Smoke Control Design Calculator

Smoke exhaust and stairwell pressurization solve different physical problems, so NFPA 92 treats them with entirely separate methods. Smoke-exhaust mode models a rising plume of hot smoke using the axisymmetric plume equation to find how much volume must be removed to hold a clear layer above occupants, while pressurization mode models air leaking through door gaps and cracks using the orifice-flow equation to find how much supply air keeps a stairwell smoke-free. There's no shared calculation between the two modes because the underlying phenomena aren't related.

Not all the energy from a fire goes into driving the buoyant plume that carries smoke upward — some is lost as radiation to surrounding surfaces. The calculator uses the common design assumption that about 70% of total heat release rate is convective (plume-driving) energy, and only that convective portion (Qc) feeds into the mass-flow equation for the smoke plume.

The plume equation raises clear height to the 5/3 power (z^(5/3)), so mass flow grows much faster than height grows — a taller clear zone lets the plume entrain far more surrounding air as it rises before reaching the smoke layer. Doubling the clear height more than triples the calculated mass flow, which is why atria with generous clear heights need proportionally larger exhaust fans.

The calculator uses 25 Pa as a typical design target that sits comfortably within NFPA 92's allowed range of about 12.5 Pa (0.05 in. w.g.) minimum to 87 Pa (0.35 in. w.g.) maximum with doors closed — high enough to resist smoke intrusion but low enough that door-opening force stays within the roughly 30 lbf limit. It's held constant so the leakage-flow calculation has a consistent basis; a real design would tune this value against actual door hardware and stack-effect conditions.