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(11) | EP 0 013 621 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Heat exchanger with increased radius on the inner row U-tubes |
| (57) A heat exchanger has U-shaped tubes mounted in a tube sheet wherein the tubes (11)
with the smaller radius U-bends are offset with respect to the other tubes (13) to
increase the radius of the U-bend in the offset tubes. |
[0001] This invention relates to heat exchangers and more particularly to heat exchangers with U-shaped tubes.
[0002] In steam generators utilized with pressurized water reactors a phenomena commonly referred to as denting has in some cases, hampered the operational performance of the U-shaped heat exchanger tubes. As a possible result of the denting process the oblong-shaped flow slot in the center portion of the tube support plates tends to become closed The effect of this closure is the deformation of the small radius U-bend which results in increased stresses in the U-bend portion of the tubes and in possible stress cracking of the tubes. The cracked tubes must be plugged, that is, removed from service. Analysis has shown that tubes having U-bends of a radius generally equal to the radius of the second smallest radius U-bend will not develop stresses which result in cracks even if the oblong-shaped flow slot is completely closed at the center portion. However, eliminating the innermost row of tubes reduces the total heat transfer surface of the tube bundle and hence the effectiveness of the steam generator.
[0003] It is therefore the principal object of the present invention to provide a steam generator with U-tubes in which relative movement of the support structure for the legs of the U-tubes does not cause cracking of the bent portion of the tubes.
[0004] With this object in view, the present invention resides in a heat exchanger comprising a plurality of U-shaped tubes mounted in a tube sheet with symmetrically arranged holes and forming a closely packed tube bundle, said U-shaped tubes being generally disposed in a symmetrical pattern, characterized in that at least some of the tubes having the smaller radius bends in the center of the tube bundle are offset with respect to the remainder of the tubes, such that the radius of their U-bends is greatly increased and, further, relative movement of the legs at these tubes causes torsional stresses in the legs rather than bending stresses in the bent portions of the tubes.
[0005] The invention will become more apparent from the following description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawings, in which:
Figure 1 is a partial plan view of two rows of U-shaped tubes as utilized in the prior art;
Figure 2 is a sectional view taken on line II-II of Figure 1;
Figure 3 is a partial plan view of two rows of U-shaped tubes disposed in accordance with this invention;
Figure 4 is a sectional view taken on line IV-IV of Figure 3;
Figure 5 is a partial plan view of two rows of U-shaped tubes disposed in accordance with this invention;
Figure 6 is a sectional view taken on line VI-VI of Figure 5; and
Figure 7 is a partial plan view of two rows of U-shaped tubes disposed in accordance with this invention.
[0006] Referring now to the drawings in detail, there are shown portions of two of the innermost rows of U-shaped tubes 11 and 13 disposed in holes 15 and 17 symmetrically disposed in a tube sheet 19. The holes 15 and 17 may be disposed on a square pitch as shown in Figures 1, 3 and 5, or the holes 15 c and 17c may be disposed on a triangular pitch as shown in Figure 7. Irrespective of whether a square or a triangular pitch is utilized, it is standard practice to place the holes 15 and 15c and 17 and 17c in a symmetrical pattern and place the tubes in symmetrical rows and columns. Rows are formed from tubes with the same radius of U-bends and columns are formed from tubes of increasingly larger radii generally disposed so that their centerline is in a plane.
Figure 1 shows the prior art arrangement wherein the inner. row of tubes 11 have a much smaller radius U-bend portion 21 than the second or next row of tubes 13 which have a larger U-bend portion 23.
Figure 2 shows the relative size of the innermost U-bends 21 and the second row of U-bends 23.
In Figure 3 there is shown a row of tubes lla offset or skewed with respect to the second row of tubes 13 and also the remainder of the tubes in the tube bundle which are not shown, but are symmetrically disposed with respect to the second row of tubes 13. The offset is one tube space, hole or pitch. This increases the radius of the U-bend portion 21a as can be seen best in Figure 3. When the inner row of tubes lla is complete, it contains only one tube less than the arrangement shown in Figure 1, however, the radius of the U-bend portion 21a as shown in Figure 4 is substantially greater than the radius of the U-bend portion 21 of the inner row of tubes 11 shown in Figure 2.
[0007] Consistent with this invention, the inner row of tubes llb may be offset more than one space as shown in Figure 5. In this embodiment, the inner row of tubes llb are offset two spaces, holes or pitches, thus increasing the radius of the U-bend portion 21b as best shown in Figure 6. When the innermost row of tubes llb is complete, it contains only two less tubes than the arrangement shown in Figure 1, however, the radius of the U-bend portion 21b is substantially greater than the radius of the U-bend portion 21 as may be-seen by comparing Figures 2 and 6.
[0008] Figure 7 shows tubes llc and 13c and holes 15c and 17c disposed on a triangular pitch. The inner row of tubes llc are offset one tube space, hole or pitch with respect to the remainder of the tubes in the tube bundle resulting in the radius of the U-bend portions 21c being substantially larger than the radius of U-bend portions of tubes lie if they were symmetrically disposed with respect to the tubes 13c and the remainder of the tubes in the tube bundle.
[0009] The radius of the offset U-bend 21a, b or c can be expressed mathematically as follows:
R = radius of offset U-bends;
R = radius of the smallest radius tube when the tubes are symmetrically disposed;
N = number of spaces or pitches offset;
P = tube pitch.
[0010] The number of tubes lost in the row where the tubes are offset is equal to the number of spaces, holes, or pitches by which the tubes are offset.
[0011] The U-shaped tubes hereinbefore described are disposed to form a closely packed tube bundle in which the tubes are arranged in a symmetrical pattern except for the tubes having the smaller radius U-bend at least some of which are offset with respect to the remainder of tubes forming the tube bundle, whereby the tubes having the smallest radius bends are of much greater radius than would be possible if all of the tubes were symmetrically disposed in the tube bundle. This eliminates very small radius U-bend tubes in a closely packed tube bundle without any substantial decrease in the number of tubes in the tube bundle. Also, movement of the support plates toward or away from each other causes relatively small torsional stresses in the long legs rather than high bending stresses in the relatively short U-bent portions.