Principle of finned tube heat exchanger

Principle of finned tube heat exchanger

Finned tube, also known as finned tube or finned tube, English name is “fin tube” or “found tube”, also sometimes called “extended surface tube”, namely extended surface tube. As the name suggests, finned tube is to process many fins on the surface of the original tube (no matter the outer surface or the inner surface), so that the original surface can be expanded to form a unique heat transfer element.

So why use finned tube heat exchanger? Compared with other kinds of heat exchanger, what is the difference between finned tube?

Because the heat transfer between the solid surface and the fluid in contact with it is called convective heat transfer. We are most familiar with the convective heat transfer is the heat transfer between the outer surface of the radiator and the air.

Life experience tells us: the larger the radiator area, the higher the surface temperature (that is, the greater the temperature difference between the surface temperature and the air), and the longer the heating time, the greater the heat exchange and the warmer the room. This shows that convective heat transfer is directly proportional to heat transfer area, temperature difference and time. In order to compare the strength of convective heat transfer under different conditions, we need to define a physical quantity called “heat transfer coefficient”. Heat transfer coefficient refers to unit area, unit temperature difference (temperature difference between wall and fluid) and convective heat transfer per unit time. The unit is J / (s.m2. ℃) or w / (m2. ℃).

The heat transfer coefficient mainly depends on the following factors:

1. Types and physical properties of fluids: for example, water and air are quite different, and their heat transfer coefficients are quite different;

2. Whether phase change occurs in the process of heat transfer, that is, whether boiling or condensation occurs. If phase transition occurs, the heat transfer coefficient will be greatly increased;

3. It is also related to the velocity of fluid and the shape of solid surface. wait.

4. The size of convective heat transfer coefficient is mainly determined by experimental research.

A set of numerical ranges under common conditions are given as follows:

Condensation of water vapor: H = 10000-20000 w / (M2 * ℃)

Boiling of water: H = 7000-10000 w / (M2 * ℃)

Convection of water: H = 3000-5000 w / (M2 * ℃)

Forced convection of air or flue gas: H = 30-50 w / (M2 * ℃)

Natural convection of air or smoke: H = 3-5 w / (M2 * ℃)

It can be seen that the difference of heat transfer coefficient under different conditions is very huge. Please keep in mind the value range of the above heat transfer coefficient, which is very useful for the understanding and selection of finned tubes in the future.

A specific example of heat transfer equipment will be discussed as follows:

There is a heat exchanger that uses hot water to heat the air. Hot water flows in the pipe and air flows outside the pipe. For example, the hot air curtain for heating or the radiator on the car belong to this type of heat transfer, that is, the heat of hot water is transferred to the cold fluid air outside the pipe through the pipe wall. It can be seen that the heat transfer process is closely related to the two convective heat transfer processes on both sides of the partition wall.

For the above example, the convective heat transfer coefficient of the water side inside the tube is about 5000, while the convective heat transfer coefficient of the air side outside the tube is about 50, with a difference of 100 times. Because the “capacity” of the air side is far lower than that of the water side, the “capacity” of the water side is limited, which makes the air side become the “bottleneck” of the heat transfer process and limits the increase of heat transfer. In order to overcome the “bottleneck” effect on the air side, it is a more sensible choice to install fins on the outer surface of the air side. After the installation of fins, the original heat transfer area of the air side has been greatly expanded, which makes up for the low heat transfer coefficient of the air side and greatly improves the heat transfer, as shown in the following figure:


a: B: after adding fins, the bottleneck is eliminated and the heat flux is increased

The function of adding fins can also be illustrated by the following more vivid examples:

“At the exit and entry point of a border port, it is assumed that Party A’s port has ten inspection ports, which can release 5000 people per hour, while Party B’s port has only one ticket gate, which is very slow, and can only release 50 people per hour. In this way, Party B’s side has become the bottleneck of passenger clearance, making Party A’s “ability” unable to play. In order to improve the flow of customs clearance, the most effective way is to open more inspection ports on Party B’s side. This is the same principle as adding fins. “

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