Note: Descriptions are shown in the official language in which they were submitted.
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FIRETUBE HEAT EXCHANGER
BACKGROUND OF THE INVENTION
100011 Firetube heat exchangers are well known for converting heat from hot
gases of combustion to a material, typically a liquid, exposed to the outside
surface of the
firetube. Such heat exchangers are described in U.S. Patent Nos. 5,913,289 and
6,675,746. These as well as other previously described firetube heat
exchangers have
been relatively expensive or difficult to manufacture. In addition, some
firetube heat
exchangers have been less effective in transferring heat from the hot gases of
combustion
passing through the interior of the firetube to the outside surface for
heating the liquid. It
is to an improved, highly efficient, and relatively economically manufactured
firetube
design that the apparatus described herein is directed.
SUMMARY OF THE INVENTION
[00021 Embodiments of the firetube heat exchanger described herein comprise
an elongated cylindrical shell having a fluid inlet end, a fluid outlet end
and a fin
assembly secured on the inner surface of the shell. The fin assembly comprises
a plurality
of circular rows of elongated U-shaped fins, each fin having a bottom surface
secured to
the inner surface of the shell with two generally flat, planar sides extending
upwardly
from the bottom fin surface. The fins in each row are aligned substantially
parallel along
the axis of the cylindrical shell, and the fins of one or more rows of fins
may be offset
angularly from the fins of an adjacent row of fins. In some embodiments, the
flat, planar
sides of the fins are substantially parallel and the fins in each row of fins,
respectively, are
substantially identical in fin height, length and width. In other embodiments,
the
dimensions of fins in at least two of the rows are different in height, and/or
width, and/or
length. In yet another embodiment, three or more different fin heights are
used within the
firetube heat exchanger. These as well as other variations in designs and
embodiments of
the fins and the firetube heat exchanger design will be described hereinafter.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0003] Fig. 1 is an isometric view of one embodiment of a firetube showing a
semi-transparent cylindrical tube shell.
[0004] Fig. 2 is an cross-sectional isometric view taken across the line 2-2
of
one embodiment of a firetube that illustrates the interior fin arrangement and
design.
[0005] Fig. 3 is an end view of a fin and illustrates three different fin
heights.
[0006] Fig. 4 is a side view of a fin shown in Fig. 3, also illustrating three
different fin height designations.
[0007] Fig. 5 is a cross-sectional view of the split firetube of Fig. 2 with
an
interior ceramic core plug installed.
DETAILED DESCRIPTION
[0008] One embodiment is a firetube heat exchanger that includes an outer
shell. Disposed along the interior surface of the shell is a fin assembly
having a plurality
of circular rows of elongated U-shaped fins. In one embodiment, each fin has a
bottom
surface that is secured to an inner surface of said cylindrical shell. Each
fin may also have
two sides extending upwardly from said bottom surface and defining an
elongated interior
channel. The sides may be planar and flat. In addition, in one embodiment, the
fins in
each row may be aligned substantially parallel along the axis of the
cylindrical shell. In
one embodiment, the sides of the fins in different rows have differing
heights.
[0009] In Fig. I a firetube heat exchange assembly 10 is illustrated with the
cylindrical shell 11 shown in semi-transparency for viewing the interior fins.
Reference is
also made to the cross-sectional view of Fig. 2 in which the cylindrical shell
has been
sectioned to show more particular features of the fin assembly.
[0010] As illustrated, the fin assembly is secured circumferentially around
the
inner surface of the cylindrical shell and comprises a plurality of circular
rows of
elongated U-shaped fins. In the illustrated embodiment, the fins in each row,
respectively, are substantially identical and using fins of three different
heights in
different rows of fins. The first row of fins nearest to the fluid inlet end
13 of shell 11
comprises substantially identical fins 12, the second row comprising
substantially
identical fins 14 and the third row and the remaining rows made up of
substantially
identical fins 16. In this embodiment, the difference between fins 12, 14 and
16 is in the
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height of their upwardly extending sides. In this embodiment, the fin sides
are lower in
the front of the firetube where gas temperatures are hottest.
[0011] The length of the fins of all the rows may be the same, although
different fin lengths in the different rows may be used. However, all of the
fins in any
single row may have substantially the same length. Similarly, the width of the
fins in any
row may be the same, although different fin widths may be used. However, in
some
embodiments, all of the fins in a row have substantially identical widths. In
other
embodiment, all of the fins in all of the rows of the firetube have
substantially identical
widths.
[0012] The difference in the heights of the sides of the fins of the different
rows is further illustrated in Figs. 3 and 4. The heights of opposite sides 22
and 24 of fin
20 are the same. However, the upper edge 21 of all fins 12 in the first row of
fins is
shorter than the height of the sides of the fins in rows 14 and 16.
Specifically, the upper
edge 23 of the sides 22, 24 of all second row fins 14 is greater than the
height of the fins
in row 12 and shorter than the height of fins in the third row of fins 16 and
the remaining
rows of fins all having an upper edge 25.
[0013] In one embodiment, the height of the fins differs by between 10% and
50%. In another embodiment, the height of the fins differs by between 15% and
35%. In
yet another embodiment, the height of the fins differs by between 20% and 30%.
In one
embodiment, one row of fins is 0.5 inches tall, the second row of fins are 5/8
inches tall
and the fins in the third and remaining rows are 0.75 inches tall. In one
embodiment, each
row of fins from the first row to the third row is 25% taller than the
preceding row.
[0014] As previously described, and illustrated particularly in Fig. 3, all of
the
fins have substantially the same width and are U-shaped with a bottom surface
26. The
bottom fin surface is generally flat or is arched or curved preferably on a
radius (radiused)
to better match the radius or curvature of the inner cylindrical surface of
the shell
underlying the bottom fin surface. Such a radiused bottom surface will also
facilitate
brazing of the fin and cylinder surfaces. Such a flat or curved bottom also
provides a
surface for tack welding or spot welding each fin in place during assembly of
the firetube
heat exchanger.
[0015] In another embodiment, the opposite fin sides are parallel and extend
upwardly substantially perpendicular (normal) from the bottom surface.
However, the
opposite sides may also be somewhat angled at obtuse or acute angles from the
bottom
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surface. Such angles may be selected, depending on the desired number of fins
in each
row, as well as the desired spacing of the fins in each row. It will also be
understood that
the specific number of fins in each row will depend on the width of the fins
and the radial
dimensions or circumference of the cylindrical shell.
[0016] In this embodiment, the fins in each respective row are aligned
lengthwise with their upwardly extending sides aligned substantially parallel
along the
axis of the cylindrical shell. As previously noted, the shortest fins or fins
in rows of fins
are at the inlet end of the firetube, and fins in succeeding rows have higher
sides. The
specific number of different heights of fins in the firetube may be selected,
but at least
two different heights may be used. In another embodiment, at least three
different heights
of fins are used, although more different heights may also be used without
departing from
the invention. In the embodiment illustrated, three different heights of fins
are used, as
previously described and shown in Figs. 1-4.
[0017] The fins in adjacent rows of fins may be aligned angularly along the
length of the firetube or fins of adjacent rows of fins may be offset
angularly from one
another. Of course, if the fins of adjacent rows of fins are of different
widths, the
upwardly extending sides of the fins in adjacent rows will present an offset
of fin sides
from inlet to outlet along the length of the firetube. In one embodiment, with
the fins
being of substantially the same width, the fins may be aligned angularly
without offset, or
they may be offset angularly up to one-half of the fin width.
[0018] The specific number of rows of fins will depend on the length of the
firetube, and the length of the fins in the different rows of fins. The number
of rows of
fins of between 2 and about 20 rows is preferred and more preferred is between
about 4
and about 12 rows of fins, fewer fins results in more heat stress along the
firetube. By
way of example, for a firetube of about 2 feet in length, 10 rows of fins
having an equal
fin length in each row is shown in the drawings.
[0019] The upper edges of the upwardly extending fins defines an elongated
interior channel in which is secured a heat resistant insert, often referred
to as a core plug,
and which is typically made of a heat resistant ceramic material. The length
of the insert
may extend between the second row of fins from the inlet end and the last rows
of fins at
the outlet end, as illustrated in Fig. 5. The shape of the insert is such that
the diameter
gradually increases from the forward end, closest to the fluid inlet of the
firetube, leaving
a space between the surface of the insert and the upper edges of the fin sides
for a portion
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of its length in and then contacting the fin edges along a successive portion
of the firetube
length. Such shape of the insert, its dimensions, and placement are well
understood by
those skilled in the art.
[0020] In another embodiment, the firetube heat exchanger assembly includes
copper rings extending between rows of fins and the firetube surface. The
copper rings
may be mounted between all rows of fins, with each ring contacting the
interior of the
surface of the firetube as well as the ends of fins in adjacent rows. At least
one ring may
be mounted at the end of the last row of fins. In another embodiment, a
plurality of
copper rings is mounted at the end of the last row of fins. In Fig. 2, copper
rings 30, 31,
32, 33, 34, 35 are illustrated. The copper rings are shown only between every
other row
of fins by way of example and for simplicity, but again, a ring may be
disposed between
every row of fins. The copper rings may be mounted using vacuum brazing or
brazed in a
hydrogen furnace, or otherwise installed by brazing techniques known to those
skilled in
the art.
[0021] In one embodiment, the rings comprise high purity (above 98%) copper
because of its ductility and conductivity. However, the use of mixtures of
copper with
another conductive metal, for example nickel, is not precluded. It is to be
understood that
when the copper ring is brazed, it will melt and flow to both rows of fins and
the tube
interior surface creating a conductive and ductile bond therebetween. Since
the rings are
to be brazed, their cross-sectional shape prior to brazing is not critical.
[0022] The firetube heat exchanger described herein is useful in any heat
exchange apparatus for directing heat from hot gases of combustion passing
along the
inside of the firetube to heat liquids contacting the outside surface of the
firetube. The
firetube is especially useful in a boiler or stripping section of the
generator of an aqua-
ammonia absorption system, for example, a GAX absorption system, such as
described in
U.S. Patent Nos. 6,487,875, 6,427,478, 6,718,792, 6,735,963 and 6,748,752. The
firetube
heat exchanger described herein has advantages of being cost effective to
manufacture,
reliable, and efficient as compared to other firetubes used and known in the
prior art.
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