A heat exchanger transfers heat from one medium to another.Shell and tube heat exchangers are comprised of a series of tubes. One set contains a fluid meant to be either heated or cooled; a second fluid runs over the tubes that are being heated or cooled to either provide the heat or absorb the heat required.

Shell and tube heat exchangers are the most common type of heat exchanger in higher-pressure applications. Boilers are examples of two-phase heat exchangers; they work with both the gaseous and liquid states of the fluid used.