Heat exchange – Resilient vibration damper isolating exchanger element
Reexamination Certificate
1998-11-16
2001-06-12
Leo, Leonard (Department: 3743)
Heat exchange
Resilient vibration damper isolating exchanger element
C165S082000, C165S162000, C122S510000
Reexamination Certificate
active
06244330
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to tube banks subjected to fluid crossflow in heat exchangers, steam generators, and similar environments. More particularly, the present invention relates to an apparatus and method for tying together two or more of the individual tubes in such a bank in order to lessen the incidence and impact of flow-induced vibrations.
BACKGROUND OF THE INVENTION
Banks of fluid-carrying tubes often experience flow-induced vibrations when subjected to external fluid crossflow in tube-in-shell heat exchangers, nuclear fuel bundles, steam generators and the like. The excitation mechanisms typically involved are vortex shedding, turbulence and fluidelastic instability. The first two vibrations are generally present throughout the anticipated load range, but the tube structure is typically designed to be capable of withstanding their long-term effects. However, fluidelastic type vibration is characterized by large amplitude displacements and often has a structurally damaging effect. A heat exchanger must, therefore, be designed to eliminate or minimize the damaging effects of fluidelastic vibration.
To reduce the impact of fluidelastic instability within the operating flow range, it is preferable to design the tube bank so that the critical crossflow fluidelastic velocities are sufficiently greater than the maximum design crossflow velocity. A minimum safety factor of 1.5, representing the ratio between the critical and the maximum crossflow velocities, is typically sought to assure the absence of fluidelastic vibration. In order to obtain the necessary critical velocities, sufficiently high tube natural frequencies are needed. The required tube frequencies are typically achieved by shortening tube spans between adjacent supports or tube support plates and/or by providing reasonably close tolerances in tube-to-support clearances.
In the cases of flexible tube banks or banks exposed to high flow velocities, a large number of supports or support plates may be needed in order to maintain the desired ratio between the critical and actual flow velocities. This can increase costs, increase pressure drops, complicate erosion/corrosion considerations, complicate overall design, and undermine performance. Thus, it is desirable to utilize simpler designs with a minimum of main supports. This reduction in the number of supports may require the addition of tube-to-tube ties to achieve the required vibration resistance of the tube bank.
Many attempts have been made to address these vibration concerns by means of tube-to-tube ties. For example, U.S. Pat. No. 5,213,155, to Hahn, attempts to dampen vibrations resulting from temperature changes and fluid flow through and outside the tubes. The tubes
15
,
19
,
23
are clamped via metal tie fasteners
30
,
40
between parallel strips
2
,
3
of a U-shaped stake. Each strip portion
2
,
3
has a “soft V” cross section and a plurality of longitudinally-spaced “saddles”
14
,
16
,
18
,
20
,
22
,
24
that engage the tubes.
Another example, U.S. Pat. No. 5,136,985, to Krowech, is an attempt to increase mechanical stability and decrease vibrations due to gas flow around tubes in a heat exchanger. A plurality of the tubes
16
are interconnected by a support
20
. A tie bar
30
, with a plurality of spaced apart fingers
32
, is positioned so that one boiler tube
16
fits between each pair of adjacent fingers
32
. A locking bar
44
is wedged between the row of tubes
16
and a series of retainer pins
40
of the tie bar, clamping the tie bar to the tubes. The locking bar is then welded to either the tie bar or one of the tubes.
U.S. Pat. No. 3,708,142, to Small, shows that in order to suppress vibration of tubes
2
arranged in rows and columns in, for example, a tube-in-shell heat exchanger, the tubes are held together in three-dimensional bundles. Support rods
16
,
26
extend across the rows and columns, respectively, of tubes, with one rod positioned between each tube row or column. Each successive rod is laterally spaced a common distance along the length of the tubes. Securing means
36
,
38
, such as metal bands, are attached to the ends of each rod to maintain and urge the rods together to form a unitary tube bundle.
U.S. Pat. No. 4,550,777, to Fournier, et al., shows an attempt to reduce stresses from weight distribution, vibrations, and thermal expansion in tubes in a heat exchanger, in which the tubes are suspended vertically in a serpentine arrangement from inlet and outlet ends. Lengths of the tube are interlocked to transfer the weight of the middle tubes to the outer tubes depending from the inlet and outlet. Complementary interlocking members
12
,
13
are welded to adjacent tubes and interlocked as shown in
FIG. 2. A
stop
22
is welded to one of the tubes to prevent the interlocking members from disengaging.
U.S. Pat. No. 3,929,189, to Lecon, shows another attempt to reduce the vibration of tubes in a heat exchanger. A structural framework is formed of a plurality of flat bars
66
interconnected at their respective upper and lower ends by tie bars
68
. Adjacent bars
66
are drawn tightly against a row of the tubes and interconnected by second retaining members
72
. The framework preserves the spacing between the tubes and prevents direct contact between the tubes.
While the known designs may help to control the intertube motion in the tube banks, most do so by tightly locking the tubes in a bundle. Thus, these approaches do not provide for maintaining a reasonably flexible tube bundle. These limitations can produce undesirable restrictions on the thermal expansion of the individual tubes. They can also create a rigid multiple-tube structure that has additional, inherent vibration concerns itself.
SUMMARY OF THE INVENTION
The present invention addresses the foregoing needs in the art by providing tube-to-tube ties that permit a degree of relative rotation, expansion, and transverse motion of the tubes in the region adjacent to the ties.
The present invention relates, in one aspect, to a method of reducing vibration in a bank of tubes due to fluid crossflow. Based on the particular tube bank and flow characteristics, a plurality of tubes (generally, two to six) is selected from the bank of tubes and interconnected. The tubes are interconnected so as to restrain motion of the selected tubes relative to one another in at least one direction, transverse' to the longitudinal axes of the tubes, while permitting each of the selected tubes to rotate on its longitudinal axis and expand and contract in a region adjacent to the interconnection.
Preferably, the interconnection is accomplished without providing any additional connection to an external support. In many cases, the interconnection should be located approximately mid-span between adjacent external supports.
As will be seen, often it will be preferable to select tubes that are aligned, i.e., all intersect a common imaginary line. In most cases, it will be preferable to select tubes that are aligned approximately parallel to the crossflow. The interconnection will generally restrain motion of the selected tubes at least in a direction that is approximately perpendicular to the line of tubes.
In another aspect, the present invention relates to an apparatus for reducing vibration in a bank of tubes due to fluid crossflow, where the tubes have substantially parallel longitudinal axes. The apparatus includes a tube-to-tube tie, interconnecting a plurality of the tubes, and a motion limiter. The tie includes complementary lateral restraints cooperating to restrain lateral motion of the plurality of tubes relative to one another, wherein the tie permits each of the plurality of tubes a degree of freedom to rotate on its longitudinal axis and expand and contract in a region adjacent to the tie. The motion limiter is affixed to at least two of the plurality of tubes so as to limit longitudinal motion of the tie relative to the plurality of tubes.
Where each of the plurality of tubes has a substantially equal nominal outer diameter,
Eisinger Frantisek L.
Rao Madhava M.
Fitzpatrick ,Cella, Harper & Scinto
Foster Wheeler Corporation
Leo Leonard
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