Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IMPROVED FURNACE FLOOR
FIELD OF THE INVENTION
The present invention relates generally to the field of industrial furnaces
and boilers
and, more particularly, to an improved decanting floor design for kraft
recovery boilers.
BACKGROUND OF THE INVENTION
Kraft recovery boilers are used in the pulp and paper industry to recover
usable
energy from byproducts of the pulp making process. Kraft recovery boilers are
similar to
conventional fossil-fuel fired boilers. Black liquor fuel is introduced into
the furnace along
with combustion air. Inside the furnace, residual water is evaporated from the
black liquor,
and the organic material from the black liquor is combusted. The inorganic
portions of the
black liquor are recovered as sodium/sulfur compounds.
Gases generated by the black liquor combustion rise out of the furnace and
flow
across convection heat transfer surfaces. The vertical enclosure walls of the
furnace are
formed from heat transfer surfaces made of interconnected water tubes.
Typically, feedwater
enters the recovery boiler at the bottom of a first pass economizer, in which
the water is
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heated as it flows to a steam drum. Saturated water is routed from the steam
drum through
pipe downcomers to lower furnace enclosure wall and floor inlet headers and a
boiler bank.
Natural circulation flow in the tubes is induced and driven by heat input to
the vertical water
cooled enclosure walls of the furnace from the combustion process.
Decanting floors in kraft recovery boilers are known for collecting and
directing
molten smelt from the black liquor combustion process to discharge openings in
the boiler
walls. The water tubes forming the floor are cooled by the circulation of
water and/or a
water/steam mixture through the tubes.
The floors of many known decanting recovery furnaces are essentially flat
across the
entire surface. Flat floors are subject to minor humping of the tubes, causing
domes which
form in the upper surfaces of the floor tubes. Steam can become trapped in
these humps or
domes (steam blanketing) which can cause the tubes to overheat and fail. More
particularly,
steam blanketing is where steam bubbles are not effectively entrained in the
water moving
through the tubes. In the flat floor tubes adj acent the furnace sidewalls,
the heat input to the
tubes may be lower. Since flow through the tubes is by natural circulation,
the low heat
input to these tubes results in lower fluid velocities and poor circulation in
the tubes, which
in turn causes steam blanketing.
Currently, the only known method for correcting this defect in flat floor
furnaces is
to replace the entire tube floor with a new floor. The new floor is sloped to
increase the
tolerance for heat absorption at lower fluid velocities and to permit venting
of the minor
humps which would otherwise trap steam and lead to tube overheat and failure.
However,
replacing the entire furnace floor is both time consuming and expensive, and a
cost-effective
solution would be welcomed by industry.
SUMMARY OF THE INVENTION
One aspect of the present invention is to provide a method for upgrading a
recovery
boiler furnace floor without replacing the entire floor.
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Another aspect of the present invention is to provide a method and furnace
floor for
preventing burst tubes which can be implemented relatively inexpensively and
efficiently as
compared to known solutions.
Accordingly, an improved floor for a recovery boiler furnace having a flat
floor has
a number of replacement tubes in the floor immediately adjacent each of the
sidewalls of the
furnace oriented at an oblique angle to horizontal.
In a first embodiment of the invention, immediately adjacent each of the
sidewalls,
the ends of the replacement tubes at each of the front and back walls are
above the remainder
of the tubes forming the flat floor and angled to slope downwardly towards a
center of the
furnace to the level of the tubes forming the flat floor. At the center of the
floor, the other
ends of the tubes are joined to a central header below the tube floor. The
sloped tubes may
have a central flat portion adjacent the center of the floor. The radius of
curvature of the
connection between the flat tube section ends and the central header may be
varied to
improve fluid flow through the connection, and to join up with the existing
flat floor tubes.
In a second embodiment, the ends of the replacement tubes at one of the
furnace front
and rear walls then are connected to a collection header and are lower than
the remainder of
the tubes forming the flat floor. The other ends of the replacement tubes at
the opposite
furnace wall are level with or higher than the remainder of the tubes in the
floor. The
replacement tubes may be substantially straight along their length. The
collection header
may be positioned below either the front or back wall of the furnace.
In a method of improving circulation in the floor tubes of a flat floor
furnace, several
floor tubes immediately adj acent the sidewalk in a flat floor furnace are
removed. A series
of curved replacement tubes adjacent to the side walls positioned such that
the tubes are
oriented at an oblique angle to horizontal are connected to the front and back
walls of the
furnace.
The improved floor design of the invention helps reduce the occurrence of
steam
bubbles being trapped in humped areas in the furnace floor tubes or forming a
steam blanket
by improving the fluid flow through the tubes adjacent the sidewalk of the
recovery boiler
furnace.
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The various features of novelty which characterize the invention are pointed
out with
particularity in the claims annexed to and forming a part of this disclosure.
For a better
understanding of the invention, its operating advantages and specific benefits
attained by its
uses, reference is made to the accompanying drawings and descriptive matter in
which a
preferred embodiment of the invention is illustrated.
BRIEF DESCRIPTION OF THE DRA WINGS
In the drawings:
Fig. 1 is a side elevational view of a first embodiment of an improved
replacement
furnace floor of the invention;
Fig. 2 is an end elevational view of a floor tube arrangement for the furnace
floor
shown in Fig. 1, viewed in the direction of arrows 2-2;
Fig. 3 is a top plan view of a portion of the furnace floor shown in Fig. 1;
Fig. 4 is a side elevational view of an alternate connection to a central
headers for
the floor tubes;
Fig. 5 is a side elevational view of a second embodiment of an improved
replacement furnace floor;
Fig. 6 is a top plan view of a portion of the furnace floor shown in Fig. 5;
and
Fig. 7 is an end elevational view of a floor tube arrangement for the furnace
floor
of Fig. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, in which like reference numerals are used to
refer to
the same or functionally similar elements, Fig. 1 shows the lower portion 10
of a recovery
boiler furnace having front wall 12, back wall 14 and flat floor 50. Front and
back walls 12,
14 are water tube walls. Floor 50 is also comprised of a plurality of water
tubes.
Adjacent the sidewalk on either side of flat floor 50, a series of water tubes
30 are
sloped downwardly from the front and back walls 12, 14 toward the center of
the furnace
lower portion 10. The sloped water tubes 30 are joined to central headers 40.
The upper
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ends of sloped water tubes 30 are connected to the front and back furnace
walls at weld
points 32.
As seen in Figs. 2 and 3, sloped tubes 30 and flat floor 50 form a continuous
surface,
with gaps between each tube sealed. Sloped tubes 30 are angled more relative
to horizontal
immediately adjacent a sidewall 20 and the angle gradually decreases the
farther the tube 30
is located from the sidewall 20 until the tubes are consistent and level with
the flat floor 50.
Between 3 and 9 water tubes 30 could be used for the sloped tubes 30.
Preferably, six water
tubes 30 are used for the sloped tubes 30, with the sixth tube 30 being
relatively flat and
unsloped. (See Fig. 2). The angles which the tubes 30 make with the horizontal
plane are
between 0 ° and 10 ° .
The sloped water tubes 30 may be 2~/2 or 3 inch outer diameter, internally
ribbed
tubes, with Inconel~ 625 outside cladding to enhance their resistance to the
furnace
environment.
By providing the sloped tubes 30 adjacent the sidewalls 20, the path which the
water
and/or steam takes through these tubes 30 is more gradual and upwards adjacent
the front and
back walls 12, 14. The gradual, upward path improves the fluid flow through
the sloped
tubes 30, which would otherwise be hampered by poor thermal conduction near
the sidewalls
20 of the furnace 10. As a result, the effect of humping and steam blanketing
is greatly
reduced.
The sloped tubes 30 are easily installed in existing flat floor recovery
boiler furnaces,
as only 12 tubes total need to be replaced and welded back to the flat floor
50. Preferably,
the existing tubes are removed adjacent each sidewall 20 between the front and
back walls
12, 14. Then, the new sloped tube sections are connected to the front and back
walls 12, 14
with welds 32 at the appropriate heights on the front and back walls 12, 14
for the desired
angle the tube will make to horizontal, gradually decreasing to the level of
the existing flat
floor 50. The other ends of the sloped tubes 30 are connected to the central
headers 40.
In the embodiment of Figs. 1-4, the sloped tubes 30 have a flat portion,
generally
designated 35, at the center of the lower portion 10 of the recovery boiler
furnace. The flat
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portion 35 of the sloped tubes 30 from both the front wall 12 and back wall 14
meet at this
location, and a seal must be provided at this point.
Fig. 4 shows an alternate configuration for the flat portion 35 connection to
the
central headers 40 which improves the seal between the sloped tube 30, flat
portion 35 and
the floor 50 at the central header 40. An elbow section 45 connects the flat
portion 35 of
section 30 to a bent tube 46 connected to the central headers 40. The elbow
section 45 bends
about 90 ° and preferably has a radius of 5'/2 inches.
In a second embodiment of the improved furnace floor, shown in Figs. 5-7,
sloped
tubes 60 adj acent the sidewalls 20 are only provided at one of the front or
back walls 12, 14.
A supply header 65 is provided at the end wall 12 or 14 where a lowest portion
of the sloped
tubes 60 are located.
As seen in Fig. 5, the sloped tubes 60 are provided at the back wall 14. In
this
embodiment, the sloped tubes 60 are oriented at angles below the level of the
flat floor 50,
as seen in Fig. 7. Preferably six tubes 60 would be used; between 3 and 9
tubes 60 could be
used as well. The tubes 60 are oriented at angles of between 0 ° and 10
° below the
horizontal, with the greatest slope being found in the tube 60 closest to the
sidewall 20 and
the angle each tube 60 makes decreasing to 0 ° where they are again
even with the flat floor
50. The ends of the sloped tubes 60 adjacent the front wall 12 are level with
the flat floor
S0. The gaps between the tubes are sealed, membraned, as above to form a
continuous floor
50.
The floor of a flat floored furnace would be renovated according to this
embodiment
in a manner similar to that described above, except that a new header 65 must
be installed
at the end wall 12 or 14 where the sloped tubes 60 are located. Another new
header 70 also
is required. Header 70 feeds those rear wall tubes adj acent to the sidewall
20 that were
previously fed with the floor.
While specific embodiments of the invention have been shown and described in
detail to illustrate the application of the principles of the invention, those
skilled in the art
will appreciate that changes may be made in the form of the invention covered
by the
following claims without departing from such principles. For example, the
present invention
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may be applied to new construction involving kraft recovery boilers, or to the
replacement,
repair, or reconstruction of existing kraft recovery boilers. In some
embodiments of the
invention, certain features of the invention may sometimes be used to
advantage without a
corresponding use of the other features. Accordingly, all such changes and
embodiments
properly fall within the scope and equivalents of the following claims.
