UNLOADED HEAT EXCHANGER TUBE SHEET

Abstract: An arrangement in a heat exchanger having a heat exchanger vessel with a shell (1) and, disposed inside the shell, at least one tube sheet (8) to which a number of heat exchanger tubes (9) are connected and whose edge portion is sealingly connected to the shell by means of a flexible connecting member (23). The connecting member is designed as part of a toroid having a cross-section of arched configuration, which exhibits a continuous curvature and has the shape of a quarter circle, such that loads deriving from pressure differences across the connecting member can be taken up in the form of general membrane stresses in the connecting member and such that thermal expansion between the shell and tube sheet can be taken up as bending of the connecting member.

Description: TITLE OF THE INVENTION Unloaded heat exchanger tube sheet FIELD OF THE INVENTION The present invention relates to an arrangement in a heat exchanger having a heat exchanger vessel with a shell and, disposed inside the shell, at least one tube sheet to which a number of heat exchanger tubes are connected and whose edge portion is sealingly connected to the shell. The invention more specifically relates to an arrangement in which the tube sheet is connected to the shell by means of a flexible connecting member. BACKGROUND OF THE INVENTION In a conventional heat exchanger, the tube sheet (or tube sheets, since there generally are two such sheets) normally has the largest wall thickness of the construc¬ tion. The reason for this is that the tube sheet edge portion fixed to the shell is subjected to substantial loads as a result of differences in thermal expansion between the shell and the tubes mounted in the tube sheet. Moreover, the edge portion of the tube sheet must be capable of withstanding pressure from one side. The load of such a pressure, which may be an external pressure or an internal pressure, is taken up in the form of bending stresses in the edge portion of the tube sheet, which necessitates a considerable wall thickness of the tube sheet. With a view to reducing the stresses due to expan- sion, certain heat exchangers have been provided with expansion bellows in the shell side. Although this means that the problems linked with thermal expansion are over¬ come, there instead arise other problems in that the load caused by internal overpressure increases. It has also been suggested to connect the tube sheet to the shell by means of a flexible connecting member, such as bellows means or a flat membrane, so as to obviate the expansion problems. This does however not solve the problems of pressure exerted from one side. OBJECT OF THE INVENTION The object of the present invention is to provide an arrangement of the type stated in the introduction to this specification, which substantially overcomes the problems discussed above, while simultaneously yielding further essential advantages in constructional respects. SUMMARY OF THE INVENTION According to the invention, the object stated above is achieved by an arrangement having the features recited in the accompanying claims. According to the invention, there is thus provided an arrangement in a heat exchanger having a heat exchanger vessel with a shell and, disposed inside the shell, at least one tube sheet to which a number of heat exchanger tubes are connected and whose edge portion is sealingly connected to the shell by means of a flexible connecting member, such that thermal expansion between the shell and the tube sheet can be taken up as bending of the connect¬ ing member which is designed as part of a toroid having a cross-section of arched configuration, said arrangement being characterised in that the connecting member is in the form of a thin membrane shell of metallic material, generally steel and preferably stainless steel, and of continuous curvature, said membrane shell having substan¬ tially the shape of a quarter circle, at least in the unloaded state, such that loads deriving from pressure differences across the connecting member can be taken up as general membrane stresses in the connecting member. The quarter-circle shape imparts to the connecting member the essential property of being able to withstand substantial- ly the same pressure exerted from one side, against either the concave or the convex side thereof, while also en¬ suring good resilience. Further, this construction entails economy of material. In other words, the arrangement according to the invention means that thermal expansion can readily be taken up as bending of the pliable connecting member, while at the same time pressure exerted from one side can be taken up as general membrane stresses well distributed over the membrane of the connecting member. The expression "general membrane stresses" as used herein has the current meaning adopted in the art. The arrangement according to the invention makes it possible to cope with thermal displacement and pressure loads from one side without the occurrence of any major forces or moments in the construction. In particular, the tube sheet itself will not be subjected to any substantial forces or moments from its edge portion. It can therefore be made much thinner than the tube sheets of conventional constructions. Typically, it has been found that when using the arrangement according to the invention, the thickness of the tube sheet can be reduced down to 10-20% of normal thickness. This reduction in thickness by up to 90% entails very substantial cuts of the costs for mate¬ rial and, in many cases, also of the manufacturing costs. Thus, since the tube sheet is thin, the holes made in it for mounting the tubes can be punched instead of drilled. The number of tubes mostly being very large, this means that the operation of forming holes in the tube sheet is highly facilitated and becomes considerably less costly. The thin, shell-like connecting member can easily be fixed to the edge of the tube sheet and to the shell of the heat exchanger vessel, e.g. by welding. The connecting member can also be designed as a continuous extension of the tube sheet or a part of the shell, in which case such an extension may have a tube sheet or shell wall thickness reduced to a suitable extent. Integrating the connecting member with the tube sheet or the shell wall permits using highly simplified and less expensive manufacturing tech¬ niques. The invention also yields a very compact construe- tion with a small gap between the edge of the tube sheet and the shell wall, which means that the interior of the heat exchanger can be utilised more efficiently. The connecting member according to the invention pre- ferably has a uniform thickness throughout its membrane surface. In addition to the toroid membrane, the connect¬ ing member may of course comprise special edge fixing means for fixing to the tube sheet and to the shell. In order to further reduce the loads on the tube sheet in the tubed area and to bridge minor untubed areas at the edge portion of the tube sheet, a stiffening ring or flange can be provided at the edge of the tube sheet outside the tubes. This ring or flange may be rigidly or elastically fixed to the tube sheet. It may also form a continuous extension of the tube sheet. Other features and advantages of the arrangement according to the invention will appear from the following description of exemplifying embodiments thereof with refe¬ rence to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic, vertical longitudinal section of a horizontal heat exchanger including an arrangement according to the invention. Fig. 2 is a schematic part- sectional view showing an embodiment of a connecting mem¬ ber according to the invention. Figs 3-9 are views similar to Fig. 2, but illustrating alternative embodiments of a connecting member according to the invention. DESCRIPTION OF EMBODIMENTS Fig. 1 schematically shows a vertical longitudinal section of a horizontal heat exchanger incorporating an arrangement according to the invention. The heat exchanger comprises in conventional manner a cylindrical shell 1; two bonnets 2 and 3 fixed to the shell by means of flange joints 5; two tube sheets 7, 8, the planes of which are perpendicular to the axis of the shell 1 and which are disposed at each end of the shell; tubes 9 fixed in holes in the tube sheets and extending between the tube sheets parallel to the axis of the shell; baffles 11, 12, 13 parallel to the tube sheets 7, 8 and distributed along the tubes 9; an inlet 15 at the top of the shell 1 at one side thereof for supplying a first heat exchanger fluid; an outlet 17 at the bottom of the shell at the other side thereof for evacuating the first heat exchanger fluid; an inlet 19 centrally disposed in the bonnet 3 for supplying a second heat exchanger fluid to pass through a first set of tubes 9 to the space inwardly of the bonnet 2; and an outlet 21 disposed at the bottom of the bonnet 3 for eva¬ cuating the second heat exchanger fluid after it has been recycled through a second set of tubes 9 to the space inwardly of the bonnet 3. At their peripheral outer edges, the tube sheets 7 and 8 are flexibly fixed to the shell 1 by means of special connecting members 23, which will be described in more detail below with reference to Figs 2-9. The central baffle 12 is fixed to the shell, and the tubes 9 in turn are fixed to the baffle 12 at the point where they pass through it. The baffles 11 and 13 are fixed to the baffle 12 by means of spacer rods 25, but are movable relative to the shell. The tubes 9 pass through holes in the baffles 11 and 13, these holes being slightly larger than the tubes, such that the tubes 9 and the baffles 11 and 13 are movable in relation to each other. The baffles 11, 12, 13 do not extend over the entire cir¬ cular cross-section and are so alternatingly adjoining the shell 1 that the first heat exchanger fluid is caused to follow an alternating downward and upward flow path about the tubes 9, as indicated by the arrows 27. The second heat exchanger fluid is supplied to the first limited set of tubes 9 from the inlet 19 by means of a tubular duct 26. This consists of two telescopic tube parts 29, 30. The part 29 is connected to the inlet 19, and the part 30 is connected to the tube sheet, around the associated tubes. The part 29 can be displaced within the part 30 in connection with thermally-conditioned displace¬ ments of the tube sheet 8, a seal 31 ensuring adequate sealing with respect to the space inwardly of the bonnet 3. It is understood that the seal 31 may be a simple gland-type seal, since the pressure exerted across it does not exceed the pressure drop of the second heat exchanger fluid during its passage through the tubes 9. The flow path of the second heat exchanger fluid is indicated by the arrows 28. The inventive flexible connection of the tube sheets 7, 8 to the shell 1 by means of the connecting member 23 appears more clearly from Figs 2-9 drawn on a larger scale. In these Figures, the same reference numerals as in Fig. 1 are used for the same or equivalent constructional elements. Figs 2-9 all schematically illustrate the provision of a metallic, shell-like connecting member 23, viz. in an enlarged partial section taken along a diameter, parallel to the axis of the shell 1. In other words, the section is taken at right angles to the centre axis of the connecting member 23 designed as a shell portion of an annular toroid. In Fig. 2, the connecting member 23 is a thin mem¬ brane shell of metal having the cross-sectional shape of a quarter circle, i.e. a 90° circular arc (in the unloaded state). The edges of the membrane shell are extended and fixed to the lower, i.e. "outer" edge surface 31 (in the Figure) of the tube sheet 8 and to the inner surface of the shell 1. Said edges are connected tangentially both to the inner surface of the shell 1 and to the tube sheet 8. Here, as in the other embodiments, the connection is per¬ formed by welding, as shown in the respective Figures. In this case, the membrane shell is located below the plane of the tube sheet 8, i.e. on the outside thereof. An "external" pressure exerted from one side is readily taken up as membrane stresses, as indicated by arrows 33. How¬ ever, an "internal" pressure from one side can just as easily be taken up as membrane stresses thanks to the configuration of the membrane shell. In the embodiment of Fig. 3, the membrane shell is connected orthogonally both to the tube sheet 8 and to the shell 1, and the extended fixing portion 37 of the mem- brane shell 23 is connected tangentially to a shell flange 5 and provided with a gasket 39 on its underside. More¬ over, the fixing portion of the membrane shell at the tube sheet 8 is extended so as to project on the opposite side of the tube sheet 8. The thus projecting portion 35 has been found to improve the mounting strength and stability. As shown in the Figure, the membrane shell 23 and the ex¬ tended portion 35 thereof are welded to the edge of the tube sheet 8 at its upper and lower side, respectively. An "internal" pressure exerted from one side is indicated by arrows 34. Fig. 4 shows an embodiment of the same general type as that in Fig. 2, but the connecting member 23 here is a continuous extension of the tube sheet 8. This extension has a reduced wall thickness and protrudes from the upper side (i.e. "inner side") of the tube sheet 8. The free end surface 41 of the tube sheet is concave in conformity with the curvature of the membrane shell 23 so as to provide a smooth transition. In this manner, it is possible to avoid detrimental stress concentrations. For the same purpose, the opposite end of the membrane shell 23, which is welded to the shell 1, is thickened by the curvature of the mem¬ brane shell continuing a slight distance after the 90° arc. The weld joint 43 is located in the plane of the inner wall of the shell 1. Fig. 5 shows an embodiment which is analogous with that of Fig. 2, although it is here a matter of an "inter¬ nal" pressure exerted from one side. The curvature or arch of the membrane shell 23 thus is reversed, facing down¬ wards, i.e. outwards. Also in this embodiment, the pres- sure thus acts within the concave, arched membrane shell to achieve the optimum membrane stress effect. It should however be emphasised that the membrane shell according to the invention can also take up pressure exerted only from the opposite, convex side. The connecting member 23 here is a continuous exten¬ sion of the shell 1. The transition to the shell 1 is designed in the same way as the transition to the tube sheet 8 in Fig. 4. At the tube sheet 8, the membrane shell 23 is welded to the lower part of the end edge surface. The end edge surface 45 otherwise is concave with a smooth transition from the curvature of the membrane shell 23, whereby to avoid detrimental stress concentrations. Fig. 6 shows an embodiment which is analogous with that of Fig. 1, although modified in two respects. First, the membrane shell 23 is welded to the lower part of the end edge surface of the tube sheet (as in Fig. 5), and the end edge surface is formed with a continuously connected, circumferential groove 47. Second, an annular cylindrical stiffening ring 49 is rigidly mounted (in this case weld¬ ed) on the upper side of the tube sheet 8 directly at the circumferential edge of the sheet. The ring 49 projects inwards, i.e. in the opposite direction with respect to the membrane shell 23, orthogonally to the tube sheet 8. Fig. 7 shows an embodiment which is analogous (al¬ though reversed) with respect to that of Fig. 3. It is however intended for "internal" pressure and includes a stiffening ring 49 as in Fig. 6. In this case, the stif¬ fening ring is however resiliently mounted by being welded to the projecting portion 35 of the membrane shell 23. Further, a circumferential groove 51 eliminating stress concentrations is provided on the underside of the tube sheet 8 in the immediate vicinity of the weld 53 between the tube sheet 8 and the membrane shell 23. The groove 51 and the weld 53 provide a smooth, curved transition be¬ tween the membrane shell 23 and the tube sheet 8. Fig. 8 shows an embodiment"which is analogous with that of Fig. 5, but, additionally, provided with a stiff¬ ening ring 49, as in Fig. 6. Fig. 9, finally, shows an embodiment in which the membrane shell 23 is tangentially connected by welding to the shell 1 at one end, while its other end is welded to the free end surface 55 of a fixed stiffening ring 49 pro- jecting downwards from the lower edge portion of the tube sheet 8. The ring does not project at right angles from the tube sheet, but obliquely inwardly towards the centre of the heat exchanger, i.e. away from the shell 1. The lowermost portion 57 of the ring is bent backwards so as to be perpendicular to the plane of the tube sheet 8. An annular concave groove 59 is connected to the upper weld between the membrane shell 23 and the ring portion 57, whereby to avoid detrimental stress concentrations. As is readily appreciated, this arrangement means that the gap 61 between the shell 1 and the edge of the tube sheet 8 can be reduced, thus improving the utilisation of the interior of the heat exchanger. The heat exchanger shown in Fig. 1 uses two movable tube sheets, which entails a horizontal heat exchanger, as well as the illustrated, preferred arrangement of the baffles. Other embodiments are however conceivable within the scope of the inventive concept. Thus, it is possible to provide one stationary and one movable tube sheet, in which case the heat exchanger can be arranged vertically with the tubes hanging or standing in the stationary tube sheet. Movable baffles are then mounted in conventional manner by means of spacer members in the stationary tube sheet.

Claims: CLAIMS 1. Arrangement in a heat exchanger having a heat ex- changer vessel with a shell (1) and, disposed inside the shell, at least one tube sheet (7, 8) to which a number of heat exchanger tubes (9) are connected and whose edge por¬ tion is sealingly connected to the shell (1) by means of a flexible connecting member (23), such that thermal expan- sion between the shell (1) and the tube sheet (7, 8) can be taken up as bending of the connecting member which is designed as part of a toroid having a cross-section of arched configuration, c h a r a c t e r i s e d in that the connecting member (23) is in the form of a thin mem- brane shell of continuous curvature, which has substan¬ tially the shape of a quarter circle, at least in the unloaded state, such that loads deriving from pressure differences across the connecting member (23) can be taken up as general membrane stresses in the connecting member. 2. Arrangement as claimed in claim 1, c h a r a c ¬ t e r i s e d in that the connecting member (23) is con¬ nected substantially orthogonally both to the shell (1) and to the tube sheet (8). 3. Arrangement as claimed in claim 2, c h a r a c - t e r i s e d in that the connecting member (23) is con¬ nected to an edge surface (31) of the tube sheet (8), the connecting member being extended (35) so as to project on the opposite side of said edge surface with respect to the main extent of the connecting member. 4. Arrangement as claimed in claim 1, c h a r a c ¬ t e r i s e d in that the connecting member (23) is con¬ nected substantially tangentially both to the shell (1) and to the tube sheet (8). 5. Arrangement as claimed in claim 4, c h a r a c - t e r i s e d in that the connecting member is a con¬ tinuous extension of the shell wall (1). 6. Arrangement as claimed in claim 4 or 5, c h a r ¬ a c t e r i s e d in that the connecting member is a continuous extension of the tube sheet. 7. Arrangement as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that an annular or flange-shaped stiffening means (49) is fixed at the edge portion of the tube sheet (8). 8. Arrangement as claimed in claim 7, c h a r a c ¬ t e r i s e d in that said stiffening means (49) is elas- tically fixed to the tube sheet (8). 9. Arrangement as claimed in claims 3 and 8, c h a r a c t e r i s e d in that said stiffening means (49) is fixed to the extended portion (35) of the connect¬ ing member (23).

Inventors: MALM SUNE, MALM, SUNE
Applicants: MALM SUNE, MALM, SUNE
Application year: 1990
Main classification: F28F9
IPC codes: F28F9