WO1998016791A1

WO1998016791A1 - A tube sheet segment, a heat exchanger, and a method of manufacturing a tube bundle for a heat exchanger

Info

Publication number: WO1998016791A1
Application number: PCT/DK1997/000441
Authority: WO
Inventors: Niels Joergen Hyldgaard
Original assignee: Aalborg Industries A/S
Priority date: 1996-10-11
Filing date: 1997-10-10
Publication date: 1998-04-23
Also published as: AU4452697A

Abstract

For supporting the tubes (1) in a tube heat exchanger there is provided a modular tube sheet segment adapted to fit around one tube and provided with support surfaces for engagement with similar support surfaces of adjacent segments. The outer dimensions and shape of the segment are adapted to precisely provide the desired axis spacing and the desired tube arrangement upon mutual engagement among uniformly shaped segments. Together the segments provide a sheet (12) which meets all the functional requirements of a tube support sheet. The invention further provides a heat exchanger comprising a tube bundle which is put together by arranging tubes with attached segments individually or in layers, and it provides a method of manufacturing a tube bundle which may form part of a heat exchanger.

Description

A tube sheet segment, a heat exchanger, and a method of manufacturing a tube bundle for a heat exchanger

The present invention relates components for tube heat exchangers and to tube heat exchangers in which water flowing inside tubes is heated by a hot flow of gas flowing around the exterior of the tubes. The concept relates more specifically to heat exchangers provided with a tube bundle, i.e. a considerable number of tubes that are placed with parallel axes, and in which the gas flows transversely to the axes of the tubes, and where the exterior sides of the tubes may be fitted with fins.

The invention further relates to a method of manufacturing a tube bundle for a heat exchanger.

Ordinarily such heat exchangers have a box-shaped design where the tubes are supported transversely by support walls, also referred to as tube sheets, which are situated perpendicularly to the axes of the tubes. The purpose of the tube sheets is to support the tubes in the transverse direction and to confine the gas flow within fixed limits. Tube sections in many designs span unsupported from tube sheet to tube sheet, and U-shaped bends interconnect the tube sections into coiled or serpentine patterns, wherein the water flows successively through a number of tube sections, the bends being typically arranged on the exterior sides of the tube sheets .

The holes for the tubes in the tube sheets are normally provided with such large diameter that tubes fitted with fins can pass through them, thereby allowing the tubes to be mounted from the ends and allowing individual tubes to be pulled out axially for repair, if necessary. Consequently the tubes have to bear on the outer edges of the fins. Due to the limited structural strength of the fins, the tube sheet must exhibit a substantial thickness to provide support area for a sufficient number of fins. On account of the large dimensions, the tube sheet may represent a large, solid structure which is extremely costly in manufacturing and tranport. Furthermore, the subsequent attachment by welding of the U-shaped bends is a costly and very time consuming process which cannot be easily automated and which involves a great number of potential errors.

US patent 4 570 704 discloses a support for a heat exchanger tube comprising a member providing four engagement surfaces. The engagement surfaces engage surfaces of adjacent supports within the heat exchanger. The support comprises an elastomeric member stretched around a portion of a heat exchanger tube where the fins have been removed. In this prior art design no provisions have been taken to allow motions as may arise for reasons of thermal expansion. The support member must precisely match the gap between the fins in order to prevent short circuiting the air flow through any small gaps left between the remaining fins and the support member.

US patent 4 245 694 discloses a tube sheet for supporting closely spaced heat exchanger tubes, which tube sheet encloses a plurality of tubular members through which the heat exchanger tubes pass, each tubular member being welded to the tubular member adjoining it. Each tubular member comprises a hole with an inside diameter sized to permit finned heat exchanger tubes to be easily passed through the tubular member.

The invention provides a modular tube sheet segment for engaging a tube of a heat exchanger tube bundle, comprising an inner flange portion presenting a surface adapted for supporting, guiding, sliding and generally sealing engagement around a heat exchanger tube, an outer flange portion presenting exterior engagement surfaces adapted for supporting, sliding and generally sealing engagement with similar engagement surfaces of other similar tube sheet segments, which adjoin it, said outer flange portion providing a sleeve extending peripherally around at least part of the tube and extending axially over a predetermined sleeve length, said sleeve being adapted to block or at least to check flow of gas in a direction radially towards the surface of the tube, and a web portion, integral with said inner flange portion and with said outer flange portion and adapted to block or at least check flow of gas in the direction along the tube axis and within an area generally delimited by the contours of the engagement surfaces of the outer flange portion.

Thus, the invention provides a modular tube sheet segment adapted to fit around one tube and provided with exterior support surfaces for engagement with similar support surfaces of the adjacent segments. The outer dimensions and the shape of the segment are adapted to precisely provide the desired tube axis spacing and the desired tube arrangement upon mutual engagement among uniformly shaped segments. Together the segments provide a consolidated sheet which meets all the functional requirements of a tube support sheet. The new segment minimizes the cost of raw material and allows great flexibility due to the fact that a standardized segment can be utilized for building heat exchangers of all sizes and dimensions. The new segment may be manufactured by a simple, standardized process suitable for automation. The new segment may be manufactured from a raw metal sheet in coil form which is rational in transport and storage and fair in procurement cost.

The segments may be closed around the tube, permitting them to be threaded over the tube from the end or they may be U-shaped, permitting them to be pushed laterally over the tube.

According to a preferred embodiment this sleeve extends peripherally around at least half the tube circumference. This permits the sleeve to provide together with similar adjoining sleeves a baffle to the flow of gas in all the directions transverse to the tube axis in order to prevent gas from bypassing any finned sections of the heat exchanger tubes .

According to a preferred embodiment the engagement surfaces of the outer flange portion has a polygonal contour. This permits the formation of the heat exchanger layouts with different grid patterns and ensures effective engagement between adjoining tube sheet segments .

According to a preferred embodiment the sleeve is adapted to enclose fins of the tube peripherally over a predetermined sleeve length. This extends the length of the path which must be followed by any gas before it can bypass the finned tube sections thereby throttling this kind of flow sufficiently to ensure effective heat exchange throughout the heat exchanger. This is achieved without sacrificing the possibility of accommodating relative sliding, e.g. due to differences in thermal expansion, thus making this design very advantageous for heat exchangers designed for elevated temperatures. According to a preferred embodiment, the outer flange portion comprises hook projections adapted for engaging similar hook projections of other similar segments which support adjacent tubes. By this design a bundle of tubes may be installed in a pendant fashion suspended e.g. from upper transverse beams.

Claim 7 further provides a heat exchanger comprising a tube bundle which is put together by arranging tubes with attached segments individually or in layers. This heat exchanger is far more rational to manufacture than a conventional heat exchanger.

In an expedient embodiment the tubes are supported at their naked surfaces and not at the outer edges of the fins. Thus when the fins need not meet maximum structural requirements this allows of a higher degree of freedom in the design of the fins which more readily can be optimized for optimum heat transfer.

The invention further provides a method of manufacturing a tube bundle which may form part of a heat exchanger. The segments are adapted expediently so as to bear directly on unfinned sections of the tubes. Individual tubes may still be removed for repair as it is possible to pull out one tube section length with attached segments .

The new design allows bending of long serpentine tubes, thereby avoiding the operation of attaching U-bends by welding. For this purpose an expedient embodiment provides a long serpentine tube fitted with fins and with naked sections properly sized for the U-bends with the new tube sheet segments attached prior to the fitting of the fins by welding. Due to the resilience of the tube, the bending operation involves bending the tube slightly more than 180°, which is possible if the naked tube section is provided with a small extra length allowing room for axial displacement of the segments, as one of the two segments to be situated on either side of the intended U-bend is displaced axially to such extent that the segments will not interfere during the bending process .

The tube sheet segments preferably have a hexagonal or quadrangular contour. Optionally the segments may have guiding projections, or they may have mutually engaging hooks for suspended installation. The segments may be manufactured by casting or by a combination of pressing, bending and welding operations.

Further embodiments and advantages of the invention will appear from the description of preferred embodiments given below with reference to the drawings wherein

Fig. 1 shows a tube sheet segment acccording to a first embodiment of the invention, depicted in Fig. la in a plan view as viewed in the direction along the tube axis, in Fig. lb in a plan view perpendicular to the tube axis, and in Fig. lc in a section along the tube axis and with a section of a heat exchanger tube inserted,

Fig. 2 shows a tube sheet segment according to a second embodiment of the invention, depicted as in Fig. 1,

Fig. 3 shows a tube sheet segment according to a third embodiment of the invention, depicted as in Fig. 1, Fig. 4 shows a tube sheet segment according to a fourth embodiment of the invention, depicted as in Fig. 1,

Fig. 5 shows a tube sheet segment according to a fifth embodiment of the invention, depicted as in Fig. 1,

Fig. 6 shows a tube sheet segment according to a sixth embodiment of the invention, depicted as in Fig. 1,

Fig. 7 shows a tube sheet segment according to a seventh embodiment of the invention, depicted as in Fig. 1,

Fig. 8 is a sectional view along the tube axis of a part of a tube bundle, where the tubes are provided with tube sheet segments according to the seventh embodiment,

Fig. 9 is an isometric view of a part of a tube bundle, where the tubes are provided with tube sheet segments according to the first embodiment and also with tube sheet segments according to the fifth embodiment,

Fig. 10 is an isometric view of a part of a tube bundle, where the tubes are provided with tube sheet segments according to the third embodiment and also with tube sheet segments according to the sixth embodiment, depicted as in Fig. 9,

Fig. 11 is an isometric view of a part of a tube bundle similar to the bundle of Fig. 9, but fitted with U-bends for interconnecting the tubes and with one tube section partially pulled out,

Fig. 12 shows a tube section under various stages of the manufacturing process of a serpentine tube,

Fig. 13 is a vertical sectional view of a heat exchanger acccording to the invention, and

Fig. 14 is a horizontal sectional view of the heat exchanger of Figure 13.

All figures are schematic, not necessarily drawn to scale, and simplified showing only details considered important for the understanding of the invention while other details are omitted from the figures for simplicity reasons. Throughout the figures identical or similar parts are identified by the same references.

Reference is first made to Figure 1 showing a tube sheet segment 21 according to a first embodiment of the invention, depicted in Fig. la plan view along the tube axis, in Fig. lb in plan view transverse to the tube axis, and in Fig. lc in sectional view parallel to the tube axis, respectively. Figure lc shows a section of a heat transfer section length 1 of which the tube axis 4 is indicated by a dash-dot line.

The tube sheet segment 21 comprises an inner flange 18 adapted so as to provide an opening matched for fitting engagement around the heat transfer section length 1 on a naked surface 5 of the tube, i.e. a tube section without fins 6. The inner flange and the tube are adapted with sufficient play to permit the tube sheet segment to be easily shifted along the tube under varying temperature conditions throughout the design temperature range but still being guided by the tube so that it cannot tilt significantly relative to the tube. In case of tubes of an external diameter of 38 mm, a segment with an opening diameter from 39 to 40 mm has been found to perform well in an installation designed for gas temperatures ranging from ambient temperatures to about 1000 °C .

The peripheral portion of the tube sheet segment 21 according to the first embodiment comprises an outer flange 19 exhibiting, as will appear from Figure la, a regular hexagonal outer contour. The outer flange 19 is connected to the innner flange 18 by the web section 20 providing a radial wall. The outer flange 19 extends in the axial direction somewhat beyond the inner flange so as to provide a kind of shield or a sleeve which encloses the fins 6 peripherally. The purpose of the sleeve is to check the flow of gas to prevent it from bypassing the fins by flowing past the naked tube section.

The exterior side of the outer flange 19 provides support surfaces by which the tube sheet segment may be supported. The tube sheet segment according to the first embodiment may be manufactured expediently by a casting process .

Figure 2 shows a tube sheet segment 22 according to a second embodiment of the invention. This tube sheet segment 22 has a contour generally similar to that of the segment 21 according to the first embodiment, and by and large it fulfils the same functional requirements. The sheet segment 22 differs particularly from the sheet segment 21 in that its shape is slightly modified with the purpose of allowing it to be manufactured by a combination of bending, pressing, and welding operations. For this reason the segment 22 in fact is an asymmetric creation as will appear from Fig. 2b; however, two similar segments may be arranged back-to-back as depicted in Fig. 2c and possibly interconnected by welding to provide a symmetric creation.

The segment 22 according to the second embodiment effectively is adapted for enclosing tube fins peripherally to one side only, whereas the double version depicted in Fig. 2c is capable of enclosing tube fins to both sides. The one-sided version is preferred for end wall installations, where no finned tubes are located to one of the sides, whereas the double-sided version is preferred for intermediate tube sheets.

The outer diameter and the axial length of the outer flange should be matched to the particular dimensions of the tube and the fins in question in order to provide a satisfactory throttling of any air flow past the naked tube section. It is recommended that the outer flange should enclose an axial length generally at least equivalent to the axial spacing between the fins. In case of helical fins, the outer flange should enclose an axial length equivalent to the pitch.

In a practical installation with tubes of an outer diameter of 38 mm provided with helical fins extending 16 mm radially from the tube surface and spiralled around the tube with a pitch of 3 mm, segments have been used wherein the axial length of the inner flange is 19 mm, the axial length of the outer flange is 38 mm, and the radial extension of the web is adapted to ensure a mimimum radial clearance around the peripheral edge of the fins of 1 mm. This segment is capable of overlapping a 19 mm axial section of fins. This will cover the fin pitch 3 mm, 1 mm of fin thickness and still allow an extra 15 mm for axial movement of the segment.

The allowance for axial movement is of major importance as the wall in the gas conduit are likely to assume higher temperatures than the tubes and therefore likely to expand more than the tubes, tending to shift the segments along the tubes away from the finned sections. The extra axial length allows for substantial thermal expansion without losing the air throttling enclosure effect of the outer flange.

Figure 3 shows a tube sheet segment 23 according to a third embodiment of the invention. This segment also comprises an inner flange, an outer flange and a web and is adapted for fitting engagement around the tube in the same manner as the segments of Figures 1 and 2. The significant difference resides in the outer contour of the segment which, as will appear from Figure 3a, has a quadrangular contour. The segment according to the third embodiment is suitable for manufacture by a casting process .

Figure 4 shows a tube sheet segment 24 according to a fourth embodiment of the invention. This segment has a contour similar to that of the segment of Figure 3 but its shape is modified with the purpose of allowing it to be manufactured by a combination of bending, pressing and welding operations similarly to the segment according to the second embodiment.

The segment 24 according to the fourth embodiment is asymmetric as evident from Fig. 4a and 4b, however, two of them may be arranged back-to-back similarly as explained above concerning the segment of the second embodiment . Reference is now made to Figure 5 showing a tube sheet segment 25 according to a fifth embodiment of the invention. As will appear from Figure 5a, this segment has a U-shaped contour, generally resembling that of the segment according to the first embodiment, but with an opening 30 provided therein. This opening is adapted to allow the tube sheet segment 25 to be pushed laterally over a heat transfer section length 1. As a result of the opening, the segment according to the fifth embodiment does not baffle the flow of gas in axial direction to the same extent as the segments of the above mentioned embodiments, and it is therefore preferably deployed at positions where this function is less critical.

Figure 6 shows a tube sheet segment 26 according to a sixth embodiment. This tube sheet segment has a quadrangular contour, but like the segment according to the fifth embodiment it is also provided with an opening which allows it to be pushed laterally over a heat transfer section length 1.

Figure 7 shows a tube sheet segment 27 according to a seventh embodiment. This tube sheet segment also has an inner flange and an outer flange and a web, but the top and the bottom of the segment are further provided with hook projections 28, while the sides provide flat support surfaces 29. The hook projections allow for axial displacement of the tubes together with the respective tube sheet segments, e.g. for the purpose of pulling out one tube from a tube bundle and replacing it.

Figure 8 shows a portion of a tube bundle, the tubes being supported by tube sheet segments according to the seventh embodiment 27, i.e. provided with hook projections. Figure 8 shows how the hook projections can cooperate for providing a pendant installation.

Figure 9 shows a section of a tube bundle 11 in which each of the heat transfer section lengths 1 is provided with hexagonal tube sheet segments of two different types. The ends are provided with closed tube sheet segments 21 according to the first embodiment, while the middle of the tubes are provided with tube sheet segments 25 according to the fifth embodiment. The closed tube sheet segments together build tube sheets 12, i.e. more or less continuous walls which are generally closed to gas flow. The tube sections between the tube sheets are referred to as heat transfer section lengths 1, and in the embodiment shown they are fitted with fins 6 except for short naked sections at both ends and at the middle.

The U-shaped segments attached on the middle of the tubes serve to support the tubes. The U-shaped segments together build a tube sheet intermediate the tube sheets 12. Due to the open shape of the U-shaped segments the intermediate tube sheet is not completely closed to gas flow across the sheet, however, this is of minor importance, as no substantial gas pressure gradient prevails across the intermediate tube sheet. The tube arrangement of Figure 9 has the advantage that the flow of gas is deflected several times during its passage due to the staggered positioning of the individual tubes, thereby resulting in an excellent exchange of heat.

Figure 10 shows a section of a tube bundle comprising quadrangular tube sheet segments, partly closed segments 23 according to the third embodiment fitted at the ends for providing together generally gas tight tube sheets 12, and partly U-shaped segments 26 according to the sixth embodiment fitted in the middle for supporting the tubes. The quadrangular outline of the tube sheet segments allows the arrangement of the tubes in horizontal layers and vertical columns, also referred to as an inline arrangement. The rectangular tube pattern in the arrangement of Figure 10 does not agitate the gas as much as the pattern of Figure 9, but it is advantageous in that it is easier to access the tubes through the duct, e.g. for blowing in order to purge the heat exchanger.

Even though the Figures only show embodiments of tube sheet segments having hexagonal or quadrangular contours, it is evident that the tube sheets may also be manufactured in other shapes, e.g. triangular or trapezoid shapes, allowing the same functional requirements to be met.

Even though the Figures show embodiments where the heat transfer section lengths are fitted with fins, it is evident that the invention may just as well be applied in connection with other types of heat transfer section lengths. In applications where the tubes are exposed to particularly high temperatures, or where the tubes are subjected to heavy fouling, it may e.g. be advantageous to use completely naked heat transfer section lengths.

Figure 11 shows a portion of a tube bundle more or less similar to that of Figure 9, but Fig. 11 furthermore depicts U-shaped connecting bends 7 so as to show how the tubes are interconnected in a coil or a serpentine pattern. Figure 11 shows the tubes without any central support as generally used for sections where the spacing between the tube sheets is not very large. In the section between the tube sheets 12, the tubes are fitted with fins while they are naked adjacent to and outside the tube sheets. Figure 11 shows a single tube where the U- bend has been cut off and in which the tube provided with the tube sheet segment has been partially pulled out, as might be the case during a repair operation.

Figure 12 shows steps during the manufacture of a bundle of tubes according to a different method, where one length of tube 2 through several bends is shaped to provide a serpentine lay-out. The left side of Figure 12a shows a section of naked tube 3, onto which a suitable number of tube sheet segments 21 according to the first embodiment are attached. Fins 6 are welded onto sections, the naked tube alternating with intermediate sections left naked. All sections are provided in precisely determined lengths, two tube sheet segments being threaded onto each naked section. Further to the right Fig. 12a depicts a set of bending tools comprising jaws 17, placed to the right and to the left of a circular bending abutment 16, which engage around the tube on a naked section between the tube sheet segments in an early stage of an operation to bend the tube.

Figure 12b shows the last step of a bending operation where the tube has been bent slightly more than 180°, the tube sheet segments being shifted axiaily relative to each other so as to make room for the bending. The bending operation is matched to the resilience of the tube so that upon opening of the jaws and release of the tube, it reverts to a configuration corresponding to a serpentine with a bend of 180°. In this position the sheet segments are shifted back into adjacent, coplanar relationship so as to bear on each other.

The bending operation shown in Figure 12b is repeated similarly for the other naked sections, alternating from one side to the other. The bends may extend in the same plane or they may be arranged in angled planes by a controlled twisting of the tube around the tube axis between the individual bending steps so as to provide a coiled tube or a serpentine tube which may constitute part of an arrangement, e.g. as the one shown in Figure 11.

With the tools available today it is considered economically viable to bend tubes of a total length of 50 m so as to provide serpentine lay-outs, the length of the heat transfer section lengths ranging from 3 to 12 meters .

Reference is now made to Figures 13 and 14 showing vertical and horizontal sectional views, respectively, of a heat exchanger according to the invention. This heat exchanger 32 is integrated into the stack of a furnace and designed for gas flow vertically from the bottom and upwards. Figure 13 shows how a serpentine tube 2 extends from the bottom header 10 via a number of serpentine bends and intermediate sections up to a top header 9. Figure 14 shows four serpentine tubes arranged in parallel. The serpentine-shaped portions of the tubes provide a tube bundle 11 while the connections to the top and bottom headers, respectively, comprise elbows and connecting tubes 8. The individual heat transfer section lengths 1, i.e. the rectilinear lengths, extend between two tube sheets 12 which are composed by tube sheet segments according to the invention. The tube sheets are supported at the bottom by carrier beams 33.

The consolidated tube sheets 13 are interconnected as shown in Figure 14 by transverse walls 14 providing a flow conduit or a central duct 34 directing the gas past the heat transfer section lengths. The transverse walls 14 extend out beyond the tube sheets 12 to the end walls 15 which connect them so as to provide a closed conduit or casing 13 having a rectangular cross section and divided into the central duct 34 and two end ducts 35. The conduit bounded by the transverse walls and the end walls 15 is designed so as to be substantially gas tight. This design implies that the tube sheets 12 comprising the tube sheet segments are in fact not required to be completely gas proof.

In the structure shown in Figure 13 the tube bundle 13 is supported substantially by the underlying carrier beams 33. As the flow of hot gas enters from the bottom, these carrier beams are thus exposed to comparatively high temperatures .

For installations designed for high gas temperatures it may be advantageous to modify the design by using tube sheet segments according to the embodiment shown in Figure 7 which are built together as indicated in Figure 8, and anchoring the tube bundle to be suspended from carrier beams mounted above. Thereby the carrier beams are arranged in the low temperature section in the heat exchanger .

Although specific elements and methods have been described above in specific contexts, such elements and methods are not excluded from application in other contexts, from combinations in other ways, and from being independently patentable. The preceding description serves only to illustrate the invention, and it is not intended to limit the scope of the invention which is exclusively defined by the appended claims.

Claims

1. A modular tube sheet segment for engaging a tube of a heat exchanger tube bundle, comprising -an inner flange portion presenting a surface adapted for supporting, guiding, sliding and generally sealing engagement around a heat exchanger tube,

-an outer flange portion presenting exterior engagement surfaces adapted for supporting, sliding and generally sealing engagement with similar engagement surfaces of other similar tube sheet segments, which adjoin it, said outer flange portion providing a sleeve extending peripherally around at least part of the tube and extending axially over a predetermined sleeve length, said sleeve being adapted to block or at least to check flow of gas in a direction radially towards the surface of the tube, and

-a web portion, integral with said inner flange portion and with said outer flange portion and adapted to block or at least check flow of gas in the direction along the tube axis and within an area generally delimited by the contours of the engagement surfaces of the outer flange portion.

2. The tube sheet segment according to claim 1, c h a r a c t e r i z e d in that the sleeve extends peripherally around at least half of the tube circumference .

3. The tube sheet segment according to claim 1, c h a r a c t e r i z e d in that the engagement surfaces of the outer flange portion provide a polygonal contour, and in particular a triangular, a trapezoid, a quadrangular or a hexagonal contour.

4. The tube sheet segment according to claim 1, c h a r a c t e r i z e d in that the sleeve is adapted to enclose fins of the tube peripherally over a predetermined sleeve length.

5. The tube sheet segment according to claim 1, c h a r a c t e r i z e d in that it comprises an opening, permitting it to be pushed laterally over a tube.

6. The tube sheet segment according to claim 1, c h a r a c t e r i z e d in that the outer flange portion comprises hook projections, adapted for engaging similar hook projections of other similar segments, which support adjacent tubes.

7. A heat exchanger comprising a bundle of parallel tubes and adapted for exchanging heat between a gaseous thermal medium flowing around the exterior of the tubes, in a direction generally transverse of the tubes and a different thermal medium, flowing internally of the tubes,

-wherein each of the tubes are supported by respective tube sheet segments, according to any of the claims 1 through 6 in such a way that the segments of adjacent tubes are in mutually engaging contact, and wherein the segments provide at least two generally continuous tube sheets extending generally transversely to the tubes and serving to guide the flow of gas, and wherein transverse walls combine with the tube sheets in order to delimit a flow conduit for the gaseous thermal medium.

8. The heat exchanger according to claim 7, c h a r a c t e r i z e d in that the tubes comprise sections provided with exterior, axially spaced fins, placed so as to leave tube sections without fins at the positions intended for the placing of the tube sheet segments, and in that the sleeves are adapted to enclose fins of respective tubes peripherally over an axial length of the respective tubes extending at least equivalent to the axial spacing between the fins.

9. The heat exchanger according to claim 7, c h a r a c t e r i z e d in that the tube sheets are spaced sufficiently to allow for thermal expansion of the tubes by the tubes sliding axially within the respective segments, the sleeves being sized axially to extend over at least the adjacent fin interspace at any operational temperature .

10. The heat exchanger according to claim 7, c h a r a c t e r i z e d in that the transverse walls are interconnected by end walls in order to provide a generally rectangular gas conduit, said tube sheets being situated within said conduit, dividing it into a central duct and two end ducts.

11. The heat exchanger according to claim 7, c h a r a c t e r i z e d in that the tubes extend generally in parallel across the central duct, whereas U- bends and fittings adapted for providing flow connections to the tubes are arranged in the end ducts.

12. The heat exchanger according to claim 7, c h a r a c t e r i z e d in that the tubes are provided with fins over respective sections extending between the tube sheets, whereas they are naked at the sections traversing the tube sheets.

13. A method of manufacturing a tube bundle for a heat exchanger, said tube bundle comprising a number of generally parallel tubes and at least two tube sheets for lateral support of the tubes, extending generally transversely to the tube axes and serving to guide the flow of gas, said method comprising

-determining individual, heat transfer section lengths of heat exchanger tubes and placing two tube sheet segments onto each heat transfer section length, each tube sheet segment being adapted according to claim 1, -arranging the individual heat transfer section lengths of heat exchanger tubes in parallel, with the tube sheet segments of the respective tubes together composing the two tube sheets, each heat transfer section length extending across both tube sheets.

14. The method according to claim 13, c h a r a c t e r i z e d by comprising providing the tube section lengths with exterior fins, placed so as to leave tube sections without fins at the positions intended for the placing of the tube sheet segments.

15. The method according to claim 13, c h a r a c t e r i z e d by comprising providing the exterior fins by spiraling an elongate strip around the tube in a helical pattern and so as to extend radially from the tube surface and welding the strip along one edge to the tube surface.

16. The method according to claim 13, c h a r a c t e r i z e d by comprising selecting a predetermined tube length equivalent to several tube section lengths, providing said tube length with a number of tube sheet segments, welding fins onto sections of said tube length, which sections are spaced by intermediate naked sections of said tube length, each naked section carrying two tube segments, and bending the predetermined tube length into U-shape at each of said naked sections in order to provide a serpentine tube layout.