An atrium (internal courtyard, lobby, shopping mall) traps smoke due to its large volume. Long evacuation times let smoke fall into sleeping areas. NFPA 92 (Smoke Control Systems) defines smoke reservoirs, exhaust flow, makeup air, pressurization and CFD simulation requirements.
Smoke Reservoir
- Purpose: separate smoke layer at top from clean air below
- Depth: 20-25 percent of atrium height
- Smoke curtain: auto-deploying (2-hour rating)
- Minimum clear layer: 2.5 m above floor (egress free)
Exhaust Flow Calculation
NFPA 92 Annex A:
- m = 0.071 × Q^(1/3) × (z - z0)^(5/3)
- Q = fire HRR (kW) - typical 2-5 MW hotel lobby
- z = clear layer height (m)
- z0 = virtual origin
- m = plume mass flow (kg/s)
25 m atrium, 5 MW fire, z=2.5 m: m = 80-100 kg/s ~= 240,000 m3/h exhaust fan.
Makeup Air
- Flow: 85-95 percent of exhaust (slight negative pressure)
- Velocity: <1 m/s at floor - avoid disrupting plume
- Source: atrium floor level, doors and grilles
- Temperature: conditioned outdoor air
Pressurization
- Stair: +50 Pa (upward airflow 1-2 m/s)
- Elevator shaft: +25 Pa
- Lobby/corridor: -25 Pa (smoke outflow)
- Test: pressure maintained with doors open
- Control: VFD fans
CFD Validation
- FDS (Fire Dynamics Simulator, NIST) free
- Pathfinder + Pyrosim egress simulation
- Verify clear layer maintained for 15 min
- Sprinkler activation time, smoke spread
- AHJ often requests CFD (IBC Sec 909, NFPA 92 Annex D)
Atrium smoke with MEP Calc
MEP Calc runs NFPA 92 plume mass flow, exhaust fan sizing, makeup air and pressurization for atrium smoke control.
Download MEP Calc on the App Store
Sources & Further Reading
NFPA 92, NFPA 101, IBC Section 909, NIST FDS User Guide, SFPE Handbook. NFPA 92 Smoke Control standard.