US20120312514A1

US20120312514A1 - Dense twisted bundle heat exchanger

Info

Publication number: US20120312514A1
Application number: US13/507,166
Authority: US
Inventors: Donald Charles Erickson
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-06-13
Filing date: 2012-06-11
Publication date: 2012-12-13

Abstract

A shell and tube heat exchanger is disclosed that is comprised of a dense hexagonal bundle of tubes with tube spacing maintained by spacers on the individual tubes. Exchanger performance is enhanced by any or all of three types of bends of the tubes: bundle twisting; bundle enlargement at the tubesheets; and shell bending. Referring to FIG. 7, the spacing enlargement bending is illustrated.

Description

BACKGROUND Of THE INVENTION

One cross-cutting requirement for increased energy efficiency is more and better heat exchange for energy conversion cycles. This invention is directed toward higher performing and lower cost heat exchangers.
Shell and tube heat exchangers are generally low in cost, but they have certain constraints that limit their performance. Good performance is achieved when the thermal boundary layer thickness is very thin on both sides of the transfer surface. Two conditions necessary for that are good fluid velocity and small hydraulic diameter. It is also important that both of those parameters be reasonably uniform throughout the exchanger. For low viscosity liquids a typical “good” velocity is the range 2 m/s (7 fps) to 3 m/s (10 feet/sec). Smaller diameter tubes can achieve the desired performance on the tube side. However on the shell side there are two serious constraints—the baffles and the tubesheets. Segmental baffles create lower than average velocity zones (low heat transfer) and higher than average velocity zones (high pressure drop). The latter causes the overall average velocity to be decreased to where pressure drop is acceptable.
Various techniques have been proposed to overcome the disadvantages of segmental baffles: rod baffles, twisted tubes, tubes with spacers, etc. Each of those introduces other potential problems.
The tubesheet for each end of the tube bundle has a strength-related code requirement that the minimum tube-to-tube spacing be 1.25 tube diameters. That is equivalent to a minimum gap between tubes of 0.25D (25% of the tube diameter). With straight tubes, that spacing is maintained throughout the bundle. This constrains the shell-side hydraulic diameter to be larger than the tube-side hydraulic diameter. Furthermore, the bundle geometry and/or the type of baffle frequently causes local regions where there are shell-side gaps larger than the gaps between the tubes. Shell flow will preferentially go to those gaps, where little transfer occurs, and hence reduce velocity in the properly spaced areas.

SUMMARY OF THE INVENTION

A shell and tube heat exchanger is disclosed that is comprised of a hexagonal bundle of round tubes in triangular pitch configuration inserted in a shell with a tubesheet at each end of the shell, into which the individual tubes are inserted. The tubes are spaced from one another by a minimum gap of between 5% and 25% of the tube diameter. A majority of the tubes are bent so as to enhance performance.
The invention contemplates three different types of performance enhancing bends of the tubes. They can be applied all three together or singly.
One type of bend is the result of twisting the bundle after it is inserted into the shell. The center tube remains straight, but the other tubes are biased into a helicoidal shape around the center tube, with a modest angle to the center tube axis that increases the further out the tube and the greater the twist. The twisting action causes more tubing to enter the shell, filling it out so as to remove the insertion clearance and the non-circular gaps of the hexagon. Thus the shell-side hydraulic diameter becomes more uniform without the need for insertion of blocking devices. Also the outer tubes are biased against the shell wall to prevent their vibration.
A second type of bend is tube bundle flaring at the two ends of the bundle. That is required when the tube spacing in the bundle is less than 1.25 D, whereas the tubesheet spacing is 1.25 D or greater. The center tube is not required to be bent, but all the other tubes receive an S bend: an outward bend to get them to the required location, then an inward bend to get them back to parallel for tubesheet insertion.
A third type of bend is done to the entire assembly of shell plus bundle. Any desired bundle twisting would first be done. Then the shell plus bundle is bent. This reduces the overall dimension of the finished heat exchanger and provides additional advantages described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the cross section of a circular shell with a seven-tube (two ring) hexagonal bundle inserted, including three spacers, round tubes, and triangular pitch.

FIG. 2 illustrates the cross-section of a shell with a nineteen-tube (three ring) hexagonal bundle, including seven spacers.

FIG. 3 illustrates the cross-section of a circular shell with a thirty-seven tube (four ring) hexagonal bundle plus twelve spacers (note no spacer on the center tube).

FIG. 4 illustrates a helical wire spacer coiled or wrapped around a tube.

FIG. 5 illustrates a helical strip spacer coiled around a tube.

FIG. 6 illustrates the cross section of a seven ring hexagonal bundle inserted into a closely conforming circular shell, where approximately every other tube in the bundle has a spacer, and adjoining spacers are wrapped in opposite directions.

FIG. 7 illustrates a cross section of one end of the bundle plus shell, where the tube spacing is enlarged by tube bending to allow the tubes to enter the tubesheet.

FIG. 8 a illustrates a portion of a seven tube hexagonal bundle without inserts that has been twisted.

FIG. 8 b illustrates a portion of a seven tube hexagonal bundle with inserts that has been twisted.

FIG. 9 illustrates a shell and hexagonal tube bundle heat exchanger wherein the tube spacing has been enlarged at each end to fit into tubesheets, and wherein the composite shell plus bundle has been bent into a coil.

FIG. 10 illustrates a shell and tube heat exchanger wherein the composite shell plus bundle has been bent into a serpentine shape by a series of 180° bends of the shell.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a seven-tube (two ring) hexagonal bundle of round tubes (1-7) is illustrated, inserted into a closely conforming circular shell 8. Three of the tubes (2, 4, 6) in the second ring of the hexagon have helical spacers 9 attached. The tubes in the bundle are in triangular pitch configuration. The spacers have a thickness of 0.15 tube diameter, setting that as the minimum tube-to-tube spacing in the bundle.
Referring to FIG. 2, a three ring hexagonal bundle of tubes is illustrated, where the third ring has three tubes (for example 10, 11, 12) on each of the six sides of the hexagon. The 19 tube bundle is inserted into a closely conforming circular shell 13. This bundle has one spacer 14 on the center tube and six spacers 14 on every other tube in the third ring. The spacers are illustrated as 0.138D thickness, thus maintaining that tube-to-tube gap throughout the interior of the bundle.
Referring to FIG. 3, a four ring hexagonal bundle of round tubes is illustrated in cross section, where the fourth ring has four tubes (for example 17, 18, 19, 20) on each of the six sides, there are a total of 18 tubes in the fourth ring, and a total of 37 tubes in the entire bundle. This bundle does not have a helical spacer on center tube 22. It has three spacers 23 in the second ring, three spacers 24 in the third ring, and six spacers 25 in the fourth ring, for a total of twelve spacers. “Closely conforming” means there is a slight clearance between the shell ID and the bundle OD, sufficient for the assembled bundle to be inserted into the shell. That clearance would typically range from 0 to 0.6 tube diameters, with the larger clearances in the larger tube count bundles.
The spacers that maintain the spacing for the tubes in the bundle can be of various configurations. Two preferred configuration are illustrated in FIGS. 4 and 5. In FIG. 4, a wire spacer 26 is disclosed that is helically wrapped or coiled around tube 27. It is possible to only wrap portions of the tube. However given the amount of performance bending of the tubes that is disclosed, and the need to maintain the tube spacing in bent regions, it is preferred that the circular wire be wrapped around the entire length of the tube except for the two ends where shell fluid enters and exits the bundle. In addition to the winding length, the two other geometrical parameters are the wire diameter and the pitch of the wrap. FIG. 4 illustrates a wire diameter of 0.16 tube diameter, thus fixing the tube-to-tube minimum gap. Also, the pitch is two tube diameters, i.e. each wrap advances two diameters along the tube.
FIG. 5 illustrates a spacer 28 comprised of a strip of rectangular cross-section that is helically wrapped around tube 29. The strip has 0.2D thickness and 2.5D pitch as illustrated. This cross section provides more bearing surface against the tube, so as to resist local deformation. Other cross sections are also possible for the helically wound spacers, e.g. oval.
With the helical spacers, the pitch should be small enough that the contact points are frequent enough to maintain tube-to-tube spacing even where the tubes are bent. The pitch should also be large enough that the spacer does not impede good fluid contact with the tube. Helical pitches from 1D to 20D are contemplated, with the range of 2D to 4D generally preferred.
Other types of tube spacers that are known in the art and that would serve in this application include: individual discs or washers spaced along the tube (e.g. U.S. Pat. Nos. 4,386,456 and 2,774,575); protruding fins or convolutions that are made as part of the tube, especially helical protrusions (e.g. fluted tubing or grooved tubing) (e.g. U.S. Pat. No. 5,181,560); and highly perforated sheet metal strips between the tubes. For example, every tube in FIGS. 1-3 shown having a spacer could be a fluted tube, with the others being bare smooth tubes.
The general idea of the spacers is that each tube with a spacer physically touches all of its neighboring tubes (through the spacer). Thus all tubes are fixed in position by physical contact. That is in contrast to tubes inserted through segmental baffles, where there is normally some clearance between the baffle and the tubes that allows some vibration. Hence the “spacered” bundle is highly resistant to vibration if the contact is maintained.
FIGS. 1-3 illustrate a “sparse” distribution of spacers, i.e. the minimum density of spacers necessary to ensure every tube is contacted. Every interior tube with a spacer is in contact with its neighboring six bare tubes. Every bare tube in the interior of the bundle is in contact with three spacered tubes. No spacered tube contacts another spacered tube. In this sparse configuration, approximately every third tube has a spacer, i.e. is “spacered”.
Two distinguishable sparse configurations of spacers are possible for each size hexagon bundle (i.e. number of rings). They differ in whether or not the center tube (ring 1) has a spacer. In FIG. 1, a two-ring hexagon, the center tube does not have a spacer, and also for the four ring hexagon of FIG. 3. However the three-ring hexagon of FIG. 2 does have a spacer on the center tube. It has been discovered to be advantageous when using sparse spacers to have a spacer on the center tube when the hexagon has an even number of rings, and no spacer on the center tube for an odd number of rings. The difference affects the outermost ring of the hexagon, and when done as disclosed it becomes marginally more compact and closer to circularity.
It is possible to provide a greater density of spacers in applications where that would be useful. FIG. 6 is one example, a five ring hexagon of tubes where approximately every second tube has a spacer, vs every third tube with the sparse configuration. Wherever two tubes with spacers are adjoining, the spacers overlap. Therefore if the spacers are helically wrapped around the tubes, any adjoining spacers must be wrapped in opposite direction. That is illustrated in FIG. 6 by denoting right-hand wrap with R and left-hand wrap with L.
It is also possible to have spacers on every tube. Then it is not possible to have the spacers nest into one another as shown in FIG. 6, and therefore the spacers must be half the thickness of the desired minimum tube-to-tube gap.
Enlargement—One key advantage of using the sparse spacers is that the shell-side spacing can easily be reduced below 1.25D, thus achieving desirable small hydraulic diameter. However the tubes must still be inserted in the tubesheets, which will have spacing of at least 1.25D. That is accommodated as illustrated in FIG. 7. Five tubes are shown in cross section where they insert into tubesheet 35. All except the center tube have an S bend, first to enlarge the spacing beyond that of most of the bundle, and then to return the tubes to parallel at the new spacing. Two of the tubes have wire wraps 36. The wraps end before the enlargement. Whereas bundle tube densities are contemplated down to about 1.05 tube diameters of tube-to-tube spacing (5% gaps between tubes), the generally preferred densities are 1.1 to 1.25 diameters.
Twist—The hexagonal bundle of parallel tubes is assembled with spacered tubes appropriately distributed among the bare tubes. Then the bundle is inserted into an unbent circular shell. At that point there are two issues to be addressed—the gaps and the loose outer tubes. To facilitate insertion, the bundle diameter is slightly less then the shell internal diameter. Thus there is appreciable clearance between the sides of the hexagon and the shell. These clearances result in shell-side gaps (non-uniform hydraulic diameter). The peripheral tubes along the sides are only supported from one side, and can move and vibrate.
The prior art discloses measures to deal with these two problems such as inserting blockers in the gaps, tie rods, weld bars, etc. See for example U.S. Pat. No. 7,117,935.
It has been discovered that for smaller tubes (no more than about ¾ inch diameter) a simpler technique is possible. The bundle is twisted after insertion in the circular shell. Twisting the hexagonal bundle causes it to expand to a more circular shape and better fill the shell. Each tube except the center one is caused to assume a helicoidal position in the bundle, with the outer tubes curving at greater angles than the inner tubes. The angle of curvature relative to the bundle axis results in the tube cross section being slightly elliptical in the plane of the bundle, more so at the periphery. The tube gaps are retained (by the spacers). The peripheral tubes are pushed out to contact with the shell wall. Since the tubes are no longer straight, the assembly can much better withstand differential thermal expansion and thermal shock. The twisting can be accomplished by loosely inserting the tubes at each end of the bundle into a tubesheet that has not yet been affixed to the shell, and then rotating at least one of the tubesheets relative to the other. The tubesheet(s) are affixed to the shell after rotation, and the tubes are rolled and/or welded into the tubesheets after that (also after shell bending if that is done).
In other words, the bundle of tubes is twisted within the shell by at least one rotation relative to when the shell axis and all tube axes are parallel, i.e. one tubesheet with tubes is rotated relative to where it would be in the parallel configuration, and relative to the other tubesheet.
Since bundle twisting causes the tubes to be curved along their entire length, it is important that the spacers be full length also, except not at the ends of the bundle where the shell side fluid enters and exits the bundle.
The degree of twist of the bundle does not need to be very severe. For example, a bundle that is one inch in diameter and twenty feet long might advantageously be twisted about six revolutions. The one-inch bundle would thus undergo one full revolution every 40 inches, for a bundle pitch of 40D. A three-inch bundle would only require two revolutions over twenty feet for the same 40D pitch. The bundle pitch will generally be in the range of 10D to 200D. FIG. 8 illustrates a twisted seven tube hexagonal bundle, with a bundle pitch of approximately 2D (i.e. exaggerated twist for ease of illustration). Note that the shell-side flow continues to be fully countercurrent to the tubeside flow even with this twisting. That is in contrast to conventional shell and coil designs wherein the shell-side flow flows across individual tubes of the coils.
Hexagonal wrap—The disclosed bundle twisting technique is advantageous for smaller diameter tubes and for smaller hexagon bundles, i.e. no more than about ten rings in the hexagon. Beyond those constraints another technique is preferred to eliminate the gaps and the loose tubes. A sheet metal wrap is provided around the bundle that is hexagonal in shape, the same size as the bundle. The wrap extends the full length of the bundle except the ends where the shell fluid enters and exits. The pressure is maintained approximately the same on both sides of the hexagonal wrap so it can be constructed from quite thin sheet and still has no tendency to balloon out. The bundle is first fitted with the hexagonal wrap, and then that assembly is inserted into the shell. In order to prevent flow of shell side fluid outside the wrap, a seal is made between at least one end of the wrap and the shell wall. For lower shell-side pressure applications, the hexagonal wrap could actually be the shell, and could also be non-metallic, e.g. plastic.
Shell bending—When the bundle of tubes plus the shell is very long and of small diameter, it is advantageous to bend the shell into shorter shapes. Especially preferred is a helical coil shape as illustrated in FIG. 9. When this coiling is done there is a strong tendency for the tube bundle to be biased against the inner shell wall of the bend, thus possibly leaving large gaps at the outer wall. Thus it is very important to use a hexagonal bundle and to have properly fitted spacers—either sparsely distributed, or if two helical spacers are in contact, they must be oppositely wound. It is also important to twist the bundle, so if there is any oversize gap at the outer wall, the fluid traversing it keeps contacting different tubes. Prior art examples of helically coiled shells are found in U.S. Pat. No. 4,398,567 and NASA Technical Note D-5092, May 1969.
The shell bending can also be “interrupted”, i.e. bends interspersed with straight sections, resulting in serpentine or racetrack shapes—see FIG. 10. The shell bending provides three important benefits—the bent tubes have more support points, and hence are almost impervious to vibration; they will accommodate differential expansion such as due to rapid temperature cycling or temperature extremes; and the fluid flow is more turbulent. The key is to not allow the tube spacing in the bundle to collapse due to the shell bending, and thus lose all the benefit.
One preferred combination is the triple helix heat exchanger, comprised of helical wrap spacers on some of the tubes in the hexagonal bundle with sparse spacers; a twisted bundle comprised of helicoidal shaped tubes; and a helically coiled shell. The tube spacers, the bundle twist, and the shell coiling are all helical. This combination can have the same tube-to-tube spacing in the bundle and the tubesheets, or the bundle spacing can be denser than the tubesheet spacing, thus also including appropriate enlargement of the spacing by tube bending at each end of the bundle. Other preferred combinations have only a single bend of the shell (U-shaped shell), or no shell bend at all.
With smaller tube sizes in the range of 3/16 inch to ⅜ inch diameter, and dense spacing less than 1.25D, this geometry can achieve heat exchange surface densities in the range of 50 to 150 ft2/ft3. The tubesheet spacing will frequently be desired to be larger than 1.25D with these small diameter tubes to provide welding room, and that can be accommodated with the disclosed enlargement technique. The fully countercurrent flow with no significant interruptions or blockages is resistant to fouling.
The disclosed heat exchange geometry is particularly advantageous in applications requiring high number of transfer units and similar flow area for both streams. Examples are feed/effluent exchangers, recuperators, and solution heat exchangers.

Claims

1. A heat exchanger comprised of:

a. a hexagonal bundle of tubes in triangular pitch configuration;

b. helically wound spacers on some of the tubes;

c. a closely conforming shell containing the bundle; and

d. a tubesheet at each end of the shell plus bundle;

e. wherein at least some of the tubes are bent.

2. The heat exchanger according to claim 1 wherein the spacers maintain tube-to-tube spacing of less than 1.25 tube diameters along most of the bundle, and wherein the tubes are bent at each end of the bundle to increase the tube-to-tube spacing to the tubesheet spacing which is no less than 1.25 tube diameters.

3. The heat exchanger according to claim 1 wherein at least some of said tube bends are due to the bundle being twisted inside said shell, whereby individual tubes assume a helicoidal shape.

4. The heat exchanger according to claim 2 wherein at least some of said tube bends are from bending the shell plus bundle.

5. The heat exchanger according to claim 1 wherein the tube bends are due to a combination of bundle twisting and shell bending.

6. The heat exchanger according to claim 1 wherein said spacers are sparsely distributed on approximately every third tube, and wherein the hexagonal bundle has an odd number of rings of tubes, and wherein the center tube of the hexagonal bundle has a spacer.

7. The heat exchanger according to claim 1 wherein said spacers are sparsely distributed on approximately every third tube, and wherein the hexagonal bundle has an even number of rings of tubes, and wherein the center tube of the hexagon does not have a spacer.

8. A heat exchanger comprised of a circular shell that contains a twisted bundle of heat exchange tubes, plus a tubesheet for each end of the bundle.

9. The heat exchanger according to claim 8 additionally comprised of spacers around some of the tubes, and wherein the bundle of tubes is twisted at least one revolution

10. The heat exchanger according to claim 9 wherein the spacers are helically wrapped around some of the tubes, wherein for any adjoining tubes with spacers the spacers are wound in opposite direction, and wherein the bundle of tubes is hexagonal with triangular pitch.

11. The heat exchanger according to claim 10 wherein the shell plus bundle is bent into a helical coil.

12. The heat exchanger according to claim 10 wherein the shell plus bundle has at least one bend and at least two straight sections.

13. The heat exchanger according to claim 10 wherein the bundle and the tubesheets have the same tube-to-tube spacing, and that spacing is at least 1.25 tube diameters.

14. The heat exchanger according to claim 10 wherein the tube-to-tube spacing of the bundle is less than the tube-to-tube spacing of the tubesheets, and additionally comprised of tube bends at each end of the bundle to transition from the one spacing to the other.

15. A process for manufacturing a heat exchanger comprising:

a. assembling a hexagonal bundle of parallel tubes;

b. inserting said bundle into a circular shell;

c. loosely inserting the tubes into tubesheets at each end of the bundle;

d. twisting one of the tubesheets at least one revolution relative to the other tubesheet;

e. attaching the tubesheets to the shell; and

f. attaching the tubes to the tubesheets.

16. The process according to claim 15 additionally comprising placing helically wrapped spacers on some of the tubes before step a.

17. The process according to claim 15 additionally comprising bending the shell between steps e. and f.

18. The process according to claim 15 additionally comprising bending the tubes at each end of the bundle to match tubesheet spacing, between steps b. and c.

19. The heat exchanger according to claim 2 wherein at least some of said tube bends are due to the bundle being twisted inside said shell, whereby individual tubes assume a helicoidal shape, and wherein said heat exchanger has no baffles for the tubes.

20. The heat exchanger according to claim 1 wherein at least some of said tube bends are from bending the shell plus bundle, and wherein said heat exchanger has no baffles for the tubes.