Finned Tube Bundles

The previous sections covered the principles of heat transfer and structural features of individual finned tubes. This section focuses on finned tube bundles, a topic closer to practical applications of finned tubes. We will use many photos of finned tube heat exchangers to enhance understanding and prepare for future discussions on designing these exchangers.

1. Definition of Finned Tube Bundles:

A finned tube bundle (Finned Tube Bundle) consists of multiple finned tubes arranged systematically to form a heat exchange unit. A single finned tube heat exchanger can comprise one or more finned tube bundles. Understanding finned tube bundles aids in designing and applying finned tube heat exchangers.

2. Components of Finned Tube Bundles:

  • Finned Tubes (Multiple): Basic elements for heat transfer.
  • Headers (or Headers Plates): Boxes or plates connecting the ends of the finned tubes. Once connected, the spacing between the finned tubes is fixed, and headers create continuous flow channels within the tubes.
  • Structure: Supports and fixes the entire finned tube bundle.

Below are two photos of finned tube bundles. The first photo shows headers or bends connecting the finned tubes. The second photo lacks headers but instead arranges the finned tubes systematically. Why? Because this is a heat pipe bundle, which doesn’t require headers, as we will discuss later.

The first bundle has two headers—one for the inlet and one for the outlet, connected by bends.

The finned tube bundle shown below is part of a heat pipe air preheater. It comprises seven rows of tubes without visible headers but requires header plates to position the heat pipes (finned tubes).

3. Arrangement of Finned Tubes:

In a bundle, the arrangement of finned tubes is crucial. There are two main arrangements: staggered and inline. As shown in the following diagram:

  • Staggered Arrangement: Tubes are crisscrossed in the direction of airflow.
  • Inline Arrangement: Tubes are aligned sequentially in the direction of airflow.

For different fin structures like rectangular fins or plate fins, there can also be staggered and inline arrangements.

The arrows indicate the flow direction of fluid outside the tubes, S1 represents the transverse tube spacing, and S2 represents the longitudinal tube spacing.

Advantages and Disadvantages of Inline and Staggered Arrangements:

  • Inline: Smaller disturbance to fluid flow outside the tubes, resulting in lower heat transfer coefficients but reduced pressure drop.
  • Staggered: Greater disturbance to fluid flow outside the tubes, resulting in higher heat transfer coefficients but increased pressure drop.

If there are no strict limitations on pressure drop, staggered arrangement should be preferred. If minimizing pressure drop is critical, an inline arrangement should be chosen.

The sizes of tube spacings S1 and S2 significantly impact heat transfer and pressure drop. These are typically expressed as relative values S1/Db and S2/Db, where Db is the base tube diameter of the finned tube. Sometimes, they are expressed relative to the fin outer diameter Df.

For staggered arrangements, equilateral triangular patterns are commonly used, sometimes isosceles triangular patterns. Below are examples of equilateral triangular spacing used in air coolers.

4. Header Structures:

While the arrangement of tubes (inline or staggered) mainly considers external fluid heat transfer requirements, header design focuses on internal fluid pressure and heat transfer needs. Generally, the following principles apply:

  • High Internal Fluid Pressure: Large-diameter round tubes are typically used as headers, as shown in Figure (a). In boiler applications, round tubes are almost always chosen as headers.
  • Air Cooler Applications: Square headers are preferred, as shown in Figure (b). These allow connections to multiple rows of finned tubes. When dealing with steam condensation, large steam spaces are required, necessitating headers connected to multiple rows of tubes, as shown in Figure (c).
  • Significant Temperature Difference Between Inlet and Outlet: Headers may deform due to differential thermal expansion. In such cases, segmented headers are recommended, as shown in Figure (d).
  • First and Last Rows: Headers must connect the first and last rows directly. Other rows should ideally be connected via individual bends. Advantages include:
    • Improved heat transfer efficiency due to preventing mixing of fluids between rows.
    • Reduced fluid flow resistance by maintaining consistent cross-sectional areas.
    • Bends absorb thermal expansion deformation.

 

Here are a few photos of finned tube bundles with comments:

  • Typical Finned Tube Bundle: First and last rows use large-diameter round headers, while intermediate rows use individual bends. Excellent!
  • Equipment with Seven Rows: Each row contains ten tubes, divided into two passes, totaling 14 passes. Each pass has a draw-off tube connected via bends to other passes. Multiple passes might be due to small fluid flow rates.
  • Beautiful Finned Tube Bundles: Consist of four rows with round headers at the inlets and outlets, and individual bends elsewhere. This is a rational and standard bundle structure.
  • Square Headers: Both bundles use square headers, indicating relatively low internal fluid pressure. Fluid enters and exits on opposite sides, possibly for heating applications?
  • Clearly Divided Passes: Each row uses square headers to create two passes, connected via thick bends.

The photo above shows a finned tube bundle for an air cooler. It includes square headers, round headers, individual bends, and connections using headers.