A heat exchanger comprising: a duct comprising: a wall having a first and a second end spaced along a central axis, the first end to be in fluid communication with an upstream first fluid source, and the second end being connected with a downstream destination for the first fluid source; an inlet manifold positioned within a downstream portion of the duct at a radially outward location, the inlet manifold to be connected to a second fluid source; an outlet manifold positioned within an upstream portion of the duct at a radially outward location; the duct being enclosed, and having a circumference, wherein at least one of the inlet and outlet manifolds extend at least 10 degrees around the circumference of the duct; a central manifold disposed between the inlet and outlet manifolds , and radially inwardly of the inlet and outlet manifolds ; and wherein heat exchanger entrance elements extend radially inward from the inlet manifold to the central manifold , and heat exchanger exit elements extend radially outward from the central manifold to the outlet manifold , the heat exchanger entrance and exit elements extending over the circumference of the duct, wherein the heat exchanger entrance elements and the heat exchanger exit elements extend on an arc from the central manifold to the inlet and outlet manifolds , respectively, wherein one of the heat exchanger entrance and exit elements extend to form a clockwise arc-shape and the other of the heat exchanger entrance and exit elements extend to form a counterclockwise arc-shape, characterised in that the heat exchanger entrance and exit elements are each passages formed into a solid element . a wall having a first and a second end spaced along a central axis, the first end to be in fluid communication with an upstream first fluid source, and the second end being connected with a downstream destination for the first fluid source; an inlet manifold positioned within a downstream portion of the duct at a radially outward location, the inlet manifold to be connected to a second fluid source; an outlet manifold positioned within an upstream portion of the duct at a radially outward location; the duct being enclosed, and having a circumference, wherein at least one of the inlet and outlet manifolds extend at least 10 degrees around the circumference of the duct; a central manifold disposed between the inlet and outlet manifolds , and radially inwardly of the inlet and outlet manifolds ; and wherein heat exchanger entrance elements extend radially inward from the inlet manifold to the central manifold , and heat exchanger exit elements extend radially outward from the central manifold to the outlet manifold , the heat exchanger entrance and exit elements extending over the circumference of the duct, wherein the heat exchanger entrance elements and the heat exchanger exit elements extend on an arc from the central manifold to the inlet and outlet manifolds , respectively, wherein one of the heat exchanger entrance and exit elements extend to form a clockwise arc-shape and the other of the heat exchanger entrance and exit elements extend to form a counterclockwise arc-shape, characterised in that the heat exchanger entrance and exit elements are each passages formed into a solid element . <EOS>