While the layout configuration of the tube bundle (in-line or staggered arrangements, as well as tube pitch selection) is primarily determined based on the shell-side (external) fluid thermal requirements, the type and structure of the headers (tube boxes) are predominantly governed by the tube-side (internal) fluid pressure and heat transfer demands.

In engineering practice, the following design principles are generally observed:

(1) High-Pressure Applications: If the tube-side fluid is under high pressure, a large-diameter cylindrical pipe is typically utilized as the cylindrical header (pipe header), as illustrated in Figure (a). For instance, in boiler applications, cylindrical headers are almost exclusively selected.

(2) Air-Cooled Exchangers (ACEs): In air-cooled heat exchanger applications, rectangular headers (box headers) are preferred, as shown in Figure (b). The primary advantage of a box header is its capability to connect multiple rows of finned tubes simultaneously. Furthermore, when the tube-side medium undergoes vapor condensation, a substantial vapor space is required, making it essential for a single header to interface with multiple tube rows, as depicted in Figure (c).

(3) Thermal Expansion & Differential Stress: When there is a significant temperature difference between the tube-side fluid inlet and outlet, the header may undergo structural distortion due to differential thermal expansion between the tube rows. Under such operating conditions, a split header (decomposed header) should be adopted, as illustrated in Figure (d).

(4) U-Bend / Return Bend Connections: Except for the first and last rows of the tube bundle, which must be connected to their respective inlet/outlet headers, the intermediate rows are ideally interconnected on a one-to-one basis using return bends (U-bends). This configuration offers the following engineering advantages:

• A. Maximized Thermal Efficiency: Thermodynamic theory demonstrates that one-to-one bend connections prevent fluid cross-mixing (maldistribution) between different tube rows, which would otherwise degrade the logarithmic mean temperature difference (LMTD) and overall heat transfer efficiency.

• B. Reduced Pressure Drop: One-to-one return bends maintain a constant cross-sectional flow area, effectively eliminating the fluid resistance caused by continuous expansion and contraction losses within a common header space.

• C. Thermal Stress Relief: The geometric flexibility of return bends allows them to act as expansion loops that safely absorb the structural deformations induced by thermal expansion.