Ducts are defined as air tight conduits that convey air or flue-gas under positive or negative pressure. They may or may not be exposed to high temperatures. Ducts may be circular or rectangular in cross section, or sometimes they may be very unusual in shape as they transition into a piece of major equipment, such as fan, precipitator or scrubber. To accommodate temperature changes, duct are usually independent structures which “float” on slide bearing plates or are suspended by a hanger system, depending of the size of a plant.