Tunnel critical velocity calculator for Android
Critical velocity and required airflow for smoke control in a tunnel fire, by the SEDH Handbook method or NFPA 502 (2017, 2020 or 2026 edition).
For Android phones, Android 8.0 and later. No account, no ads, no internet permission.
What Critical Velocity Calculator is
Critical Velocity Calculator is a free Android app by Sonicon Software for tunnel ventilation and fire-safety engineers. It computes the critical velocity and required airflow for smoke control in a tunnel fire using the SEDH Handbook method or NFPA 502 (2017, 2020 or 2026 edition), shows each iteration, and exports a PDF or CSV report. It works offline.
Four methods, each with its own equations
Pick the SEDH Handbook method or one of three NFPA 502 editions, and the form changes to that method's inputs. SEDH and NFPA 502 2017 share one iteration, 2020 is a closed-form calculation, and 2026 iterates critical velocity and plume temperature together. Every equation is listed in the app's Formula reference and on this site.
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SEDH Handbook
Iterative critical velocity from the convective heat release you enter, with your own K1.
Iterative -
NFPA 502 2017
The same iteration from total heat release: the app takes 70% as convective and interpolates K1 from the edition's table.
Iterative -
NFPA 502 2020
Comparison factors F1 and F2 pick the R1 or R2 ratio, with an optional backlayering length.
Closed-form -
NFPA 502 2026
A coupled iteration of critical velocity and plume temperature rise, using fire size and tunnel geometry factors. No grade input.
Iterative
What a calculation gives you
Each calculation gives the required airflow in m³/s and the critical velocity at the tunnel section and in the annulus around a train, in m/s. Derived values update as you type, the iteration list shows each pass, and Export turns the case into an A4 PDF or a CSV file.
- Derived values as you type: grade factor Kg and blockage ratio BR, the 2020 F1, F2, R1 and R2 steps, and the 2026 fire and geometry factors.
- Iteration list for SEDH, NFPA 502 2017 and 2026, ending at the last pass, marked CONVERGED when it settles.
- Formula reference: a screen with every equation the app uses.
- Export: an A4 PDF or a CSV with the inputs, results and iterations, under an optional project name. Save it to a folder you pick, or share it.
- Annulus Velocity (VA)
- 3.18 m/s
- Volume Flow Rate (Q)
- 46.60 m³/s
- Tunnel Velocity (VT)
- 1.89 m/s
Converged on pass 10
Built for site work
The app has no internet permission, so it works with no signal. There is no account and no ads. It does not save your inputs: closing the app clears them, so export a PDF or CSV report if you need to keep a record of a case.
- Internet permission
- Not requested
- Account, ads, analytics
- None
- Inputs after you close the app
- Cleared
- Units
- SI; temperature in K or °C
Check before you rely on it
Critical Velocity Calculator is a calculation aid. It applies the equations shown in the app to the numbers you enter; it does not know your tunnel, your ventilation system or the edition your authority adopts. Check every result, and the edition your project must follow, before you use it in a design.
See the terms for this app and the inputs and limits.
What happens to your data
Nothing leaves your phone through the app: it has no internet permission, account, ads or analytics. Inputs and results stay in memory and are gone when you close it. A report you export goes only where you save or share it. Android backup is on, but the app stores no data for it to copy.
Screens from the app
Get Critical Velocity Calculator