A Shell and Tube Heat Exchanger is a class of heat exchanger designs. It is the most common type of heat exchanger in oil refineries and other large chemical processes, and is suited for high-pressure applications. As its name implies, this type of heat exchanger consists of a shell (a large pressure vessel), with a bundle of tubes inside it. One fluid runs through the tubes, and another fluid flows over the tubes (through the shell), to transfer heat between the two fluids. The set of tubes is called a tube bundle, and the body enclosing it is called the shell.

Two fluids, of different starting temperatures, flow through the heat exchanger. One flows through the tubes (the tube side) and the other flows outside the tubes but inside the shell (the shell side). Heat is transferred from one fluid to the other through the tube walls, either from tube side to shell side or vice versa. The fluids can be either liquids or gases on either the shell or the tube side. In order to transfer heat efficiently, a large heat transfer area should be used, leading to the use of many tubes. In this way, waste heat can be put to use. This is an efficient way to conserve energy.