Modern underground parking under malls/residences use impulse jet-fan ventilation instead of ducted systems. Ideal for low ceilings (2.2-2.5 m), large footprint (10,000 m2+) and congested duct routes. NFPA 88A and EN 50272 define smoke zoning, CFD validation and fan layout.
Jet Fan vs Ducted
- Jet fan: ductless impulse under ceiling
- Ducted: duct + diffuser, higher cost
- Advantage: 40 cm ceiling saving
- Disadvantage: CFD mandatory, complex airflow
- Thrust: 50-100 N per fan (jet momentum)
CFD Design Criteria
- Fire HRR: 4-8 MW car, 30 MW truck
- Critical velocity: 2.5-3 m/s backlayering prevention
- Hot-gas temp: 400-600 C brief under ceiling
- Visibility: 10 m for egress
- CO: 200 ppm egress limit
- Software: FDS, ANSYS Fluent, PHOENICS
Fan Layout
- Row spacing: 20-30 m between fans
- Wall distance: 1.5 x fan diameter
- Ceiling gap: 300 mm clear
- Around columns: 1 m clear
- Nozzle direction: toward exhaust shaft (parallel)
- Acceleration zone: ramp exits need attention
Exhaust and Make-up
- Exhaust rate: 6-10 ACH normal, 10-12 fire
- Make-up air: ramp mouth, natural intake
- Exhaust shaft: 10 m2 min cross-section
- Fan: 400 C 2h F400 class min
- Damper: zone isolation, coordinated mode
CO and Gas Detectors
- CO detector: 25 ppm alarm, 50 ppm exhaust start
- NO2 detector: for diesel, 3 ppm
- Detector spacing: 1 per 400 m2
- Automation: BMS with VFD control
- Energy: partial operation (VFD) - 70 percent saving
Jet fans with MEP Calc
MEP Calc computes jet-fan count, thrust, critical velocity, exhaust flow and NFPA 88A zoning for underground parking.
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Sources & Further Reading
NFPA 88A, EN 50272, BS 7346-7, PIARC 2019, AIJ 2013. NFPA 88A Parking code.