A heat exchanger has many functions. They are critical to the process industries as they used to condense, heat, cool, or chill a fluid. It also can be used to change the phase of a fluid (liquid to vapor or vapor to liquid) like a boiler. Heat exchangers can be a direct contact HEX or an indirect contact type HEX.
In a direct contact heat exchanger, heat transfer continuously occurs by the mixing of two process streams. An example of this is a cooling tower, commonly found in power plants where a stream of hot air is consistently mixed with cooler water. The water cools the hot air and exits the top of the cooling tower. Warm water exits the bottom of the tower, is recollected, cooled and used again.
An indirect contact type heat exchanger transfers heat by maintaining a separation between the two process streams. Examples of an indirect contact type HEX are Shell and Tube Heat Exchangers and Air Cooled Heat Exchanger (also known as a fin-fan)