finned, fin tube, finned tube radiator

Basic knowledge of finned tube heat exchanger

In the heat exchanger, the heat transfer coefficients of the fluids on both sides of the heat transfer are often unbalanced. Usually, we add fins on the side with the smaller heat transfer coefficient.

finned, fin tube, finned tube radiator

What is a finned tube?

As the name suggests, the finned tube is to process a lot of fins on the surface of the original tube (whether the outer surface or the inner surface), so that the original surface is expanded to form a unique heat transfer element.

Why use finned tubes?

What is the function of machining fins on the original surface?

As the name suggests, the finned tube is to process a lot of fins on the surface of the original tube (whether the outer surface or the inner surface), so that the original surface is expanded to form a unique heat transfer element. 

The structure of the finned tube heat exchanger is basically the same as that of the general shell and tube heat exchanger, except that the finned tube is used instead of the light tube as the heat transfer surface. This makes its structure more compact, the heat exchange area increases, and the heat exchange can be enhanced.

When should finned tubes be used?

There are several principles:
(1) If the heat transfer coefficients on both sides of the tube are very different, fins should be installed on the side with the smaller heat transfer coefficient.

Example 1: Boiler economizer, water runs in the tube, flue gas flows out of the tube, and fins should be used on the flue gas side.

Example 2: For an air cooler, the liquid flows inside the tube and the air flows out of the tube, and the fins should be added on the air side.

Example 3: Steam generator, the water is boiling inside the tube, the flue gas goes outside the tube, and the fins should be added on the flue gas side.
It should be noted that in the design, the side with the small heat transfer coefficient should be placed outside the tube to facilitate the installation of fins.

(2) If the heat transfer coefficient on both sides of the tube is very small, in order to strengthen the heat transfer, fins should be installed on both sides at the same time. In this case, if fins are only added on one side, the increase in heat transfer will not have a significant effect.

Example 1: A traditional tubular air preheater, with air inside the tube and flue gas outside the tube. Because it is gas-to-gas heat transfer, the heat transfer coefficients on both sides are very low, and it is very difficult to add fins to the tube, so the tube has to be used.

Example 2: Heat pipe type air preheater, although the flue gas heats the air, the flue gas and air flow outside the tube, so the finned tube can be conveniently used on both the flue gas side and the air side to make the transmission Heat is greatly increased.

(3) If the heat transfer coefficients on both sides of the tube are large, it is not necessary 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 high enough, so there is no need to use finned tubes. However, in order to further enhance heat transfer, threaded or corrugated pipes can be used instead of bare pipes.

Example 2: Condenser in a power plant, water vapor is condensed outside the tube, and water flows inside the tube. The heat transfer coefficients on both sides are very high, and in general, finned tubes are not required.
Finned tube bundle

1 What is a finned tube bundle?
The heat exchange unit composed of multiple finned tubes arranged according to a certain rule is called a finned tube bundle. A finned tube heat exchanger can consist of one or more finned tube bundles.

The structural composition of the 2 finned tube bundle includes?
Finned tube (multiple): the basic element of heat transfer.
Tube box (header) or tube sheet: a box, elbow or steel plate connecting both ends of the finned tube. After the finned tubes are connected to the box or tube sheet, the spacing between the finned tubes is fixed, and at the same time, the tube box forms a continuous flow channel for the fluid in the tube.
Framework: enables the entire finned tube bundle to be supported and fixed.

3 How are the finned tubes arranged?
In a tube bundle, the selection of the arrangement of finned tubes is very important. There are two arrangements: cross row and in-line row.

The so-called cross permutation refers to the cross arrangement of pipes in the air flow direction, while the in-line permutation refers to the sequential arrangement of pipes in the air flow direction.

The figure below shows that for finned tubes with different fin structures, such as rectangular fins or integral plate fins , there are also cross row and in-line row.

The arrows in the figure represent the flow direction of the fluid outside the tube, S1 represents the transverse tube spacing, and S2 represents the longitudinal tube spacing.

Advantages and disadvantages of cross row and in-line row:

Cross row: when the fluid flows around the pipe, it is subject to less disturbance and low heat transfer coefficient, but the advantage is low resistance.

In-line row: The disadvantage is that when the fluid is disturbed by the external flow, the resistance around the in-line row is high.

When there is no strict limit on the resistance drop, the fork row arrangement should be preferred; When the required resistance drop is very small, the sequential arrangement scheme shall be selected.

  1. Structural form of pipe box

If the arrangement form of the tube bundle (in-line or cross row, and the selection of tube spacing) is mainly determined by considering the heat exchange requirements of the fluid outside the tube, the form and structure of the tube box mainly consider the pressure and heat exchange requirements of the fluid inside the tube.

Generally, the following principles shall be followed:

  • If the fluid pressure in the pipe is high, the large-diameter circular pipe is generally selected as the pipe box.

For example, in boiler applications, almost all round tubes are selected as tube boxes.

(2) In the application of air cooler, I like to use square box.

The advantage of the square box is that multiple rows of fin tubes can be connected at the same time. When there is steam condensation in the pipe, a large steam space is required, and it is necessary to connect one pipe box with multiple rows of pipes.

(2) When the inlet and outlet temperatures of the fluid in the pipe vary greatly, the pipe box may be deformed due to different thermal expansion of the pipe row. At this time, the decomposition pipe box should be used.

(a) Thermal deformation of multi-pass tube bundles (b)Separate tube box

(4) Except that the first and last rows of tube bundles must be connected by corresponding pipe boxes, other rows are best connected one-to-one by elbows.

Advantages:

  1. Improve heat exchange efficiency. The theory shows that one-to-one connection can avoid the mixing of fluid in each row of pipes, and the mixing of fluid reduces the heat transfer temperature difference and heat transfer efficiency.
  2. Can reduce the flow resistance of fluid. Because the one-to-one connection ensures the constant flow cross-sectional area and avoids the continuous expansion and contraction of the fluid.
  3. the bend can “absorb” the deformation caused by thermal expansion.

Example :

This is a typical finned tube bundle. The first and last rows adopt large-diameter circular tube boxes, while each row in the middle adopts one-to-one elbow connection.

This equipment has seven rows of fin tubes, ten tubes in each row, divided into two tubes, a total of 14 tubes. Each pipe side has an outlet pipe, which is connected with other pipe sides through elbow. The reason why so many pipe passes are selected may be that the flow of fluid in the pipe is too small.

This group of finned tube bundles is composed of four rows of tubes. Except for the round tubular inlet and outlet pipe boxes, others are connected by one-to-one elbows. It is a reasonable and standard tube bundle structure.

These two groups of finned tube bundles are square tube boxes. It seems that the fluid pressure in the tube is not too high.

Obviously, each row of finned tubes of this equipment is divided into two tube sides with a square tube box, and each tube side is connected with a thick elbow.

The figure above shows the finned tube bundle of a group of air coolers. It can be seen that there are square pipe boxes, round pipe boxes, one-to-one elbow connections, and some are all connected with pipe boxes.

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