A heat exchanger composed of finned tubing.

Finned Tube----Finned tube heat transfer principles and selection principles

Finned tubes, also known as fin tubes or finned tubes, the English name is "Fin Tube" or "Finned Tube", also sometimes called "Extended Surface Tube", which extended surface tube. As the name implies, is a finned tube on the original surface of the tube (irrespective of the outer surface or inner surface) of processing of a number of fins, the original surface is extended to form a unique heat transfer components.

Why use finned tubes? Processing of the original surface of the fin can play what role? To answer this question, but also from some of the basic principles of heat transfer process to start.

First,To introduce a heat transfer on the definition: solid surface in contact with the fluid, and it is called convection heat transfer between the heat exchanger.

We are most familiar with is the convective heat radiator and air between the outer surface of the heat exchanger. Life experience tells us: Radiators area is larger, the higher the surface temperature (i.e., surface temperature and the greater the temperature difference between the air), the longer the heating time, the greater the heat exchange,

Room more warm. This shows that heat and convective heat transfer area is proportional to the temperature difference and the proportional and proportional to the time. To compare the strength of convective heat transfer case, we need to define a physical quantity: called "heat transfer coefficient."

Is the heat transfer coefficient per unit area and temperature (the temperature difference between the wall and the fluid), the convective heat transfer per unit time. The unit is J / (s. m. ℃) or W / (m. ℃). Convective heat transfer coefficient h is commonly used symbols.

Heat transfer coefficient depends mainly on the following factors:

 the type of fluid and physical properties: such as water and air is totally different, the heat transfer coefficient vary considerably;

 whether the process fluid in the heat exchanger phase change, that is, whether the boiling or condensation occurs. If the phase transition occurs, then it will greatly enhance the heat transfer coefficient;

 also, and fluid velocity and solid surface shape. And so on.

The size of the convective heat transfer coefficient is mainly through experimental studies to determine, given below a set of common cases

Range of values:

 water vapor condenses: h = 10000 --- 20000 W / (m * ℃)

 boiling water: h = 7000 --- 10000,,,,,

 water convection: h = 3000 --- 5000,,,,

 forced convection of air or gas: h = 30 --- 80,,,,,

 natural convection of air or gas: h = 3-10

,,,,,

Thus,

The heat transfer coefficient under different circumstances the difference is huge. Keep in mind that the range of values of the heat transfer coefficient, which is the understanding and subsequent selection of finned tubes is a great useful.

The following will discuss a concrete example of the heat transfer device:

With a heat exchanger with hot water heated air, hot water flowing in the pipe, the air flow in the tube. Such as heating with hot air curtain or automobile radiator (radiator) belong to this type of heat transfer that heat hot water to pass through the wall of the tube cold fluid - air. Thus, the heat transfer process with the partition of the two sides of convective heat transfer process tightly linked.

For the above example: pipe water side convective heat transfer coefficient is approximately 5000, while the tube air-side heat transfer coefficient is about 50, a difference of 100 times. As the air-side heat exchanger "capacity" is far below the water side, which limits the water side heat exchanger "capacity" of the play, such that the air-side heat transfer process in a "bottleneck", limiting the increase in heat transfer. In order to overcome the air side of the "bottleneck" effect, in the installation of air-side outer surface of the fin would be a wise choice. After the installation of the fins, the air-side heat transfer area of the original has been greatly expanded, Mi fill the air-side heat transfer coefficient is low, and makes heat transfer greatly improved, as shown in the following figures.

The role of the fins on the installation can also use the following example to illustrate more vividly: border crossings in an immigration office, assuming there are ten ports Party Inspection mouth, release 5,000 people per hour, while the B port is only one wicket mouth, and run very slow release of 50 persons per hour only. Thus, the B-side passenger clearance has become a bottleneck, making the party's "ability" can not play. In order to improve customs clearance flow, the most effective way is to open a few B-side inspection port. This principle is the same as the installation of the fins.

Finned tubes in the understanding of the principles and the role of the future, and in what situations selection finned tubes, are the following principles:

(1)

If both sides of the heat transfer coefficient of the tube a big difference, you should install the side heat transfer coefficient is small fins.

Example 1: boiler economizer tube walking through water, drain pipe flue gas side fins should be used.

Example 2: air coolers, tube go liquid, drain the air tube, the fins should be added to the air side.

Example 3: steam generator tube is water boiling, take the gas tube, fins should be added in the flue gas side.

It should be noted, in the design, the heat transfer coefficient should be placed on the side of the tube in order to facilitate the installation of the fins.

(2) If the tube side heat transfer coefficient is very small, in order to enhance heat transfer fin should be installed on both sides at the same time, if the structural difficulties, the two sides can not add fins. In this case, if only the side add fins to increase heat transfer effect is not obvious.

Example 1: The traditional tubular air preheater, take the air tube, the tube take the fumes. Because it is a gas on gas heat exchanger, the heat transfer coefficient is very low on both sides, plus finned tube very difficult, had run out of the tube.

Example 2: heat pipe air preheater, although still smoke heated air, but the gas and air are flowing in the tube, so the gas side and air side can easily adopt finned tube, so that the heat transfer greatly increased.

(3) If the tube side heat transfer coefficient is large, it is not necessary to use finned tubes.

Example 1: water / water heat exchanger, heated with hot water, the two sides are sufficiently high heat transfer coefficient, it is not necessary to use the finned tubes.

However, in order to further enhance the heat transfer, can be used instead of threaded pipe or bellows fluorescent tubes.

Example 2: plant condenser tube condensation of water vapor, the water pipe to go. Both sides of the heat transfer coefficients are high, in general, without the use of finned tube.

Key Works: Finned Tube, Fin Tubes, Tubos Aletados, heat transfer

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