A first header for a high-pressure heat exchanger, the header comprising: a first high-pressure inlet configured to connect to a source of high-pressure fluid and through which the high-pressure fluid flows therethrough to enter the first header; a first high-pressure tube extending from the high-pressure inlet to a first high-pressure transition section; the first high-pressure transition section configured to divide the high-pressure fluid from the first high-pressure tube into multiple first high-pressure flow channels extending in an axial direction, the first high-pressure transition section having inlets of the multiple first high-pressure flow channels spaced from one another in a radial direction and collectively arranged in a substantially circular shape, the inlets of the multiple first high-pressure flow channels on a radially outer edge of the first high-pressure transition section being spaced further apart in a circumferential direction from adjacent inlets of the multiple first high-pressure flow channels than radially inward inlets are spaced from adjacent radially inward inlets of the multiple first high-pressure flow channels; the multiple first high-pressure flow channels extending from the first high-pressure transition section to a second high-pressure transition section; the second high-pressure transition section being configured to be adjacent to a core of the heat exchanger and configured to divide each of the multiple first high-pressure flow channels into at least two first high-pressure sub-flow channels, the first high-pressure sub-flow channels being configured to connect to high-pressure flow passages of the core, wherein each flow channel of the first high-pressure flow channels and each sub-flow channel of the first high-pressure sub-flow channels have a round cross-sectional shape; and a low-pressure flow path configured to extend from the core to a component able to accept low-pressure fluid. a first high-pressure inlet configured to connect to a source of high-pressure fluid and through which the high-pressure fluid flows therethrough to enter the first header; a first high-pressure tube extending from the high-pressure inlet to a first high-pressure transition section; the first high-pressure transition section configured to divide the high-pressure fluid from the first high-pressure tube into multiple first high-pressure flow channels extending in an axial direction, the first high-pressure transition section having inlets of the multiple first high-pressure flow channels spaced from one another in a radial direction and collectively arranged in a substantially circular shape, the inlets of the multiple first high-pressure flow channels on a radially outer edge of the first high-pressure transition section being spaced further apart in a circumferential direction from adjacent inlets of the multiple first high-pressure flow channels than radially inward inlets are spaced from adjacent radially inward inlets of the multiple first high-pressure flow channels; the multiple first high-pressure flow channels extending from the first high-pressure transition section to a second high-pressure transition section; the second high-pressure transition section being configured to be adjacent to a core of the heat exchanger and configured to divide each of the multiple first high-pressure flow channels into at least two first high-pressure sub-flow channels, the first high-pressure sub-flow channels being configured to connect to high-pressure flow passages of the core, wherein each flow channel of the first high-pressure flow channels and each sub-flow channel of the first high-pressure sub-flow channels have a round cross-sectional shape; and a low-pressure flow path configured to extend from the core to a component able to accept low-pressure fluid.