What Is a Heat Exchanger?
A heat exchanger is a device that moves heat from one material, usually a fluid, to another through conduction. The fluids may be either liquids or gases depending on the type of heat exchanger used, and are sealed in a series of pipes that allow them to circulate. Heat exchangers work according to the principles of thermal equilibrium, which govern the transfer of heat between objects.

How Does a Heat Exchanger Work?
Heat exchangers are basically two chambers separated by a wall, with each chamber containing one type of fluid substance. Heat passes from a hot object such as a furnace to the first fluid, which warms the fluid and cools the object. The circulating fluid then transfers its heat to the second fluid through the separating wall, warming the second fluid and taking heat from the first. The net effect is to cool the primary hot object in stages.

Where Are Heat Exchangers Used?
Heat exchangers are used in nuclear power plants and other large-scale industrial applications. In a nuclear reactor, uranium and other materials produce tremendous amounts of heat which must be carried away to avoid overheating the reactor. A primary fluid, which may be water or a liquefied metal such as sodium picks up the heat from the reactor. Pumps send the primary fluid to a heat exchanger, which transfers heat to water, boiling it to steam. The steam, in turn, drives a turbine which powers an electric generator. The steam condenses back to water which passes back to the heat exchanger, repeating the cycle.

Heat exchangers are used in all sorts of industrial processes. The size and type of heat exchanger for each process can be customized, as they all follow similar principles. Instead of using unnecessary coolant, many industries instead pipe in a fluid or chemical that needs to be heated anyway, saving heat energy that would otherwise be wasted.

Not all heat exchangers are found in industrial settings, one example being the human lungs. Cool air comes in and presses up against the tissue, behind which is hot blood. The heat from the blood transfers to the air, helping to cool the body.