Heat exchanger issues
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Recently, I came across a problem related to heat exchangers, specifically a single-pass tubular heat exchanger in which the solution flows inside the tubes, while saturated water vapor condenses on the shell side to form water at the same temperature. If the total number of tubes remains unchanged and the number of flow passes becomes 4, then in a turbulent regime the convective heat transfer coefficient α is proportional to the 0.8th power of the flow velocity u. The explanation for this is that with 4 flow passes, the flow velocity inside the tubes increases by a factor of 4? I don’t understand. Also, when calculating the tube length L, should the outer diameter of the tube be used? I don’t quite understand the rules for selecting pipe diameters. I appreciate your guidance.
This has nothing to do with the heat transfer coefficient; it merely examines the relationship between area and velocity at a certain volume flow rate.
This post was last edited by f9kx on 2015-12-18 at 16:13. The number of passes can be understood as the number of times the fluid travels back and forth through the pipes of the heat exchanger. This question can be understood as follows: There are 4 pipes inside the heat exchanger. When these 4 pipes are connected in parallel (equivalent to forming one pipe with a larger diameter), the fluid flows through the heat exchanger in one direction only, without returning, which is known as a single-pass flow ； If 4 pipes are connected in series with 180° elbows at the pipe ends (which function as elbows under actual operating conditions), the fluid will flow back and forth inside the heat exchanger twice; this is what constitutes a 4-pipe circuit ； If they are connected in parallel pairs and then in series, the fluid will flow back and forth, resulting in 2 tube passes. Once you understand what the number of pipes means, it becomes easy to comprehend changes in flow rate. As for whether to use the outer diameter or the inner diameter, it depends on whether the overall heat transfer coefficient is referenced to the inner surface or the outer surface; if Ki is used for the overall heat transfer coefficient, then the inner diameter should be used ； If the overall heat transfer coefficient is denoted by Ko, then the outer diameter is used ； Ko is generally used