Hotel Atrium Smoke Control: NFPA 92 Exhaust, Pressurization and Calculation

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)

Example: for a 5 MW fire (Q = 5000 kW), neglecting z0, z = 2.5 m gives m = 0.071 × 5000^(1/3) × 2.5^(5/3) ≈ 5.6 kg/s (about 17,000 m3/h). Flow grows with z^(5/3): keeping the smoke layer interface 15 m above the floor of a 25 m atrium requires m ≈ 111 kg/s, roughly 330,000 m3/h of exhaust (assuming ρ ≈ 1.2 kg/m3). At that scale the fan room and shafts must be coordinated with the architect at the earliest stage.

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 the smoke layer interface stays above the highest unprotected opening to adjoining spaces (or 1.83 m above the highest floor level of exit access open to the atrium) for 1.5 times the calculated egress time or 20 min, whichever is greater (NFPA 101-2024 8.6.7(5))
• Sprinkler activation time, smoke spread
• AHJ may request 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

SprinkCalc — Fire Sprinkler Design Across Three Standards

SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report. One of the few tools that unifies all three major fire protection standards.

Download SprinkCalc on the App Store