Q&A on heat exchanger technology

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1. How are heat exchange equipment classified? Answer: According to the \"Equipment Classification Catalog of the Petrochemical Corporation\", they can be classified as follows: (1) Shell-and-tube heat exchangers, (2) Double-pipe heat exchangers, (3) Water-cooled heat exchangers, (4) Spray-type heat exchangers, (5) Rotary (coiled-tube) heat exchangers, (6) Plate heat exchangers, (7) Plate-fin heat exchangers, (8) Tube-fin heat exchangers, (9) Waste heat boilers, (10) Others. 2. How do heat exchangers transfer heat? Answer: In the most common partitioned heat exchangers, there are mainly two modes of heat transfer: conduction and convection. The hot fluid first transfers heat to one side of the pipe wall via convective heat transfer; then, the heat is conducted from one side of the pipe wall to the other side. Finally, the other side of the pipe wall transfers the heat to the cold fluid through convective heat transfer, thus completing the heat transfer process in the heat exchanger. 3. What is the impact of medium flow velocity on heat transfer efficiency? Answer: The higher the flow rate of the medium within the heat exchanger, the greater its heat transfer coefficient. Therefore, increasing the flow rate of the medium within the heat exchanger can **improve the heat exchange efficiency. However, the negative consequence of increasing the flow rate is that it raises the pressure drop across the heat exchanger, thereby increasing the energy consumption of the pumps; hence, there must be an appropriate range for such flow rates. 4. What is the impact of the surface structure of the heat exchange tubes on the heat exchange efficiency? Answer: Specially designed surface structures for heat exchange tubes, such as finned tubes, stud-covered tubes, and threaded tubes, not only increase the heat transfer area but also, due to the turbulence-inducing effect of these special surfaces, enhance the turbulence level of the fluid outside the tubes. Both factors contribute to improving the overall heat transfer efficiency of the heat exchanger; therefore, these surface structures offer superior performance compared to those of plain tubes. 5. What are the commonly used methods for descaling the surface of heat exchange tubes at present? Answer: Common methods for descaling the surface of heat exchange tubes include: Mechanical descaling: manual scaling removal using steel rods, and scaling removal using pressurized water. 6. What methods are there for preventing scaling on the surface of heat exchange tubes? Answer: (1) Nickel-phosphorus plating (2) Chemical coating, 847 paint. 7. What are the common methods for enhancing heat transfer in heat exchange equipment? Answer: The main methods for enhancing heat transfer in heat exchange equipment are as follows. First, structures that increase the heat transfer surface area can be used, such as finned tubes, stud tubes, threaded tubes, corrugated tubes, etc.; the tube surface can also be mechanically processed to create structures like helical tubes, spirally grooved tubes, or threaded tubes. Additionally, using tubes with smaller diameters allows for an increased number of tubes per unit area of the tube sheet, thereby increasing the heat transfer area. Second, increasing the flow velocity of the fluid within the heat exchanger can raise its heat transfer coefficient; this can be achieved by introducing elements that create turbulence, such as spiral strips inserted inside the tubes, baffle plates placed outside the tubes, or dummy tubes. 2 Increase the number of tubeside or shellside passes. Furthermore, using materials with good thermal conductivity to manufacture heat exchangers, taking measures to prevent corrosion and scaling in these devices, and removing scale promptly are all ways to improve heat transfer efficiency. 8. What are the requirements regarding the number of blocked tubes during the maintenance of shell-and-tube heat exchangers? Answer: In the event of corrosion and perforation in individual tubes within a bundle, it is permissible to plug them using metal plungers with a machined taper of 3–5°. Generally, within the same pipe bank, the number of blocked pipes does not exceed 10% of the total number of pipes, but this ratio can be increased appropriately depending on process requirements. 9. Why must the gaskets on both sides of the tube sheet be made of the same material? Answer: Because the flange fastening bolts on both sides of the tube sheet are the same bolts, the specific pressure applied to the gaskets on both sides of the tube sheet is identical. If different materials are used for the gaskets on both sides, it will inevitably result in insufficient compressive force on one of the gaskets, leading to seal failure, or excessive compressive force on the other gasket, also causing seal failure. Therefore, the gaskets on both sides of the tube sheet must be made of the same material. 10. Why does scale form in the cooling water heat exchanger? Answer: Scale is formed when dissolved salts in water crystallize and adhere to the walls of heat exchanger tubes. It is characterized by being dense, hard, firmly attached, and difficult to remove. The large number of suspended particles present in water can serve as seed crystals; other impurity ions, bacteria, and rough metal surfaces all have a strong catalytic effect on the crystallization process, **reducing the supersaturation required for crystal formation. As a result, water scale easily forms in cooling water heat exchangers. 11. What are the main components of a floating head heat exchanger? Answer: The main components include: tube bundle, baffles, impact plates, tie rods, spacing tubes, shell, tube box, tube sheet, inlet flange, outlet flange, floating tube sheet, float head flange, float head cover, float head hook ring, float head gasket, outer head flange, outer head side flange, outer head, outer head gasket, vent port, drain port, tube box flange, tube box side flange, tube box gasket, tube box side gasket, fixed saddle, and movable saddle. 12. What are the main components of a fixed-tube-sheet heat exchanger? Answer: The main components include: tube bundle, baffle plates, tie rods, spacers, casing, tube box (top cover), tube sheet, inlet flange, outlet flange, tube box flange, tube box gasket, fixed saddle, movable saddle, ear support, and expansion joint. 13. What are the main components of a U-tube heat exchanger? Answer: The main components include: U-tube bundles, baffle plates, anti-scour plates (inner guide cylinders), tie rods, spacing tubes, casing, tube boxes, tube sheets, inlet flanges, outlet flanges, tube box flanges, tube box side flanges, tube box gaskets, tube box side gaskets, fixed saddles, and movable saddles. 14. What are the main components of a shell-and-tube heat exchanger? Answer: The main components of a shell-and-tube heat exchanger are: the inner tube, the outer tube, and the elbow. 15. What are the main components of a submerged heat exchanger? Answer: The main components of a submerged heat exchanger are: inlet pipe, outlet pipe, collector pipe, coiled tube, and cooling water tank. 16. What are the main components of a spray-type heat exchanger? Answer: The main components of a spray-type heat exchanger are: the tube bundle, fan, water nozzles, drain pipe, and feed water pump. 17. What are the characteristics of fixed-tube-sheet heat exchangers, U-tube heat exchangers, and floating-head heat exchangers? Answer: The fixed-tube-sheet heat exchanger is characterized by a compact and simple structure, low cost; it allows for the maximum number of tubes to be installed within the same shell diameter. It is convenient to replace or repair individual tubes, as well as to clean the inside of the tubes, but cleaning the outside of the tubes is more difficult. Additionally, there are significant temperature differences between the tubes and the shell, resulting in high stress. The U-tube heat exchanger is characterized by a simple structure, no issue with temperature difference stress, high fluid flow rates, and low metal consumption. It is suitable for use with high-temperature and high-pressure fluids. The tube bundle can be removed to facilitate cleaning of the shell side and between the tubes; however, the bends inside the tubes are difficult to clean. There are few tubes per tube sheet, the tube spacing is large, and there is a gap at the center of the tube bundle, which allows the fluid outside the tubes to take a shortcut. The floating-head heat exchanger is characterized by a tube bundle that can move freely, eliminating the problem of thermal stress; the tube bundle can be easily removed for cleaning the outside of the tubes as well as the bundle itself. However, its structure is complex and its cost is high. Sealing requirements at the floating head are stringent, and leaks are likely to occur there during operation, making them difficult to detect. 18. Where are fixed-tube-sheet heat exchangers suitable for use? Answer: Fixed-tube-sheet heat exchangers are suitable for applications where the fluid in the shell side is clean, does not tend to form scale, and there is a relatively small temperature difference between the fluids. 19. Where are U-tube heat exchangers suitable for use? Answer: U-tube heat exchangers are suitable for applications where there is a large temperature difference between the shell and the tubes, and where a clean fluid flows inside the tubes under high temperature and pressure conditions. 20. Where are floating-head heat exchangers used? Answer: Floating-head heat exchangers are suitable for applications where there is a large temperature difference between the tubes and the shell, the fluid is dirty, and frequent cleaning is required. 21. The arrangements of shell-and-tube heat exchangers include triangular arrangement and square arrangement rotated by 45°. Why is this so? Answer: The triangular arrangement and the square arrangement rotated by 45° have their respective advantages and disadvantages. The advantage of a triangular arrangement is its compactness and high heat transfer efficiency; it allows for the maximum number of tubes per unit area of the tube sheet, about 15% more than in a square arrangement. However, it is difficult to clean the outer surface of the tubes ； Arranging the cleaning tubes at a 45° angle in a square pattern makes it easier to clean the outer surface of the tubes, but the number of tubes used is much less compared to the triangular arrangement. 22. What are the common materials used for tubes in shell and tube heat exchangers? Answer: Common materials for tube shells include: 10#, 20#, 12CrMo, 15CrMo, 0Cr13, 1Cr13, 1Cr5Mo, 0Cr18Ni9Ti, 1Cr18Ni9Ti, titanium tubes, 410, 321, and so on. 23. In shell-and-tube heat exchangers, why are the tube diameters chosen as φ32, φ25, φ19, and φ16? Answer: The diameter of the pipe directly affects the performance of the heat exchanger. It has a small diameter and a high heat transfer coefficient, resulting in a large effective heat transfer area within the same volume. This not only makes the structure compact but also saves materials. However, too small a pipe diameter also has adverse effects. For fluids with the same flow rate, the smaller the pipe diameter, the greater the resistance encountered during flow, and thus the pressure loss increases as well. In addition, overly narrow pipes are prone to getting clogged with debris, making cleaning difficult. Therefore, the pipe diameter of heat exchangers is generally between 16 millimeters and 32 millimeters. 24. Why are the bolt holes in heat exchanger supports circular, as well as oblong circular? Answer: The bolt holes on the fixed supports are circular, so as to firmly secure the housing to the foundation. The bolt holes on the movable support are elongated circular in shape; this design allows the housing to expand and contract freely as temperature changes, thereby preventing the generation of excessive stress and protecting the equipment. 25. What are the common gaskets used in heat exchangers? Answer: Commonly used heat exchanger gaskets include oil-resistant asbestos gaskets, iron-clad gaskets, wave-patterned gaskets, and metal gaskets. 26. What considerations should be taken when selecting bolts for the small floating head in a floating-head heat exchanger? (1) Length (2) Wet H2S stress corrosion (3) Temperature. 27. What is the function of baffle plates (baffle rods) in shell-and-tube heat exchangers? Answer: The baffle plates (baffle rods) in heat exchangers can be used to change the flow direction of the fluid in the shell side, increase the flow velocity of that fluid, enhance the turbulence level of the medium, improve heat transfer efficiency, and support the tube bundle. 28. Why do shell and tube heat exchangers have single-pass, double-pass, four-pass, six-pass, and eight-pass configurations? Answer: When the total number of tubes in the heat exchanger is the same, increasing the number of tube passes can increase the flow velocity within each pass, thereby increasing the heat transfer coefficient and reducing the required heat transfer area. But at the same time, it increases the pressure drop, prevents the fluid from exchanging heat entirely in a counter-current manner, and makes the heat exchanger structure more complex. Therefore, the number of pipe stages generally used is not less than 2 and not more than 8; the specific value should be chosen based on the actual process requirements. 29. What are the causes of internal leakage in shell and tube heat exchangers? Answer: The internal leakage in a heat exchanger may be caused by the following reasons: a. Corrosion and perforation, or breakage of the heat exchange tubes; b. Leakage due to corrosion and thinning at the tube ends; c. Looseness at the expansion joints between the heat exchange tubes and the tube sheet; d. Cracks, pores, or corrosion-related perforations at the welds between the heat exchange tubes and the tube sheet; e. Looseness or breakage of the small floating head bolts; f. Damage to the small floating head gaskets; g. Damage to the seals of the small floating head or floating tube sheet. 30. Why is it necessary to perform a hydrostatic test after repairing a heat exchanger? Answer: The purpose of conducting a water pressure test after the maintenance of a heat exchanger is to check whether the heat exchanger has the ability to safely withstand the design pressure (i.e., its pressure resistance), its tightness, the quality of its connections and joints, the quality of its welds, and the tightness of its sealing structures. In addition, it is also possible to observe the residual deformation of the base metal welds in containers and pipes after being under pressure, thereby enabling the timely detection of any issues with the material. 31. Why are some tubular heat exchangers installed vertically while others are installed horizontally? Answer: Some tubular heat exchangers are vertical while others are horizontal, and this decision is based on the following factors: ① Process requirements: For example, certain reboilers require a certain liquid level height; if a horizontal heat exchanger is used, it is not possible to achieve this required liquid level, so a vertical heat exchanger must be chosen ； ②Larger scale: If a process unit requires a heat exchange area of several thousand square meters, using horizontal exchangers with heat pipe lengths of 6 meters may necessitate the use of multiple exchangers, which takes up a lot of space and makes it difficult to arrange the equipment efficiently. On the other hand, with vertical exchangers having heat pipe lengths of 12 meters, one unit is sufficient to meet the requirements ； ③Reduce pressure drop: Some production processes require minimizing the pressure drop during the transfer of the medium; therefore, vertical heat exchangers are chosen and placed alongside the towers, thereby shortening the connection pipelines to the towers and reducing the pressure drop. 32. Why are shell-and-tube heat exchangers and water-cooled heat exchangers used in some places, while tube-type heat exchangers are used in others? Answer: At present, the heat exchange equipment used in oil refining and chemical production plants is mostly tube-type heat exchangers; however, a small number of shell-and-tube heat exchangers and water-cooled heat exchangers are still in use in some of these plants. Although shell-and-tube heat exchangers have a compact structure and high heat exchange efficiency, their small heat exchange tubes can easily lead to blockages when used with media containing solid particles. Therefore, in media containing solid particles, shell-and-tube heat exchangers or submerged heat exchangers are generally used.