A shell‑and‑tube heat exchanger consists of a shell, heat‑transfer tube bundle, tube sheets, baffles, channel boxes and other components. The shell is mostly cylindrical and houses the tube bundle inside. Both ends of the tube bundle are fixed onto the tube sheets.
Of the two hot and cold fluids undergoing heat exchange, one flows inside the tubes and is known as the tube‑side fluid; the other flows outside the tubes and is called the shell‑side fluid. To raise the shell‑side fluid's heat‑transfer coefficient, a number of baffles are generally fitted inside the shell. Baffles increase the velocity of the shell‑side fluid and force it to flow transversely across the tube bundle repeatedly along designated paths, intensifying fluid turbulence.
Heat‑transfer tubes can be arranged on tube sheets in either equilateral‑triangle or square patterns. The equilateral‑triangle layout offers compact arrangement, high turbulence of shell‑side fluid and a high heat‑transfer coefficient. The square layout facilitates external tube‑side cleaning and is suitable for fluids prone to scaling.
One pass of fluid through the tube bundle is defined as one tube pass; one pass of fluid through the shell is defined as one shell pass. To boost the velocity of fluid inside tubes, partition plates can be installed in the channel boxes at both ends to divide all tubes into several groups. In this way the fluid passes through only part of the tubes each time and travels back and forth across the tube bundle for multiple cycles - this is known as multi‑tube‑pass design. Similarly, longitudinal baffles can be mounted inside the shell to increase shell‑side velocity, forcing fluid to pass through the shell space multiple times, which forms a multi‑shell‑pass configuration. Multi‑tube‑pass and multi‑shell‑pass structures can be used in combination.
Due to the temperature difference between fluids inside and outside the tubes, temperatures of the shell and tube bundle of the heat exchanger differ. A substantial temperature difference will generate considerable thermal stress inside the equipment, which may cause tube bending, rupture or pull‑out from tube sheets. Therefore, proper compensation measures shall be adopted to eliminate or mitigate thermal stress when the temperature difference between the tube bundle and shell exceeds 50 °C. Based on compensation measures employed, shell‑and‑tube heat exchangers are classified into the following major types:
① Fixed‑tube‑sheet heat exchanger Tube sheets at both ends of the tube bundle are integrally connected with the shell. It features simple structure, yet it is only applicable to heat‑exchange operations with small temperature difference between hot and cold fluids and where mechanical cleaning on the shell side is not required. When the temperature difference is moderately large and shell‑side pressure is relatively low, an elastic expansion joint can be fitted on the shell to reduce thermal stress.
② Floating‑head heat exchanger The tube sheet at one end of the tube bundle is free to float, which completely eliminates thermal stress. Moreover, the entire tube bundle can be pulled out from the shell for convenient mechanical cleaning and maintenance. Floating‑head heat exchangers are widely applied, but they have relatively complex structures and high costs.
③ U‑tube heat exchanger Each heat‑transfer tube is bent into a U‑shape. The two ends of each tube are fixed respectively in upper and lower sections of the same tube sheet. Partition plates inside the channel box separate inlet and outlet chambers. This type of heat exchanger fully eliminates thermal stress and has a simpler structure than the floating‑head type. However, cleaning on the tube side is difficult.
