A heat exchanger is a device used to transfer heat between the fluids. The fluids may be separated by a solid wall to avoid mixing. They are widely used in space heating, power stations, chemical plants, air conditioning, petrochemical plants, petroleum refineries, refrigeration, and sewage treatment. The classic example of a heat exchanger is found in an internal combustion engine. Inside this engine is enclosed with a circulating fluid known as engine coolant flows through out
For the efficient operation of the compressor, the temperature inside the gas should be low. At the lower temperature, the energy will be very less and it is used to compress the gas for the given application of final temperature and pressure. However, the gas exhausted from both separators as well as the compressor will be relatively hot. When gas inside it is compressed, the thermodynamic balance remains same, which means that pressure nkT exchangers consist opposite direction exchangers 
When designing the process, the thermal energy balancing should be very important . oil train
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