A double pipe heat exchanger is one of the simplest form of Shell and Tube Heat Exchangers. Here, just one pipe inside another larger pipe. To make an Unit very Compact, The Arrangement is made Multiple Times and Continues Serial and Parallel flow. One fluid flows through the inside pipe and the other flows through the annulus between the two pipes. The wall of the inner pipe is the heat transfer surface.This is also called as a hairpin heat exchanger. These are may have only one inside pipe, or it may have multiple inside tubes, but it will always have the doubling back feature shown. In some of the Special Cases the Fins also Used in Tube side.