Tube heat exchange unit for internals of heat exchangers reactors

Abstract

Tube-bundle heat exchange unit (1) for internals of heat exchangers or reactors, comprising: at least one tube bundle (2); a plurality of baffles (3) associated with said tube bundle and defining through-openings according to a predefined arrangement, each opening being passed through by one of more tubes of the tube bundle, and a shell (6) which surrounds said tube bundle and said baffles, wherein the assembly of the tube bundle and the shell can be disassembled and the shell is structurally collaborating with the tube bundle through said baffles.

Claims

1. A tube-bundle heat exchange unit for internals of heat exchangers or reactors, comprising: a tube bundle, which in turn comprises a plurality of tubes and a plurality of baffles supporting the tubes; and a shell which surrounds said tube bundle, wherein said baffles are perpendicular to a longitudinal axis of the tube bundle and define through-openings for the tubes, according to a predefined scheme; wherein the assembly formed by the tube bundle and the shell is demountable; the heat exchange unit being characterized in that: said shell is structurally cooperating with the tube bundle through said baffles, so as a stress acting on the shell is partially withstood by the tube bundle itself.

2. The heat exchange unit according to claim 1, wherein the baffles of the tube bundle comprise respective peripheral edge rings, and the structural cooperation between the shell and the tube bundle is given by the shell resting on said rings.

3. The heat exchange unit according to claim 2, wherein the shell rests on said rings of the baffles of the tube bundle along at least 50% of their perimeter and preferably along the entire perimeter.

4. The heat exchange unit according to claim 1, wherein the baffles comprise bars substantially acting as struts of said shell.

5. The heat exchange unit according to claim 1, wherein the shell is not self-supporting, the shell thickness being smaller than a minimum thickness required by the stress acting on the shell in operation, and the shell resistance to said stress being given by the structural collaboration with the tube bundle.

6. The heat exchange unit according to claim 1, wherein the shell is formed by one or more sectors wrapped around the bundle, the rims of said one or more sectors being connected along one or more longitudinal joints.

7. The heat exchange unit according to claim 6, wherein the sector or the sectors of the shell are represented by substantially plane metal sheets which are bended for surrounding the tube bundle.

8. The heat exchange unit according to claim 1, wherein said shell comprises a plurality of longitudinal sections.

9. The heat exchange unit according to claim 8, wherein the shell comprises at least two longitudinal sections, the heat exchange unit comprises at least one blind baffle between two consecutive sections of said shell, said blind baffle forming a gas-tight barrier in the shell side of said apparatus, the shell side of the apparatus thus being divided into at least two separate gas passages.

10. The heat exchange unit according to claim 8, wherein said longitudinal sections have a length, in the longitudinal direction, substantially equivalent to the distance or pitch between said baffles of the tube bundle.

11. The heat exchange unit according to claim 1, wherein the shell has a cross-section chosen among: a cross-section with the form of a regular or irregular polygon; a stepped cross-section; and a cross-section comprising at least one straight side and at least one curvilinear side.

12. The heat exchange unit according to claim 1, wherein said shell comprises means for constraining the baffles, acting in the axial direction parallel to said tube bundle, said baffles being therefore axially retained in a predefined position by said shell.

13. The heat exchange unit according to claim 1, characterized by a substantially fluid-tight joint between said baffles and said shell.

14. The unit according to claim 1, comprising at least one impermeable sealed partition which is longitudinal and parallel with respect to the tube bundle.

15. A pressurized apparatus, comprising an outer shell resistant to a predefined operating pressure, and at least one internal tube heat exchange unit, according to claim 1.

16. The pressurized apparatus according to claim 15, wherein said apparatus is a chemical reactor of the multi-bed type comprising a plurality of adiabatic catalytic beds, and the at least one tubular heat exchange unit inside the reactor is a heat exchanger for intercooling between two catalytic beds.

17. A method for assembling a tube heat exchange unit for internals of heat exchangers or reactors according to claim 1, comprising: providing a tube bundle having a plurality of baffles, the baffles being freely movable at least in the axial direction with respect to said tube bundle, providing a shell around the tube bundle, and wherein said shell, once positioned around the tube bundle, axially retains the baffles with respect to the tube bundle in respective operating positions, and the shell structurally cooperates with the tube bundle through said baffles, so as a stress acting on the shell is partially withstood by the tube bundle itself.

18. The method according to claim 17, wherein the shell is formed by one or more portions which are bended and wrapped around the tube bundle during the assembling.

19. The heat exchange unit according to claim 1, wherein the shell has a cross-section chosen among: a cross-section with the form of a regular polygon; a stepped cross-section; and a cross-section comprising at least one straight side and at least one circle arc.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a diagram of a tube heat exchange unit according to a first embodiment of the invention.

(2) FIG. 2 is a perspective view of a portion of a tube bundle with a shell fixed to baffles of the tube bundle, according to one of various embodiments of the invention.

(3) FIGS. 3 and 4 are details of FIG. 2.

(4) FIG. 5 is a perspective view of a portion of tube bundle with a cylindrical shell, provided with a longitudinal joint and shown open during construction.

(5) FIG. 6 is a detail of a preferred embodiment for providing a longitudinal joint of the cylindrical shell shown in FIG. 5.

(6) FIG. 7 shows the shell according to FIG. 5 closed by the respective longitudinal joint.

(7) FIG. 8 shows a detail of a preferred way of supporting the baffles by the shell.

(8) FIG. 9 shows an embodiment with a cylindrical shell formed by longitudinal sections.

(9) FIG. 10 shows the detail A of FIG. 9.

(10) FIG. 11 shows an example of embodiment comprising baffles without a frame and rods fixed directly to the shell.

(11) FIG. 12 illustrates an example of a step for assembly of a tube heat exchange unit according to an embodiment of the invention.

(12) FIG. 13 shows an embodiment with U-shaped tubes.

(13) FIGS. 14, 15, 16 and 17 are diagrams similar to that of FIG. 1, showing a number of other embodiments of the invention.

(14) FIG. 18 shows a multi-bed catalytic reactor of the conventional type.

(15) FIG. 19 shows a multi-bed catalytic reactor comprising a heat exchange unit according to the invention.

DETAILED DESCRIPTION

(16) FIG. 1 shows a heat exchange unit 1 for internals of reactors, comprising a tube bundle 2 (only the tube axes are indicated for the sake of simplicity) and a series of baffles 3. The example shows straight tubes between two tube plates 4 and 5, but in other embodiments the tube bundle 2 may be for example U-shaped.

(17) The baffles 3 prevent vibration of tubes, promote heat exchange and define passing-through openings, each passed through by one or more tubes, for example a row of tubes. Adjacent baffles may support tubes in different directions, in accordance with the rod baffle construction technique or other known techniques.

(18) The exchanger 1 comprises a shell 6 which surrounds the tube bundle 2 and which, according to the invention, is structurally integral with the tube bundle 2 through the baffles 3 and can be disassembled. The arrow G of FIG. 1 denotes a fluid passing through the shell side of the exchanger 2, entering near the plate 4 and exiting near the plate 5.

(19) The baffles 3 are spaced from each other by a pitch p, which is preferably constant.

(20) The flow G exchanges heat with another flow passing inside the tubes of the bundle 2. In some applications, the exchanger 1 is a gas-to-gas exchanger; the gas G contains reagents and reaction products and is the effluent of a catalytic bed; the flow inside the tubes for example consists of reagents which are preheated. In other applications the fluid inside the tubes may consist of water, steam, etc.

(21) With reference now to the constructional aspects, FIG. 2 shows one of the possible embodiments in which the shell 6 is formed by a wall 7 with a stepped polygonal shape. Said wall 7 is removably fixed to the frames 10 of the baffles 3 by means of pins 8. The reference number 9 denotes the tubes which form the bundle 2.

(22) The apparatus comprises a plurality of baffles 3 which are spaced by a pitch p in a similar manner to that shown in FIG. 1. The baffles are collectively denoted by the reference number 3. FIG. 2 shows two baffles 3.1 and 3.2 with a different arrangement of the openings for the tubes, in particular oriented at 90 degrees.

(23) The form of the baffles 3 and the structural connection to the wall 7 (forming the shell 6) are more clearly visible in FIGS. 3 and 4.

(24) A baffle 3 comprises essentially a peripheral frame or edge ring 10 and straight and parallel elements in the form of bars 11 which define openings 12 for the tubes 9. Each opening 12 receives a certain number of tubes. The openings 12 in adjacent baffles may be differently oriented in a plane perpendicular to the axis of the tubes, for example they are oriented at 90 degrees, so as to support the tubes in complementary support planes, as can be understood by comparing the details shown in FIGS. 3 and 4.

(25) It should be noted that the rods 11 represent one of the constructional forms of the baffles 3 and that there are equivalent embodiments with baffles of a different type such as grid baffles, which are known per se and therefore not described in detail here.

(26) In the example shown the frame 10 has a stepped polygonal form, substantially like that of the wall 7; in other embodiments the frame 10 has different forms, for example it is a regular or irregular polygon or a circumference.

(27) The wall 7 of the shell 6 may be formed by different longitudinal sections and/or by different portions which together surround the tube bundle 2.

(28) The pins 8 engage eyelets 13 of the wall 7 and holes 15 of corresponding lugs 14 of the frames 10 of the baffles 3, ensuring positioning and axial support of the said baffles 3. One of the holes 15 is visible in FIG. 4.

(29) It can be understood that, by means of the releasable connection with the pins 8, the shell 6 is structurally cooperating with the baffles 3. The shell directly supports the baffles 3 and no specific framework or structure is necessary, as is instead required in the prior art. The same baffles 3 act as transverse ribs for the shell 6, cooperating to the strength of the shell which can be made particularly light and thin.

(30) Owing to the stepped polygonal form, the potential bypass space of the tubes, denoted by the symbol S.sub.BP in FIG. 2, is very small. This is because the stepped wall 7 remains very close to the peripheral tubes 9 of the bundle 2 and matches their arrangement much better than a circular cross-section. Moreover, as can be noted in FIGS. 3 and 4, the small thickness of the frame 10 (owing to the structural collaboration) helps to reduce said bypass space S.sub.BP.

(31) The amount of play typically present between the outer periphery of the baffles and the inner wall of the shell is also removed, resulting in an increased heat exchange efficiency. A sealing gasket may be provided between the baffles 3 and the wall 7, although it is not essential and normally not present.

(32) FIG. 2 shows an embodiment in which the tube bundle 2 has an annular configuration and the heat exchange unit also comprises an inner wall 7 with the function of an inner tube, for example for conveying the flow upwards after a passage through the shell side. Preferably said inner wall 7 has the same configuration as the outer wall 7, for example the stepped configuration shown in FIG. 2 or a polygonal or circular configuration.

(33) Advantageously the shell 6 comprises one or more joints arranged longitudinally, i.e. Parallel to the direction of the tubes 9.

(34) FIG. 5 shows an example of a circular shell 6, formed by a metal sheet 16 wrapped around the tube bundle 2, and with a single longitudinal joint 17. Also the FIG. 5 shows two baffles 3, denoted by the symbols 3.1 and 3.2. It should be noted that the metal sheet 16 may be bended and wrapped around the tube bundle, forming a cylinder, as shown in FIG. 5, owing to the small thickness of the metal sheet itself, made possible by the structural collaboration.

(35) Details of the longitudinal joint 17, according to a preferred embodiment, are shown in FIG. 6. The frame 10 of each baffle 3 has a seat 19, for example in the form of a dovetail, for receiving the ends 18 of the sheet 16. The ends 18 of said sheet 16 are advantageously configured to engage the seats 19, for example they are folded in a hook shape. The sheet 16 is wrapped around the tube bundle 2 as shown for example in FIG. 6 and locked by means of a shaped profile 20.

(36) FIG. 7 shows the assembled shell with the longitudinal joint 17. It can be noted that in this example an annular tube bundle 2 with central tube 21 is also shown.

(37) The joint 17 is described solely by way of example; other types of joint are possible, for example joints with eyelets and wedges or joints of the conventional type with overlapping portions. In order to simplify wrapping of the shell 6 around the tube bundle 2, the shell 6 may be advantageously formed by a plurality of sections as in the embodiment of FIG. 9.

(38) The detail in FIG. 8 shows an example of supporting means associated with the shell and suitable for supporting the baffles 3 in the desired positions. In the embodiment shown, by way of example, said support means are formed with pairs of cleats 22 which are fixed to the inner wall of the shell 6 (i.e. the sheet 16). The structural cooperation of the shell 6 which itself supports the baffles 3 is therefore readily understood.

(39) It should also be noted that, owing to the construction with the shell 6 fixed to the baffles 3, said baffles 3 expand longitudinally in relation to the tubes 9 following the shell 6. As a consequence, the heating and in particular the temperature transients (e.g. during start-up of a chemical reactor inside which the unit 1 is inserted) do not induce tensions between the shell and baffles.

(40) The longitudinal joints such as the joint 17 described above and any circumferential joints may also be non-releasable, for example they may be welded and/or riveted. Non-releasable joints may be preferred when the shell removal feature is not necessary or is not required. Also in this case, the welded portions will be configured in order to ensure easy removal of the welding without causing any damage to the parts, so as these parts may be utilized again. Also regarding this aspect, easily demountable portions are contemplated. The structural unity between the shell 6 and the baffles 3 is in any case ensured.

(41) FIG. 9 shows an embodiment in which the shell 6 comprises longitudinal sections. The example shows a construction similar to that of FIG. 7, i.e. with a cylindrical shell, where two sections 16.1 and 16.2 are visible. Preferably the length L of a section is equivalent to the pitch p of the baffles 3, shown in FIG. 1, or to a multiple pitch.

(42) The example in FIG. 9 also shows a welded structure, as can be understood from the detail shown in FIG. 10. The baffles 16.1, 16.2, etc., which form the shell 6 are welded to the baffles 3. In FIG. 3 it is possible to see the edge of the baffle 3 shaped to receive the ends of the sheets 16.1, 16.2.

(43) FIG. 11 shows a variant, where the elements 11 which define the openings 12 for the tubes are directly fixed to the shell 6, i.e. the baffles 3 do not have the frame 10.

(44) The figure shows an example in which substantially circular rods 11 are welded inside holes 23 in the wall 7. In variants with the sleeve formed by several sections, said holes 23 are advantageously formed by half-cavities formed in the edges of the sections. It should be noted that other forms of the rods 11 and the respective seats in the shell (equivalent to the holes 23) are possible.

(45) It should also be noted that FIG. 11 shows a shell 6 formed by a stepped wall 7, as in FIG. 2, but it must be considered that said variant comprising frameless baffles is applicable also to all the other embodiments, such as those comprising a circular shell as shown in FIGS. 5-9.

(46) A central duct, if present (as for example shown in FIG. 7 or FIG. 9), shall be provided with suitable blind seats for said elements.

(47) FIG. 12 illustrates an example of the manufacturing method which also forms an aspect of the invention. The figure shows the main components, i.e. the bundle 2 of tubes 9, the plates 4 and 5, and the baffles 5. The baffles 3 are movable axially, i.e. in a direction parallel to the tubes 9, and in case also transversely, i.e. in the direction of the openings 12. The figure shows the baffles 3 staggered as a result of this transverse mobility. The transverse mobility is possible in certain cases, for example with baffles which define slotted openings, as long as the peripheral tubes are not mounted; once all the tubes are mounted or in the case of other types of baffle, for example grid baffles, said transverse mobility may be prevented.

(48) It can be understood from FIG. 12 that introduction of tubes 9 into the openings 12 of the baffles 3 during assembly is greatly facilitated. In the prior art, in fact, the tubes are inserted when the baffles 3 are already locked in their definitive position, which means that a tube must engage exactly with the series of openings 12 which have minimum play. This gives rise to major assembly problems. With the invention, on the other hand, the mobility of the baffles 3 makes insertion of the tubes easier and quicker. The baffles 3 are then positioned as desired, spaced at a pitch p, and locked by means of a template or other auxiliary means; the shell 6 is then mounted for example using the procedure illustrated in FIGS. 5-9. The shell 6, once mounted, keeps the baffles 3 in position for example by means of the cleats 22 described above. This example illustrates even more clearly the structural cooperation between the shell and the baffles.

(49) FIG. 13 shows another constructional variant suitable in particular for a U-shaped tube bundle 2. In this variant the shell 6 (which may be divided into longitudinal sections) is formed by half-shells 24.1 and 24.2 joined together by longitudinal flanges 25. Said half-shells are advantageously shaped with an arc-shaped portion and a flat portion 26 terminating in the flanges 25 and support a longitudinal partition 27 which is clamped between said flanges 25.

(50) It is known that U-tube exchangers normally require a longitudinal partition in order to obtain a shell side passage in counter-flow relative to the tubes; FIG. 13 shows that a longitudinal partition 27 may be supported directly by the shell 6, which in turn is fixed to the baffles 3.

(51) Partitioning of the shell side in U-tube exchangers in other words is particularly simple and advantageous because it ensures absolute sealing of the central partition 27, increasing efficiency and reducing costs. As can be seen from the figure, the shape of the two half-shells 24.1 and 24.2 terminating with flat portions 26 is such that the longitudinal flanges 25 remain within the external dimensions of a conventional cylindrical shell. Several longitudinal divisions of the shell side are possible; for example a shell with three longitudinal passages is obtained with two partitions (which may be constructionally similar to the partition 27 shown in the figure).

(52) FIGS. 14-17 show some of the numerous configurations which are possible thanks to the invention.

(53) FIG. 14 shows an exchanger with straight tubes, in which the shell 6 is formed substantially by three longitudinal sections 6.1, 6.2 and 6.3. The ends of the sections are spaced so as to leave openings for gas inlet and outlet. Furthermore the exchanger comprises blind baffles 3.c between a longitudinal section and the following section. Said baffles 3.c, unlike the baffles 3 (indicated by a thin line) do not allow the gas passage in the shell side. Three shell side passages are then obtained, as shown by the arrows in the figure.

(54) FIG. 15 shows a diagram with U-shaped tubes instead of straight tubes and, therefore, with a single tube plate 4. A longitudinal partition is also present and the shell side is divided into six sections.

(55) The fluid which flows in the shell side may undergo a given process step between one passage and another. Said process step may include for example a heat exchange and/or a chemical reaction stage. The exchanger shown in FIG. 14 or FIG. 15, in other words, may perform essentially the same function as three different exchangers, while being constructionally simpler and less costly than three separate exchangers, since it has only one or two tube plates, depending on whether the tubes are U-shaped or straight.

(56) For example, the exchanger shown in FIG. 14 may be inserted in a reactor for ammonia synthesis, and the three passages on the shell side perform intermediate cooling between catalytic beds.

(57) FIGS. 16 and 17 show two configurations in which the shell side is divided into two passages. In FIG. 16 there is a seal between the blind baffle 3.c and the shell section 6.1, with the first passage outlet alongside the inlet. Said configuration is advantageous in certain applications, for example vertical exchangers inside reactors, where it is desirable that both the inlet and outlet of the shell side gas are located at the top.

(58) The invention may be applied to new reactors, in the form of new equipment, or may be used to modernize existing reactors.

(59) One of the applications of the invention relates to modernization of the reactors of the type shown in FIG. 18. By replacing one or more pre-existing tube exchangers with tube exchangers according to the invention, advantages may be obtained, including: greater efficiency, recovery of useful volume for the catalyst, possibility of reducing the internal piping.

(60) Said FIG. 18 shows schematically a multi-bed reactor 30 for the high-pressure synthesis of ammonia or methanol, comprising catalytic beds 31, 32, 33 and two intermediate cooling exchangers 34, 35. The reactor 30 is equipped with an outer shell 36 able to withstand the operating pressure. The operating principle of the reactor 30 is known from the art and does not require a detailed description. A flow of reagent gases passes radially through the first bed 31, increasing in temperature owing to the exothermic reaction; the hot flow leaving the bed 31 is cooled when passing through the shell side of the exchanger 34 and passes into the bed 32 for an ensuing reaction stage; upon leaving the bed 32, the gas cools when passing into the shell side of the exchanger 32 and enters into the bed 33 for the final conversion stage.

(61) FIG. 19 shows the modified reactor, indicated by the reference number 30. The heat exchange between the catalytic beds is performed by a unit 1 according to the invention with segmental shell formed by two sections 6.1 and 6.2 which replaces the two exchangers 34, 35. The path of the shell side gas is regulated by means of suitable seals 37.

(62) One of the advantages of the invention consists in the fact that, owing to the segmented structure of the shell, a single tube bundle is sufficient and therefore only two tube plates are needed. The conventional structure according to FIG. 18 instead requires two tube bundles, each having two respective plates.

(63) It should be noted that the elevated reaction pressure can be withstood by the shell 36. The shell 6.1, 6.2 of the internal unit 1 is subject to a pressure difference substantially due to pressure losses and therefore limited to a few bars.

(64) Referring to FIG. 19, it can be seen that the pressure inside the shell of unit 1 is lower than the pressure outside said shell. The stress which the shell is subjected to is substantially equal to that of a cylinder being subjected to a pressure from the outside. Owing to the structural collaboration, the stress is partially withstood by the baffles and the tube bundle, and the shell is constructed with a small thickness.