A heat exchanger comprising a plurality of first fluid channels a plurality of linear conduits for interconnecting the first fluid channels a plurality of second fluid channels a plurality of linear conduits for interconnecting the second fluid channels wherein the heat exchanger comprises a three dimensional portion of a heat exchanger core wherein the portion is configured as a plurality of repeating units, wherein each repeating unit comprises three mutually orthogonal reference axes x, y and z; wherein the y-dimension corresponds to height, the x-dimension corresponds to width and the z dimension corresponds to depth, each repeating unit comprising: a rectangular planar base plate which is parallel with the zx plane; the base plate further comprising a pair of openings (218a, 218b); and, a pair of linear conduits (208a, 208b) extending from the base plate, wherein the pair of openings and the pair of linear conduits are arranged such that their footprints in the base plate define a rectangle; wherein the pair of linear conduits are arranged diagonally opposite each other on the base plate; wherein the pair of linear conduits extend from the base plate at an incline of 45 degrees with respect to the yx plane; wherein one of the pair of linear conduits extends at an angle of -45 degrees with respect to the yx plane and the other of the pair of linear conduits extends at an angle of +45 degrees with respect to the yx plane; such that the pair of linear conduits are arranged in rotational symmetry about an axis extending perpendicularly through the centroid of the base plate; and wherein the shape of the opening corresponding to the shape of the conduits. a plurality of first fluid channels a plurality of linear conduits for interconnecting the first fluid channels a plurality of second fluid channels a plurality of linear conduits for interconnecting the second fluid channels wherein the heat exchanger comprises a three dimensional portion of a heat exchanger core wherein the portion is configured as a plurality of repeating units, wherein each repeating unit comprises three mutually orthogonal reference axes x, y and z; wherein the y-dimension corresponds to height, the x-dimension corresponds to width and the z dimension corresponds to depth, each repeating unit comprising: a rectangular planar base plate which is parallel with the zx plane; the base plate further comprising a pair of openings (218a, 218b); and, a pair of linear conduits (208a, 208b) extending from the base plate, wherein the pair of openings and the pair of linear conduits are arranged such that their footprints in the base plate define a rectangle; wherein the pair of linear conduits are arranged diagonally opposite each other on the base plate; wherein the pair of linear conduits extend from the base plate at an incline of 45 degrees with respect to the yx plane; wherein one of the pair of linear conduits extends at an angle of -45 degrees with respect to the yx plane and the other of the pair of linear conduits extends at an angle of +45 degrees with respect to the yx plane; such that the pair of linear conduits are arranged in rotational symmetry about an axis extending perpendicularly through the centroid of the base plate; and wherein the shape of the opening corresponding to the shape of the conduits. a rectangular planar base plate which is parallel with the zx plane; the base plate further comprising a pair of openings (218a, 218b); and, a pair of linear conduits (208a, 208b) extending from the base plate, wherein the pair of openings and the pair of linear conduits are arranged such that their footprints in the base plate define a rectangle; wherein the pair of linear conduits are arranged diagonally opposite each other on the base plate; wherein the pair of linear conduits extend from the base plate at an incline of 45 degrees with respect to the yx plane; wherein one of the pair of linear conduits extends at an angle of -45 degrees with respect to the yx plane and the other of the pair of linear conduits extends at an angle of +45 degrees with respect to the yx plane; such that the pair of linear conduits are arranged in rotational symmetry about an axis extending perpendicularly through the centroid of the base plate; and wherein the shape of the opening corresponding to the shape of the conduits.