NFPA 92-2024 is the global benchmark for smoke control engineering. It governs three system types: stairwell pressurization, atrium smoke exhaust, and zoned smoke control. Each has its own performance criterion. For Indian high-rises and atrium-mall projects, NFPA 92 in conjunction with NBC 2016 Pt 4 and IS 15493 sets the design framework.
This guide unpacks the design intent for each system, the math the AHJ expects to see, and the tenability concepts (ASET vs RSET) that justify acceptance of a performance-based design.
NFPA 92 system types
| System | Purpose | Typical application |
|---|---|---|
| Stairwell pressurization | Maintain stair as smoke-protected egress route | High-rise > 4 storeys, basement stairs |
| Atrium smoke exhaust | Remove smoke from atrium, maintain clear-layer height | Malls, hotels, hospitals with multi-storey atria |
| Zoned smoke control | Pressurize fire floor / depressurize adjacent floors | Specialized — hospitals, data centres |
| Smoke barrier | Vertical/horizontal compartmentation | Subdivides smoke compartments per NBC |
For Indian projects, stairwell pressurization is universal in high-rise; atrium smoke exhaust is required wherever a Class I building has an atrium >9 m or volume > 1,800 m³.
Stairwell pressurization design intent
NFPA 92 §4.4 specifies the performance criterion: the stair must be tenable for the time required for occupants to evacuate. Practically:
- Pressure with stair door closed: 50-87 Pa across door, with door-opening force ≤ 110 N
- Air velocity through open stair door at fire floor: ≥ 0.75 m/s (some editions require 1.0 m/s for tall buildings)
- Pressure must remain stable even with multiple doors open simultaneously (stairs at fire and discharge floor)
The supply duty for the pressurization fan = sum of all leakage paths × √(ΔP/50) at design ΔP, plus the open-door make-up. Typical 20-storey stair:
- Closed-door leakage at 50 Pa: 6-8 m³/s
- Open-door make-up (1 door open, 0.75 m/s through 0.9×2.1 m door): 1.4 m³/s
- Total fan duty: 7.5-9.5 m³/s = ~16,000-20,000 cfm
Distributed injection (every 4-6 floors instead of single rooftop) keeps pressure uniform and prevents over-pressurization at top floors due to stack effect. NFPA 92 strongly recommends distributed for buildings > 30 m height.
Atrium smoke exhaust design
For an atrium, NFPA 92 §6 requires maintaining a smoke layer above the heads of occupants. Two approaches:
Approach 1: Steady-state exhaust (NFPA 92 §6.2)
Exhaust at a rate equal to the design fire’s plume mass flow. The plume entrains room air as it rises; mass at the smoke layer ≈ entrainment + fire mass.
For axisymmetric plume with Heat Release Rate Q (kW) and clear-layer height z (m):
m_plume = 0.071 × Q^(1/3) × (z - z_v)^(5/3) (kg/s, Heskestad correlation)
Where z_v is virtual origin height ≈ 0.083 × Q^(2/5) – 1.02 D (D = fire diameter in m). For most atrium fires, z_v ≈ 0.
For a 4 MW design fire (typical atrium application, retail occupancy) with clear layer at 5 m above the floor:
m_plume = 0.071 × (4000)^(1/3) × (5)^(5/3) = 0.071 × 15.87 × 14.62 = 16.5 kg/s
At 25 °C (cold smoke, conservative):
V_exhaust = m / ρ = 16.5 / 1.18 = 14.0 m³/s = ~29,700 cfm
For 6 MW (mall main concourse): ≈ 18 m³/s = 38,000 cfm.
Approach 2: Computational simulation (NFPA 92 §7)
For complex geometries (sloped roofs, restricted exhaust at one corner), Approach 1’s steady-state may not apply. CFD simulation (FDS, OpenFOAM) shows actual smoke movement, layer formation, and tenability metrics.
For Indian projects, Approach 1 is universally accepted by AHJs; Approach 2 is reserved for atrium > 20 m height or when initial sizing fails the simple correlation.
Tenability: ASET vs RSET
The performance test of any smoke-control system is whether occupants survive long enough to evacuate. Two times matter:
- ASET (Available Safe Egress Time): time before tenability fails. Measured by ISO 13571 criteria — visibility, irritant gas concentration, FED toxicity, radiant + convective heat.
- RSET (Required Safe Egress Time): time for slowest occupant to reach safety. Measured by movement studies, occupant-density calcs, queue formation.
Acceptance: ASET > RSET with a safety factor (typ 1.5-2.0 in performance-based engineering).
For a typical Indian commercial building:
- Pre-movement time (alarm to start moving): 60-180 seconds
- Travel time (typical office floor to exit stair): 30-90 seconds
- Stair travel (per floor): 15-30 seconds
- Total RSET: 6-15 minutes for 20-storey
- Required ASET: ≥ 12-30 minutes with safety factor
ASET > 12 minutes typically requires either pressurization (keeping stairs clear) or aggressive smoke exhaust (delaying smoke layer descent). Without smoke control, ASET in many enclosed atria is < 5 minutes — insufficient for any high-rise evacuation.
Make-up air: the perpetual neglect
Every NFPA 92 system needs make-up air. Without it:
- Atrium exhaust pulls negative pressure → smoke flows into adjacent compartments
- Stair pressurization fan starves → ΔP drops → smoke leaks in
- Smoke barriers fail their differential pressure spec
Make-up air requirements:
| System | Make-up | Sourcing |
|---|---|---|
| Stairwell pressurization | 100% of leakage + open-door | Outdoor air, dedicated fan, low-level |
| Atrium smoke exhaust | 80-95% of exhaust | Outdoor air via mechanical fan + opening egress paths |
| Zoned smoke control | Pressure-balanced | From adjacent zone or dedicated fan |
Make-up fans must start with or just before exhaust fans. 30-60 second lag = depression > 100 Pa = compromised system.
Worked example: 25-storey hotel with 12 m atrium
Building: 25 floors above grade, single stair shaft, atrium at lobby (12 m height, 600 m² floor area).
Stair pressurization:
- Stair leakage at 50 Pa (single-leaf doors per floor, 25 levels): ~7.5 m³/s
- Open-door make-up (1 door open, 0.9×2.1 m at 0.75 m/s): 1.4 m³/s
- Total fan duty: 8.9 m³/s = 18,860 cfm, distributed at floors 1, 7, 13, 19, 25
Atrium smoke exhaust (4 MW design fire, clear layer at 4.5 m above floor):
- m_plume = 0.071 × (4000)^(1/3) × (4.5)^(5/3) = 0.071 × 15.87 × 12.8 = 14.4 kg/s
- V_exhaust at 25 °C = 14.4 / 1.18 = 12.2 m³/s = 25,800 cfm
- 4 fans at 6,500 cfm each, distributed at atrium ceiling
Make-up air: 90% × 25,800 = ~23,200 cfm via outdoor air through ground-floor egress paths during system operation.
Tenability check:
- Without smoke exhaust at atrium fire: layer descends to 2 m in ~4 min (insufficient)
- With smoke exhaust: layer stabilises at 4.5 m for 12+ min (acceptable for 25-storey RSET ~10 min)
Five smoke control mistakes that cost approval
1. No air-flow check at door opening. The 0.75 m/s through-door requirement is easy to miss — verify with single-door-open scenario.
2. No make-up air fan modelled. AHJ catches it during commissioning; add late = expensive retrofit.
3. Single-injection pressurization on tall buildings. Stack effect over-pressurizes upper floors → doors won’t open. Distribute every 4-6 floors.
4. Smoke exhaust without controlled make-up paths. Negative pressure at atrium pulls smoke into adjacent retail; tenability there is degraded.
5. Fans rated for 250 °C but supports / dampers / flex connectors not. Whole exhaust path must match the fire rating.
Quick checklist
- [ ] System type identified (stairwell pressurization / atrium exhaust / zoned)
- [ ] ΔP target 50-87 Pa stair, force ≤ 110 N
- [ ] Open-door air velocity 0.75 m/s minimum at fire floor
- [ ] Plume mass flow correlation (Heskestad axisymmetric) for atrium exhaust
- [ ] Make-up air sized 80-100% with fan-to-fan interlock
- [ ] Tenability assessed: ASET > RSET with safety factor
- [ ] Smoke barriers and fire-rated dampers verified at every penetration
- [ ] Fans rated 250 °C / 2 hr; supports + dampers + connectors match rating
References: NFPA 92-2024 Standard for Smoke Control Systems; NBC 2016 Pt 4 §6; ISO 13571 Life-threatening components of fire; IS 15493 Code of Practice for Smoke Management; SFPE Handbook of Fire Protection Engineering 5th Ed.
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