A double-walled pipe includes an inner wall, which bounds a flowpath for a first fluid such as air, an outer wall radially spaced from the inner wall so that there is an annular gap between the inner wall and the outer wall; and walls extending in the annular gap between the inner wall and the outer wall, such that flow passages are formed in the annular gap, through which a temperature-controlled fluid can be circulated to control the temperature of the inner wall. The double-walled pipe is formed by an additive manufacturing process. The walls may be helical walls, defining helical flow passages.