Abstract: A heat exchanger for exchanging heat between two different temperature fluids. The heat exchanger includes a sealed housing having first and second pairs of inlets and outlets. A first conduit is disposed within the housing. A second conduit is concentrically disposed within and spaced from the first conduit to define a first flow path between the spaced wall surfaces thereof which is disposed in fluid flow communication with the first inlet and first outlet for flow of a first fluid in a first direction through the housing. A second fluid flow path including first and second serially connected sections is provided for directing a second fluid between the second inlet and the second outlet over the exterior wall surface of the first conduit and through the second conduit in a second direction opposite to the direction of flow of the first fluid. In a preferred embodiment, a sealed chamber is formed interiorly within the housing about the first conduit.