Have you taken a look at the diagram of heat exchangers shown at http://www.pp2k.co.uk? It can be quite confusing to those who are not familiar with this stuff. But only those who have been working in the heavy industries know exactly when such a device like the heat exchanger is used. Even households have this device installed in one of the machines they use at home, without them being aware of it.
First of all, what is a heat exchanger? Based on the definition, this is a device utilised in transferring heat in between one, two or more fluid sources, like gas or liquid. The fluids are divided by a hard wall to avoid it mixing or get in direct contact. The heat exchangers are used widely in various places such as in chemical plants, air conditioning, refrigeration, space heating, sewage treatment, natural gas processing, petroleum refineries and petrochemical plants. A fine example of where you can find the heat exchanger is within the internal combustion engine. This kind of engine makes use of an engine coolant that flows through the coils of the radiator. Air flows through the radiator coils, which then cools the off the coolant then heats up the incoming air. You can find more examples of this at http://www.pp2k.co.uk.
There are various places and machines that make use of the heat exchangers; this usually works with cooling or heating buildings and others helping with machines and engines to work more productively. For example, air-conditioners and refrigerators use the heat exchangers in an opposite manner that the central heating systems do. These two machines remove the heat from a room or compartment where it is not wanted and then put it out away in a fluid to another place where it is going to be dumped out.
In another case such as with engines or power plants, the exhausts gases sometimes contain heat that is heading to open air, which many consider as wasting more energy. This is something that the heat exchanger can still utilize and reduce the wasted energy. It may not entirely contain it, but at least only a small part of it gets lost. The only way to solve this situation is using the heat exchangers and place it inside the smokestacks or tailpipes. When the hot exhaust gases go upward, they go past the copper fins that has water flowing through them. The water then carries away the heat then goes back inside the plant. In there, it might get recycled or warm up the cold gases that are feeding into the furnace or engine. Thus this saves energy that would have been needed otherwise be needed for heating it up. It could also be put to other good uses as well, such as heating up an office that is located near the smokestack.
Vehicles also make use of the heat exchangers, which are the type that is also available in http://www.pp2k.co.uk. A good example would be buses; wherein it uses the fluid to cool its diesel engine. The car radiator makes use of a different type of heat exchanger as well.