Characteristics of gas flow distribution in the shell side of a scaled tube heat exchanger

Thread Content

Over 10 years ago, while working in a factory, I encountered a situation where the heat transfer performance of the coil tube heat exchanger did not meet the required standards. The intermediate heat exchange was modified, and the designed overall heat transfer coefficient was 31.5; at the beginning of operation it was around 25, but later it dropped to 12! The designer of the aforementioned scaled tube heat exchanger also came twice to inspect the SO2 bypass of the intermediate heat and cold exchangers (no problems were found). It was later found that the gas flow distribution in the shell side of the scaled tube heat exchanger was uneven; the heat transfer load of the tube banks near the center was low, while that of the tube banks near the heat exchanger shell was higher. An important detail is that all the tubes were arranged on the fin plate of the scale tube heat exchanger at that time; there was no baffle design in the shell side (thus the shell-side resistance was low), and the inlet and outlet gases on the shell side were both arranged circumferentially. 1. Gases are fluids and tend to take the shortest path. For the SO2 gas in the shell side, the path through the tube bundle near the heat exchanger shell is shorter, while the path through the tube bundle near the center is longer. Therefore, the flue gas flow rate in the shell side of the tube bundle near the heat exchanger shell is higher. 2. For the gas to flow through the central tube bundle, it is necessary to wash the tube bundle laterally; however, the resistance to such lateral washing is high, which makes it difficult for the gas in the shell side to reach the central tube bundle. 3. As the gas passes through the tube bundles near the cylinder via lateral flushing and reaches the central tube bundle, it is also heated; as a result, the heat transfer temperature difference in the central tube bundle decreases. Taking these three factors into account, in a ceiling-mounted fully-piped shell-and-tube heat exchanger, the heat transfer load of the central tube bank is much lower than that of the tube banks located near the heat exchanger shell, and the wall temperature of the central tube bank is relatively higher as well. When a scaled tube heat exchanger is used for cooling, condensate acid will precipitate on the inner wall of the tubes because the wall temperature in the tube side near the lower tube sheet is below the flue gas dew point. When the aforementioned condensed acid comes into contact with catalyst powder, it adheres to it, and over time this can cause blockages in the pipes. The central tube bank is less prone to clogging due to its relatively high wall temperature, whereas the tube bank near the heat exchanger shell has a lower wall temperature and is more susceptible to clogging. When the aforementioned tube bundle becomes blocked, the shell side is not blocked simultaneously, but SO2 gas still flows through it, acting as a bypass. This is the main reason why the heat transfer efficiency of the scaled tube heat exchanger drops significantly after blockage occurs. Back then, it was suggested to the heat exchanger designer that some of the tube bundles should be removed and a passage created in order to improve the airflow distribution around the central tube bundle in the shell side. A few years later, I happened to attend a sulfuric acid industry conference, where this designer talked at length about multi-channel flare tube heat exchangers.