<P> A heat exchanger is a device used to transfer heat between a solid object and a fluid, or between two or more fluids . The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact . They are widely used in space heating, refrigeration, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural - gas processing, and sewage treatment . The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air . Another example is the heat sink, which is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant . </P> <Ul> <Li> <P> Fig. 1: Shell and tube heat exchanger, single pass (1--1 parallel flow) </P> </Li> <Li> <P> Fig. 2: Shell and tube heat exchanger, 2 - pass tube side (1--2 crossflow) </P> </Li> <Li> <P> Fig. 3: Shell and tube heat exchanger, 2 - pass shell side, 2 - pass tube side (2 - 2 countercurrent) </P> </Li> </Ul>

What is the difference between heater and heat exchanger
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