A heat exchanger includes a heat exchanger body arranged along an axis and having an external surface in contact with a surrounding first fluid. The body defines a first fluid passage centered on and extending along the axis. The body also defines a second fluid passage extending parallel with respect to the axis and spaced away from the axis by a second passage distance, and a third fluid passage extending parallel with respect to the axis and spaced away from the axis by a third passage distance. The first, second, and third passages are parallel to one another, while the third passage distance is greater than the second passage distance. Each of the first and third passages is configured to accept a flow of a second fluid and the second fluid passage is configured to hold a volume of air to thermally insulate the first passage from the third passage.