Tunnel Fire Ventilation Calculator

Calculate tunnel fire ventilation critical velocity to prevent backlayering per NFPA 502

Input Parameters

Results

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Critical Velocity
m/s
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Critical Velocity
ft/min
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Required Airflow
m3/s
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Peak Smoke Temperature
C
Note: NFPA 502 Standard for Road Tunnels. Critical velocity prevents smoke backlayering upstream of the fire. Typical design fires: passenger car 5-10 MW, bus/truck 20-30 MW, HGV 50-100 MW, tanker 200-300 MW. Longitudinal ventilation with jet fans is common for tunnels under 2 km.

Reference

NFPA 502 Critical Velocity (Kennedy): Vc = Kg * Kf * [g*Qc / (rho*Cp*A*Tf)]^(1/3) Kg = 1 + 0.0374 * grade^0.8 (grade correction) Tf = Tamb + Qc / (rho*Cp*A*Vc) (iterative) Qc = 0.7 * Q (convective fraction) Design fires: cars 5-10 MW, trucks 30-100 MW, tankers 100-300 MW