Abstract: A heater drum has an outer cylindrical wall centered on and rotatable about an axis, an inner cylindrical wall fixed concentrically within the outer wall and defining therewith an annular chamber, and a body of a heat-transmitting fluid in the chamber in heat-transmitting contact with both walls. A core body fixed nonrotatably about the axis inside the inner wall is heated to a high treatment temperature. This heat is transmitted radially from the core body to the inner wall. The walls are rotated jointly about the axis relative to the core body to distribute the heat of the inner wall via the fluid body to the outer wall. The chamber is at subatmospheric pressure and the fluid is partly vaporized and partly liquid at the high treatment temperature. The core body is of part-cylindrical section generally centered on the axis and has an outer surface spaced slightly radially inward of the inner wall.