A heat exchanger is a device used to transfer heat between two or more fluids.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.They are widely used in space heating, refrigeration, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment.A solid wall to prevent mixing may separate the fluids or they may be in direct contact.