Ducting fire protection

A fire protection system for horizontally disposed ducting comprising:

• a length of fiberglass duct of uniform diameter and having a continuous inner wall and having a flow path therethrough from an inlet to an outlet;
• and
• a thermoplastic sleeve of uniform diameter and having a continuous wall and positioned entirely within said duct resting against said continuous inner wall of said duct as a liner coaxial with said duct in self-supporting relation, with the end of said sleeve closest to said outlet affixed to said duct and with said sleeve maintaining its position in contact with said duct inner wall prior to heat softening without any sleeve internal support other than that which affixes said sleeve outlet end to said duct;
• said sleeve being of a material characterized by having a softening temperature substantially lower than the softening temperature of said fiberglass duct whereby flow from said inlet to said outlet automatically effects permanent collapse of said sleeve onto itself into the flow path of said duct when said sleeve is heated to its softening temperature by flame or fluid flow in said duct, with said sleeve being of sufficient length to block flow through said duct upon collapse of said sleeve.