Process requirements for shell and tube heat exchangers
In the manufacturing process of the shell and tube heat exchanger, including the container manufacturing, the connection between the tube-and-tube heat exchanger tube and the tube sheet, and the manufacture of the tube sheet baffle, these three aspects have high requirements for the welding process.
1. Most of the shell and tube heat exchanger containers are welded.
The weld must be subjected to hydrogen elimination treatment and post-weld heat treatment. During the welding process, hydrogen from the welding rod, flux and air is decomposed into atomic state at high temperature and dissolved in liquid metal. When the weld is cooled, the solubility of hydrogen in the steel drops sharply. Because the weld is cooled quickly, hydrogen is too late. Escapes, stays in the weld metal and forms delayed cracks over time. After welding, the weld was heated to 200 ° C for 16 hours and subjected to hydrogen elimination treatment. The post-weld heat treatment heats the weldment integrally or locally to a temperature below the A line (phase change point) for heat preservation, and then the furnace is cooled or air cooled. As long as the purpose is to eliminate and reduce the stress generated during the welding process, to avoid cracks (hydrogen cracks) in the welded structure, and to recover the mechanical properties lost by cold work. It should be noted that in order to ensure the sealing of the pipe box equipment flange, it is often required to process the sealing surface after the overall heat treatment.
2. Welding method of shell and tube heat exchanger
In the manufacturing process, common welding methods include manual arc welding, submerged arc automatic welding, gas shielded welding (argon arc welding, CO2 protective welding). According to different materials, different thicknesses, different groove openings, different welding processes are adopted. Manual arc welding is a widely used welding method. It is flexible in operation and simple in equipment. It can be welded at all positions, but the welding quality depends largely on the technical level of the welder. The submerged arc automatic welding arc has high heat utilization and welding. Faster speed, high productivity, saving metal and improving working conditions, but limited by it, generally used only for welding straight welds and large circumferential ring welds. For example, the longitudinal seam and circumferential seam welding of the cylinder (δ≥18mm) can be firstly grounded by manual arc welding. After passing the test, the submerged arc is automatically welded and welded. Because the heat exchange tube is relatively thin, the tube is The welding of the plate and the heat exchange tube is performed by argon arc welding, and then expanded by the expander.
3. The connection between the tube-and-tube heat exchanger tube and the tube sheet is as follows:
Strength welder paste b, strength expansion plus seal welding c, strength welding d, strength expansion. At present, extensive use of expansion welding is used. This method can improve the fatigue resistance of the joint and eliminate stress corrosion and crevice corrosion. For the inspection of welded pipe joints, the surface penetration or magnetic particle inspection method is generally used to control the joint quality, and there is no inspection requirement for the internal defects of the welded joint.
4. The manufacture of tube sheets and baffles in shell and tube heat exchanger is a prominent problem in the manufacturing process.
The tubesheet is machined and its hole diameter and hole spacing are subject to tolerances for different tube bundles. Drilling holes can be drilled with holes and drilled with drill holes. CNC machines can be used for advanced ones. However, when drilling with a scribe line, due to the poor precision, when drilling the baffle tube hole, the tube plate and the baffle plate must be overlapped and matched with the drill. After the drilling, the baffle plate is sequentially numbered and oriented to facilitate assembly. . The baffle should be drilled according to the whole circular plate, and the hole is drilled and then cut into the desired shape.
In the manufacture of heat exchangers, the manufacturing process of the cylinder, the head and other parts is the same as that of the general container manufacturing, but the requirements are different. The key points are to check the material, the tube plate, the baffle tube hole, and the barrel. Welding, flange processing, etc. Throughout its manufacturing process, most of the traditional processes are used, in which the proportion of welding is high, so it is necessary to strictly follow the welding process and weld the weld.