A heat exchanger is a device capable for transferring thermal energy between two or more fluids at different temperatures. In most cases, the fluids in a heat exchanger are separated by a heat transfer surface and thus are not mixed. Heat exchangers have wide applications in a variety of industries, such as power generation, petroleum production, transportation, etc. Although heat exchangers are often referred to as devices to warm a cold fluid by a hot fluid, they can also be used for air-conditioning and refrigeration. Common examples of heat exchangers include evaporators, condensers, coolers, and radiators. Industrial heat exchangers have been classified by flow arrangement, type of construction, and other criteria.