Working Principle of Normally Closed Fire Door
he core principle of normally closed fire doors is "closed under normal conditions, self-retaining in fire". Their fire resistance is not permanent; instead, they maintain compartmentation within the rated fire endurance of Class A, B and C fire doors.
If kept open for extended periods during daily operation, door closers may fail and sealing strips may get damaged, rendering the entire fire compartmentation system ineffective. Only when the door remains intact and closed can the door leaf, core material, intumescent sealing strips and hardware work in synergy to achieve three functions simultaneously: fire containment, heat insulation and smoke blocking, forming a life-saving compartment barrier inside buildings.
Only when the normally closed fire door is shut can a complete fire compartment be formed. The door leaf, door frame and fire-resistant core material constitute a fire-resistant barrier to divide fire compartments and restrict fire spread. If the door is left open, the fire compartmentation fails, and flames can penetrate through the door opening into evacuation areas, resulting in rapid fire propagation.
losing the door blocks smoke and saves lives: most fire fatalities are caused by toxic smoke. When closed, the door leaf closely fits the frame. Upon fire exposure, intumescent sealing strips expand to seal gaps, and the labyrinth rebate of the door frame also blocks smoke. If the door remains open, heavy smoke flows directly into stairwells and antechambers, and toxic fumes block escape routes.
Door closers and sequence coordinators function reliably only when the door is normally closed. Prolonged door opening damages hardware, so the door cannot self-close during a fire. The closed state also restrains the door leaf and reduces gaps formed by high-temperature warping.
