[0001] The present invention relates to a heat exchanger, and particularly but not exclusively
to a heat exchanger having a tube matrix which reduces thermal stress experienced
by the heat exchanger.
[0002] Heat exchangers are widely used to transfer heat from a relatively hot fluid to a
relatively cold fluid without direct contact between the fluids.
[0003] A conventional tube heat exchanger is shown in Figures 1 and 2. The heat exchanger
2 comprises an inlet manifold 4 and an outlet manifold 6. The inlet and outlet manifolds
4, 6 are fluidically coupled by a plurality of tubes 8 which together form a tube
matrix 10. The tubes 8 of the tube matrix 10 are coupled at one end to the inlet manifold
4 and are coupled at the other end to the outlet manifold 6.
[0004] The tubes 8 of the tube matrix 10 are arranged such that their longitudinal axes
are perpendicular to the longitudinal axes of the inlet and outlet manifolds 4, 6.
A plurality of the tubes 8 are aligned in a plane of the longitudinal axes of the
inlet and outlet manifolds 4, 6 to form a row, and several rows are disposed side-by-side
to form columns of the tube matrix 10.
[0005] As shown in Figures 1 and 2, the tubes 8 of the tube matrix 10 are straight. Each
tube 8 therefore follows a direct path from the inlet manifold 4 to the outlet manifold
6 without deviating from a longitudinal axis between its connection point with the
inlet manifold 4 and its connection point with the outlet manifold 6.
[0006] The inlet and outlet manifolds 4, 6 and tube matrix 10 form a conduit for the passage
of a first fluid through the heat exchanger 2. Accordingly, the first fluid flows
into the heat exchanger 2 via the inlet manifold 4, passes through the tubes 8 of
the tube matrix 10 and exits the heat exchanger 2 via the outlet manifold 6.
[0007] A second fluid flows over exterior surfaces of the tubes 8 of the tube matrix 10.
The first and second fluids have different temperatures and therefore heat is transferred
between the first and second fluids.
[0008] Figure 3 shows a simulated stress distribution for the heat exchanger 2 under large
thermal and pressure loads. In this simulation, the ends of the inlet and outlet manifolds
4, 6 are assumed to have a fixed position.
[0009] As shown in Figure 3, the heat exchanger 2 experiences large stresses throughout
as a result of thermal expansion of the inlet and outlet manifolds 4, 6 and the tubes
8 of the tube matrix 10. Increased loads are seen across those tubes 8 which are located
towards the ends of the inlet and outlet manifolds 4, 6 due to the fixed position
of the inlet and outlet manifolds 4, 6 at these locations.
[0010] Various tube matrix geometries have been proposed to reduce the stress experienced
by heat exchangers under large thermal and pressure loads.
[0011] For example, Figures 4 and 5 show a heat exchanger 102 which has a tube matrix 110
which is formed of two portions 110a, 110b comprising U-shaped tubes 108. Each U-shaped
portion 110a, 110b is connected at one end to the inlet manifold 104 and at the other
end to the outlet manifold 106. The U-shaped portions 110a, 110b extend in opposite
directions from the inlet and outlet manifolds 104, 106 to form an oval.
[0012] Figure 6 shows a simulated stress distribution for the U-shaped heat exchanger 102
under large thermal and pressure loads. As can be seen, the stress levels are far
reduced for the tube matrix 102, since the U-shaped tubes 108 are not constrained
by the inlet and outlet manifolds 104, 106. Accordingly, the U-shaped heat exchanger
102 is insensitive to the displacement constraints.
[0013] However, the U-shaped heat exchanger 102 requires more space and is heavier than
the straight tube matrix 2.
[0014] Figures 7 and 8 show another example of a known heat exchanger 202. The heat exchanger
202 has an S-shaped tube matrix 210, with the tubes 208 of the tube matrix 210 following
a serpentine path between the inlet manifold 204 and the outlet manifold 206.
[0015] Specifically, the tubes 208 of the S-shaped tube matrix 210 comprise first and second
straight portions 212a, 212b adjacent the inlet and outlet manifolds 204, 206 respectively,
and first and second curved portions 214a, 214b disposed between the first and second
straight portions 212a, 212b. The first and second curved portions 214a, 214b deviate
in opposite directions from the axis of the first and second straight portions 212a,
212b in a plane defined by the longitudinal axes of the inlet and outlet manifolds
204, 206 to form the S-shape.
[0016] The S-shaped nature of the tube matrix 210 acts to reduce the thermal stress placed
on the heat exchanger 202, without considerably increasing the size and weight of
the heat exchanger.
[0017] However, as shown in Figure 9, the gap between adjacent tubes 208 is reduced over
the first and second curved portions 214a, 214b of the S-shaped tube matrix 210. Consequently,
the tubes 208 must be spaced further from one another at the inlet and outlet manifolds
204, 206 in order to prevent the tubes 208 from contacting one another. This reduces
the number of tubes 208 in the tube matrix 210 for a fixed size heat exchanger 202
or increases the size of the heat exchanger 202 for a fixed number of tubes 208. Furthermore,
the curved portions 214a, 214b increase the complexity of the manufacturing process,
thus increasing the cost of the heat exchanger.
[0018] Further tube matrix geometries are known which use tubes with additional curved portions;
for example, see
US Patent No. 5,058,663. However, although these matrices may reduce thermal stress, they exacerbate the
reduction in the gap between the tubes and the increased complexity and cost of manufacturing.
[0019] Accordingly, it is desirable to provide a heat exchanger with a tube matrix which
overcomes some or all of the problems described above.
[0020] In accordance with a first aspect of the invention there is provided a heat exchanger
comprising: an inlet manifold; an outlet manifold; and a tube matrix comprising a
plurality of tubes, each tube being fixedly connected at one end to the inlet manifold
and at the other end to the outlet manifold; wherein each tube extends generally along
a longitudinal axis defined between the connection of the tube with the inlet manifold
and the connection of the tube with the outlet manifold; and wherein a single portion
of each tube is offset to one side of the longitudinal axis.
[0021] The tubes may be fixedly connected between the inlet manifold and outlet manifold
independently from each other.
[0022] The tubes may each be separated from an adjacent tube by a substantially similar
gap at corresponding portions along the longitudinal axis.
[0023] The tubes may be generally C-shaped.
[0024] The offset portion may comprise a curved portion which curves away from the longitudinal
axis.
[0025] The curved portion may have a constant curvature.
[0026] The offset portion may comprise a straight portion and pair of angled portions which
offset the straight portion from the longitudinal axis.
[0027] The offset portion may be offset in a plane defined by a longitudinal axis of the
inlet and outlet manifolds.
[0028] The tubes may be arranged in one or more rows in a plane defined by a longitudinal
axis of the inlet and outlet manifolds.
[0029] A plurality of rows may be disposed side-by-side to form columns.
[0030] A minimum gap between adjacent tubes over the offset portion may be greater than
2/3 of the maximum gap between adjacent tubes at the inlet and outlet manifolds.
[0031] The offset portion may be 30-70% of the total length of the tube.
[0032] The offset portion may allow deformation of the tubes during thermal expansion, thereby
reducing thermal stress experienced by the heat exchanger. Furthermore, the offset
portion does not considerably affect the gaps between adjacent tubes. Consequently,
the size of the heat exchanger is not significantly increased, if at all. This may
make the heat exchanger of the present invention particularly suitable for installation
in an aero-engine, where space is at a premium.
[0033] In addition, the manufacturing process for the single offset portion is simple and
requires only one bending process. Accordingly, the manufacturing costs are minimised.
[0034] The present invention results in a high efficiency, high temperature, high pressure,
lightweight and compact heat exchanger design.
[0035] According to another aspect of the invention there is provided a heat exchanger comprising
an inlet manifold, an outlet manifold, and a tube array comprising a plurality of
tubes, wherein the tube array generally extends along a longitudinal axis between
the inlet manifold and the outlet manifold, and wherein the tube array comprises an
offset portion that is offset from the longitudinal axis. There may be a single offset
portion. Each tube may comprise an offset portion that is offset to the same side.
The tube array may comprise at least one longitudinally extending portion and an offset
portion. The tube array may comprise first and second longitudinally extending portions
coupled to the inlet and outlet manifolds respectively, with the offset portion disposed
between the first and second longitudinally extending portions. The single offset
portion may be directly between the first and second longitudinally extending portions.
The offset portion may be generally C-shaped.
[0036] For a better understanding of the present invention, and to show more clearly how
it may be carried into effect, reference will now be made by way of example to the
accompanying drawings, in which:
Figure 1 is a cross-sectional view of a conventional heat exchanger having a straight
tube matrix in a plane defined by a longitudinal axis of the tubes;
Figure 2 is a cross-sectional view of the heat exchanger of Figure 1 in a plane defined
by a longitudinal axis of the manifolds;
Figure 3 is a simulated stress distribution for the heat exchanger of Figures 1 and
2;
Figure 4 is a cross-sectional view through a conventional heat exchanger having a
U-shape tube matrix in a plane defined by a longitudinal axis of the tubes;
Figure 5 is a cross-sectional view of the heat exchanger of Figure 5 in a plane defined
by a longitudinal axis of the manifolds;
Figure 6 is a simulated stress distribution for the heat exchanger of Figures 4 and
5;
Figure 7 is a cross-sectional view through a conventional heat exchanger having a
S-shaped matrix in a plane defined by a longitudinal axis of the tubes;
Figure 8 is a cross-sectional view of the heat exchanger of Figure 7 in a plane defined
by a longitudinal axis of the manifolds;
Figure 9 is an enlarged view of a portion of Figure 8;
Figure 10 is a cross-sectional view through an embodiment of a heat exchanger in a
plane defined by a longitudinal axis of the manifolds;
Figure 11 is an enlarged view of a portion of Figure 10;
Figure 12 is a simulated stress distribution for the heat exchanger of Figure 11;
Figure 13 is a comparative graph of thermal and thermo-mechanical stress for conventional
heat exchangers and the heat exchanger of the present invention; and
Figure 14 is a cross-sectional view through another embodiment of a heat exchanger
in a plane defined by a longitudinal axis of the manifolds.
Detailed Description
[0037] Figure 10 shows a heat exchanger 302 according to an embodiment of the invention.
The heat exchanger 302 comprises an inlet manifold 304 and an outlet manifold 306.
The inlet and outlet manifolds 304, 306 are fluidically coupled by a plurality of
tubes 308 which together form a tube matrix 310. The tubes 308 of the tube matrix
310 are fixedly connected, for example, by welding, at one end to the inlet manifold
304 and are coupled at the other end to the outlet manifold 306.
[0038] A plurality of the tubes 308 are aligned in a plane defined by the longitudinal axes
of the inlet and outlet manifolds 304, 306 to form a row, and several rows are disposed
side-by-side to form columns of the tube matrix 310 arranged along a common plane.
[0039] Each tube 308 comprises first and second straight portions 312a, 312b adjacent the
inlet and outlet manifolds 304, 306 respectively, and a single curved portion 314
disposed between the first and second straight portions 312a, 312b. The curved portion
314 deviates from a longitudinal axis of the tube 308 between its connection point
with the inlet manifold 304 and its connection point with the outlet manifold 306.
As shown in Figure 10, the curved portion 314 is offset in the plane defined by the
longitudinal axes of the inlet and outlet manifolds 304, 306. The size of the offset
from this longitudinal axis is defined as the offset length. The curved portion 314
follows a single curvature between the first straight portion 312a and the second
straight portion 312b. Accordingly, the tube matrix 310 is generally C-shaped. Further,
each of the tubes is separated from adjacent tubes by a similar gap at corresponding
portions along the length of the tubes and are held between the manifolds independently
of each other in the present embodiment, although supporting members may be incorporated
between each of the tubes to help maintain a common gap therebetween.
[0040] The inlet and outlet manifolds 304, 306 and tube matrix 310 form a conduit for the
passage of a first fluid through the heat exchanger 302. Accordingly, the first fluid
flows into the heat exchanger 302 via the inlet manifold 304, passes through the tubes
308 of the tube matrix 310 and exits the heat exchanger 302 via the outlet manifold
306.
[0041] As shown in Figure 11, whilst the gap between adjacent tubes 308 is reduced over
the curved portion 314, the size of this reduction is minimised. Consequently, the
spacing between the tubes 308 at the inlet and outlet manifolds 304, 306 is not significantly
effected.
[0042] The curved portion 314 absorbs thermal expansion by elastically deforming. Thus,
the curved portion 314 reduces the thermal stress experienced by the heat exchanger
302.
[0043] The geometry of the tube matrix 310 is optimised in order to minimise the thermal
stress experienced by the heat exchanger 302. Accordingly, a design of experiment
(DOE) analysis was performed using the Central Composite Design method and sensitivity
analysis and response surface analysis was performed using the results of the DOE
analysis.
[0044] From the results of the sensitivity analysis it was shown that the offset length
and the length of the straight portions 312a, 312b (straight length) were found to
be the most significant factors in reducing the thermal stress.
[0045] The response surface analysis was performed in order to find the optimum values for
the offset length, the straight length and the curvature of the curved portion 314
which minimise the thermal stress, whilst maintaining a minimum gap between adjacent
tubes over the curved portion 314 of 2/3 the maximum gap at the inlet and outlet manifolds
304, 306.
[0046] The result of this process showed that the thermal stress at the ends of the heat
exchanger 302 (i.e. adjacent the inlet and outlet manifolds 304, 306) decreases as
the straight length and the offset length increase. Furthermore, the thermal stress
at the centre of the heat exchanger 302 (i.e. midway between the inlet and outlet
manifolds 304, 306) was shown to decrease with increasing offset length and decreasing
straight length.
[0047] The thermal stress was found to be at a minimum when the:
- inside diameter of the tubes 308 is approximately 0.9 times the outside diameter;
- the length of the tubes 308 is approximately 107 times the outside diameter;
- the offset length is approximately 13.3 times the outside diameter;
- the length of the straight portions 312a, 312b is approximately 23.3 times the outside
diameter; and
- the radius of curvature between the straight portions 312a, 312b and the curved portion
314 is approximately 13.3 times the outside diameter.
[0048] Accordingly, the dimensions of the heat exchanger are preferably as follows:
- the diameter of the tubes 308 is approximately 1.0 to 5.0mm;
- the length of the tubes 308 is approximately 100-500mm;
- the length of the curved portion 314 is approximately 30-70% of the total length;
and
- the offset length is approximately 10-100mm.
[0049] Figure 12 shows a simulated stress distribution for the heat exchanger 302 under
large thermal and pressure loads. As shown, the heat exchanger 302 experiences larger
stresses at its centre over the portion 314 as a result of thermal expansion and deformation
of the tubes 308. However, as shown in Figure 13, the stress experienced at the centre
and at the ends of the heat exchanger 302 is far lower than for the straight tube
heat exchanger 2, and comparable to the U-shaped heat exchanger 102.
[0050] Figure 14 shows a heat exchanger 402 according to another embodiment of the invention.
The heat exchanger 402 comprises an inlet manifold 404 and an outlet manifold 406.
The inlet and outlet manifolds 404, 406 are fluidically coupled by a plurality of
tubes 408 which together form a tube matrix 410. The tubes 408 of the tube matrix
410 are coupled at one end to the inlet manifold 404 and are coupled at the other
end to the outlet manifold 406.
[0051] A plurality of the tubes 408 are aligned in a plane of the longitudinal axes of the
inlet and outlet manifolds 404, 406 to form a row, and several rows are disposed side-by-side
to form columns of the tube matrix 410.
[0052] Each tube 408 comprises first and second straight portions 412a, 412b adjacent the
inlet and outlet manifolds 404, 406 respectively, and a single offset portion 416
disposed between the first and second straight portions 412a, 412b. The offset portion
414 deviates from a longitudinal axis of the tube 408 between its connection point
with the inlet manifold 404 and its connection point with the outlet manifold 406.
As shown in Figure 14, the curved portion 414 is offset in a plane of a longitudinal
axis of the inlet and outlet manifolds 404, 406. The size of the offset from this
longitudinal axis is defined as the offset length. The offset portion 416 comprises
a third straight portion 418 which is connected to the first and second straight portions
412a, 412b by first and second angled portions 420a, 420b. The tubes 408 are curved
at the intersections between the first/second straight portions 412a, 412b and the
first/second angled portions 420a, 420b, and between the first and second angled portions
420a, 420b and the third straight portion 418. The third straight portion 418 is arranged
such that it is offset from, but parallel with, the first and second straight portions
412a, 412b. Accordingly, the tube matrix 410 is generally C-shaped.
[0053] Again, whilst the gap between adjacent tubes 408 is reduced over the curved intersections
of the offset portion 416, the size of this reduction is minimised. Consequently,
the spacing between the tubes 408 at the inlet and outlet manifolds 404, 406 is not
significantly effected.
[0054] The offset portion 416 absorbs thermal expansion by elastically deforming. Thus,
the offset portion 416 reduces the thermal stress experienced by the heat exchanger
402.
[0055] The first and second straight portions and the offset portion may be integrally formed
or may be separate components which are subsequently joined together to form the tube
308,408.
[0056] It should be noted that the tubes need not have a circular cross-section and could
have any other cross-section, so long as they provide a conduit for the passage of
a fluid from the inlet manifold to the outlet manifold.
1. A heat exchanger comprising:
an inlet manifold;
an outlet manifold; and
a tube matrix comprising a plurality of tubes, each tube being fixedly connected at
one end to the inlet manifold and at the other end to the outlet manifold;
wherein each tube extends generally along a longitudinal axis defined between the
connection of the tube with the inlet manifold and the connection of the tube with
the outlet manifold, wherein a single portion of each tube is offset to one side of
the longitudinal axis; and wherein the tubes are arranged in one or more rows in a
plane defined by a longitudinal axis of the inlet and outlet manifolds and a plurality
of rows are disposed side-by-side to form columns.
2. A heat exchanger matrix as claimed in claim 1, wherein the tubes are fixedly connected
between the inlet manifold and outlet manifold independently from each other.
3. A heat exchanger matrix as claimed in claims 1 or 2, wherein the tubes are each separated
from an adjacent tube by a substantially similar gap at corresponding portions along
the longitudinal axis.
4. A heat exchanger as claimed in any of claims 1 to 3, wherein the tubes are generally
C-shaped.
5. A heat exchanger as claimed in claim 1 or 4, wherein the offset portion comprises
a curved portion which curves away from the longitudinal axis.
6. A heat exchanger as claimed in claim 6, wherein the curved portion has a constant
curvature.
7. A heat exchanger as claimed in claim 1 or 4, wherein the offset portion comprises
a straight portion and pair of angled portions which offset the straight portion from
the longitudinal axis.
8. A heat exchanger as claimed in any preceding claim, wherein the offset portion is
offset in a plane defined by a longitudinal axis of the inlet and outlet manifolds.
9. A heat exchanger as claimed in any preceding claim, wherein a minimum gap between
adjacent tubes over the offset portion is greater than 2/3 of the maximum gap between
adjacent tubes at the inlet and outlet manifolds.
10. A heat exchanger as claimed in any preceding claim, wherein the offset portion is
30-70% of the total length of the tube.