What are the selection criteria for heat exchanger tubes and their structural parameters

1. Selection of heat exchanger tube structural parameters: Heat exchanger tubes can be plain tubes, threaded tubes, spiral grooved tubes, etc. The following factors should be considered when selecting heat exchanger tubes:
(1) Tube diameter of heat exchanger tubes: Smaller tube diameters result in more compact and cheaper heat exchangers, and can achieve a better ratio of heat transfer coefficient to resistance coefficient. However, smaller tube diameters lead to greater pressure drop. Generally, φ19mm heat exchanger tubes are recommended while meeting allowable pressure drop requirements. For fluids prone to scaling, φ25mm outer diameter heat exchanger tubes are used for easier cleaning. For process fluids with gas-liquid two-phase flow, larger tube diameters are generally selected. For example, in reboilers and boilers, φ32mm and φ51mm diameter tubes are commonly used. Heat exchanger tubes directly heated by fire often use φ76mm diameter tubes. 
(2) Length of heat exchanger tubes: In the absence of phase change heat transfer, a longer heat exchanger tube increases the heat transfer coefficient. For the same heat transfer area, a longer tube results in a smaller flow cross-sectional area, higher flow velocity, and fewer tube passes, thus reducing the number of bends in the heat exchanger and consequently lowering the pressure drop. Furthermore, the cost per square meter of heat transfer surface area is lower with longer tubes. However, excessively long heat exchanger tubes pose manufacturing difficulties. Therefore, a tube length of 4-6 m is generally selected. For heat exchangers with large heat transfer areas or no phase change, a tube length of 8-9 m can be used.
(3) Arrangement and center-to-center distance of heat exchanger tubes: The arrangement of heat exchanger tubes in the tube. There are two main tube arrangement patterns on the heat exchanger plate: square and triangular. Triangular arrangements are beneficial for achieving turbulent flow in the shell side and allow for a larger number of tubes. Square arrangements are beneficial for shell-side cleaning. To compensate for their respective drawbacks, square arrangements rotated at a certain angle (i.e., transposed square arrangements) and triangular arrangements with cleaning channels have been developed. Concentric circle arrangements are less commonly used and are generally employed in small-diameter heat exchangers. The tube spacing is the distance between the centers of two adjacent tubes. Smaller tube spacing results in a more compact device but leads to thicker tube sheets, more difficult cleaning, and increased shell-side pressure drop. Therefore, a range of (1.25~1.5) is generally selected. do (do is the outer diameter of the tube).

2. Selection of the Number of Tube Passes and Shell-Side Type of Heat Exchanger Tubes
Heat exchanger tubes have 1 to 8 passes, with 1, 2, or 4 passes being commonly used. Increasing the number of tube passes increases the flow velocity inside the tubes and also increases the heat transfer coefficient. However, the flow velocity inside the heat exchanger tubes is limited by factors such as the pressure drop across the tubes. Commonly used flow velocities in industrial production are as follows: Water and similar fluids generally have a velocity of 1 to 2.5 m/s; for cooling water in large condensers, the velocity can be increased to 3 m/s. Gas and steam velocities can be selected within the range of 8 to 30 m/s.

Shell-side types can be broadly classified as follows:
Single-shell heat exchanger: The shell-side can be... Various types of baffles are added primarily to increase fluid velocity and enhance heat transfer. This is the most commonly used type of heat exchanger. In vacuum operation with single-component condensation, the tubes can be moved to the center of the shell.
A two-shell heat exchanger with longitudinal baffles: This increases shell-side flow velocity, improves thermal efficiency, and is less expensive than two heat exchangers in series.
A split-flow heat exchanger: It is suitable for high flow rates and low pressure drop requirements. The baffles can be perforated plates when used as condensers.
A double-split-flow heat exchanger: It is suitable for low pressure drop, when one fluid's temperature change is very small compared to the other, and for situations with large temperature differences or high tube-side heat transfer coefficients.