In the simplest terms, a heat exchanger is a heat transfer system. It can transfer heat between two or more two fluids. Heat exchangers can be used to heat or cool the system.
This equipment is used in a variety of industries such as refrigeration, petrochemical, natural-gas processing, air conditioning, petroleum refineries, power centers, space heating, chemical plants, and sewage treatment.
One of the best examples to understand the working of heat exchangers is internal combustion engines. A fluid, usually called coolant, circulates and flows inside the radiator coils of the internal combustion engine.