A double pipe heat exchanger, into its simplest form is currently one pipe within another larger pipe. One fluid flows through the inside pipe and the further flows through the annulus between the two pipes. The wall of the inner pipe is the heat transfer surface. The pipes are usually doubled back multiple times as shown in the diagram at the left, in order to make the overall unit more compact.
The term 'hairpin heat exchanger' is also used for a heat exchanger of the arrangement in the diagram. A hairpin heat exchanger may have only one inside pipe, or it may have multiple inside tubes, but it will always have the doubling back attribute shown. Some heat exchanger manufacturers advertise the availability of finned tubes in a hairpin or double pipe heat exchanger. These would always be longitudinal fins, rather than the more common radial fins used in a cross flow finned tube heat exchanger.