Critical velocity questions

What is critical velocity in a tunnel fire?

Critical velocity is the lowest longitudinal air speed that stops smoke from a tunnel fire moving upstream against the airflow, or holds it to a set backlayering length. Critical Velocity Calculator works it out on an Android phone from your tunnel geometry, fire heat release and air properties, using the SEDH Handbook or NFPA 502 method you pick.

Which calculation methods does the app use?

Four: the SEDH Handbook method and NFPA 502 in its 2017, 2020 and 2026 editions. SEDH and 2017 share one iterative equation; 2017 takes the total heat release, counts 70% of it as convective and looks up K1. The 2020 edition is closed-form with an F1/F2 branch, and 2026 is a coupled iteration with fire-geometry factors.

Every equation is on How it works.

How does the app get K1 for NFPA 502 2017?

From the total heat release rate. The app interpolates linearly between 10 MW 0.87, 30 MW 0.74, 50 MW 0.68, 70 MW 0.64, 90 MW 0.62 and 100 MW 0.606, rounds to three decimals and holds the end values outside 10 to 100 MW. At 15 MW it gives 0.838; below 10 MW it stays at 0.87.

Why does NFPA 502 2020 give the same velocity for a 15 MW and a 100 MW fire?

Because the 2020 method switches branch. When the comparison factor F1 is larger than F2, the app uses R2 = 0.43·(1 − BR)·decay, which contains no heat release term. In the app's sample tunnel, 10 MW takes the R1 branch at 1.7743 m/s, and 15 MW and 100 MW both take R2 at 1.7768 m/s.

Industry papers report a 2020 move to withdraw this Annex D formula; check the edition your authority adopts. See Inputs and limits.

Does roadway grade change the critical velocity?

Only downhill. A negative grade raises the result through the grade factor Kg = 1 + 0.0374·|Gf|^0.8, with Gf in percent; level and uphill grades give Kg = 1. SEDH, NFPA 502 2017 and 2020 apply it. The 2026 method has no grade input, because the app follows that edition in treating the gradient effect as negligible.

What is the difference between tunnel and annular critical velocity?

The annular velocity is the air speed in the gap around a train; the tunnel velocity is averaged over the whole section. The blockage ratio, train frontal area over tunnel area, links them. In SEDH and 2017 the iteration gives the annular value and the tunnel value is Vc·(1 − BR). The 2020 and 2026 methods report both.

What units does Critical Velocity Calculator use?

SI only. Lengths are in metres, areas in m², heat release in kW, density in kg/m³, heat capacity in kJ/kg·K (J/kg·K for NFPA 502 2026) and g in m/s². Temperature can be entered in kelvin or °C. Results are in m/s and m³/s. There is no imperial input, so convert feet, cfm or BTU first.

Does it work offline, and does it keep my inputs?

It works offline: the app has no internet permission and does every calculation on the phone. It does not keep your inputs. They stay while the app is open and are cleared when it is closed, so export a PDF or CSV report if you need a record of a case.

Can I export the results?

Yes. After a calculation, Export makes an A4 PDF report or a CSV file with the method, inputs, derived values, results and the iteration list, plus a project name if you set one. Save opens Android's file picker so you choose the folder; Share hands the file to an app you pick, such as email.

Recalculate before you export: the app keeps showing the last result after you change an input or a calculation fails.

Can I use the result as a design value?

Only after you check it. The app is a calculation aid: it applies the equations in its Formula reference to the numbers you enter. It does not know your tunnel, your ventilation system or the edition and amendments your authority adopts, and a qualified engineer has to check each result before it goes into a design.

See the terms for this app and the inputs and limits.

What does the app do with my 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.

Full details are in the privacy policy.