Heat exchanger is a device used for transfer of thermal energy between two or more fluids that are at different temperatures. Heat exchangers work because heat naturally flows from high temperature to lower temperature. Therefore if a hot fluid and a cold fluid are separated by heat conducting surface heat can be transferred from the hot fluid to cold fluid. Heat exchangers may be classified according to transfer process, construction, flow arrangement, surface compactness, number of fluids and heat transfer mechanisms or according to process functions. Heat exchangers are useful in many engineering processes like those in refrigerating and air- conditioning systems, power systems, food processing systems, chemical reactors and space or aeronautical applications.

Double pipe heat exchanger is a simplest form of a heat exchanger, for particular advantages for small thermal loads and high pressure applications. It consists of a tube or pipe fixed concentrically inside a larger pipe or tube. They are used when the flow rates of the fluids and the heat duty are small (less than 500 kW).These are simple to construction, but may require a lot of physical space to achieve the desired heat transfer area. Double pipe Heat Exchanger consists of a tube or pipe fixed concentrically inside a larger pipe or tube. They are used when the flow rates of the fluids and the heat duty are small (less than 500 kW).These are simple to construction, but may require a lot of physical space to achieve the desired heat transfer area.