Value of heat transfer coefficient during process design
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Circulating water is used to cool the hot water, with the requirement that its temperature be reduced from 75°C to 35°C. A shell-and-tube heat exchanger is employed. During design, what temperatures should generally be assumed for the inlet and outlet of the circulating water, and what value should be used for the overall heat transfer coefficient?
The temperature difference in the circulating water system depends on the process; generally, in the north it is designed to be 7–10°C (around 32→40°C), while in the south it is set at 5°C (32→37°C). Here, you need to lower the hot water to 35°C; a temperature difference of 3°C is also acceptable. 32→35℃. As for the heat transfer coefficient, for water-water systems, a value of 1000 W/(m²·K) can be used as a starting point for calculations; after determining the heat transfer area, the heat exchanger can then be designed. After the heat exchanger is designed, the heat transfer coefficient and heat transfer rate need to be recalculated based on the specific structure of the heat exchanger.
The factory where I work is in Shanghai. For shell-and-tube heat exchangers, a heat transfer coefficient of 800 is used; for plate heat exchangers, it’s 2500. Temperature differences of 32–37 or 32–42 have both been utilized. Generally, it is the owners who set the conditions, and the manufacturer provides an accurate value for the heat transfer coefficient
The heat transfer coefficient is a complex issue that is related to water quality (mainly scaling), the structure of the heat exchanger (which primarily affects convective heat transfer), the physical properties of the fluid flowing through it, and the material used in the equipment; therefore, actual calculations are necessary to determine it