You can see heat exchangers in all kinds of places, usually working to heat or cool buildings or helping engines and machines to work more efficiently.
In power plants or engines, exhaust gases often contain heat that’s heading uselessly away into the open air. That’s a waste of energy and something a heat exchanger can
certainly reduce (though not eliminate entirely—some heat is always going to be lost). The way to solve this problem is with heat exchangers positioned inside the exhaust tail pipes or smokestacks.
As the hot exhaust gases drift upward, they brush past copper fins with water flowing through them. The water carries the heat away, back into the plant. There, it might be recycled directly,
maybe warming the cold gases that feed into the engine or furnace, saving the energy that would otherwise be needed to heat them up. Or it could be put to some other good use,
for example, heating an office near the smokestack.