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, the main modes of heat transfer are conduction and convection. The hot fluid first transfers heat to one side of the tube wall through convection, then conducts the heat from that side to the other side of the tube wall, and finally the cold fluid transfers heat from the other side of the tube wall back through convection, thus completing the heat transfer process in the heat exchanger. 3. What effect does the medium flow velocity have on the 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 impact does the surface structure of heat exchange tubes have on the heat transfer effect? Answer: Specially designed surface structures for heat exchange tubes, such as finned tubes, spiked tubes, and threaded tubes, not only increase the heat transfer area but also, due to the turbulent effects created by these special surfaces, enhance the turbulence of the fluid flowing 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? Answer: Common methods for removing scale from the surface of heat exchange tubes include: Mechanical descaling: manual removal of scale using steel rods, descaling with pressurized water; Chemical descaling. 6. What methods are there to prevent scale formation on the surface of heat exchange tubes? Answer: (1) Nickel-phosphorus plating; (2) Chemical coating, 847 coating. 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, spiral-grooved tubes, threaded tubes, etc.; 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 improve its heat transfer coefficient. This can be achieved by adding turbulence-inducing elements, 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. Additionally, using materials with good thermal conductivity to manufacture heat exchangers, implementing effective anti-corrosion and anti-scaling measures, and promptly removing scale 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 case of corrosion and perforation in individual tubes within a bundle, it is permissible to seal them using metal plungers with a machined taper of 3–5°. Generally, within the same manifold, the number of blocked pipes should not exceed 10% of the total number of pipes; however, depending on process requirements, this percentage may be increased appropriately. 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 and hard, adhering firmly, and being difficult to remove. The large amount 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, baffle plates, anti-scour plates, tie rods, spacing tubes, casing, tube box, tube sheet, inlet flange, outlet flange, floating tube sheet, floating head flange, floating head cover, floating head hook ring, floating head gasket, outer cover flange, outer cover side flange, outer cover, outer cover gasket, vent port, drain port, tube box flange, tube box side flange, tube box gasket, tube box side gasket, fixed saddle, 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, spacing tubes, shell, 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 bundle, baffles, impingement plate (inner guide cylinder), tie rods, spacers, shell, tube box, tube sheet, inlet flange, outlet flange, tube box flange, side flange of the tube box, tube box gasket, side gasket of the tube box, fixed saddle, and movable saddle. 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: the inlet pipe, the outlet pipe, the collection tube, the coiled tubes, and the 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 respectively? 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 within the same shell diameter. Individual tubes can be easily replaced and repaired, and cleaning inside the tubes is straightforward, although cleaning outside the tubes is more difficult. Additionally, there are significant temperature difference stresses between the tubes and the shell. 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 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 floating-head structure is complex and costly, and strict sealing requirements apply at this section. Leaks are likely to occur at the floating head during operation, and such leaks are 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 shell-side medium is clean, prone to minimal scaling, and where the temperature difference between the media is relatively small. 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 tube-type heat exchangers include triangular arrangement and square arrangement with a 45° rotation – why? 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 tube sheet, about 15% more than in a square arrangement. However, it is difficult to clean the outer surface of the tubes ； Arranging the tubes in a square pattern rotated by 45° makes it easier to clean their outer surfaces; however, the number of tubes in this arrangement is significantly less than in a 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, etc. 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, an excessively small pipe diameter can also have adverse effects. For a fluid flowing at the same velocity, the smaller the pipe diameter, the greater the resistance encountered during flow, and thus the higher the pressure loss. Additionally, overly thin pipes are prone to clogging by impurities, making them difficult to clean. Therefore, the pipe diameter in heat exchangers generally ranges from 16 mm to 32 mm. 24. Why are some of the bolt holes in heat exchanger supports circular, while others are oblong/elliptical? Answer: The bolt holes on the fixed support are circular; this is done so that the shell can be securely fastened 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: Common gaskets used in heat exchangers include oil-resistant asbestos gaskets, iron-clad gaskets, wave-shaped 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 baffles (baffle rods) in shell-and-tube heat exchangers? Answer: The baffle plates (baffle rods) in a heat exchanger 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. However, it also increases the pressure drop, prevents the fluid from exchanging heat entirely in a counterflow 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 selected based on the actual process requirements. 29. What causes internal leaks in shell-and-tube heat exchangers? Answer: Possible causes of internal leakage in heat exchangers include: Corrosion and perforation or breakage of heat exchange tubes; Leakage caused by corrosion-induced thinning at tube ends; Loosening at the expansion joint between heat exchange tubes and tube sheets; Cracks, pores, or corrosion-induced perforations at the welded joints between heat exchange tubes and tube sheets; Loosening or breakage of small floating head bolts; Damage to the gasket of the small floating head; Damage to the seals of the small floating head or floating tube sheet. 30. Why is a hydrostatic test conducted after heat exchanger maintenance? Answer: The purpose of conducting a hydrostatic test after maintenance of a heat exchanger is to check whether the heat exchanger has the capability to safely withstand the design pressure (i.e., its pressure resistance), its tightness, the quality of its interfaces or joints, the quality of welding, and the degree of 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 arranged 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 used 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 chosen.