What is a Finned Tube Bundle?
1. What is a Finned Tube Bundle?
A finned tube bundle is a heat exchange unit formed by arranging multiple finned tubes in a specific pattern. A finned tube heat exchanger may consist of one or more finned tube bundles.
2. Composition of the finned tube bundle
1. Finned tubes (multiple units) are the fundamental of the heat transfer elements.
2. Tube header (manifold) or tube sheet:These are the boxes, bent tubes, or steel plates that connect both ends of the finned tubes.
Once the finned tubes are connected to the tube header or tube sheet, the spacing between the finned tubes is fixed. Additionally, and the tube header creates a continuous flow path for the fluid inside the tubes.
3. Frame: This supports and secures the entire fin tube bundle.
Please note that for heat tube bundles, heat tubes do not require tube header.
The first tube bundle on the left has two tube headers, one serving as the inlet box and the other as the outlet box, which are connected by a bend tube in between.
The finned tube bundles shown in the figure below is part of a heat tube air conditioner. Although there are no tube header at the ends, but tube sheets are essential to secure and position each heat tube (finned tube).
3. Arrangement of Finned Tubes
In a tube bundle, the selection of the finned tube arrangement is crucial. There are two types of arrangements: in-line (aligned) and staggered , as shown in the figure below.
Staggered arrangement refers to the tubes being arranged in a crossing pattern along the airflow direction, while in-line arrangement refers to the tubes being aligned sequentially along the airflow direction.
The following diagram illustrates that for finned tubes with different fin structures, such as rectangular fins or integral plate-like fins (also known as plate fins), staggered and in-line arrangements similarly apply.
The arrows in the diagram represent the flow direction of the fluid outside the tubes, S1 denotes the transverse tube spacing, and S2 denotes the longitudinal tube spacing.
Advantages and disadvantages of in-line and staggered arrangements:
In-line arrangement: When fluid flows around the tubes externally, it experiences relatively low disturbance, resulting in a lower heat transfer coefficient. However, its advantage is lower flow resistance.
Staggered arrangement: When fluid flows around the tubes externally, it experiences greater disturbance, leading to a higher heat transfer coefficient. However, its disadvantage is higher flow resistance.
When there are no strict limitations on pressure drop, the staggered arrangement should be preferred. When a very low pressure drop is required, the in-line arrangement should be selected.
The sizes of the tube spacings S1 and S2 also significantly impact heat transfer and flow resistance. They are typically expressed using the relative values S1/Db and S2/Db, where Db is the diameter of the finned tubes base tube. Sometimes, ratios relative to the fin outer diameter Df may also be used. For staggered tube bundles, an equilateral triangular arrangement is often employed, while an isosceles triangular arrangement is sometimes used as well.
4. Structure and Configuration of the Tube Header
If the arrangement pattern of the tube bundle (in-line or staggered, and the selection of tube spacing) is primarily determined by considering the heat transfer requirements of the external fluid, then the form and structure of the tube header are mainly determined by considering the pressure and heat transfer requirements of the internal fluid.
Generally, the following principles should be followed:
(1) If the fluid pressure inside the tubes is high, large-diameter circular tubes are typically selected as the tube header, as shown in Figure (a). For example, in boiler applications, circular tubes are almost exclusively used as tube headers.
(2) In air cooler applications, square-shaped headers are preferred, as shown in Figure (b).
The advantage of square headers is their ability to connect multiple rows of finned tubes simultaneously.
When steam is condensing inside the tubes, a large steam space is required, making it necessary for one tube header to connect to multiple rows of tubes, as illustrated in Figure (c).
(3) When there is a significant temperature difference between the inlet and outlet fluids inside the tubes, the tube header may deform due to differential thermal expansion of the tube rows. In such cases, a segmented tube header is recommended, as shown in Figure (d).
(4) Apart from the first and last rows of the tube bundle, which must be connected using corresponding tube headers, it is preferable to connect the other rows individually with U-bends. The advantages of this approach are:
A. It improves heat transfer efficiency. Theoretical studies have shown that one-to-one connections prevent fluid mixing between different tube rows, as fluid mixing reduces the temperature driving force and heat transfer efficiency.
B. It reduces fluid flow resistance. One-to-one connections maintain a constant flow cross-sectional area, avoiding repeated expansion and contraction of the fluid.
C. U-bends can "absorb" deformation caused by thermal expansion.