Tube-bundle for process boiler

Abstract

Tube-bundle for shell-and-tube process boiler wherein at the exchanging tubes inlet at least two sets of ferrules of different length are installed. The ferrules of at least a set have a layout so as to be symmetrical relative to at least an axis of symmetry of the bundle. Tube-bundle wherein the critical heat flux of the bundle, beyond which the risk of steam blanketing exists, is enhanced.

Description

[0001] MNN008WO Giovanni Manenti

[0002] TUBE-BUNDLE FOR PROCESS BOILER

[0003] DESCRIPTION

[0004] Scope

[0005] This invention applies to tube-bundles of heat exchangers and chemical reactors, of shell- and-tube type, installed in refineries and plants for production of petrochemicals, hydrogen, ammonia, methanol, nitric acid, sulfuric acid, hydrogen cyanide and others.

[0006] In particular, this invention applies to the tube-bundles of process boilers, of shell-and-tube type, installed downstream of chemical reactors and used to cool a high-temperature process gas discharged from the reactor by means of pressurized boiling water. The invention applies to process boilers where the hot gas flows on tube-side.

[0007] Technical problem to be solved

[0008] When in shell-and-tube equipment, used for indirect heat exchange between two fluids, a tube-bundle area is characterized by high heat transfer coefficients and large temperature differences between the two fluids, strong heat fluxes occur through the wall of the exchanging tube.

[0009] For example, in methanol production plants where a syngas discharged from an adiabatic oxidative reformer at about I000°C is cooled by means of a process boiler whit syngas flowing on tube-side and cooling boiling water at about 320°C flowing on shell-side, heat fluxes greater than 350kW / m2 can occur through the exchanging tube wall.

[0010] For example, in nitric acid production plants where a process gas discharged from the ammonia oxidation reactor at about 800°C is cooled by means of a vertical process boiler whit gas flowing on tube-side and cooling boiling water at about 250°C flowing on shell-side, heat fluxes greater than I50kW / m2 can occur through the exchanging tube wall.

[0011] Strong heat fluxes occurring through the tube wall of process boilers involve the risk of poor or no water supply on the external surface of the exchanging tube (steam-blanketing phenomenon), with the consequent risk of overheating, corrosion and mechanical damage.

[0012] Critical heat flux (CHF) and maximum heat flux (MHF) are fundamental parameters for the thermo-hydraulic sizing of process boilers. The CHF represents the value of the heat flux, given the geometry and operating conditions of the tube-bundle, beyond which the steam blanketing of tubes can occur; the CHF is a parameter related to the tube-bundle. The MHF represents the MNN008WO Giovanni Manenti maximum value of the heat flux, given the geometry and operating conditions of the exchanging tube, installed in the process boiler; MHF occurs at the tube inlet, where the temperature difference between hot gas and boiling water is maximum. The MHF is a parameter related to the individual exchanging tube.

[0013] The CHF and MHF installed in a process boiler must guarantee that CHF>MHF. Higher the CHF -to-MHF ratio, greater the safety margin installed in the boiler relative to the dangerous steam blanketing phenomenon.

[0014] To prevent MHF from occurring at tube-sheet and tube to tube-sheet welded joints where the hot process gas enters, in accordance with prior art, the exchanging tubes of process boilers operating with high-temperature gas (>600°C) are normally provided at inlet with ferrules inserted in the heat exchanging tube and extending into shell. The ferrules are basically pipe butts usually lined on the outside with a layer of thermally insulating ceramic paper; the ferrules, therefore, thermally insulate the tube from the hot gas and move the MHF away from the gas inlet tube-sheet, transferring the MHF to the ferrule outlet. Typically, the ferrule portion inserted into the tube and extending into the shell has a length of about 100-200mm.

[0015] Conventional ferrules installed at inlet of the exchanging tubes of a process boiler, according to major prior art, have the portion inside the exchanging tube and the shell of identical length and have the sole purpose of transferring the MHF away from the gas inlet tube-sheet. Conventional ferrules are not designed to increase the tube-bundle CHF or CHF -to-MHF ratio. In other words, conventional ferrules do not improve the performance of the boiler relative to the CHF value. Thus, the conventional configuration of tube-bundles of process boilers has a technological limitation.

[0016] Scope of the invention

[0017] The invention here disclosed has the scope to make available a tube-bundle, for shell-and- tube equipment for indirect heat exchange between two fluids, provided with ferrules at tubes inlet and apt to overcome the CHF limitation of conventional configurations.

[0018] In particular, the present invention aims to make available a tube-bundle for a shell-and-tube process boiler with gas on tube-side where the bundle CHF and CHF -to-MHF ratio are increased, keeping same bundle geometry and operating conditions, by means of an innovative configuration of the bundle provided with ferrules. This invention allows increasing the thermo-hydraulic safety margins of a process boiler relative to the dangerous steam blanketing phenomenon. MNN008WO Giovanni Manenti

[0019] Alternatively, the invention here disclosed aims to make available a tube-bundle for shell- and-tube process boilers, with hot gas flowing on tube-side, where the tube-bundle size can be reduced keeping same bundle operating conditions and CHF. In this case, the present invention allows optimizing the process boiler from a technical -economic standpoint.

[0020] Brief description of the invention

[0021] The present invention describes a tube-bundle provided with ferrules of different lengths installed at inlet of at least a portion of the exchanging tubes.

[0022] The tube-bundle is provided with at least two sets of ferrules where the ferrules of each set have the same length and where the ferrules of different sets have different lengths. For example, the tube-bundle can be provided with two sets of ferrules, where the first set has first ferrules of length LI, the second set has second ferrules of length L2, and where the length LI is different from the length L2.

[0023] According to the present invention, the ferrules of at least one set are symmetrically or substantially symmetrically distributed relative to at least one axis of symmetry of the tubebundle. For process boilers with tube-bundle substantially horizontal relative to the ground plane, the ferrules of at least one set are symmetrically or substantially symmetrically distributed relative to at least one axis of symmetry of the bundle that is preferably vertical relative to the ground.

[0024] According to a preferential alternative configuration of the present invention, the layout of ferrules of at least one set is symmetric or substantially symmetric relative to at least two axes of symmetry of the tube-bundle.

[0025] In this description, the inlet end or the inlet of the exchanging tubes refers to the end of the tubes where the hot fluid or gas enters. Similarly, the inlet end or the inlet of the ferrule refers to the end of the ferrule where the hot fluid or gas enters the ferrule; the outlet end or the outlet of the ferrule refers to the end of the ferrule where the hot fluid or gas exits the ferrule.

[0026] In this description, as better described later, the length of a ferrule refers to the portion of ferrule located inside the exchanging tube and extending into the equipment shell.

[0027] It is underlined that, according to the present invention, the ferrules are installed in at least a portion of the exchanging tubes; in addition, it is underlined that the ferrules are installed on tubeside, at inlet of the exchanging tubes, and the ferrules are at least partially inserted in the exchanging tubes and extend into the shell to a point placed between the tube ends. MNN008WO Giovanni Manenti

[0028] The outlet ends of the exchanging tubes, where the cooled tube-side fluid or gas exits, can also be connected to the tube-sheet which the inlet ends of the exchanging tubes are connected to. Such a configuration includes "U" shaped exchanging tubes or straight exchanging tubes divided into at least two groups to obtain at least two fluid passages on tube-side.

[0029] It is emphasized that the CHF of a process boiler strongly depends on the geometric parameters of the tube-bundle, such as the tube pitch and number; in other words, the CHF is highly dependent on tubular density of the tube-bundle. Larger the tube pitch, higher the CHF; smaller the tube number, higher the CHF. Therefore, lower the tubular density of the bundle, i.e. smaller the number of tubes inscribed in a given boundary (or outside tube limit - OTL), higher the CHF relative to the bundle. Correspondingly, smaller the OTL for a given tube pitch, higher the CHF. In general, higher the bundle CHF and / or CHF-to-MHF ratio, greater the safety margin with respect to the steam blanketing and better the technical -economic optimization of the boiler.

[0030] On the contrary, the MHF strongly depends on the operating and thermal parameters related to the individual exchanging tube (and indirectly on the intemal / extemal diameter of the tube) and does not depend on the geometric parameters of the tube-bundle. In other words, the MHF relative to an exchanging tube is negligibly influenced by the tube pitch or number.

[0031] In a conventional tube-bundle, where the ferrules installed at the tube inlet have an identical length, the MHF relative to each exchanging tube is released substantially in the same longitudinal position for all the bundle tubes, i.e. at the outlet of ferrules. This decreases the CHF and CHF- to-MHF ratio values for the tube-bundle: the MHF of all the exchanging tubes of a conventional bundle occurs at the same bundle cross-section leading to high local steam fractions and risk of steam engulfment and blanketing on tubes outside surface.

[0032] On the contrary, the present invention teaches that by installing sets of ferrules of different lengths LI, L2, L3, etc. at inlet of the exchanging tubes and distributing the ferrules of at least one set in a symmetrical or substantially symmetrical way relative to at least one axis of symmetry of the bundle, the thermo-hydraulic conditions related to a cross-section of the bundle are mitigated.

[0033] According to the present invention, the MHF occurring at the bundle cross-section related to the outlet of ferrules of length LI does not add up to the MHF occurring at the bundle crosssections related to the outlet of ferrules of length L2, L3, etc. The MHF occurring at the outlet of ferrules of length LI, L2, L3, etc. lie, respectively, on distinct bundle cross-sections longitudinally spaced from each other. In other words, by installing ferrules of different lengths, from a thermo- hydraulic point of view, is equivalent to increase the tube pitch or to space the tubes from one another, and thus to increase the bundle CHF and CHF-to-MHF ratio. The MHF remains MNN008WO Giovanni Manenti unchanged as it essentially depends only on the heat transfer coefficient related to the tube and on the temperature difference between the process gas and the boiling water.

[0034] The symmetrical or substantially symmetrical distribution of the ferrules of at least one set relative to at least one axis of symmetry of the bundle, as taught by the present invention, is an essential and advantageous feature. Such a configurational feature allows installing overall uniform thermo-hydraulic conditions at a bundle transversal section, preventing large imbalances of the natural draft and large shell-side areas with poor steam production and potentially stagnant or recirculating conditions.

[0035] Overall, by implementing the configuration disclosed by the present invention, the process boiler advantageously works in milder and safer thermo-hydraulic and thermo-mechanical conditions; the boiler presents an improved mechanical reliability and an extended operating life, and can also accept larger deviations from operating conditions, such as deviations from gas temperature or boiler water chemistry.

[0036] The present invention realizes to be advantageous also in case of revamping projects: by replacing at least a portion of the ferrules of an existing process boiler, based on prior art, with ferrules of different lengths distributed symmetrically or substantially symmetrically relative to at least one axis of symmetry of the tube-bundle according to the present invention, the bundle CHF and CHF-to-MHF ratio are increased; therefore, the existing boiler, as revamped according to the present invention, may accept a larger gas flow rate and / or a higher gas temperature.

[0037] State of the art

[0038] The publication "L.Presciuttini and D.Lippolis, Facing the Demand of Reformed Gas Boilers of Big Capacity, Ammonia Technical Manual, AIChE Annual Meeting, Nov. 2003 " refers to shell- and-tube process boilers with hot gas on tube-side and boiling water on shell-side. The publication generically describes a horizontal tube-bundle having, at tubes inlet, two sets of ferrules of different lengths, where the longer ferrules are installed in the upper tube rows. This publication does not disclose a symmetrical distribution of the ferrules relative to one or more axes of symmetry of the bundle.

[0039] The generic distribution of the ferrules described in the above-mentioned publication implies that for a bundle cross-section where only the longer ferrules are installed, the steam production is negligible in the upper portion of the cross-section, whereas is significant in the lower portion. This leads, in the upper bundle portion, to the risk of stagnation or recirculation conditions and, in case of natural circulation of the boiling water, to poor draft especially during start-up. In other words, the tubes of the upper rows, in correspondence with the longer ferrules, do not generate MNN008WO Giovanni Manenti steam and thus do not contribute to the natural draft; the tubes of the upper rows, in correspondence with the longer ferrules, represent a net resistance to the draft.

[0040] Patent document No. JPS5549693A describes a process boiler provided, at gas inlet, with first and second tube-sheets spaced from each other and with ferrules placed at exchanging tubes inlet and fixed to the first tube-sheet. The ferrules have same length and work as an ejector. This configuration allows preventing the entering hot gas is in direct contact with the second tubesheet.

[0041] Patent document No. W02018215102A1 describes a process boiler having ferrules (or bypass tubes) placed inside the exchanging tubes and inside the shell; these ferrules can be different from each other and / or have different dimensions [page 21, lines 23-29], However, this prior art document teaches that the ferrules, contrarily to the present invention, are inserted at the outlet end of the exchanging tubes and are not apt either to transfer the MHF or to increase the bundle CHF.

[0042] Patent document No. US4294312 describes a tube-bundle with straight exchanging tubes where ferrules longer than the tubes are inserted inside the exchanging tubes so that the ends of the ferrules extend beyond the tubes ends and into the distribution chambers.

[0043] Patent document No. JPS5549693 describes a tube-bundle of a process boiler with straight exchanging tubes equipped with ferrules at the process gas inlet. The ferrules extend into the respective exchanging tubes, inside the boiler shell; the ferrules are of identical length.

[0044] Patent document No. US4191246 describes the tube-bundle of a steam generator with "U" shaped exchanging tubes and ferrules of identical length.

[0045] Patent documents No. US4401153 and No. EP2482020B1 describe tube-bundles with "U" shaped exchanging tubes equipped with ferrules installed in a set of straight legs of the exchanging tubes. These documents do not disclose ferrules of different lengths substantially symmetrically distributed relative to at least one axis of symmetry of the bundle.

[0046] Numerous patent documents describe ferrule configurations for exchanging tubes; some patent documents are No. US2445273, No. US3707186, No. US20010040024A1, No. W02009066260A1, No. US2008202732A1 and No. EP3724590. However, none of these documents describes a tube-bundle having ferrules of different lengths substantially symmetrically distributed relative to at least one bundle axis of symmetry.

[0047] Brief description of the figures MNN008WO Giovanni Manenti

[0048] The detailed description of the present invention, hereafter given, is supported by the following figures:

[0049] Fig. l, where the inlet portion of a process boiler with gas on tube-side is schematically shown in longitudinal section according to the prior art;

[0050] Fig.2, where a detail of the inlet portion of a process boiler with gas on tube-side is schematically shown in longitudinal section according to the prior art;

[0051] Fig.3, where an inlet portion of a process boiler with gas on tube-side is schematically shown in longitudinal section according to a preferential configuration of the present invention;

[0052] Figs.4-9, where the tube-bundle of a process boiler with gas on tube-side is schematically shown in cross-section according to preferential configurations of the present invention;

[0053] Figs. 10 and 11, where the tube-bundle of a process boiler with gas on tube-side is schematically shown in cross-section according to preferential configurations of the present invention.

[0054] Detailed description of the invention

[0055] Fig. l schematically shows, in section, the longitudinal view of the gas inlet portion of a process boiler (1), according to prior art, having shell -and-tube configuration and a substantially cylindrical geometry with longitudinal axis (X). The boiler portion (1) includes a shell (2) that envelops the exchanging tubes (3) forming the tube-bundle, a baffle (21) that supports the exchanging tubes (3), a tube-sheet (4) which the inlet ends of the exchanging tubes (3) are connected to, a distribution chamber (5) connected to the tube-sheet (4) on the opposite side relative to the shell (2), an inlet connection (6) located on the chamber (5) and inlet and outlet connections (8) located on the shell (2). The distribution chamber (5) is internally lined with refractory (7) protecting the metal walls from the high temperature of the gas (G); the refractory (7) also coats and protects the tube-sheet (4) on tube-side (TS).

[0056] According to Fig. l, the process gas (G) at high temperature and pressure, discharged from a chemical reactor located upstream, flows on tube-side (TS) of the boiler (1). The gas (G) enters the chamber (5) from the inlet connection (6), distributes in the exchanging tubes (3) and indirectly exchanges heat with the cooling boiling water (W) flowing on boiler shell-side (SS) through the relevant inlet / outlet connections (8). The distribution chamber (5) is therefore in fluid communication with the exchanging tubes (3). MNN008WO Giovanni Manenti

[0057] The gas inlet portion of the boiler (1) of Fig. l is provided with ferrules at the inlet of the exchanging tubes (3).

[0058] Fig.2 schematically shows, in section, the longitudinal view of the inlet portion of an exchanging tube (3) with ferrule (10), a portion of tube-sheet (4) and a portion of refractory (7), in accordance with the prior art. Fig.2 corresponds to the detail (DET) relating to Fig. 1.

[0059] According to Fig.2, the exchanging tube (3) is connected to the tube-sheet (4) by means of a welded joint (9) of any type; more specifically, the inlet end of the exchanging tube (3) is welded to the tube-sheet (4). The tube-sheet (4), on tube-side (TS), is lined with refractory (7). The ferrule ( 10) is of tubular shape; it basically corresponds to a pipe butt, partially inserted in the exchanging tube (3) coaxially relative to the tube longitudinal axis (Y). Therefore, the ferrule (10) has a smaller external diameter than the internal diameter of the exchanging tube (3) for at least a longitudinal portion of the ferrule. The ferrule (10) has an inlet end (11) placed in the distribution chamber and, specifically, at the free surface (14) of the refractory (7) installed on the tube-sheet, and an outlet end (12) placed inside the exchanging tube (3) and beyond the tube-sheet (4) in the direction of the gas flow (G), so that the ferrule (10) extends into the shell. The ferrule (10) is externally provided with a layer of thermally insulating ceramic paper (13) which, preferably, longitudinally extends for all, or almost all, of the ferrule (10). The layer of ceramic paper (13) fdls the gap formed between the tube (3) and ferrule (10).

[0060] According to the prior art, the ferrule (10) of Fig.2 preferentially has the inlet end (11) flared or conical to reduce the pressure drops of the entering gas (G). Similarly, the outlet end (12) can be flared or conical to decrease the turbulence of the exiting gas (G). The ferrule (10) is partially embedded in the refractory (7); the ferrule (10) allows the passage of gas (G) through the refractory layer (7).

[0061] According to Fig.2, the hot process gas (G) flowing in the tube-side distribution chamber (5) enters the ferrule (10) from its inlet end (11), flows along the ferrule (10) and exits from its outlet end (12) into the exchanging tube (3). The ferrule (10), including the ceramic paper (13), is a barrier to heat exchange. The ferrule (10) protects the tube-sheet (4), the tube to tube-sheet joint (9) and the first portion of the exchanging tube (3) from the high temperature of the gas (G). The indirect heat exchange between gas (G) and water (W) along the ferrule (10) is minor; consequently, the heat flux along the ferrule (10) is also minor.

[0062] As shown in Fig.2, the ferrule (10) protrudes from the wetted surface (15) of the tube-sheet (4), located on shell-side (SS), by a protrusion length (L); in other words, the ferrule (10) extends into the shell, inside the exchanging tube (3), by a protrusion length (L). This length (L) allows the MHF to be transferred from the tube-sheet (4) to the ferrule outlet end (12). Therefore, the MNN008WO Giovanni Manenti strong indirect heat exchange between hot gas (G) and boiling water (W) and the consequent MHF occur at the ferrule outlet end (12).

[0063] Fig.3 schematically shows, in section, the longitudinal view of a gas inlet portion of a process boiler (100) in accordance with a preferential configuration of the present invention; the boiler (100) is of the shell-and-tube type, substantially cylindrical in shape with a longitudinal axis (X). The boiler portion (100) shown in Fig.3 is similar to the one already described for Fig. l; the numbering relative to Fig.3 corresponds to that of Fig. l for some details and therefore the description of Fig.3 is partially omitted referring to the description of Fig.l. The ferrules of Fig.3 have a configuration similar to the ferrule described for Fig.2, and therefore the detailed description of the ferrules of Fig.3 is omitted referring to the description for Fig.2. The hot gas (G) in Fig.3 flows as already described for Figs. l and 2.

[0064] According to Fig.3, ferrules of different lengths are installed at the inlet of the exchanging tubes (3). It is underlined that in this description the length of the ferrules, or the length of the protrusion of the ferrules, refers to the portion of the ferrule that protrudes in the shell (2) from the tube-sheet surface (15) wetted by the shell-side water, as already described above for Fig.2.

[0065] Fig.3 schematically shows two sets of ferrules installed in the bundle; the first set of ferrules has first ferrules (16) of length (LI) and the second set of ferrules has second ferrules (17) of length (L2). The first and second ferrules (16,17) are installed in the exchanging tubes (3) so as to be symmetrically or substantially symmetrically distributed relative to at least one axis of symmetry of the tube-bundle. The lengths (LI) and (L2) are different and, precisely, the length (L2) is greater than the length (LI).

[0066] Each ferrule (16,17) of Fig.3 has an inlet end (11) and an outlet end (12) placed, respectively, in the distribution chamber (5) at the free surface (14) of the refractory (7) installed on the tubesheet and in the shell (2) inside an exchanging tube (3), as already described for Fig.2. The outlet ends (12) of the first ferrules (16) and the outlet ends (12) of the second ferrules (17) are longitudinally spaced from each other due to the different protrusion length (L1,L2).

[0067] Fig.3 shows a transversal baffle (21) installed on shell-side (SS) in a longitudinal position of the tube-bundle intermediate between the outlet ends (12) of the first and second ferrules (16, 17). The boiler (100) is equipped with inlet / outlet connections on shell-side (8) for the cooling water (W) placed upstream and downstream of the transversal baffle (21) relative to the direction of the tube-side fluid flow (G).

[0068] Fig.4 to Fig.9 schematically show the transverse views of the tube-bundle of a boiler similar to the boiler (100) of Fig.3 according to the Z-Z' cross-section of Fig.3 and according to MNN008WO Giovanni Manenti preferential configurations of the present invention. Figs.4-9 refer to two sets of ferrules of different lengths installed in the bundle. The Z-Z' section of Fig.3 transversely cuts the tubebundle between the outlet ends (12) of the first and second ferrules (16,17); the baffle (21) is not shown in Figs.4-9 for descriptive simplicity.

[0069] Figs.4-9 show the layout of the exchanging tubes (3) enveloped by the shell (2) and the layout of the first and second ferrules (16,17) inside the tubes. The exchanging tubes (3) provided with the second ferrules (17) of length (L2) are marked with a solid circle, while the exchanging tubes (3) provided with the first ferrules (16) of length (LI) are marked with an empty circle.

[0070] The exchanging tubes (3) in Figs.4-9 are arranged with a triangular pitch. However, the exchanging tubes (3), according to alternative and preferential configurations of the present invention, can be arranged with a square pitch or by other geometric layouts. The present invention, therefore, is also valid for tube-bundles with a square pitch or, in general, with a layout other than a triangular pitch.

[0071] According to Figs.4, 5 and 6, the first ferrules (16) of length (LI) and the second ferrules (17) of length (L2) are symmetrically or substantially symmetrically distributed in the bundle relative to the vertically oriented axis of symmetry (V) of the bundle. The distribution of the ferrules (16,17) in Figs.4-6 is not symmetrical relative to the horizontally oriented axis of symmetry (H) of the bundle. It is underlined that as per the configurations of Figs.4-6 the upper tube rows are provided with both long and short ferrules so that some tubes generate steam and thus contribute to the natural draft, contrarily to the cited AIChE publication.

[0072] In Fig.4, the ferrules (17) of length (L2) are distributed so to substantially form a "Y" that vertically extends in the lower and upper halves of the tube-bundle; the "Y" shape can be compared to three tube rows crossing one other. According to a preferential configuration of the present invention, the ferrules of a set are arranged so to essentially form tube rows that cross or are perpendicular to one other.

[0073] In Fig.5, the ferrules (17) of length (L2) are distributed so to substantially form three columns or vertical tube rows, horizontally spaced from each other, two placed in the lower half and one placed in the upper half of the tube-bundle. According to a preferential configuration of the present invention, the ferrules of a set are arranged so to essentially form parallel and spaced tubes rows.

[0074] In Fig.6, the ferrules (17) of length (L2) are distributed so to essentially form five columns or vertical tube rows, horizontally spaced from each other, and distributed in the upper half of the tube-bundle. MNN008WO Giovanni Manenti

[0075] The preferential configurations relative to Figs.4, 5 and 6 form flow paths arranged along the main direction of the shell-side fluid flow. These configurations advantageously form preferential paths for water irrigation and steam outflow, without creating bundle zones, arranged horizontally or perpendicularly relative to the fluid flow, of stagnation or net resistance to the natural draft. More specifically, the configurations of Figs.4, 5 and 6 are appropriate for horizontal tube-bundles with a vertically placed axis of symmetry (V). In horizontal process boilers, the shell-side boiling water circulates from bottom to top of the bundle crossing the exchanging tubes (3): accordingly, the resistance to upward steam outflow is significant. The ferrules layout (16, 17) of Figs.4-6 forms substantially vertical flow paths characterized by both negligible and strong steam production, efficiently realizing the bundle irrigation by the water entering the shell (2) and the upward outflow of the steam produced in the bundle. Such ferrule layouts, forming substantially vertical flow paths distributed in the bundle, represent an advantageous thermo-hydraulic configuration.

[0076] According to Figs.7-9, the first ferrules (16) of length (LI) and the second ferrules (17) of length (L2) are distributed in the bundle in a symmetrical or substantially symmetrical way relative to two axes of symmetry of the bundle (H,V) placed perpendicularly to each other.

[0077] In Fig.7, the ferrules (17) of length (L2) are uniformly distributed in the bundle. As per a preferential configuration of the present invention, the ferrules of at least one set have a uniform or substantially uniform layout in the bundle, or at least in a portion of the bundle, so that the relevant distribution pitch is uniform. The preferential configuration of Fig.7 is advantageous as it allows obtaining a steam production, relative to a cross-section, uniform and similar to that of a conventional boiler. Consequently, for a tube-bundle as shown in Fig.7, the thermo-hydraulic engineering correlations and the shell-side operating conditions are similar to those of a conventional boiler (except for the CHF that is increased); such a similarity guarantees that design and operating principles of the tube-bundle object of the present invention are reliable.

[0078] In Fig.8, the layout of the second ferrules (17) of length (L2) substantially corresponds to two concentric rings with same center of the tube-bundle, and in particular to a first peripheral ring and a second inner ring, radially spaced from each other. According to a preferential configuration of the present invention, the ferrules of a set are arranged so as to form one or more rings, or one or more portions of a ring, having the same center of the tube-bundle; in case of two or more rings, these are concentric and radially spaced from each other. According to a preferential configuration of the present invention, the ferrules of a set form at least one peripheral ring, i.e. placed at the boundary of the tube-bundle, and having the same center of the bundle: such a preferential configuration is advantageous as the steam production of the bordering exchanging tubes, provided with ferrules of appropriate length, is eliminated and consequently the OTL inscribing the bundle is reduced from a thermo-hydraulic point of view. This means that the MNN008WO Giovanni Manenti resulting CHF, for a cross-section, is based on a smaller tube-bundle size. The configuration of Fig.8 also presents the same advantages already described for Fig.7.

[0079] With reference to Fig.8, a preferential configuration presents the first ferrules (16) shorter than the second ferrules (17), therefore at the outlet of the first ferules (16) the exchanging tubes with second ferrules (17) are inactive from steam production standpoint and consequently the OTL made by the active tubes is smaller. This means that the CHF relative to the bundle crosssection at the outlet of the first ferrules (16) is reduced.

[0080] In Fig.9, the layout of the second ferrules (17) of length (L2) basically corresponds to three parallel columns or vertical tube rows horizontally spaced from each other, and extending both in the lower and upper halves of the bundle. According to a preferential configuration of the present invention, the ferrules of a set are arranged so to substantially form a tubes row or several tube rows that are parallel to each other; in case of multiple rows, the rows are spaced from each other. Preferentially, said tube row or rows are oriented along the main direction of the shell-side fluid flow. This configuration allows forming preferential paths to efficiently distribute the water across the bundle and outflow the steam from the bundle. The configuration of Fig.9 also presents the same advantages already described for Fig.7.

[0081] Figs. 10 and 11 schematically show the transverse views of the tube-bundle of a boiler similar to the boiler (100) of Fig.3, according to the Z-Z' cross-section of Fig.3 as already described for Figs.4-9; Figs. 10 and 11 represent preferential configurations of the present invention.

[0082] The exchanging tubes (3) in Figs. 10 and 11 are arranged with a triangular pitch. However, according to alternative and preferential configurations of the present invention, the exchanging tubes can be arranged with a square pitch or with another layout.

[0083] Fig. 10 shows the exchanging tubes (3) enveloped by the shell (2) and the first ferrules (16) of length (LI) and the second ferrules (17) of length (L2), respectively represented by an empty and a solid circle, with a layout such that the second ferrules (17) are combined in clusters spaced apart and symmetrical relative to two axes of symmetry (H,V) of the tube-bundle. According to a preferential configuration of the present invention, the ferrules of at least a set are arranged so as to form one or more clusters of adjacent ferrules of the same set symmetrically or substantially symmetrically allocated relative to at least one axis of symmetry of the bundle; in case of several clusters, these are spaced from each other. Fig. 10 also shows a central area (22) of the tube-bundle without exchanging tubes. This central area (22) can allocate a bypass tube (not shown in the figure), normally with an external diameter considerably larger than the external diameter of the exchanging tubes (3), connected to the gas inlet tube-sheet (4) and characterized by negligible or limited heat exchange compared to the heat exchange of the exchanging tubes (3). According to MNN008WO Giovanni Manenti a preferential configuration of the present invention, the process boiler (100) is equipped with a bypass tube located in the central area (22) of the tube-bundle, with an external diameter greater than the external diameter of the exchanging tubes (3).

[0084] Fig. 11 shows the exchanging tubes (3) enveloped by the shell (2) and the first ferrules (16) of length (LI) and the second ferrules (17) of length (L2), respectively represented by an empty and a solid circle, with a layout such that the second ferrules (17) substantially form a ring placed in the peripheral area of the tube-bundle and symmetrical relative to two axes of symmetry (H,V) of the bundle. The ring layout formed by the ferrules of a set, with the same center of the bundle, is a preferential configuration of the present invention already described for the above Fig.8. Fig. 11 also shows a central area (23) of the tube-bundle equipped with bypass exchanging tubes (24) having an external diameter greater than that of the exchanging tubes (3). The bypass exchanging tubes (24) are provided at the inlet with third ferrules (25) of length (L3), represented in the figure by a cross; these third ferrules (25) are substantially similar to the ferrules described for above Fig.2. The bypass exchanging tubes (24) present a reduced heat exchange compared to the exchanging tubes (3) and usually have a larger external diameter than the external diameter of the exchanging tubes (3). The third ferrules (25) installed at the inlet of the bypass exchanging tubes (24) preferably have an external diameter greater than the first and second ferrules (16,17) installed in the exchanging tubes (3). It is underlined that, as per the present invention, the third ferrules (25) may have the same length as the first ferrules (16) or the second ferrules (17), or have a different length (L3) than the first and second ferrules. According to a preferential configuration of the present invention, the process boiler (100) is equipped with bypass exchanging tubes (24) located in the central area (23) of the tube-bundle and having an external diameter greater than the external diameter of the exchanging tubes (3); the bypass exchanging tubes (24) are preferably provided with ferrule (25) at their inlet. In accordance to another preferential configuration of the present invention, the bypass exchanging tubes (24) are provided with ferrules of different lengths; in other words, the bypass exchanging tubes (24) can be provided with ferrules belonging to different sets of ferrules.

[0085] As per the configurations of Figs.7, 9 and 10, it is to be noted that the upper tube rows are provided with both long and short ferrules so that some tubes generate steam and thus contribute to the natural draft, contrarily to the cited AIChE publication.

[0086] As per the configurations of Figs.8 and 11, the upper tube rows are provided with long ferrules only; however, contrarily to the cited AIChE publication, the ferrules layout is symmetrical or substantially symmetrical relative to two axes of symmetry perpendicular each other, guaranteeing an overall uniform steam production in the bundle cross-section similar to a conventional boiler where all the ferrules have same length. Moreover, the configurations of MNN008WO Giovanni Manenti

[0087] Figs.8 and 11 have a peripheral ring -type layout for ferrules, contrarily to the cited AIChE publication where the ferrules are concentrated in the upper rows only, and therefore the resulting bundle OTL of configurations in Figs.8 and 11 is uniformly reduced in radial direction, guaranteeing a well-proportioned steam production in the bundle cross-section.

[0088] It is emphasized that in Figs.4-11 the position of the first and second ferrules (16,17) can be swapped, without departing from the inventive concepts disclosed herein and keeping same advantages.

[0089] The layout of the second ferrules (17) relative to Figs.7-11 can be modified by replacing the second ferrules (17) in the lower or upper bundle half with first ferrules (16), so that the resulting ferrules layout is no more symmetrical relative to two axes (H,V) but it is symmetrical or substantially symmetrical relative to a single axis (V) of symmetry; this modification relating to Figs.7-11 leads to further preferential configurations of the present invention.

[0090] The ferrules layouts (16,17,25) relative to Figs.4-11 are advantageous because the bundle CHF is increased and the steam production installed in a bundle cross-section is overall uniformly distributed guaranteeing overall uniform natural draft, especially during operating transients. It is to be noted, anyway, the shell-side thermo-hydraulic conditions are more uniform, that is natural draft and steam outflow are better balanced in a bundle cross-section, when the ferrules layout is symmetrical relative to two axes of symmetry; therefore, one the most preferred configurations of the tube-bundle disclosed by the present invention refers to a bundle having ferrules of at least one set that are symmetrical to at least two axes of symmetry (H,V).

[0091] The overall uniform steam production relative to a transversal bundle section makes the bundle object of the present invention particularly advantageous from the engineering design standpoint because usual thermo-hydraulic correlations from literature can be adopted; such literature correlations refer to tube-bundles substantially symmetrical relative to one or two symmetry axes and assume an overall uniform steam production relative to a cross section.

[0092] It is emphasized that according to the present invention, the shell-and-tube equipment may include, at the gas inlet tube-sheet, exchanging tubes provided with ferrule (where the gas to be cooled enters) and exchanging tubes without ferrule (where the cooled gas exits), forming distinct groups of tubes with ferrule and tubes without ferrule. The ferrules layout is in any case as per the present invention.

[0093] In accordance with a preferential configuration of the present invention, all the tubes of the bundle connected to the gas inlet tube-sheet are provided with ferrule at the inlet. MNN008WO Giovanni Manenti

[0094] In this description, the terminology "substantially symmetrical" means that most of the ferrules contribute to symmetry or, in other words, that a limited number of ferrules do not participate in symmetry. A limited number of ferrules preferentially corresponds to less than 10%, more preferably to less than 5%, and even more preferentially to less than 3% of the total ferrules installed in the bundle.

[0095] Preferentially, according to the present invention, the difference between the protrusion lengths of two ferrules of different lengths is greater than 100mm, or greater than 250mm, or greater than 500mm.

[0096] According to a preferential configuration of the present invention, a shell-side transversal baffle is installed between the outlet ends of the ferrules of one set and the outlet ends of the ferrules of another set. In other words, the tube-bundle here disclosed preferentially includes an intermediate transversal baffle installed in a longitudinal position placed between the outlet ends of ferrules of length (LI) and the outlet ends of ferrules of length (L2). In a process boiler, this intermediate transversal baffle allows segregating, at least partially, the shell-side steam production relative to two consecutive longitudinal portions of the exchanging tubes. The intermediate transversal baffle object of the present invention allows segregating the shell-side thermo-hydraulic effects of the MHF that occurs at the outlet of ferrules of length (LI) from the shell-side thermo-hydraulic effects of the MHF that occurs at the outlet of ferrules of length (L2). This baffle helps to improve the thermo-hydraulic performance of the bundle of a process boiler, consolidating the safety margins relative to the steam blanketing phenomenon. Shell-side inlet and outlet connections can be located upstream and downstream of the baffle, in the direction of the tube-side fluid flow.

[0097] The shell-and-tube equipment object by this invention preferably include a bundle with straight exchanging tubes, welded at the ends to a first tube-sheet and a second tube-sheet (not shown in the figures). Alternatively, the shell-and-tube equipment include a bundle with U-shaped exchanging tubes, welded at the ends to one tube-sheet or two separate tube-sheets.

[0098] It is emphasized that the equipment which this invention refers to are preferentially provided, in addition to the ferrules, with other elements designed to protect the metal walls from the high temperature of the entering hot gas (G). For example, as already described for Figs. 1-3, the distribution chamber and / or the gas inlet tube-sheet are lined with refractory material on tubeside. Alternatively, the distribution chamber and / or the gas inlet tube-sheet can be coated on tubeside with a metal layer obtained by weld deposit and / or with a ceramic layer obtained by spray resistant to high temperature, corrosion or erosion. MNN008WO Giovanni Manenti

[0099] According to a preferential and alternative configuration of the present invention, an inner chamber (not shown in the figures) is installed inside the gas inlet distribution chamber of the equipment; such inner chamber is made of materials resistant to high temperatures or is provided with protective layers, and it is spaced from the distribution chamber so to form a passage for the tube-side fluid. In accordance with this preferential and alternative configuration, the inner chamber is connected to the tube-sheet and puts in fluid communication the gas inlet connection with the ferrules.

[0100] According to a preferential and alternative configuration of the present invention, an inner chamber and an inner tube-sheet (not shown in the figures) are installed inside the gas inlet distribution chamber of the equipment; such inner chamber and inner tube-sheet are made of materials resistant to high temperatures or are provided with protective layers, and are spaced from the distribution chamber and the tube-sheet so to form a passage for the gas. In accordance with this preferential and alternative configuration, the inner chamber is connected to the inner tube-sheet and the inner tube-sheet is connected to the inlet end of the ferrules so that the gas inlet connection is in fluid communication with the ferrules.

[0101] The tube-bundle object of the present invention preferably includes exchanging tubes with an external diameter equal to or greater than 31 ,75mm, more preferably equal to or greater than 33.70mm and even more preferably equal to or greater than 38.05mm. The tube-bundle object of the present invention preferably includes bypass exchanging tubes with an external diameter equal to or greater than 50.8mm. The tube-layout object of the present invention preferably includes a central bypass tube with an outer diameter of at least 114.3mm.

[0102] In accordance with an alternative and preferential configuration of the present invention, the tube-bundle comprises exchanging tubes of different external and / or internal diameters. In accordance with a preferential configuration of the present invention, the exchanging tubes provided at inlet with ferrule have different external and / or internal diameters.

[0103] In accordance with an alternative and preferential configuration of the present invention, ferrules of different lengths have a different external and / or internal diameter, relative to at least one longitudinal portion of the ferrule. In accordance with a preferential configuration of the present invention, the longer ferrules have an internal diameter, at least for a longitudinal portion of the ferrule, that is greater than the internal diameter of the shorter ferrules. If the longer ferrules have a larger internal diameter, for at least a longitudinal portion of the ferrule, than the internal diameter of the shorter ferrules, the pressure drops of the gas flowing in the longer ferrules and of the gas flowing in the shorter ferrules can be substantially balanced. If the gas pressure drops are substantially balanced in the ferrules of different lengths, the gas flow rate in the exchanging MNN008WO Giovanni Manenti tubes equipped with different ferrules is substantially balanced. As per a preferential configuration of the present invention, the internal diameter of two ferrules of different lengths differs, for at least one longitudinal portion of the ferrule, by at least 0.5mm, or 1.0mm, or 1.5mm, or 2.0mm on the diameter. It is underlined that as per the present invention, the difference in the internal diameter between two ferrules of different lengths can also refer to a very short longitudinal portion of ferrule; in other words, the ferrule can inside be provided with a profile or an element that forms a local narrowing or enlargement, even abrupt, such as an obstacle, apt to modify the gas flow. It is underlined that according to the present invention the difference in the internal diameter or profile of the ferrule may also refer to a different shape and / or size of the inlet and / or outlet end of the ferrule.

[0104] The tube-bundle object of the present invention can be horizontally or vertically arranged; if vertically arranged, the gas inlet tube-sheet where the ferrules of different lengths are installed can be placed at the top or bottom.

[0105] The ferrules object of the present invention can be made of any material; the ferrules are preferably metallic and / or ceramic.

[0106] The ferrules object of the present invention may include elements apt to improve the anchoring of the ferrules to the refractory and / or to the tube-sheet or exchanging tube. For example, the portion of ferrule embedded in the refractory may have an external shape suitable for better fixation of the ferrule.

[0107] It is emphasized that the outlet end of the ferrules object of the present invention is placed inside the exchanging tube, i.e. it is placed at a point between the ends of the exchanging tube.

[0108] The tube-bundle object of the present invention is preferably provided with two sets of ferrules where the ferrules of one set have the same length, where the ferrules of one set have a different length than the ferrules of the other set and where the ferrules have a symmetrical or substantially symmetrical layout relative to at least one axis of symmetry of the tube-bundle. However, it is underlined that the bundle of the present invention may include a number of ferrules sets greater than two: even in this case, the ferrules of one set have the same length, the ferrules of one set have a different length than the ferrules of another set, and the ferrules of at least one set have a symmetrical or substantially symmetrical layout relative to at least one axis of symmetry of the bundle.

[0109] In accordance with a preferential configuration of the present invention, the number of ferrules of a given length, i.e. the number of ferrules belonging to the same set of ferrules, may vary from 5% to 95% of the total ferrules installed in the bundle. MNN008WO Giovanni Manenti

[0110] The tube-bundle here disclosed, when implemented in a process boiler where the tubular density or the geometry is imposed and the CHF can be modified, allows the bundle CHF to be increased by at least 2%, or by at least 5%, or by at least 10%, or by at least 15%. Such approach is typical of rating calculations or revamping problems.

[0111] The tube-bundle here disclosed, when implemented in a process boiler where the CHF is imposed and the geometry can be modified, allows the radial size of the bundle to be reduced by at least 2%, or at least 5%, or at least 10%. The bundle dimension can be reduced since, by applying the configuration here disclosed, the tube pitch can be reduced without sacrificing the CHF. Such approach is typical of design calculations or techno-economic optimization problems.

[0112] From the above, the tube-bundle object of the present invention represents an improvement over a conventional bundle provided with ferrules of identical length or ferrules of different length but not symmetrically distributed in the bundle. The tube-bundle here disclosed, equipped with ferrules of different lengths symmetrically or substantially symmetrically distributed in the bundle, advantageously allows: increasing the CHF of a boiler at fixed bundle size, thus improving its thermo- hydraulic safety margins, reducing the boiler size at fixed CHF, thus optimizing its technical-economic design, installing overall uniform thermo-hydraulics conditions relative to a bundle crosssection.

[0113] Moreover, the tube-bundle here disclosed, provided with ferrules of different lengths symmetrically or substantially symmetrically distributed in the bundle, when implemented in a process boiler, allows advantageously obtaining an overall uniform steam production and natural water circulation relative to a bundle cross-section in correspondence of the ferrules.

[0114] The tube-bundle object of the present invention can be subjected to further modifications and variants, all attributable to the same inventive concept, as well as to include details and / or construction materials different from those described above without departing from the inventive concepts.

[0115] The scope of protection of the present invention is described in the attached claims.

Claims

MNN008WO Giovanni ManentiClaims1. Tube-bundle, part of a shell-and-tube equipment (100) essentially of cylindrical shape developed along a longitudinal axis (X) and apt for the indirect heat exchange between a high-temperature gas (G) flowing on tube-side (TS) and a cooling fluid (W) flowing on shell-side (SS), comprising exchanging tubes (3,24) enveloped by the shell (2) and connected at the ends to at least one tube-sheet (4) and in fluid communication with at least one distribution chamber (5) placed at said at least one tube-sheet (4) on the opposite side relative to the shell (2) and provided with at least one gas inlet connection (6), and comprising at least two sets of ferrules (16,17) installed at said at least one tubesheet (4) and at the inlet of at least a portion of said exchanging tubes (3,24), where said ferrules (16,17) are substantially tubular in shape each having an inlet end (11) placed in said at least one distribution chamber (5) and an outlet end (12) placed in the shell (2) inside an exchanging tube (3,24), where the length (L1,L2) of the ferrule portion extending in the shell (2) inside the exchanging tube (3) is identical for ferrules belonging to a set and different for ferrules belonging to different sets so that the outlet ends (12) belonging to ferrules of different sets result longitudinally spaced each other, said tubebundle being characterized in that the ferrules of at least a set present a symmetrical or substantially symmetrical layout relative to at least an axis of symmetry (V) of the tubebundle.

2. Tube-bundle as per claim 1, wherein the ferrules of at least a set present a symmetrical or substantially symmetrical layout relative to two axes of symmetry (H,V) of the tube-bundle.

3. Tube-bundle as per claim 2, wherein said two axes of symmetry (H,V) are perpendicular or crossing each other.

4. Tube-bundle as per any claim 1 to 3, wherein a shell-side (SS) transversal baffle (21), apt to at least partially segregate the shell-side effects of heat flux at outlet of ferrules of different length, is installed between the outlet ends (12) of ferrules (16,17) belonging to different sets.

5. Tube-bundle as per any claim 1 to 4, wherein the ferrules (17) of at least a set present a uniform or substantially uniform layout, for at least a portion of the tube-bundle, so that the related layout pitch is uniform.MNN008WO Giovanni Manenti6. Tube-bundle as per any claim 1 to 4, wherein the ferrules (17) of at least a set have a layout so to form one cluster of adjacent ferrules of the same set or more clusters of adjacent ferrules of the same set spaced each other.

7. Tube-bundle as per any claim 1 to 4, wherein the ferrules (17) of at least a set have a layout so to substantially form one or more rings, or one or more portions of ring, having same centre of the tube-bundle.

8. Tube-bundle as per any claim 1 to 4, wherein the ferrules (17) of at least a set have a layout so to substantially form one tube row or more tube rows parallel and spaced each other and / or crossing one another.

9. Tube-bundle as per any claim 1 to 8, wherein the length (LI) relative to ferrules (16) of a set differs from the length (L2) relative to ferrules (17) of another set by at least 100mm.

10. Tube-bundle as per any claim 1 to 9, wherein ferrules (16,17) belonging to different sets have different internal diameters for at least a longitudinal portion of the ferrule and / or have a different internal profile apt to change the gas flow (G).

11. Tube-bundle as per any claim 1 to 10, wherein said at least one tube-sheet (4) is lined on tube-side (TS) with refractory (7), and wherein said inlet end (11) is placed at the free surface (14) of refractory (7) so as to be in direct fluid communication with said at least one distribution chamber (5).

12. Tube-bundle as per any claim 1 to 11, wherein said exchanging tubes (3) provided with ferrule (16,17) have different external and / or internal diameter.

13. Tube-bundle as per any claim 1 to 12, having a central zone (22,23) provided either with bypass exchanging tubes (24), provided at inlet with ferrules (25) of same or different length, or with a bypass tube, wherein said bypass exchanging tubes (24) or said bypass tube have an external diameter greater than the external diameter of the exchanging tubes (3).

14. Tube-bundle as per any claim 1 to 13, wherein said equipment (100) corresponds to a process boiler installed downstream of a chemical reactor, and wherein said cooling fluid (W) is pressurized boiling water.

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