Fire damper for ventilation ducts - has material forming a foam at high temperatures in vapour proof sachets

The damper has a circular casing whch flanges into a section of the ventilation duct. Within the casing a grid is placed perpendicularly to the direction of air flow. The grid is formed of metal bars with slots along their length facing the direction of air flow. Thermally sensitive elements are inserted in these slots. Each element contains a material (14) which expands and forms a foam when it is heated. It is held between two metal foils (15) which are coated on the inside (16) with a material resistant to alkalies such as polyethylene. The thermally sensitive material is thus protected by a chemical and vapour proof barrier against deterioration with time by the action of moisture in the air.