A heat exchanger utilizes the fact that heat transfer occurs when there is a difference in temperature . In a heat exchanger, there is a cold stream and a hot stream. The two streams are separated by a thin, solid wall. The wall must be thin and conductive in order for heat exchange to occur. Yet the wall must be strong enough to withstand any pressure by he fluid.
Copper seems to be one of a common choice for construction.
Here is a simple flow diagram showing how heat transfers in a heat exchanger. This flow arrangement is called co-current . If the direction of one of the stream is reversed, the arrangement is called counter-current flow. Here are the temperature profiles along the heat exchanger. Counterflow heat exchanger provides more effective heat transfer.