A flexible manifold (14, 16, 114) adapted for being connected to a plate-fin heat exchanger core, comprising a first and second plurality of core layers, each core layer including two parting sheets, each defining a parting sheet thickness; and two closure bars, each defining a closure bar thickness, the flexible manifold comprising a plurality of individual layers, each individual layer defining a lower floor, an upper floor, and two side walls, wherein: the first plurality of core layers comprises a first flow circuit of a first medium and the second plurality of core layers comprises a second flow circuit of a second medium; each of the plurality of individual layers is adapted to channel the flow of the first medium therethrough; each of the plurality of individual layers includes a plurality of vertical members (432, 532); each of the plurality of vertical members extends vertically from the lower floor to the upper floor of the respective individual layer; characterised in that the first flow circuit of the first medium is generally perpendicular to the second flow circuit of the second medium; and each of the plurality of vertical members is configured to provide structural support for the respective individual layer; and each of the vertical members is selected from the group consisting of: vertical guide vanes dividing each of the plurality of individual layers into a plurality of discrete manifold flow passages extending at least part of a distance from a first end to a second end of the flexible manifold, wherein the vertical guide vanes further define at least one end radius configured to reduce material stress in a region of the individual layer, and reduce flow turbulence in the medium; and vertical columns, wherein the vertical columns have a hydrofoil cross section configured to control the flow of the medium through each of the plurality of individual layers. the first plurality of core layers comprises a first flow circuit of a first medium and the second plurality of core layers comprises a second flow circuit of a second medium; each of the plurality of individual layers is adapted to channel the flow of the first medium therethrough; each of the plurality of individual layers includes a plurality of vertical members (432, 532); each of the plurality of vertical members extends vertically from the lower floor to the upper floor of the respective individual layer; characterised in that the first flow circuit of the first medium is generally perpendicular to the second flow circuit of the second medium; and each of the plurality of vertical members is configured to provide structural support for the respective individual layer; and each of the vertical members is selected from the group consisting of: vertical guide vanes dividing each of the plurality of individual layers into a plurality of discrete manifold flow passages extending at least part of a distance from a first end to a second end of the flexible manifold, wherein the vertical guide vanes further define at least one end radius configured to reduce material stress in a region of the individual layer, and reduce flow turbulence in the medium; and vertical columns, wherein the vertical columns have a hydrofoil cross section configured to control the flow of the medium through each of the plurality of individual layers. vertical guide vanes dividing each of the plurality of individual layers into a plurality of discrete manifold flow passages extending at least part of a distance from a first end to a second end of the flexible manifold, wherein the vertical guide vanes further define at least one end radius configured to reduce material stress in a region of the individual layer, and reduce flow turbulence in the medium; and vertical columns, wherein the vertical columns have a hydrofoil cross section configured to control the flow of the medium through each of the plurality of individual layers.