A stress corrosion resistance enhanced pull rod structure and heat exchanger
Patent Information
- Application Number
- CN202611372993.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-09-07
- Publication Date
- 2026-10-16
AI Technical Summary
[0005]综上，现有技术缺少一种兼顾管束整体刚度提升、疏导腐蚀介质、降低端部应力、抑制流致振动的一体化管束结构，无法满足聚甲醛装置长周期稳定运行需求
[0019] Firstly, the load-bearing structure is separated, with rigid tie rods serving as independent longitudinal load-bearing frames and not bearing heat exchange functions; the heat exchange tubes are only responsible for heat exchange and do not bear the overall bending load of the tube bundle. There is no need to thicken the heat exchange tubes or sacrifice the number of tubes, and the effective heat exchange area of the equipment is not reduced; rigid tie rods, which account for only 1% to 5% of the total number of tubes in the tube bundle, can significantly improve the overall bending stiffness of the tube bundle.
Smart Images
Figure CN122920897A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the technical field of heat exchangers, specifically to a stress corrosion resistant reinforced tie rod structure and heat exchanger. Background Technology
[0002] Shell-and-tube heat exchangers are common heat exchange equipment in the petroleum and coal chemical industries. In the formaldehyde distillation section of a polyoxymethylene (POM) plant, the vertical heat exchanger undertakes the core process of heating and distilling the tripolymer feedstock. This equipment operates at high temperatures for extended periods, leading to two typical structural failure problems:
[0003] Firstly, stress corrosion cracking failure at the tube bundle ends: Conventional tube bundles rely solely on baffles for indirect constraint of the heat exchange tubes, resulting in low bending stiffness of individual tubes. The combined effects of the tube bundle's own weight, lateral fluid impact, and thermal expansion deformation due to temperature differences between hot and cold media lead to significant deflection in the middle of the tube bundle. Equipment failure testing indicates that the ends of the heat exchange tubes approximately 100mm from the upper and lower tube sheets are the high-risk area for failure. This region simultaneously bears the rigid constraint stress of the tube sheet, residual installation stress, and thermal stress from the temperature difference. Trace amounts of formic acid in the medium possess the ability to corrode austenitic stainless steel, leading to stress-corrosion coupling and stress corrosion cracking (SCC), resulting in frequent tube bundle leaks and shutdowns. Existing technologies only address this by thickening the heat exchange tubes and replacing them with corrosion-resistant alloy materials, significantly increasing material modification costs without eliminating the root cause of stress concentration. Baffle reinforcement schemes only slightly reduce vibration and cannot alleviate stress concentration at the tube bundle ends.
[0004] Secondly, flow-induced resonance fatigue wear failure: the shell-side fluid laterally scours the tube bundle, causing periodic vortex shedding. When the fluid excitation frequency coincides with the natural frequency of the heat exchange tubes, resonance occurs. Large heat exchanger tube bundles have long, unsupported spans, and resonance can cause mutual collisions and wear of the heat exchange tubes, leading to fatigue fracture and shortening the equipment's operating cycle. Existing baffle optimization schemes only add external support to the tube bundle without changing the overall stiffness of the tube bundle itself, resulting in limited vibration suppression effects.
[0005] In summary, existing technologies lack an integrated tube bundle structure that can simultaneously improve the overall rigidity of the tube bundle, facilitate the flow of corrosive media, reduce end stress, and suppress flow-induced vibration, thus failing to meet the requirements for long-term stable operation of polyoxymethylene (POM) devices. Summary of the Invention
[0006] The purpose of this invention is to provide a stress corrosion resistant reinforced tie rod structure to solve the technical problems in related technologies.
[0007] To achieve the above objectives, the present invention provides the following technical solution:
[0008] A stress corrosion resistant reinforced tie rod structure is adapted to a tube bundle consisting of multiple parallel heat exchange tubes, including at least one rigid tie rod. The rigid tie rod is a tubular member extending along the axial direction of the tube bundle. Both ends of the rigid tie rod are welded with caps to isolate the interior of the tie rod from the tube-side medium. The interior of the tie rod forms a through-flow cavity communicating with the shell-side medium. No heat exchange tubes pass through the interior of the rigid tie rod. The upper end of the rigid tie rod is fixedly connected to the upper tube sheet of the heat exchanger, and the lower end is fixedly connected to the lower tube sheet of the heat exchanger, forming a longitudinal load-bearing skeleton for the tube bundle. All rigid tie rods are distributed across the cross-section of the tube bundle, with a center-to-center distance of 1000mm to 2000mm between adjacent rigid tie rods.
[0009] As described above, a through-hole for steam discharge is opened on the inner wall of the rigid tie rod 10cm from the upper tube sheet, and a through-hole for purging is opened on the inner wall of the rigid tie rod 10cm from the lower tube sheet; the steam discharge holes and purging holes are symmetrically distributed along the circumference of the rigid tie rod; the total flow area of the through holes is 30% to 70% of the internal cross-sectional area of the rigid tie rod; except for the two opening areas, the remaining tube wall of the rigid tie rod is a continuous fully welded sealing structure.
[0010] The aforementioned rigid tie rod is a seamless steel pipe of DN50 to DN80 with a wall thickness of 5mm to 15mm. The wall thickness is determined based on the nominal diameter of the heat exchanger shell, design pressure, design temperature, and strength calculation of the total length of the tube bundle.
[0011] The steam discharge through hole and the venting through hole mentioned above are circular or square holes.
[0012] The aforementioned rigid tie rod is connected to the upper tube sheet and lower tube sheet by a fully welded rigid joint; the moment of inertia of a single rigid tie rod section is more than 10 times that of a single heat exchange tube section.
[0013] As mentioned above, multiple baffles are arranged axially at intervals inside the heat exchanger shell; the baffles are provided with support holes adapted to the outer diameter of the rigid tie rod and through holes adapted to the outer diameter of the heat exchange tube; the diameter of the support hole is larger than the diameter of the through hole; the radial clearance between the support hole and the rigid tie rod and the radial clearance between the through hole and the heat exchange tube are both 0.5mm to 1.0mm.
[0014] The present invention also relates to a heat exchanger, specifically a vertical shell-and-tube heat exchanger, comprising a shell, an upper tube sheet, a lower tube sheet, a tube bundle, and a stress corrosion-resistant reinforced tie rod structure as described in any one of claims 1 to 5; the vertical shell-and-tube heat exchanger is applied to the trioxymethylene distillation section of a polyoxymethylene production unit.
[0015] As described above, the tube-side medium of the heat exchanger is a mixture of trimer gas and trimer condensate feed, and the medium components include formaldehyde, water, methanol, formic acid, and methyl formate; the shell side is circulated with heat exchange medium to achieve heating and distillation of the tube-side medium.
[0016] The rigid tie rods mentioned above are evenly arranged in a rectangular grid; the center distance between the rigid tie rods and the surrounding heat exchange tube holes is greater than the standard center distance of the heat exchange tubes, which meets the strength requirements of the tube sheet hole bridge in GB / T151.
[0017] As mentioned above, the two ends of the heat exchange tube are sealed with the upper tube sheet and the lower tube sheet by hydraulic strength expansion; the two ends of the rigid tie rod are fully welded to the tube sheet by argon arc welding, and the weld leg height is not less than 8mm.
[0018] The beneficial effects of this invention are as follows:
[0019] Firstly, the load-bearing structure is separated, with rigid tie rods serving as independent longitudinal load-bearing frames and not bearing heat exchange functions; the heat exchange tubes are only responsible for heat exchange and do not bear the overall bending load of the tube bundle. There is no need to thicken the heat exchange tubes or sacrifice the number of tubes, and the effective heat exchange area of the equipment is not reduced; rigid tie rods, which account for only 1% to 5% of the total number of tubes in the tube bundle, can significantly improve the overall bending stiffness of the tube bundle.
[0020] Secondly, the bidirectional through-hole perforation of the cavity provides a dual mechanism to inhibit stress corrosion cracking at the tube sheet ends:
[0021] ① Stress Reduction: The rigid tie rod forms a high-strength rigid connection with the upper and lower tube sheets. The overall bending load of the tube bundle is borne by the rigid tie rod. The maximum equivalent stress at the end of the heat exchange tube is reduced by about 60% compared with the traditional tube bundle, eliminating the stress driving force for stress corrosion cracking. The rigid tie rod DN50~DN80 thick-walled steel tube is equipped with a corrosion allowance of 5~15mm, and its own corrosion resistance is far superior to that of the heat exchange tube.
[0022] ② Media Conveying: The caps at both ends of the tie rod only isolate the tube-side material. The interior of the tie rod is a through cavity. A bidirectional through-hole is opened in the high-failure area (10cm from the tube sheet). Shell-side vapor flows into the internal cavity of the tie rod through the upper steam vent and is discharged upwards. Liquid accumulated during shutdown flows into the tie rod cavity through the lower drain hole and is completely drained. The shell-side medium can freely enter and exit the interior of the tie rod, preventing the local concentration of formic acid corrosive media at the tube bundle ends, reducing the corrosion risk by more than 80%. Except for the opening locations, the tie rod wall is continuously fully welded to ensure that the overall structural strength is not weakened by the opening.
[0023] Third, the detuned structure suppresses flow-induced resonance and reduces heat exchange tube wear. The moment of inertia of the rigid tie rod section is more than 10 times that of ordinary heat exchange tubes, and the natural frequencies of the two are significantly different, which destroys the conditions for fluid excitation resonance. At the same time, the rigid tie rod divides the long-span tube bundle into multiple short support units, increasing the first-order natural frequency of the tube bundle by 3 to 4 times, completely avoiding the excitation range of 15 to 80 Hz of the shell-side fluid. The vibration stress amplitude of the heat exchange tube is reduced by 78% compared with the traditional tube bundle, which greatly reduces the risk of fatigue fracture.
[0024] Fourth, the integrated structure facilitates manufacturing and assembly. The rigid tie rod simultaneously serves as a load-bearing frame for the tube bundle, a steam guide channel for the shell side, and a liquid drainage channel, eliminating the need for additional guide pipes or support tie rods. After the tie rods are prefabricated, the overall cage frame is assembled, and the heat exchange tube insertion and expansion processes are compatible with conventional heat exchanger manufacturing processes, without adding complex processing steps. Attached Figure Description
[0025] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.
[0026] Figure 1 This is a schematic diagram of the overall longitudinal section structure of the vertical shell-and-tube heat exchanger of the present invention.
[0027] Figure 2 for Figure 1 Schematic diagram of the cross-sectional structure of the middle AA section;
[0028] Figure 3 This is a detailed view of a partial structure of the rigid tie rod of the present invention.
[0029] Explanation of reference numerals in the attached figures:
[0030] 1. Shell; 2. Upper tube sheet; 3. Lower tube sheet; 4. Rigid tie rod; 41. Steam exhaust port; 42. Drain port; 43. Tube cap; 6. Heat exchange tube; 7. Baffle plate; 71. Support hole; 72. Through hole; 8. Trimer gas inlet; 9. Trimer condensate inlet; 10. Tube-side medium outlet; 11. Shell-side medium inlet; 12. Shell-side medium outlet. Detailed Implementation
[0031] To enable those skilled in the art to better understand the technical solution of the present invention, the following will be described in conjunction with the appendix. Figure 1 To be continued Figure 3 The present invention will now be described in further detail.
[0032] This invention relates to a stress corrosion resistant reinforced tie rod structure, adapted to a tube bundle consisting of multiple parallel heat exchange tubes 6, including at least one rigid tie rod 4; the rigid tie rod 4 is a tubular member extending axially along the tube bundle, with tube caps 43 welded to both ends of the rigid tie rod 4 to isolate the interior of the tie rod from the tube-side medium, and the interior of the tie rod forming a through-flow cavity communicating with the shell-side medium, without heat exchange tubes 6 passing through the interior of the rigid tie rod 4; the upper end of the rigid tie rod 4 is fixedly connected to the upper tube sheet 2 of the heat exchanger, and the lower end is connected to the lower tube sheet of the heat exchanger. Plate 3 is fixedly connected to form the longitudinal load-bearing skeleton of the tube bundle; all rigid tie rods 4 are distributed on the cross-section of the tube bundle, with the center-to-center distance between two adjacent rigid tie rods 4 being 1000mm to 2000mm; a through-hole for steam discharge is opened on the rigid tie rod 4 10cm inside the tube wall of the upper tube plate 2, and a through-hole for venting is opened on the rigid tie rod 4 10cm inside the tube wall of the lower tube plate 3; the steam discharge holes and venting holes are symmetrically distributed circumferentially along the rigid tie rods 4; the total flow area of the holes is [missing information - likely a measurement or percentage]. The rigid tie rod 4 occupies 30% to 70% of the internal cross-sectional area; except for the two opening areas, the remaining tube wall of the rigid tie rod 4 is a continuous fully welded sealed structure; the rigid tie rod 4 is a seamless steel pipe of DN50 to DN80, with a wall thickness of 5mm to 15mm; the wall thickness is determined based on the nominal diameter of the heat exchanger shell 1, the design pressure, the design temperature, and the strength calculation of the total length of the tube bundle; the steam discharge through hole and the exhaust through hole are circular or square holes; the rigid tie rod 4 is connected to the upper tube sheet 2 and the lower tube sheet 3. The system employs fully welded rigid joint connections; the moment of inertia of a single rigid tie rod 4 is more than 10 times that of a single heat exchange tube 6; multiple baffles 7 are axially spaced inside the heat exchanger shell 1; the baffles 7 have support holes 71 adapted to the outer diameter of the rigid tie rod 4 and through holes 72 adapted to the outer diameter of the heat exchange tube 6; the diameter of the support hole 71 is larger than the diameter of the through hole 72; the radial clearance between the support hole 71 and the rigid tie rod 4, and the radial clearance between the through hole 72 and the heat exchange tube 6 are both 0.5mm to 1.0mm.
[0033] This invention also relates to a heat exchanger, specifically a vertical shell-and-tube heat exchanger, comprising a shell 1, an upper tube sheet 2, a lower tube sheet 3, tube bundles, and the stress corrosion-resistant reinforced tie rod structure described in the foregoing embodiments. The vertical shell-and-tube heat exchanger is applied to the trioxymethylene distillation section of a polyoxymethylene production unit. The tube-side medium of the heat exchanger is a mixed feed of trioxymethylene gas and trioxymethylene condensate, the medium components including formaldehyde, water, methanol, formic acid, and methyl formate. A heat exchange medium is introduced into the shell side to achieve heating and distillation of the tube-side medium. The rigid tie rods 4 are evenly arranged in a rectangular grid. The center distance between the rigid tie rods 4 and the surrounding heat exchange tubes 6 is greater than the standard center distance of the heat exchange tubes 6, meeting the GB / T151 tube sheet hole bridge strength requirements. The two ends of the heat exchange tubes 6 are hydraulically expanded and sealed to the upper tube sheet 2 and the lower tube sheet 3. The two ends of the rigid tie rods 4 are fully argon-arc welded to the tube sheets, with a weld leg height of not less than 8 mm.
[0034] Based on the above technical solutions, two sets of embodiments are provided to fully illustrate the above technical solutions:
[0035] Example 1: For a medium-sized polyoxymethylene distillation heat exchanger (DN1800 shell 1)
[0036] I. Overall Structure
[0037] The shell 1 has a nominal diameter of DN1800 and an effective tube bundle length of 6000mm. The upper part of the shell 1 is provided with two tube-side feed inlets: a tripolymer gas inlet 8 and a tripolymer condensate inlet 9. The lower part of the shell 1 is provided with a tube-side medium outlet 10. The lower part of the side wall of the shell 1 is provided with a shell-side medium inlet 11, and the upper part of the side wall is provided with a shell-side medium outlet 12. The interior of the shell 1 is uniformly arranged with bow-shaped baffles 7 along the axial direction, with a baffle spacing of 400mm and a cut ratio of 25%. The upper tube sheet 2 and the lower tube sheet 3 are welded to the upper and lower ends respectively.
[0038] II. Fluid Movement Scheme
[0039] Tube side: Trimer gas and trimer condensate enter the tube bundle and flow from top to bottom for heat exchange. After heating is completed, they are sent out from the tube side medium outlet 10.
[0040] Shell side: The heat exchange medium enters through the shell side medium inlet 11, laterally washes the tube bundle, absorbs heat and vaporizes, and the vaporized medium flows out from the shell side medium outlet 12;
[0041] The tube-side and shell-side media form a countercurrent heat exchange.
[0042] Rigid tie rod 4 fluid passage: The shell side medium can enter the internal through cavity of the tie rod through the steam discharge hole 41 and the drain hole 42 to realize the end medium guidance and liquid drainage.
[0043] III. Tube Bundle Configuration
[0044] The heat exchange tubes 6 are made of φ25×2.5mm stainless steel tubes, arranged in an equilateral triangle pattern, with a standard tube center distance of 32mm, and a total of 1680 heat exchange tubes 6; the rigid tie rods 4 are made of DN50 seamless steel tubes with a wall thickness of 6mm, and there are 32 of them; the rigid tie rods 4 are evenly arranged in a 1000mm×1000mm rectangular grid, with a center distance of 1000mm between adjacent tie rods; the center distance of the heat exchange tubes 6 around the tie rods is adjusted to 40mm to meet the tube sheet hole bridge strength standard.
[0045] IV. Rigid Tie Rod 4 Machining Structure
[0046] The rigid tie rod 4 has caps 43 welded at both ends to isolate the interior of the rigid tie rod 4 from the tube side medium; four circumferentially symmetrical square steam discharge holes 41 are opened 100mm from the inner side of the upper tube sheet 2, each hole is 20×15mm, and the total flow area is 45% of the inner cross-sectional area of the tie rod; four square drain holes 42 are opened 100mm from the inner side of the lower tube sheet 3, each hole is 18×12mm, and the total flow area is 35% of the inner cross-sectional area of the rigid tie rod 4; the interior of the rigid tie rod 4 is connected to the shell side by two through holes to form a through cavity, and except for the area with the holes, the tube wall of the rigid tie rod 4 is continuously and fully welded and sealed.
[0047] V. Assembly and Connection
[0048] First, all rigid tie rods 4 are fully argon-arc welded to the upper tube sheet 2 and lower tube sheet 3 to form an integral cage-like load-bearing frame; all heat exchange tubes 6 are inserted in sequence, and the two ends of the heat exchange tubes 6 are sealed by 180MPa hydraulic expansion joint; the baffle plate 7 passes through the rigid tie rods 4 and the heat exchange tubes 6, with a radial gap of 0.6mm between the support hole 71 and the tie rod, and a radial gap of 0.5mm between the through hole 72 and the heat exchange tubes 6.
[0049] VI. Operating Condition Verification
[0050] The first-order natural frequency of a conventional tube bundle of the same specification is 11Hz, while the first-order natural frequency of the tube bundle in this embodiment is 48Hz; the maximum equivalent stress at the end of the heat exchange tube 6 is 70MPa, with no risk of stress corrosion cracking; there is no resonance wear failure during long-term operation, the through hole of the tie rod continuously guides the shell-side medium, and there is no medium accumulation at the end.
[0051] Example 2: For a large formaldehyde distillation vertical heat exchanger (DN2400 shell 1)
[0052] I. Overall Structure
[0053] The shell 1 has a nominal diameter of DN2400 and an effective heat exchange length of 8000 mm. Two inlets at the top of the shell 1 allow trimer gas and trimer condensate to enter the tube side, with the material flowing from top to bottom. The shell-side heat exchange medium enters through the shell-side medium inlet 11, flows laterally upwards through the tube bundle, absorbs heat, vaporizes, and exits through the shell-side medium outlet 12, resulting in counter-current heat exchange between the hot and cold media. The rigid tie rod 4 communicates with the shell side through upper and lower through-holes, forming a continuous flow cavity that continuously dries the shell-side medium at the upper and lower ends of the tube bundle.
[0054] II. Tube Bundle Configuration
[0055] The heat exchange tubes 6 are φ25×2.5mm in size, arranged in an equilateral triangle pattern, with a standard tube center-to-center distance of 32mm, and a total of 2952 tubes. The rigid tie rods 4 are made of DN50 seamless steel pipe with a wall thickness of 8mm, and there are a total of 48 rods. The rigid tie rods 4 are evenly arranged in a 1200mm×1200mm rectangular grid, with a center-to-center distance of 1200mm between adjacent tie rods. The center-to-center distance of the heat exchange tubes 6 around the tie rods is 42mm.
[0056] III. Rigid Tie Rod 4-Hole Parameters
[0057] Steam discharge holes 41 are four symmetrical square holes, each 25×20mm, with a total flow area accounting for 62% of the internal cross-sectional area of the tie rod; drain holes 42 are four symmetrical square holes, each 20×15mm, with a total flow area accounting for 61% of the internal cross-sectional area of the tie rod; meeting the requirement of a flow area range of 30% to 70%, ensuring sufficient flow between the tie rod cavity and the shell-side medium.
[0058] IV. Connection and Assembly Parameters
[0059] The upper and lower tube sheets 3 are 75mm thick; the rigid tie rod 4 is fully welded to the tube sheet with a weld leg height of 8mm; the heat exchange tube 6 is hydraulically expanded with a pressure of 180MPa; the gap between the baffle plate 7 support hole 71 and the tie rod is 0.7mm, and the gap between the heat exchange tube 6 through hole 72 is 0.6mm.
[0060] V. Performance Comparison Data
[0061] Traditional tube bundles of the same specifications: first-order natural frequency 12Hz, maximum equivalent stress at the end of heat exchange tube 6 180MPa; Improved tube bundle in this embodiment: first-order natural frequency 52Hz (4.3 times higher), maximum equivalent stress at the end of heat exchange tube 6 72MPa (stress reduced by 60%); vibration stress amplitude of heat exchange tube 6 reduced by 78%, tube bundle stress corrosion failure risk reduced by more than 80%.
[0062] Complete manufacturing and assembly process:
[0063] Step 1, Pre-fabrication of rigid tie rod 4: Cut seamless steel pipes from DN50 to DN80, and fully weld pipe caps 43 at both ends to isolate the inside of the tie rod from the tube side material; symmetrically open steam exhaust holes 41 and drain holes 42 circumferentially at a position 100mm from the tube sheet assembly end face to connect the inside and outside of the tie rod tube wall; after the holes are opened, continuously fully weld all non-holeed tube walls of the tie rod to ensure the overall structural strength.
[0064] Step 2, cage frame pre-assembly: Align the upper and lower ends of all prefabricated rigid tie rods 4 with the corresponding mounting holes of the upper tube plate 2 and lower tube plate 3 respectively, and perform argon arc welding to fully weld and form an integral longitudinal load-bearing frame.
[0065] Step 3, heat exchange tube 6 insertion and assembly: Pass all heat exchange tubes 6 through the through hole 72 of the baffle plate 7 in sequence, and insert both ends into the tube holes of the upper and lower tube sheets 3 respectively to complete the tube layout.
[0066] Step 4, tube head sealing: All heat exchange tubes 6 are hydraulically expanded to complete the tube sheet sealing; the welding position of the rigid tie rod 4 is non-destructive tested and no welding defects are found.
[0067] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.
Claims
1. A stress corrosion resistant reinforced tie rod structure, adapted to a tube bundle composed of multiple parallel heat exchange tubes, characterized in that, Includes at least one rigid tie rod; The rigid tie rod is a tubular component extending along the tube bundle axis. The two ends of the rigid tie rod are welded with tube caps to isolate the inside of the tie rod from the tube-side medium. The inside of the tie rod forms a through-flow cavity that communicates with the shell-side medium. No heat exchange tubes are installed inside the rigid tie rod. The upper end of the rigid tie rod is fixedly connected to the upper tube sheet of the heat exchanger, and the lower end is fixedly connected to the lower tube sheet of the heat exchanger, forming a longitudinal load-bearing skeleton for the tube bundle. All rigid tie rods are distributed on the cross-section of the tube bundle, and the center-to-center distance between two adjacent rigid tie rods is 1000mm to 2000mm.
2. The stress corrosion resistant reinforced tie rod structure according to claim 1, characterized in that, A through-hole for steam discharge is opened on the rigid tie rod 10cm inside the pipe wall of the upper tube sheet, and a through-hole for draining is opened on the rigid tie rod 10cm inside the pipe wall of the lower tube sheet. Steam exhaust holes and venting holes are symmetrically distributed along the circumference of the rigid tie rod; the total flow area of the holes is 30% to 70% of the internal cross-sectional area of the rigid tie rod; except for the two opening areas, the rest of the rigid tie rod wall is a continuous fully welded sealed structure.
3. The stress corrosion resistant reinforced tie rod structure according to claim 1, characterized in that, The rigid tie rod is a seamless steel pipe of DN50 to DN80 with a wall thickness of 5mm to 15mm. The wall thickness is determined based on the nominal diameter of the heat exchanger shell, the design pressure, the design temperature, and the strength calculation of the total length of the tube bundle.
4. The stress corrosion resistant reinforced tie rod structure according to claim 1, characterized in that, The steam discharge port and the venting port are circular or square.
5. The stress corrosion resistant reinforced tie rod structure according to claim 1, characterized in that, The rigid tie rod is connected to the upper tube sheet and the lower tube sheet by a fully welded rigid joint; the moment of inertia of a single rigid tie rod section is more than 10 times that of a single heat exchange tube section.
6. The stress corrosion resistant reinforced tie rod structure according to claim 1, characterized in that, Multiple baffles are arranged axially at intervals inside the heat exchanger shell; the baffles have support holes adapted to the outer diameter of the rigid tie rod and through holes adapted to the outer diameter of the heat exchange tube; the diameter of the support hole is larger than the diameter of the through hole; the radial clearance between the support hole and the rigid tie rod, and the radial clearance between the through hole and the heat exchange tube are both 0.5mm to 1.0mm.
7. A vertical shell-and-tube heat exchanger, characterized in that, The heat exchanger includes a shell, an upper tube sheet, a lower tube sheet, a tube bundle, and a stress corrosion-resistant reinforced tie rod structure as described in any one of claims 1 to 5; the vertical shell-and-tube heat exchanger is applied to the trioxymethylene distillation section of a polyoxymethylene production unit.
8. The vertical shell-and-tube heat exchanger according to claim 7, characterized in that, The heat exchanger tube-side medium is a mixture of trimer gas and trimer condensate feed, and the medium components include formaldehyde, water, methanol, formic acid, and methyl formate; the shell side is circulated with heat exchange medium to achieve heating and distillation of the tube-side medium.
9. The vertical shell-and-tube heat exchanger according to claim 7, characterized in that, The rigid tie rods are evenly arranged in a rectangular grid; the center distance between the rigid tie rods and the surrounding heat exchange tube holes is greater than the standard center distance of the heat exchange tubes, which meets the strength requirements of the tube sheet hole bridge in GB / T151.
10. The vertical shell-and-tube heat exchanger according to claim 7, characterized in that, The heat exchange tubes are hydraulically expanded and sealed to the upper and lower tube sheets at both ends; the rigid tie rods are fully argon-arc welded to the tube sheets at both ends, with a weld leg height of not less than 8mm.