Abstract: A rotatable cylinder is provided having an interior cylindrical passageway defined in part by the inner surface of the outer periphery of such rotatable cylinder and the outer surface of an inner, centrally positioned cylinder. Headers having liquid passageways therethrough and applied in a liquid-tight manner to the opposed cylinder ends complete a liquid-tight chamber with the cylindrical passageway through which heated liquid may circulate from one cylinder end to the other. A helical blade disposed in the cylindrical liquid passageway comprises an impeller which assists passage of the liquid through the passageway. The blade also assures desired heat transfer from the circulating liquid to the rotatable cylinder inner surface from which the heat is conducted to the cylinder outer surface whereat the heat is utilized.