According to the small tube of the finned tube, a heat exchanger composed of a common circular tube (light tube) is known. In many cases, the heat transfer coefficient of the fluid outside the tube and the fluid inside the tube is different. The heat transfer coefficient refers to the heat exchange amount per unit heat exchange area and unit temperature difference (temperature difference between fluid and wall surface), which represents the heat exchange capacity between the fluid and the wall surface. E.g:
The heat transfer coefficient when water condenses on the wall surface is: 10000—20000 w/(m2.°C)
The heat transfer coefficient when water boils on the wall is: 5000----10000 ------
The heat transfer coefficient of water flowing through the wall is approximately: 2000---10000 ------
The heat transfer coefficient of air or smoke flow through the wall is: 20---80 --- ---
The heat transfer coefficient of natural convection of air is only: 5---10 -------
It can be seen that the difference in the heat exchange capacity between the fluid and the wall surface is very different.
Next, imagine an actual heat transfer situation: the inside of the round pipe is flowing water, the heat transfer coefficient is 5000 (---), and the flow outside the pipe is flue gas, and its heat transfer coefficient is only 50 (---) The difference between the two is 100 times. Where does the “bottleneck” or “maximum drag” of the heat transfer process occur when heat is transferred from the inside of the tube to the outside of the tube or from outside the tube to the inside of the tube? Of course, the flue gas side outside the tube, because the flue gas side heat transfer coefficient, that is, the lowest heat transfer capacity, limits the increase in heat transfer.
Here, let's take an example of a series resistor: in a series circuit composed of multiple resistors, if one of the resistors is much larger than the other resistors, the resistor will constitute the "bottleneck" of the current, only to reduce the The maximum resistance of the term can effectively increase the current flowing through the series circuit. The same is true for the heat transfer process described above.
How can we increase the heat transfer of a circular tube? One of the most effective methods is to use an extended surface on the outer surface of the tube, that is, the flue gas side, that is, to form a finned tube. Assume that the actual heat transfer area of the finned tube is several times the outer surface area of the original light pipe. Although the heat transfer coefficient of the flue gas is still very low, the heat transfer effect reflected on the outer surface area of the light pipe will be greatly increased, thereby making the whole The heat transfer process is enhanced, and the metal consumption of the device is reduced and the economy is improved when the total heat transfer amount is constant.