Shell and Tube Exchangers are common throughout the chemical process industries for heat economy. Many tubes go from a header on one side to a header on the other. The other fluid is in the space outside the tubes. Hot streams exchange energy with colder streams so that the thermal energy of the hot streams is not wasted. Furthermore, companies are not allowed to discharge hot streams that can damage the environment, so removing heat from a waste hot stream is important.
The colors in this sketch are supposed to represent heat gradients. Red is hottest, and yellow is coolest. Note that the outlets cannot reach exactly the same temperature as the inlet of the other stream except when there is infinite surface for heat transfer. The sketch is a little misleading for the shell temperatures because there should be a left to right gradient when the inlet and outlet are at different ends.