A heat exchanger allows heat from a fluid (a liquid or a gas) to pass to a second fluid (another liquid or gas) without the two fluids having to mix together or come into direct contact.

A hot fluid flows through a tube coiled inside a larger shell through which another, colder fluid is running in the opposite direction. Heat is exchanged by the fluids: the hot fluid cools down and the cold fluid warms up, without them actually coming into contact and mixing.