For the above example: the convective heat transfer coefficient on the water side of the tube is about 5000, while the
convective heat transfer coefficient on the air side is about 50, and the difference between the two is 100 times. Due to the heat exchange on the air side the “capacity” is much lower than the water side, which limits the performance of the heat exchange “capacity” of the water side and makes the air the side becomes the “bottleneck” of the heat transfer process, limiting the increase in heat transfer.
To overcome the air side for the bottleneck effect, adding fins on the outer surface of the air side will be the most sensible choice.
After installing the fins, the original heat transfer area on the air side has been greatly expanded to make up for overcome the shortcomings of low heat transfer coefficient on the air side, the heat transfer is greatly improved.
The effect of adding fins can also be illustrated by the following more vivid example:
The immigration department at the border port assumes that Party A’s port has ten inspection ports and can be released every hour
5000 people, but there is only one ticket gate at Party B’s port, and it is very slow. It can only be released every hour.
Line 50 people. In this way, Party B’s side becomes a bottleneck for passenger clearance, making Party A’s “ability”
Can’t play. In order to increase the flow of customs clearance, the most effective way is to open a few more on the side of Party B.
Inspection port. This is the same as the principle of installing fins.
When should finned tubes be used for heat exchange?
After understanding the principle and function of finned tubes, where to choose finned tubes, there are the following principles:
(1) If the heat transfer coefficients on both sides of the tube are very different, the heat transfer coefficient should be small
Fins are installed on one side.
Example 1: The boiler economizer has water in the tube and flue gas out of the tube, and fins should be used on the flue gas side.
Example 2: Air cooler, liquid is in the tube, air flows out of the tube, fins should be added to the air
Example 3: In the steam generator, water is boiled inside the tube, and flue gas flows outside the tube. Fins should be added to the
Smoke side. It should be noted that in the design, the side with the smaller heat transfer coefficient should be placed outside the tube as much as possible to facilitate the installation of fins.
(2) If the heat transfer coefficients on both sides of the tube are very small, in order to enhance heat transfer, the same
When installing fins, if there are structural difficulties, no fins can be added on both sides. under these circumstances,
If only fins are added to one side, there will be no obvious effect on the increase in heat transfer.
Example 1: The traditional tubular air preheater uses air inside the tube and flue gas outside the tube. because
It is the heat transfer of gas to gas, the heat transfer coefficients on both sides are very low, and it is difficult to add fins in the tube, so I had to use a bare tube.
Example 2: Heat pipe air preheater, although the flue gas still heats the air, but due to the flue gas and
The air flows outside the tube, so finned tubes can be conveniently used on both the flue gas side and the air side, so that
the heat transfer is greatly increased.
(3) If the heat transfer coefficients on both sides of the tube are large, there is no need to use finned tubes.
Example 1: Water/water heat exchanger, when hot water is used to heat cold water, the heat transfer coefficients on both sides are sufficiently high, there is no need to use finned tubes. But in order to further enhance heat transfer, threaded pipe or
the corrugated tube replaces the light tube.
Example 2: In the condenser of a power plant, water vapor is condensed outside the tube, and water flows inside the tube. On both sides.
The heat transfer coefficient is very high, under normal circumstances, no need to use finned tubes.
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