A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing, or they may be in direct contact. Classification of heat exchangers: 1. Classification according to construction: Tubular heat exchangers. Plate heat exchangers. Extended surface heat exchangers. Regenerative heat exchangers. Pipe in pipe heat exchangers: Hairpin heat exchangers (often also referred to as â??double pipesâ??) are characterized by a construction form which imparts a U shaped appearance to the heat exchanger. In its classical sense, the term double pipe refers to a heat exchanger consisting of a pipe within a pipe, usually of a straight-leg construction with no bends. However, due to the need for removable bundle construction and the ability to handle differential thermal expansion while avoiding the use of expansion joints (often the weak point of the exchanger), the current U-shaped configuration has become the standard in the industry.