A system comprising: an engine case; and a heat exchanger inside the engine case, wherein the heat exchanger includes an air passage and a fuel passage, wherein the air passage and fuel passage are in fluid isolation from one another, but are in thermal communication with one another for exchange of heat; wherein the heat exchanger includes a toroidal double walled tube, wherein the fuel passage is defined between inner and outer walls of the double walled tube; wherein the heat exchanger includes an outer tube outside of the double walled tube, wherein the air passage includes: a space between the outer tube and the outer wall of the double walled tube; and a space inward of the inner wall of the double walled tube; the system characterized in that an open circumferentially facing end of the outer tube is an inlet from a space inside the engine case into the space between the outer tube and the outer wall of the double wall tube and to the space inward to the inner wall of the double walled tube, and wherein an air outlet fitting connected to the opposite circumferential end of the outer tube has a single outlet for both the space between the outer tube and the outer wall of the double wall tube and for the space inward of the inner wall of the double walled tube. an engine case; and a heat exchanger inside the engine case, wherein the heat exchanger includes an air passage and a fuel passage, wherein the air passage and fuel passage are in fluid isolation from one another, but are in thermal communication with one another for exchange of heat; wherein the heat exchanger includes a toroidal double walled tube, wherein the fuel passage is defined between inner and outer walls of the double walled tube; wherein the heat exchanger includes an outer tube outside of the double walled tube, wherein the air passage includes: a space between the outer tube and the outer wall of the double walled tube; and a space inward of the inner wall of the double walled tube; the system characterized in that an open circumferentially facing end of the outer tube is an inlet from a space inside the engine case into the space between the outer tube and the outer wall of the double wall tube and to the space inward to the inner wall of the double walled tube, and wherein an air outlet fitting connected to the opposite circumferential end of the outer tube has a single outlet for both the space between the outer tube and the outer wall of the double wall tube and for the space inward of the inner wall of the double walled tube. a space between the outer tube and the outer wall of the double walled tube; and a space inward of the inner wall of the double walled tube; the system characterized in that an open circumferentially facing end of the outer tube is an inlet from a space inside the engine case into the space between the outer tube and the outer wall of the double wall tube and to the space inward to the inner wall of the double walled tube, and wherein an air outlet fitting connected to the opposite circumferential end of the outer tube has a single outlet for both the space between the outer tube and the outer wall of the double wall tube and for the space inward of the inner wall of the double walled tube.