Three processes for inserting tubes into heat exchangers

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Tube threading in heat exchangers generally falls into three types: open threading, concealed threading, and blind threading. External pass: Outside the shell, the heat exchange tubes are passed through the baffles. Process sequence: Fix the baffle to the tie rod tube sheet, and insert a small number of heat exchange tubes to adjust the coaxiality; then insert all the heat exchange tubes from the baffle in the direction of the tie rod tube sheet. Dark penetration: In a semi-closed shell, the heat exchange tubes are passed through the baffles. Depending on the direction of pipe threading, concealed threading can be further divided into forward concealed threading and reverse concealed threading. Sequence of the forward blind penetration process: ① The pull rod tube sheet is first welded to the shell component. ② All baffle plates are fixed in sequence through tie rods, settling tubes, and tie rod tube sheets, and a small number of heat exchange tubes are inserted to adjust their coaxiality. ③All the heat exchange tubes are inserted starting from the last baffle and moving in the direction of the first tube sheet. Sequence of the backward blind welding process: ① First, weld the tie rod tube sheet to the cylinder component. ② The first baffle is fixed to the tie-rod tube sheet; then all heat exchange tubes are inserted starting from the tie-rod tube sheet in the direction of the first baffle. ③ Assemble the second baffle, and insert all the heat exchange tubes from the tie rod tube sheet in the direction of the second baffle. ④ Similarly, assemble the remaining baffles and insert all heat exchange tubes. Blind passage: Within a fully enclosed cylinder, the heat exchange tubes pass sequentially through the tie-rod tube sheet, all baffle plates, and the non-tie-rod tube sheet. Process sequence for blind threading: ① First, the cylinder components are welded to the tie rod tube sheet; then baffle plates are installed inside the cylinder, and a small number of heat exchange tubes are inserted to adjust the coaxiality. ② The cylinder component is welded together with the non-rod tube sheet. ③ All heat exchange tubes pass through the baffle from the tie rod tube sheet side and then into the non-tie rod tube sheet. Analysis and comparison of three pipe insertion methods. Advantages of open insertion: The heat exchange tubes can be seen throughout the entire process of passing through each baffle; thus, the pipe insertion proceeds relatively quickly. Disadvantage: The open-through method is not suitable for applications with excessively large cylinder diameters, heavy tube bundles, or thin tube walls. Dark threading is a common and traditional method for assembling tube bundles in fixed tube sheet heat exchangers. The advantage is that for all types of fixed tube sheet heat exchangers, the tube bundle can be assembled using the blind-hole method. Typically, the tube bundle assembly for fixed-tube-sheet heat exchangers uses a forward blind-pass method. For the tube bundles of fixed-plate heat exchangers with very few special configurations, when there is uncertainty regarding the machining accuracy of the tube holes and hole spacing, or doubt regarding the coaxiality during assembly, the backward hidden-pass method is employed. The advantage is that the tube bundle assembly is the most reliable method, as it allows for the timely detection of issues such as baffle plates that can affect tube insertion, making rework easier. The disadvantage is that it requires certain tooling, and the construction progress is slow. For the cylinder diameter using the hidden passage method, it should neither be too small nor too large; the cylinder wall should not be too thick, and the tube bundle should not be too heavy. The common drawback of both open and closed configurations is that all the heat exchange tubes have to pass through the baffle twice. The greater the distance between the two tube sheets, the larger the mass of the tube bundle inside the cylinder, and it becomes more difficult to maintain the coaxiality of these two tube sheets; as a result, there is a higher risk of damage to components such as the heat exchange tubes. The ease of assembling large tube bundles has a significant impact on the overall manufacturing schedule. The blind threading method is a tube threading technique used in recent years for fixed-tube-sheet heat exchangers that require overall heat treatment. Its advantages include the ability to thread all the heat exchange tubes through the baffle at once, easier control of the coaxiality between the two tube sheets, and less damage to the heat exchange tubes or the inner surface of the shell. The disadvantage is that the assembly of the tube bundle involves higher risks, and there are strict requirements regarding the tolerances of the pore size and pore spacing of the baffle plates (or grids), as well as the coaxiality during assembly.