Abstract: Described herein is a damper for controlling the passage of hot gases through a duct. The damper has a frame through which are passages. Along these passages coolant is passed to cool the frame and so reduce its thermal expansion as a result of the passage of hot gases through the duct. One or more damper blades are positioned within the opening in the frame, these blades being mounted on shafts rotatably journalled in a bearing cooled by the coolant passing through the frame, a seal being provided where the shafts extend through the frame to allow flow of coolant from the frame into the duct and prevent flow in reverse.