front covers removed.
[0020] FIG. 3 is a rear view of the same boiler in FIG. 2, with
rear cover removed.
[0021] FIG. 4 is a side view of the same boiler in FIG. 2 without
the front and rear covers.
[0022] FIG. 5 is section view along section line A-A.
[0023] FIG. 6 is a perspective view of the same boiler in FIG.
2.
[0024] FIG. 7 is a front view of the tube sheet 31 or 32.
[0025] Identical reference numerals throughout the figures identify
common elements.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] For the purpose of this invention, a burner shall mean a
device to produce one or more flames in the firetube boiler of the
current invention in a controlled manner, taking inputs from at
least one fuel source and an oxidizer source such as air. The two
stages of the burner disclosed in this invention could arguably be
referred to as two separate burners by anyone skilled in the art.
Such change of nomenclature does not create a new invention outside
the scope of the current invention.
[0027] FIG. 1 shows a schematic view of an apparatus for the
current invention. A boiler 5 has a cylindrical shell 30, which is
welded to a front tube sheet 31 and a rear tube sheet 32 to form a
pressure vessel 40. Furnace tubes 33 and 38 are positioned in the
shell 30 to extend the length of the shell from tube sheet 31 and
to tube sheet 32, and sealingly attached (typically welded) to
these tube sheets per firetube boiler codes. A plurality of
firetubes 35 and 39 are also positioned in the shell 30 to extend
the length of the shell from tube sheet 31 to tube sheet 32. These
firetubes are sealingly attached to these tube sheets, commonly by
tube expansion (also called "tube rolling"). The boiler 5 in FIG. 1
could be a horizontal boiler or a vertical boiler.
[0028] The boiler 5 has a front end 6 near tube sheet 31, and a
back end 7 near tube sheet 32. Feed water is supplied into the
boiler through water inlet 42. When necessary, water can be drained
through drain outlet 43. Steam is collected in the vapor space
within the pressure vessel 40 and above the water level 41, and
discharged through steam outlet 48 when pressure is higher than a
desired pressure setpoint.
[0029] A burner 10 has a first stage 11 and a second stage 12. Each
of these two stages of the burner 10 comprises means for supplying
a fuel and combustion air in a proper air/fuel ratio so that
combustion can be sustained, a means of ignition, and a means for
flame monitoring to ensure safety. For clarity and simplicity of
illustration, some details of these two stages of the burner are
omitted in FIG. 1. The combustion air to these two stages of the
burner may be supplied by two separate blowers, or may preferably
be supplied by a single blower to take full advantage of this
invention. The fuel supplied to these two stages of the burner may
be taken from the same source, or two separate sources of fuel.
Fuel supplies to these two stages (11 and 12) may be modulated or
shutoff. Means for flame monitoring may include but are not are
limited to UV, IR, flame rod and any other method.
[0030] The first stage 11 of the burner 10 produces a flame in a
furnace tube 33. The flue gas from the first stage is referred to
as the first flue gas. The furnace tube 33 is also called the first
pass. The first flue gas flows in the first pass in the direction
from the front end 6 to the back end 7, then exits from the first
pass into a chamber 50 affixed to the back end 7. The first flue
gas then goes through a plurality of firetubes 35 (only one
firetube 35 shown in FIG. 1, but this schematic illustration is for
illustrating the concept, not to be taken literally), which are
referred to as the second pass of the boiler, in the direction from
the back end 7 to the front end 6, and discharges into a chamber 20
affixed to the front end 6. The second stage 12 of the burner is
disposed in chamber 20 to produce a flame in a furnace tube 38. The
first flue gas in chamber 20 is injected into the flame of the
second stage 12 to reduce the peak flame temperature, and thus
reduce NOx emissions from the second stage 12 of the burner 10.
Using flue gas to reduce NOx emissions is well understood in the
combustion community.
[0031] The advantages of the current invention include three
aspects: 1) the first flue gas does not go through the wheel of any
blower, thus reducing the motor horsepower requirement and
electricity consumption of the motor; 2) the use of the first flue
gas helps reduce the oxygen level in the second flue gas while
achieving the desired NOx levels, thus improves the thermal
efficiency of the boiler; 3) there is no need for an external FGR
pipe or a separate FGR blower. Elimination of the external FGR pipe
or the separate FGR blower is advantageous as is discussed in the
section of "Description of Related Art".
[0032] The second stage 12 produces a second flue gas in the
furnace tube 38, which is referred to as the third pass of the
boiler. The second flue gas exits the third pass and discharges
into a rear chamber 60, and goes through a plurality of firetubes
39, which are the fourth pass of the boiler. The second flue gas
exits the fourth pass, and discharges into a flue gas collection
chamber 70, and is vented out of the boiler through flue gas outlet
80. The rear chambers 60 and 50 are separated by a divider 81,
which is made out of a refractory material since the flue gases on
both sides are at elevated temperatures. The chambers 20 and 70 are
separated by a divider 82, which could be made out of steel, since
the flue gases on both sides are at relatively low temperatures
(for example, 250-400 degree Fahrenheit).
[0033] The stages 11 and 12 of the burner 10 are both located in
the vicinity of the front end 6. The observation ports 52 and 62
are located in the vicinity of the back end 7. Ports 52 and 62
allow manual observation of the flames in furnace tube 33 and
furnace tube 38, respectively. For simplicity, insulation and
refractory materials commonly used for boilers are not shown in any
figures in this invention.
[0034] It is well known that burners can be classified as premix
type or diffusion type (also known as non-premix type), depending
on whether the fuel and air is mixed well before combustion is
initiated. Each of the stages 11 and 12 in FIG. 1 can be either a
premix type or diffusion type. The first flue gas from the first
stage 11 is an intermediate fluid that is not vented out of the
boiler directly. In a sense, high NOx or CO in the first flue gas
has no consequence if these pollutants are subsequently destroyed
in the second stage 12. From a practical standpoint, NOx, once
formed in the first flue gas, will be difficult to destroy in the
second stage 12. Therefore it is desirable to keep the NOx level as
low as possible in the first flue gas. In contrast, CO and oxygen
levels can be high in the first flue gas, without causing any
adverse effects on the overall performance of the boiler and the
burner system, since CO and oxygen can be consumed by the flame of
the second stage 12. This fact adds to the design and operational
flexibility of the burner.
[0035] In a particular embodiment, a single blower 1 supplies
combustion air to both stages 11 and 12 of burner 10. Combustion
air is drawn in from inlet 2 by the blower 1, goes through air duct
3A and 3B to the first stage 11 and the second stage 12,
respectively. A fuel, such as natural gas, propane or fuel oil, is
supplied from a single source (not shown) through fuel lines 4A and
4B to stage 11 and 12 of burner 10, respectively. The fuel flows
through 4A and 4B are modulated by modulation valves and can be
shut off by safety shutoff valves (not shown). Combustion air flow
through 3A and 3B are modulated by two dampers and a variable
frequency drive (not shown) on the motor of the blower. One air
damper is installed in the air inlet duct 2, controlling the total
amount of combustion air supplied to both stages 11 and 12. The
other air damper is installed in the air duct 3B to control the
percentage of air supplied to stage 12. The air supplied through
air duct 3A to the first stage 11 has to go through more passes
than the air supplied through air duct 3B. Therefore there is a
natural tendency for combustion air to prefer the path of 3B. For
this reason, an air damper in the air duct 3B is preferred. Stage
11 and stage 12 are both equipped with independent and separate
means for ignition and flame monitoring systems (not shown).
[0036] The first stage 11 is generally rated for a smaller fraction
of the total heat input of the burner 10 than the second stage 12.
The heat input of the first stage 11 as a percentage of the total
heat input of burner 10 depends on the need of flue gas for the
second stage. The more flue gas is needed for NOx suppression in
the second stage, the larger fraction the first stage needs to be.
There is an upper limit on how much flue gas the second stage can
take before the second stage becomes unstable. In general, the
first stage 11 should account for 10-33% of the total heat input of
the burner 10, and the second stage accounts for the balance of the
heat release.
[0037] In one particular embodiment, the first stage of the burner
11 utilizes the "lean premix" technique commonly used in
conventional ultra low NOx burners. The high excess air was used to
lower the peak flame temperature, which in turn suppresses
formation of thermal NOx. The higher oxygen level in the first flue
gas allows the NOx emissions in the first flue gas to reach ultra
low NOx levels. In one embodiment, the first stage could be
operated with 5-11% oxygen (dry volume based) in the first flue
gas, depending on the requirement of NOx emissions. For example, at
9-11% oxygen (dry volume based) in the first flue gas, 4-6 ppm NOx
(dry volume based, corrected to 3% oxygen) can be achieved with
less than 50 ppm CO. Lean premix associated with "surface
combustion" type burners are well known for lower efficiency, due
to the increased heat loss to the stack exhaust. This shortcoming
is overcome in the current invention. The higher level of oxygen in
the first flue gas does not result in lower efficiency of the
boiler, because at least some of the oxygen in the first flue gas
will be consumed in the second stage of the burner. The oxygen
level in the second flue gas is maintained at lower levels such as
1-5% (dry volume based), preferably at 1-3% (dry volume based) to
achieve high thermal efficiency of the boiler.
[0038] Burner 10 can be operated in two modes. In a first mode, the
first stage 11 and second stage 12 are both in operation,
converting fuel and air into flue gas and generating heat. This is
the normal mode of operation where low and ultra low NOx emissions
are desired. In a second mode, the first stage 11 is in operation,
and the second stage 12 is turned off. In this mode of operation,
the flue gas from the first stage 11 still goes through the first,
second, third and fourth passes of the boiler, but the fuel supply
to the second stage is turned off. The combustion air supply to the
second stage of the burner is kept on but modulated to a minimal
flow rate, just to prevent the flue gas from the second pass of the
boiler to back flow into the second stage and cause damages to the
burner head. This is the mode of operation when extremely high
turndown (24:1 to 30:1) is desired for the boiler. Caution should
be used to limit the maximum turndown to avoid condensation in the
firetubes, if the boiler is not designed as a condensing
boiler.
[0039] FIGS. 2 and 3 show the front and rear views of an embodiment
of the firetube boiler according to the current invention. The four
passes are generally divided into four quadrants. FIG. 3 shows how
the divider 81 seperated a rear smokebox into two chambers 50 and
60. A refractory insulated backcover (not shown), as is commonly
seen in firetube boilers, when installed, should seal tightly
against this divider 81, to prevent the first flue gas from the
chamber 50 to go directly to chamber 60, bypassing the second
pass.
[0040] FIG. 4 shows a side view of the boiler in FIG. 2. Note the
side opening 71 is for easy access to the second stage 12. The
cover for the side opening 71 is not shown.
[0041] FIG. 5 shows a section view along section line A-A in FIG.
4. It shows the first flue gas moving from the lower section of the
front chamber 20 to the upper section of front chamber 20. The
upper and lower sections of chamber 20 are fluidically
communicating with each other. They can be partially divided, to
guide the flow pattern of the first flue gas. It also shows the
second flue gas flows in chamber 70 to the exhaust stack 80. The
divider 82 seperates chamber 20 from chamber 70.
[0042] FIG. 6 shows a perspective view of the boiler in FIG. 2.
FIG. 7 shows a front view of the tube sheets 31 or 32. Tube sheets
31 and 32 are identical.
[0043] Some common elements such as handholes and ports for water
level control were omitted in these figures for clarity of
illustration.
[0044] The third and fourth passes (furnace tube 38 and firetubes
39) of the boiler in FIG. 1, correspond to the first and second
passes of a two-pass boiler, if the first stage 11, furnace tube 33
and firetubes 35 are thought of as a dedicated flue gas generator.
The third and fourth passes can arguably be referred by anyone
skilled in the art as the first and second passes associated with
the second stage 12 of the burner, since fuel and air from the
second stage 12 is indeed making a first and second passes through
the boiler. However, during normal operation, the first flue gas
from the first stage 11 of the burner have already made two passes
(furnace tube 33 and firetubes 35) through the boiler when they go
through furnace tube 38 and firetubes 39, and therefore making a
third and fourth passes through the boiler. Calling furnace tube 38
and firetubes 39 as first and second passes for the second stage 12
of the burner by anyone skilled in the art is simply a choice of
nomenclature, and does not create a new invention outside the scope
of this invention.
[0045] It is common in the firetube boiler industries to have
one-pass, two-pass, three-pass and four-pass conventional firetube
boilers.
[0046] Additional passes can be added to the boiler in FIG. 1. For
example, a fifth pass could be added to the boiler in FIG. 1
allowing flue gas from the fourth pass to flow in the direction
from front end to the back end, and the exhaust outlet 80 would
move to the back end of the boiler, similar to a conventional
3-pass boiler. Similarly, a sixth pass could be added, and the
exhaust outlet 80 would stay at the front end of the boiler,
similar to a conventional 4-pass boiler.
[0047] It is common in the firetube boiler industries to have dry
back and wet back designs. FIG. 1 shows a dry back design. But a
wet back design could be easily implemented for the current
invention by anyone skilled in the art, after reviewing the current
disclosure.
[0048] As is well understood in the boiler industry, if hot water
production is desired instead of steam, steam outlet 48 in FIG. 1
would be replaced by a hot water outlet located at a proper
location on the shell 30.
[0049] The foregoing description, for purposes of explanation, used
specific nomenclature to provide a thorough understanding of the
invention. However, it will be apparent to one skilled in the art
that the specific details are not required in order to practice the
invention. In other instances, well known devices are shown in
block diagram form in order to avoid unnecessary distraction from
the underlying invention. Thus, the foregoing descriptions of
specific embodiments of the present invention are presented for
purposes of illustration and description. They are not intended to
be exhaustive or to limit the invention to the precise forms
disclosed. Obviously many modifications and variations are possible
in view of the above teachings. The embodiments were chosen and
described in order to best explain the principles of the invention
and its practical applications, the thereby enable others skilled
in the art to best utilize the invention and various embodiments
with various modifications as are suited to the particular use
contemplated. It is intended that the scope of the invention be
defined by the following claims and their equivalents.
* * * * *