Double pipe heat exchangers are important heat exchange devices. They are widely used in industries such as chemical, petrochemical, and energy. Their main function is to exchange heat between two fluids while keeping them separate. To ensure the equipment operates efficiently and remains stable over time, strict control over the manufacturing process is required. This is especially important for geometric dimensions, pipe installation, welding, and testing. This article details the manufacturing and assembly process for double pipe heat exchangers. It covers geometric dimension control, pipe fitting, assembly and welding, expansion, and leak and pressure testing.
Geometric Dimension and Orientation Control
In the manufacturing of double pipe heat exchangers, controlling geometric dimensions and orientation is crucial. All component dimensions must meet strict tolerances according to the relevant standards. For example, edge misalignment, angularity, and non-destructive testing should meet the GB 150 standard. Circumference, roundness, and straightness must follow the GB 151 standard. The following points must be considered during shell fabrication.
Shell Length and Precision: The shell length must match the design drawing exactly to ensure proper fitting with other components.
End Face Parallelism: The two end faces of the shell must be parallel to each other and perpendicular to the shell axis. This can be verified by calibrating the cross-center and orientation lines.
Concentricity and Orientation Line Control: During installation, ensure that the orientation lines are parallel to the shell axis. This will guarantee that the tube bundle is installed accurately.
Shell Inner Diameter and Baffle Fitting
The fit between the shell's inner diameter and the baffles is especially important in the design and assembly of the heat exchanger. To ensure smooth assembly, templates matching the outer diameter of the baffles are used to check the shell's inner diameter. This helps ensure proper clearance between the baffles and the shell's inner wall, preventing installation issues due to incorrect inner diameters.
Drilling and Assembly of Tube Plates and Baffles
The tube plates in a double pipe heat exchanger consist of an outer tube plate and an inner tube plate. The outer plate is usually at the end of the heat exchange tubes and functions as a flange. The inner tube plate is located at the other end of the tubes. During the drilling and assembly of the tube plates and baffles, attention must be paid to the following:
Drilling Precision: CNC drilling machines are used to drill the tube holes. This ensures that hole diameter, perpendicularity, and spacing meet the design requirements.
Chamfering and Cleaning of Tube Holes: Special care should be taken to chamfer and clean the tube holes, especially on the inner tube plate. This prevents spiral or longitudinal scratches that could interfere with tube bundle assembly.
Baffle Installation: Baffles should be stacked in the correct order, with clear markings to indicate the front and back. The tube holes on each baffle must be chamfered and deburred to prevent damage during tube insertion.
Assembly and Welding of Double Tube Plates
The assembly and welding of double tube plates are crucial steps in manufacturing the heat exchanger. Accurate assembly ensures the equipment's seal and stability. The main steps are as follows.
Positioning and Adjustment: Positioning plates are used to connect the two sets of double tube plates. Adjustments are made to ensure concentricity, parallelism, and proper alignment.
Pre-insertion of Heat Exchange Tubes: After positioning the tube plates, heat exchange tubes are pre-inserted one by one. This ensures that the tube bundle will fit properly into the shell.
Welding Fixation: Once the heat exchange tubes are pre-inserted, welding is performed to fix the tubes and plates in place. This ensures the sealing and stability of the welds.
Expansion Process and Testing
Tube expansion is a key method for connecting the heat exchange tubes to the tube plates. A two-stage expansion process is used to ensure a strong and leak-proof connection.
First Expansion: Hydraulic expansion is performed using a specific pressure to ensure that the tube and tube plate are tightly connected.
Second Expansion: A slightly higher pressure is applied during the second expansion to enhance the connection's stability and seal.
Expansion Sequence: The expansion is performed using a continuous flexible chain method, starting from the top, to ensure uniform quality across all rows.
Welding and Leak Testing
Welding is crucial for ensuring the seal of various components in the double pipe heat exchanger. The following points should be considered during welding.
Tungsten Inert Gas (TIG) Welding: TIG welding is used to ensure precision and reliability.
Leak Testing: After welding, a leak test is performed to detect any gas leaks. Leak detection paper is used to confirm that the equipment meets the required gas-tightness standards.
Hydraulic Pressure Testing: The heat exchanger undergoes a hydraulic pressure test to ensure that the tubes and welded areas do not leak under working pressure.
Final Testing and Quality Inspection
Once the double pipe heat exchanger is fully assembled, a series of final tests and quality inspections are conducted to ensure the equipment's performance and safety.
Ammonia Permeation Test: This test checks the seal integrity of the isolation chambers.
Hydraulic Pressure and Leak Testing: Water pressure and leak tests verify the overall seal integrity of the heat exchanger, ensuring it will perform reliably over time.
Conclusion
The manufacturing and assembly of double pipe heat exchangers is a precise and complex process that involves several interconnected steps. Strict control of geometric dimensions, accurate drilling and assembly, effective expansion and welding techniques, and comprehensive leak and pressure testing all contribute to ensuring the equipment's efficiency and long-term stability. These meticulous processes not only guarantee the performance of the heat exchanger but also provide reliable assurance for its use in various industrial applications.
