In a heat exchanger composed of ordinary steel pipes (smooth tubes), in many cases, the heat transfer coefficient of the fluid outside the tube and the fluid inside the tube to the tube wall is different. The so-called heat transfer coefficient refers to the heat transfer amount per unit heat transfer area and unit temperature difference (the temperature difference between the fluid and the wall). It represents the heat transfer capacity between the fluid and the wall. For example: The heat transfer coefficient when water condenses on the wall is: 10000-20000w/(m2. ℃) The heat transfer coefficient when water is boiling on the wall is: 5000----10000---- when the water flows through the wall The heat transfer coefficient is approximately: 2000----10000---- The heat transfer coefficient when air or flue gas flows through the wall is: 20----80---- The heat transfer coefficient when air is naturally convective is only: 5----10----
It can be seen that the difference in heat transfer capacity between the fluid and the wall is very different. Next, imagine an actual heat transfer situation: the inside of the tube is full of flowing water, its heat transfer coefficient is 5000 (----), and the outside of the tube is flue gas, its heat transfer coefficient is only 50 (-- --), the difference between the two is 100 times. When heat is transferred from the inside of the tube to the outside or from the outside to the inside of the tube, where does the "bottleneck" or "maximum resistance" of the heat transfer process occur? Of course, it is the flue gas side outside the tube, because the heat transfer coefficient of the flue gas side, that is, the lowest heat exchange capacity, limits the increase in heat transfer.
How can we increase the heat transfer of the round tube? One of the most effective methods is to use an extended surface on the outer surface of the tube, that is, on the flue gas side, that is, to make a finned tube. Assuming that the actual heat transfer area of the finned tube is several times the outer surface of the original smooth tube, although the heat transfer coefficient of the flue gas is still very low, the heat transfer effect reflected on the outer surface of the smooth tube will be greatly increased, so that the entire The heat transfer process is enhanced, and under the condition of a certain total heat transfer, the metal consumption of the equipment is reduced and the economy is improved.