text: Expansion joints are required in large ducted air systems to allow fixed pieces of piping to be largely free of stress as thermal expansion occurs. Bends in elbows also can accommodate this. Expansion joints also isolate pieces of equipment such as fans from the rigid ductwork, thereby reducing vibration to the ductwork as well as allowing the fan to “grow” as it comes up to the operating air system temperature without placing stress on the fan or the fixed portions of ductwork.
  An expansion joint is designed to allow deflection in the axial (compressive), lateral (shear), or angular (bending) deflections. Expansion joints can be non-metallic or metallic (often called bellows type). Non-metallic can be a single ply of rubberized material or a composite made of multiple layers of heat and erosion resistant flexible material. Typical layers are: outer cover to act a gas seal, a corrosion-resistant material such as Teflon, a layer of fiberglass to act as an
  insulator and to add durability, several layers of insulation to ensure that the heat transfer from the flue gas is reduced to the required temperature and an inside layer.
  A bellows is made up of a series of one or more convolutions of metal to allow the axial, lateral, or angular deflection.