Similar to traditional shell-and-tube heat exchangers, U-tube heat exchangers also follow the principle of indirect heat exchange: Two fluids at different temperatures flow in the tube side (inside the U-tube) and the shell side (inside the shell outside the U-tube), respectively. Heat is transferred from a high-temperature fluid to a low-temperature fluid through the wall of the U-shaped tube, achieving process objectives such as heating, cooling, condensation, or evaporation.