Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02593907 2007-07-18
HEAT EXCHANGER FRAMEWORK
Field and Background of the Invention
[0001] The present invention relates, in general, to serpentine type tubular
heat
exchangers located in the vertical gas passageways of steam generators, and
more
particulariy, to the structure and support of the serpentine oriented tubes to
cause the
drainage of fluid coitected therein when the steam generators are shutdown.
[00027 It is common practice in the design of a rnodern high capacity steam
generator to provide heat exchange surfaces in the form of closely spaced
serpentine
metal tube rows disposed in a vertical passageway through which combustion
gases
at relatively high velocities are conveyed, with the metal tubes in each row
having
horizontally extending segments conveying the fluid being heated, and being
arranged
in spaced parallel rows distributed transversely of the direction of gas flow.
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[0003] Whenever the steam generator is shutdown, water collects along the
horizontally extending segments of the heat exchanger tubes. The retention of
water in
the tubes after a steam generator has been shutdown, if unremoved, will lead
to metal
pitting corrosion and eventual tube failure if their walls become too thin as
a result of
the corrosion. Such an event is very costly since it requires the replacement
of the
weakened or failed tubes thereby resulting in down time of the steam
generator.
[00041 Pitting corrosion is a localized form of corrosion by which cavities or
holes
are produced in a metal. Pitting is coi=rxrnonly observed on surfaces with
little or no
general corrosion. Pitting corrosion is generally of greater concern than
uniform
corrosion because it is more difficult to detect and protect against.
Corrosion products
often cover the pits, making them difficult to identify. . Apart from
localized loss of
thickness at the tube metal surface, corrosion pits can also be harmful by
acting as
stress risers. Corrosion pits are commonly the starting points for cracking
and fatigue.
[0005] An extremely corrosive microenvironment typically forms within a
corrosion
pit that varies considerably from the bulk corrosive environment. This
corrosive
microenvironment can hasten growth of pits once initialty formed. Pitting
corrosion can
produce pits in a variety of configurations. For example, open pits may be
formed, or
pits may be covered with a semi-permeable skin comprising corrosive products.
Pits
can be hemispherical or cup-shaped, ftat=wat(ed, or completely irregular in
shape. Pits
may also reveal the crystal structure of the tube metal. Through-shaped pits
may be
narrow and deep or shallow and wide. Sideways pits may be sub surface,
undercutting, or attack the grain of the metal horizontally.
[0006] The corrosion problem is exacerbated at locations where attachment
welds
are made to the heat exchanger tubing, such as with the supports used to
maintain the
tubes and tube rows of a heat exchanger tube bank in coplanar spaced-apart and
parallel relation. The heat affected zones of the attachment welds are prone
to pitting
corrosion. Weld attachments are also known to restrict the thermal expansion
and
contraction of the tubing and thus cause it to distort or sag, such that low
spots are
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formed in the horizontal runs of the tubing. These low spots coilect water
after the
steam generator is shutdown and are susceptible to pitting corrosion.
Sumrrtary of the Invention
10007] The pitting corrosion experienced as a result of the retention of water
in the
horizontal segments of the heat exchanger tubes, subsequent to the shutdown of
a
steam generator, is largely overcome by the novel heat exchanger structural
framework of the present invention. This framework is applicable to a heat
exchanger
having at least one bank of spaced serpentine fluid conveying tube rows
disposed
within a vertical passageway of the steam generator in side-by-side parallel
relation
across the gas stream which is flowing through the passageway. Each of the
tube
rows includes spaced and substantially coplanar elongate sloped-tube segments
and
retum-bend tube segments. The sloped-tube segments replace the prior art
horizontal
tube segments and thus greatly facititate the drainage of water from the heat
exchanger when the steam generator is shutdown.
j0008] In accordance with the heat exchanger structural frameworks of the
present
invention, the bank of serpentine tubes is supported in a manner which will
allow
thermal expansion and contraction to take place without causing the tubing to
distort or
sag and thus form fow spots that are apt to collect water when the steam
generator is
shutdown. The structural frameworks include paired groups of vertically and
diagonally
extending first support members contiguously straddling the sloped-tube
segments.
The vertical structural frameworks are formed of paired vertically extending
first
support members that are rigidly connected by second support members or
lateral
cross bars which extend between the straddled sloped-tube segments. The
diagonal
structural frameworks are formed of paired diagonally extending first support
members
that are rigidly connected by second support members or lateral compression
supports
which extend between the straddled sloped-tube segments. The diagonal
structural
frameworks transmit the heat exchanger support foads to the walls of the
vertical
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passageway housing the heat exchanger. The structural frameworks preserve the
spacing between the sl+npetl-tube segments and prevent direct contact between
adjoining tube surfaces, but are not a#tached to the straddled tube segments
and thus
provide a tube supporting fit that is loose enough to perrnit the tubes to
move freely
due to thermal expansion and contraction at different rates from that of the
structural
frameworks, thereby preventing the distortion and sagging of tubes and the
formation
of water collecting low spots.
{0009} These and other features and advantages of the present invention will
be
better understood and its advantages will be more readily appreciated from the
detailed description of the preferred embodiment, especially when read with
reference
to the accompanying drawings.
Brief Description of the Drawings
j001 0] FIG. I is a schematic sectional side view of a steam generator
including a
heat exchanger embodying the present invention;
[0011] FIG. 2 is a fragmented, sectional side view of the heat exchanger, and
its
tube banks and support members;
j0013] FIG. 3 is a fragmented, perspective view of the heat exchanger tube
banks
and support members;
[0013] FIG. 4 is a fragmented, enlarged secfional view of the vertical support
members and the lateral cross bars taken along line 4- 4 of F)G. 3;
[0014] FIG. 5 is a fragmented, enlarged side view of the vertical support
members
and the lateral cross bars taken along line 5- 5 of F(G. 4;
100151 FIG. 6 is a fragmented, enlarged sectional view of the diagonal support
members and the lateral compression supports taken along line 6 -6 of FIG. 3 ;
and
10016] FIG. 7 is a fragmented, enlarged side view of the diagonal support
members and the lateral compression supports taken along line 7- 7 of FIG. 6.
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t}eacrxp#ion of the Preferred Fmbodr'rne f
[00171 Reference will hereinafter be made to the accompanying drawings wherein
like reference numerals throughout the various figures denote like elements.
[0018] Referring to FIG. 1, there is shown a steam generator 10 including
water
cooled tubular walls 12 that define a furnace chamber or combustion space 14
to
which a fuel and air mixture is supplied by burners as schematically shown at
16. After
combustion has been completed in the furnace chamber 14, the hot gases flow
upwardly and around the furnace chamber nose portion 18, and across through
the
horizontal section 22 of the convection passageway 23, and thence downwardly
through the vertical section 24 of the convection passageway 23 which is
defined by
walls 25 and includes a heat exchanger such as the primary superheater 26.
Usually,
the gases leaving the vertical section 24 of 'the convection passageway 23
flow
through an air heater, not shown, and thence through a gas clean-up system,
not
shown, and are thereafter discharged through a stack, not shown.
[00191 It will be understood that in accordance with well known practice, the
heat
exchanger 26 includes banked rows 27 of spaced serpentine tubes 28, as shown
in
FIG. 3, extending across the width of the vertical section 24 of the
convection
passageway 23, and arranged for fluid flow therethrough and in indirect heat
exchange with the combustion gases flowing through the vertical section or
passageway 24.
[0020] Referring to FIGS. 2 and 3, there are shown fragmented sectional side
and
perspectirre views, respectively, of a heat exchanger 26 including a plurality
of
serpentine tubes 28 disposed in side-by-side parallel relation to one another,
as
shown in FIG. 3, and across the gas stream which is flowing through the
vertical
section 24 of the convection passageway 23, as shown in FIGS. I and 2, and
with
each serpentine tube 28 having elongate sloped-tube segments 30 and retum-bend
tube segments 32 forming the rows 27 of tube banks 34. In accordance with the
invention, the elongated tube segments 30 of the serpentine tubes 28 extend at
an
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angle downwardly from the horizontal to cause fluid to be drained from the
serpentine
tubes 28 when the steam generator 10, shown in FIG. 1, is shutdown.
10021] The serpentine tubes 28 of heat exchanger 26 are supported by
vertically
extending structural frameworks 39 and diagonally extending structural
frameworks
41. The heat exchanger 26 is itself supported by the walls 25, shown in FIGS.
1 and 2,
by way of the first support lugs 35 which are rigidly connected, preferably by
welding,
to the lower end of the outermost return-bend tube segment 32 of each tube
bank 34
and slidably engaged with the second support lugs 37, the latter being rigidly
connected, preferably by welding, to the walls 25. The diagonally extending
structural
frameworks 41 are located closest to the return-bend segments 32 and transmit
the
heat exchanger 26 support loads to the first support lugs 35 which, in turn,
transmit
the support loads to the second support tugs 37 and thence onto the walls 25
of
vertical passageway 24.
[00221 The vertically extending structural framevirork 39 includes first
support
members 36 which are generally in the form of vertically extending bars or
plates that
are paired to contiguously straddle the sloped-tube segments 30, and second
support
members 38, shown in FIG. 3, which are generally in the form of laterally
extending
cross bars or plates which run between the straddled sloped-tube segments 30.
10023] The vertically extending first support members 36 are rigidly attached
to or
connected with the laterally extending second support members 38, preferably
by
welding, to insure that the latter remain tightly drawn against the straddled
sloped tube
segments 30, while preserving the spacing between the sloped-tube segments 30
and
preventing direct contact between adjoining tube surfaces. In accordance with
the
invention, the rigidly interconnected first and second support members 36 and
38,
respectively, are not welded or otherwise attached to the serpentine tubes 28,
thereby
creating a structural framework 39 which provides a tube supporting fit that
is loose
enough to permit relative thermal expansion and contraction of the. serpentine
tubes
28 and the framework 39, the lat#er being comprised of the first and second
support
members 36 and 38, respectively.
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(00241 The diagonally extending structural framework 41 includes first support
members 42 which are generally in the form of diagonally extending bars or
plates that
are paired to contiguously straddle the sloped-tube segments 30, and second
support
members or compression supports 44, shown in FIG. 3, which are generally in
the
form of laterally extending blocks that run between the straddled sloped-tube
segments 30, and are contoured to engage the adjacent surfaces of the sloped-
tube
segments 30.
[00251 The compression supports 44 are rigidly secured to the paired
diagonally
extending first support members 42, preferably by welding, to insure that the
latter
remain tightly drawn against the straddled sloped-tube segments 30, while
preserving
the spacing between the sloped-tube segments 30, and preventing direct contact
between adjoining tube surfaces, and also transmitting the heat exchanger 26
support
loads from the first support lugs 35 to the second support lugs 37 and hence
to the
walls 25 which form the vertical section 24 of the convection passageway 23.
In
accordance with the invention, the rigidly interconnected first and second
support
members or compression supports 42 and 44, respectively, are not welded or
otherwise attached to the serpentine tubes 28, thereby creating a structural
framework
41 which provides a heat exchanger and tube supporting fit that is loose
enough to
petmit relative thermal expansion and contraction of the serpentine tubes 28
and the
framework 41, the latter being comprised of the first and second support
members 42
and 44.
100261 Referring to FIGS. 4 and 5 there are shown fragmented, enlarged
sectfonal
and side views of the vertically extending first support members 36 and the
laterally
extending second support members or cross bars 38. The paired first support
members 36 straddle the sloped-tube segments 30 of the serpentine tubes 28.
The
first support members 36 are drawn tightly against the sloped-tube segments 30
and.
are rigidfy maintained in that position by the second support members or cross
bars
38, which are rigidly attached to the first support members 36 by welds 40.
The
vertically extending structural framework 39 formed by the first support
members 36
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and the second support members 38 preserves the spacing between the sloped-
tube
segments 30 and prevents direct contact between adjoining tube surfaces, but
is not
attached to the straddled sloped-tube segments 30 and is loose enough to allow
the
serpentine tubes 28 and the structural framework 39 to move freely in response
to
thermal expansion and contraction.
[0027] Referring to FIGS. 6 and 7 there are shown fragmented, enlarged
sectional
and side views of the diagonally extending first support members 42 and the
laterally
extending second support members or compression supports 44. The first support
members 42 are drawn tightly against the sloped-tube segments 30 and are
maintained in that position by the second support members or compression
supports
44, which are rigidly attached to the first support members 42 by welds 43.
The
second support members or compression supports 44 are formed as a solid block
of
concavo-concave cross-sectional configuration so as to have the concave
recesses 45
engage the contiguous portion of the adjacent sloped tube segments 30, as
shown in
FIG. 7, and thereby support the serpentine tube 28 off the flrst support
members 42,
while maintaining the spacing between adjacent sloped-tube segments 30. The
diagonally extending framework 41 formed by the first support members 42 and
the
second support members or compression supports 44 maintains the spacing
between
the sloped-tube segments 30 and prevents direct contact between adjacent tube
.
surfaces, while also supporting the serpentine tubes 28 off the first support
members
42, but is not attacheri to the straddled sloped-tube segments 30, and is
loose enough
to allow the serpentine tubes 28 and the strtactural framework 41 to move
freely in
response to thermal expansion and contraction,
[0028] Although the present invention has been described above with reference
to
particular means, materials, and embodiments, it is to be understood that this
invention may be varied in many ways without departing from the spirit and
scope
thereof, and therefore is not limited to these disclosed particulars but
extends instead
to all equivalents within the scope of the following claims.
