Two fluids undergo heat exchange within the tubular heat exchanger: one flows inside the tubes, its path being called the tube side; the other flows outside the tubes, its path known as the shell side. The wall of the tube bundle constitutes the heat transfer surface. To enhance the heat transfer coefficient of the fluid outside the tubes, a certain number of transverse baffle plates are typically installed within the shell. These baffles not only prevent short-circuiting of the fluid, increase its velocity, but also force the fluid to cross the tube bundle multiple times in a prescribed path, greatly increasing turbulence. The commonly used baffles are crescent-shaped and disc-shaped, with the former being more widely applied. Each time the fluid passes through the tube bundle once is termed a tube pass, and each time it passes through the shell once is termed a shell pass. To increase the fluid velocity inside the tubes, appropriate baffles can be set in the ends of the headers to evenly divide all the tubes into several groups. This way, the fluid can pass through only a portion of the tubes each time and traverse the bundle multiple times, known as multi-tube passes. Similarly, to increase the fluid velocity outside the shell, longitudinal baffles can be installed within the shell to make the fluid pass through the shell space multiple times, known as multi-shell passes. Within the tubular heat exchanger, due to the different temperatures of the fluids inside and outside the tubes, the temperatures of the shell and the tube bundle are also different.
