9.7.- HEAT EXCHANGERS

The direct contact heat exchangers are those in which the heat is transferred by friction of two currents. This type of exchanger is used when, after the transfer process, the fluids are easily separable, for example in cooling towers.

The indirect contact heat exchangers have a dividing wall that separates the two fluids, producing heat transfer by convection and conduction. There are two subgroups within the indirect contact exchanges:

• Alternative: the fluids go through the same space alternately. The hot fluid transfers the heat to the surface and is absorbed by the cold when it comes into contact with the surface.
• Surface: the heat is transferred through the solid barrier that separates the fluids. They are the most widespread in all types of systems. Depending on the separating surface, they can be:

• Plate heat exchangers: the separation surface is a flat wall.
• Tube exchangers: the separation surface is the wall of a tube. A fluid circulates through the interior and another tube is placed outside through which the other fluid circulates. According to the direction of the currents, there are cocurrent and countercurrent flows.

According to their construction, there are innumerable heat exchangers configurations, but the most common are the shell tube, concentric tube and plate heat exchangers.

A part from the air conditioning systems, heat exchangers are found in any industry. Some examples are the maritime industry, surface treatment, food industry, chemical industry and nuclear power plants.