Q&A on Heat Exchanger Technology

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Q&A on Heat Exchanger Technology 1. How are heat exchange devices 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) Tubular 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 shell-and-tube 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 is the effect of medium flow velocity on heat transfer efficiency? Answer: The higher the flow velocity 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 effect of increasing the flow rate is that it raises the pressure drop across the heat exchanger, thereby increasing the energy consumption of the pump; 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 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: manual cleaning with steel picks, mechanical cleaning, cleaning with pressurized water, and chemical cleaning. 6. What are the methods for preventing 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, spiked tubes, threaded tubes, corrugated tubes, etc.; the tube surface can also be mechanically processed to create spiral grooves or threads. Second, 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. Third, 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, baffles, or dummy tubes placed outside the 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 is the difference between the nominal heat exchange area and the actual heat exchange area in heat exchange equipment? An example can be given to illustrate 9. Is it true that the lower the temperature of the cooling water, the better? 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.