A heat transfer device includes a tube with a first fluid inlet and outlet and is arranged to form a plurality of parallel elongated segments. An enclosure defines a second fluid flow path over an exterior surface of the tube from a second fluid inlet to a second fluid outlet. A wall in the enclosure defines a first section of the second fluid flow path, which first section leads from the second fluid inlet and extends across a first portion of a length of all of the elongated segments of the tube. The wall further defines a second section of the second fluid flow path in the enclosure, which second section extends across a second portion of the length of all of the elongated segments of the tube and leads to the second fluid outlet. The interior wall is shaped and arranged such that a cross sectional area of the second fluid flow path in the enclosure changes along its length from the second fluid inlet to the second fluid outlet.