Heat exchangers are used to transfer heat between two sources. The term exchanger applies to all equipment used to transfer heat between two streams. However, the term is commonly used to equipment in which two process streams exchange heat with each other. This research paper explains the basics of heat exchangers, covering such topics as: classification; design methods; pressure drop; analysis of extended surfaces; typical designs of double pipe, shell and tube compact, plate and frame, boilers, condensers and evaporators; and fouling of heat exchangers.