double pipe heat exchanger
Double Pipe Heat Exchanger are operating in true counterflow, they can operate with a temperature cross, that is, where the cold side outlet temperature is higher than the hot side outlet temperature. The term 'hairpin heat exchanger' is also used for a heat exchanger of the configuration in the diagram. A hairpin heat exchanger may have only one inside pipe, it may have multiple inside tubes, but it will always have the doubling back feature shown. Some heat exchanger manufacturers advertise the availability of finned tubes in a hairpin / double pipe heat exchanger. These would always be longitudinal fins, rather than the more common radial fins used in a crossflow finned tube heat exchanger.