A heat exchanger comprising: a core having a nonrectangular cross-sectional area; a plurality of cold flow layers centered about a centerline, each of the plurality of cold flow layers separated by corresponding walls; a plurality of hot flow tubes corresponding to each of the plurality of cold flow layers; wherein the core has a circular cross-sectional area and the plurality of cold flow layers are annular in shape, coaxial about the centerline, and are separated by corresponding annular walls; wherein the plurality of cold flow layers comprises: a first cold flow layer having a first annular wall on a radially inner boundary; a second cold flow layer radially outward from the first cold flow layer and having a second annular wall between the first cold flow layer and the second cold flow layer; and a third cold flow layer radially outward from the second cold flow layer and having a third annular wall between the second cold flow layer and the third cold flow layer and a fourth annular wall radially outward from the third cold flow layer; and characterized in that the plurality of hot flow tubes comprises: a first set of hot flow tubes radially inward from the first cold flow layer with each hot flow tube being incorporated into the first annular wall; a second set of hot flow tubes radially between the first cold flow layer and the second cold flow layer with each hot flow tube being incorporated into the second annular wall; a third set of hot flow tubes radially between the second cold flow layer and the third cold flow layer with each hot flow tube being incorporated into the third annular wall; and a fourth set of hot flow tubes radially outward from the third cold flow layer with each hot flow tube being incorporated into the fourth annular wall. a core having a nonrectangular cross-sectional area; a plurality of cold flow layers centered about a centerline, each of the plurality of cold flow layers separated by corresponding walls; a plurality of hot flow tubes corresponding to each of the plurality of cold flow layers; wherein the core has a circular cross-sectional area and the plurality of cold flow layers are annular in shape, coaxial about the centerline, and are separated by corresponding annular walls; wherein the plurality of cold flow layers comprises: a first cold flow layer having a first annular wall on a radially inner boundary; a second cold flow layer radially outward from the first cold flow layer and having a second annular wall between the first cold flow layer and the second cold flow layer; and a third cold flow layer radially outward from the second cold flow layer and having a third annular wall between the second cold flow layer and the third cold flow layer and a fourth annular wall radially outward from the third cold flow layer; and characterized in that the plurality of hot flow tubes comprises: a first set of hot flow tubes radially inward from the first cold flow layer with each hot flow tube being incorporated into the first annular wall; a second set of hot flow tubes radially between the first cold flow layer and the second cold flow layer with each hot flow tube being incorporated into the second annular wall; a third set of hot flow tubes radially between the second cold flow layer and the third cold flow layer with each hot flow tube being incorporated into the third annular wall; and a fourth set of hot flow tubes radially outward from the third cold flow layer with each hot flow tube being incorporated into the fourth annular wall. a first cold flow layer having a first annular wall on a radially inner boundary; a second cold flow layer radially outward from the first cold flow layer and having a second annular wall between the first cold flow layer and the second cold flow layer; and a third cold flow layer radially outward from the second cold flow layer and having a third annular wall between the second cold flow layer and the third cold flow layer and a fourth annular wall radially outward from the third cold flow layer; and characterized in that the plurality of hot flow tubes comprises: a first set of hot flow tubes radially inward from the first cold flow layer with each hot flow tube being incorporated into the first annular wall; a second set of hot flow tubes radially between the first cold flow layer and the second cold flow layer with each hot flow tube being incorporated into the second annular wall; a third set of hot flow tubes radially between the second cold flow layer and the third cold flow layer with each hot flow tube being incorporated into the third annular wall; and a fourth set of hot flow tubes radially outward from the third cold flow layer with each hot flow tube being incorporated into the fourth annular wall. a first set of hot flow tubes radially inward from the first cold flow layer with each hot flow tube being incorporated into the first annular wall; a second set of hot flow tubes radially between the first cold flow layer and the second cold flow layer with each hot flow tube being incorporated into the second annular wall; a third set of hot flow tubes radially between the second cold flow layer and the third cold flow layer with each hot flow tube being incorporated into the third annular wall; and a fourth set of hot flow tubes radially outward from the third cold flow layer with each hot flow tube being incorporated into the fourth annular wall.