Field of the invention
[0001] The invention relates to the field of shell-and-tube equipment comprising a vessel
and a tube bundle, for example shell-and-tube heat exchangers or shell-and-tube chemical
reactors. The invention relates in particular to a baffle structure to support the
tubes within the bundle.
Background art
[0002] A known technique to support tubes of a tube bundle against vibrations is the rod-baffle
cage which is disclosed inter alia in
US 4,342,360.
[0003] The rod baffle design provides a precise guidance and support of tubes. A disadvantage,
however, is the need to weld a large number of rod elements and related support rings.
Another disadvantage is that the conventional rod-baffle design calls for tubes with
a square pitch, being unsuitable for a triangular layout pattern of tubes. Another
drawback of this prior art concerns the pressure drop of the shell side. In most cases,
the shell side of the equipment is traversed by a heat exchange medium, such as condensing
steam or a gas. It has been observed that the support means of the rod baffles induce
a disturbance to the flow of said medium, affecting the overall coefficient of heat
exchange.
[0004] A number of alternatives and improvements to said rod-baffle design have been proposed
in the art.
[0005] US 2008/0245515 discloses a shell-and-tube device comprising slotted baffles having elongated parallel
slots formed therein. The elongated slots are sized to receive a certain number of
aligned tubes. Said slotted baffles can be made in a relatively cheap way, by cutting
a metal sheet for example. However, this embodiment has a disadvantage in that the
thin strips of metal formed between the slots are prone to vibration and instability,
especially under the severe stress occurring during a transitory, for example during
a startup when hot gases cause a relevant thermal stress. Indeed, the thickness of
the baffle is generally smaller than the width of said strips, making them unstable
under mechanical stress (especially a compression stress) and unable to provide a
firm support to the tubes. To avoid this risk, the thickness of the metal sheet may
be increased, which however increases also cost and weight and makes the cutting more
difficult. Another partial solution to this problem is the provision of additional
support means such as tie bars or reinforcing rings, which however require additional
works inside the vessel, make the manufacturing process more difficult and more expensive
and introduce a further source of disturbance for a gaseous medium traversing the
shell side.
[0006] DE 10 2011 107 122 discloses a grid baffle layout where each one of the tubes passes through a respective
four-sided window. Accordingly, the bundle is supported and guided in an effective
manner, however a very precise construction and alignment of the baffles is necessary
since each single tube must meet a corresponding small window of the baffle. Hence,
assembly may result difficult or impossible due to tolerances of the tube bundle,
especially when tubes have a considerable length, e.g. of some meters. Furthermore,
this design causes a relevant pressure drop of the shell side, since the design of
the baffles leaves a small passage area for the medium flowing in the shell side.
Summary of the invention
[0007] The problem underlying the invention is to provide a supporting structure for a bundle
of tubes which overcomes the above drawbacks of the prior art, and in particular:
provides a secure support against vibrations of tubes; allows using also a triangular
layout of tubes; avoids an expensive realization, reduces to a minimum the negative
influence on a gaseous medium flowing in the shell side of the equipment.
[0008] The problem is solved by a shell-and-tube equipment comprising:
- a vessel
- a tube bundle inside the vessel,
- a plurality of support baffles for supporting the tubes of said bundle, said support
baffles having spaced apart slots formed therein, said slots being sized to receive
a plurality of tubes therethrough,
characterized in that:
said slots have the shape of polygons with a number N of sides, and
said slots are arranges in such a way that each slot receives at least a number N
of tubes, which are located at corner regions of the polygonal slot.
[0009] Said corner regions are to be intended as sharp corners or rounded (beveled) corner
regions. In some embodiments, the slots have beveled corners to avoid their overlapping,
for example with triangular slots. In one embodiment of the invention, for example,
slots designed to receive tubes with a triangular pitch have beveled corners, so that
they actually have 6 sides, including three long sides and three short sides (chamfers)
opposite to the long sides. Nevertheless, they are referred to as triangular slots,
since they appear substantially as triangles.
[0010] Preferably, the baffles are arranged according to a repetition of a pattern along
a longitudinal direction of the bundle. The slots of a generic baffle, according to
said pattern, are staggered relative to the slots of the adjoining baffles. Said pattern
hence comprises at least a first baffle and a second baffle, and the slots of the
second baffle are staggered relative to the slots of the first baffle.
[0011] The baffles arranged according to said pattern provide a complementary support for
each tube of the bundle. For example, a tube is prevented against a lateral displacement
in a given direction X by the first baffle of the pattern, and is prevented against
a displacement in another direction Y by the second or another baffle of the pattern.
More generally, a displacement of a tube in a certain direction is always prevented
by at least one, or more, of the baffles forming the pattern.
[0012] Preferably, the slots of a baffle of the pattern are staggered relative to the slots
of the previous or next baffle in the pattern by a distance which is equal to the
pitch of the tubes in the tube bundle. The slots of adjoining baffles may be staggered
relative to each other in one direction or in two directions.
[0013] An embodiment of the invention provides that the tubes are arranged according to
a square pitch, and said baffles have square slots or, more generally, four-sided
slots. Each slot receives four tubes located at the four corner regions of the slot,
and said pattern of the baffles is formed by a first baffle and a second baffle, which
means that first baffles and second baffles are alternated. The slots of the second
baffles are staggered relative to the slots of the first baffle, by a distance equal
to the pitch of the tubes, according to a first direction and a second direction in
a plane perpendicular to the tubes.
[0014] In another embodiment, the tubes have a triangular pitch, and said baffles have triangular
slots. Each slot receives three tubes at the corner regions. Preferably said slots
are equilateral triangles.
[0015] In another embodiment, the tubes have a triangular pitch, and said baffles have hexagonal
slots, each slot receiving six tubes at corner regions of the slot plus a seventh
tube in the middle.
[0016] It must be noted that in all the above, a pattern of only two different baffles,
with suitably spaced slots, can support the tubes in all directions in a plane perpendicular
to the tubes themselves.
[0017] According to a preferred application of the invention, said equipment is a gas-gas
heat exchanger. The exchanger may also contain a catalyst in the tubes or in the shell
side, if required. A more preferred application of the invention relates to a reactor
for the synthesis of ammonia.
[0018] An aspect of the invention is also a structure for supporting a tube bundle of a
shell-and-tube equipment, according to the attached claims.
[0019] Thanks to the invention, the tubes are guided and supported against vibrations in
a way similar to a rod-baffle cage, but avoiding the drawbacks of the conventional
rod-baffle design, including the large number of welded joints. Indeed, the slotted
baffles of the invention may be prepared from metal sheets and with a technique such
as cutting, laser cutting, water cutting or equivalent, reducing the number of the
welded junctions.
[0020] Another advantage of the invention is that a pattern of only two different baffles
can support the tubes in all directions, thanks to the tubes being received at the
corner regions of the slots. The rod-baffle design, on the contrary, needs the repetition
of a pattern of four different baffles, in order to achieve the same effect, since
each set of the conventional rods acts in one direction only.
[0021] A further and important advantage is that the invention allows using a triangular
pitch, while the rod-baffle design requires a square pitch. Tubes in a triangular
pitch are closer each other, thus providing a better use of the available volume compared
to the square pitch layout. For example when tubes are filled with a catalyst the
triangular pitch provides more room for said catalyst. Hence, the heat exchanger is
more compact and less expensive for a given duty.
[0022] The invention does not use elongated slots which may suffer vibrations or instability
under stress. There is no need to increase the thickness of the sheet in order to
keep the baffles from the above problems, or provide expensive additional support
means.
[0023] Additional advantages relate to the flow through the shell side. Firstly, the baffle
structure according to the invention leaves a large passage area in the shell side,
reducing the pressure drop. Furthermore, the applicant has noted the following advantage:
the staggered slots avoid the formation of preferred "lanes" for the gaseous medium
traversing the shell side. For example a gaseous flow line, after a passage through
a slot of a generic baffle, impinges on the surface of the next baffle and is divided
into a plurality of flow lines, passing through the slots of said next baffle. This
effect provides for a certain mixing and, hence, a good heat exchange coefficient
but, on the other hand, does not cause a high pressure drop. The applicant has found
that the prior art solutions tend to generate helical flows or linear flows with contractions
and expansions which are less preferable, since they increase the pressure drop and
do not provide optimum heat exchange.
[0024] The above advantage is of particular importance for gas-gas heat exchangers, when
the shell side is normally traversed by a gaseous flow having a considerable speed.
[0025] Another advantage of the invention is that the free space between the boundary of
the bundle and the vessel is reduced to a minimum. This space does not contribute
to the heat exchange; accordingly, the reduction of this free space means that the
available internal volume of the equipment is best exploited.
[0026] Yet another advantage is that the structure of the invention is tolerant with respect
to the unavoidable dimensional errors of the tubes. Each baffle provides a support
for a tube according to two directions, leaving the tube free to flex to a certain
extent in the other directions, to compensate for working tolerance.
[0027] In some embodiments, the shell-and-tube equipment may comprise a central duct, e.g.
to collect a medium flowing in the shell side, or for another purpose. The tube bundle
is arranged in the annular space between the central duct and the outer vessel, and
the baffles are ring-shaped as a consequence. In this case, the invention provides
additional advantages. A conventional rod-baffle design would need an inner ring as
a support means for the rods. The invention does not need this inner support ring,
since the slotted baffle can be prepared, as above, by cutting a metal sheet of the
desired shape, including a central aperture for the duct. Hence, the realization is
simpler and less expensive.
[0028] These and other advantaged will be more evident with the help of the following description
of a preferred embodiment, with the help of the enclosed drawings.
Brief description of the figures
[0029]
Fig. 1 illustrates a shell-and-tube equipment according to an embodiment of the invention.
Fig. 2 is a scheme of tubes and baffles according to a square pitch.
Fig. 3 is a scheme of tubes and baffles according to a triangular pitch.
Fig. 4 is a cross section of an equipment according to an embodiment with a central
duct.
Detailed description of a preferred embodiment
[0030] Fig. 1 illustrates a shell-and-tube equipment 1 comprising a vessel 2, a bundle of
tubes 3, a plurality of support baffles 4, 4a for supporting said tubes 3. The tubes
3 are fixed to tubesheets 6. The baffles are spaced each other by a suitable distance
d.
[0031] The equipment 1 processes a first fluid flowing in the tubes 3 and a second fluid
flowing in the shell side, which means around the tube bundle. The first fluid (tube
side) enters at 10 and leaves at 11, while the second fluid (shell side) enters for
example at 12 and leaves at 13. In a preferred application, said first fluid and second
fluid are gaseous, i.e. the equipment 1 is a gas-gas heat exchanger. In some applications
the equipment 1 may carry out a chemical reaction and to this purpose, it may contain
a suitable catalyst. In a preferred application, the equipment 1 is a reactor for
the synthesis of ammonia, where synthesis of ammonia takes place in the tube side.
[0032] Said support baffles 4, 4a are made of a metal sheet of a suitable thickness. A number
of slots are formed in the baffles, which are sized to receive a plurality of tubes
3 therethrough.
[0033] Fig. 2 shows an embodiment where tubes 3 are arranged according to a square pitch
p, said pitch being the distance between centers of adjacent tubes (see Fig. 2) and
the slots 5 are four-sided, e.g. square. Four tubes 3 are received at corner regions
of each slot 5.
[0034] Fig. 2 also shows the slots 5' of a second baffle 4', next to said baffle 4 (see
Fig. 1). Said slots 5' of the second baffle 4' are depicted with dotted lines and
Fig. 2 shows they are staggered relative to the slots 5 of the first baffle 4. In
a preferred embodiment, the baffles are staggered in both directions X and Y by a
first distance s1 and a second distance s2. Preferably said first and second distances
are equal (s1 = s2) and more preferably they are equal to said tube pitch p, as shown
in Fig. 2. Said directions X and Y are in a plane perpendicular to the tubes 3 and
to the axis of the vessel 2.
[0035] The baffles are arranged according to a pattern where first baffles 4 and second
baffles 4' alternate, as shown in Fig. 1. Hence, slots 5 and 5' of adjoining baffles
are always staggered.
[0036] It can be seen that any generic tube, such as tube 3* of Fig. 2, is embraced by an
upper corner of a slot 5 and a lower corner of an adjoining slot 5, and so on according
to the pattern of alternate baffles 4, 4'. Basically, a generic slot keep the tube
from displacement in two directions, e.g. left and down, while the next slot keep
the tube from displacement in the other two, e.g. right and up. Accordingly, the tube
is secured against vibrations in all directions of plane X-Y.
[0037] A gaseous medium traversing the shell side of equipment 1, during operation, passes
through the slots 5, 5' of the baffles, around the tubes 3, for example in the region
7 of Fig. 2. Due to the staggered arrangement of the slots, the flow passing through
a baffle will impinge on the next baffle, providing a certain mixing of the flow and
improvement of the heat exchange coefficient.
[0038] Said slots 5 or 5' can be obtained by cutting, laser cutting or another technique,
starting from a metal sheet of a suitable thickness.
[0039] Fig. 3 relates to an embodiment where tubes are arranged according to a triangular
pitch. In this embodiment, the baffles 4, 4' have substantially triangular slots 5,
5', each slot 5 or 5' receiving three tubes 3 at the corners. It may be noted that
the slots 5, 5' in this embodiment appear to have six sides, namely three long sides
8 and three short sides 9; said short sides 9 however are formed by the beveled corners.
Each long side 8 is opposite to a short side 9, so that the shape of the slot globally
resembles to a triangle. Accordingly, the slot is considered three-sided.
[0040] In both embodiments of Fig. 2 or Fig. 3, a pattern of only two different baffles
4, 4' can effectively support the tubes 3 in all directions, thanks to the fact that
the tubes are received at the corner regions of the slots 5, 5'. The conventional
rod-baffle design, on the contrary, needs the repetition of a pattern of four different
baffles, in order to achieve the same effect, since each set of the conventional rods
acts in one direction only.
[0041] Fig. 4 relates to an embodiment where the shell-and-tube equipment comprises a central
duct 10 coaxial to the shell 2. The tubes 3 are arranged in the annular region between
the duct 10 and the outer shell 2, and the baffles 4, 4' are ring-shaped as a consequence.
Fig. 4 shows tubes with a square pitch but the same embodiment with central duct 10
may use triangular pitch as in Fig. 3.
[0042] In this case, the invention is advantageous in that it does not need an inner ring
for supporting the baffles. Provided that the baffles have a sufficient thickness,
also an outer ring may not be necessary.
1. A shell-and-tube equipment (1) comprising:
- a vessel (2),
- a bundle of tubes (3) inside the vessel,
- a plurality of support baffles (4, 4') for supporting the tubes of said bundle,
said support baffles having spaced apart slots (5, 5') formed therein, said slots
being sized to receive a plurality of tubes therethrough,
characterized in that:
said slots (5, 5') have the shape of polygons with a number N of sides, and
said slots (5, 5') are arranges in such a way that each slot receives at least a number
N of tubes, which are located at corner regions of the polygonal slot.
2. An equipment according to claim 1, characterized in that said plurality of baffles are arranged according to a repetition of a pattern along
a longitudinal direction of the bundle, where the pattern is formed by at least a
first slotted baffle (4) and a second slotted baffle (4'), and where the slots (5')
of the second baffle (4') are staggered relative to the slots (5) of the first baffle
(4).
3. An equipment according to claim 2, wherein the slots of the second baffle are staggered
relative to the slots of the first baffle in one direction or in two directions, by
a distance (s1, s2) which is equal to the pitch (p) of the tubes (3) of said tube
bundle.
4. An equipment according to claim 3, wherein said tubes are arranged according to a
square pitch, and:
- said baffles have four-sided slots, each slot receiving four tubes located at the
four corner regions of the slot,
- said pattern of the baffles is formed by first baffles (4) alternated to second
baffles (4'), wherein the slots (5') of the second baffles are staggered relative
to the slots (5) of the first baffle, by a distance equal to the pitch of the tubes,
according to a first direction (X) and a second direction (Y) in a plane perpendicular
to said tubes (3).
5. An equipment according to claim 4, the slots (5, 5') being square.
6. An equipment according to claim 2 or 3, wherein the tubes (3) have a triangular pitch,
and said baffles (4, 4') have slots with a substantially triangular shape, each slot
receiving three tubes at said corner regions of the slot.
7. An equipment according to claim 2 or 3, wherein the tubes (3) have a triangular pitch,
and said baffles have hexagonal slots, each slot receiving six tubes at corner regions
of the slot plus a seventh tube in the middle.
8. An equipment according to any of the previous claims, said slots (5, 5') being formed
with any of cutting, laser cutting, water cutting.
9. An equipment according to any of the previous claims, comprising a central duct (10)
and the tubes (3) being arranged in the annular space between said central duct and
the vessel, and the baffles (4, 4') being ring-shaped.
10. An equipment according to any of the previous claims, for use as a gas-gas heat exchanger
or for use as a reactor for the synthesis of ammonia.
11. A structure for supporting a tube bundle of a shell-and-tube equipment, comprising
a plurality of support baffles (4, 4') for supporting the tubes of said bundle, said
support baffles having spaced part slots (5, 5') formed therein, said slots being
sized to receive a plurality of tubes therethrough,
characterized in that:
said slots (5, 5') have the shape of polygons with a number N of sides, and
said slots (5, 5') are arranges in such a way that each slot receives at least a number
N of tubes, which are located at corner regions of the polygonal slot.