Model tank blowdown using ideal or real assumptions. Enter pressure, temperature, nozzle, and flow inputs. Visualize results instantly with exports and practical design insight.
This sample case demonstrates a typical compressed air vessel with adiabatic depressurization and a small outlet nozzle.
| Case | Model | Volume (m³) | Initial Pressure (bar abs) | Final Pressure (bar abs) | Ambient (bar abs) | Temperature (K) | Nozzle (mm) | Cd | γ | R (J/kg·K) | Z |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Example A | Ideal | 0.250 | 50.0 | 5.0 | 1.013 | 300.0 | 6.0 | 0.92 | 1.40 | 287.0 | 1.00 |
| Example B | Real | 0.250 | 50.0 | 5.0 | 1.013 | 300.0 | 6.0 | 0.92 | 1.40 | 287.0 | 0.96 |
The calculator uses a lumped-parameter tank model with either ideal gas behavior or a constant-compressibility real gas correction.
Tank mass: m = (P × V) / (Z × R × T)
Choked flow criterion: (Pdown / Pup) ≤ (2 / (γ + 1))^(γ / (γ - 1))
Choked mass flow: ṁ = Cd × A × P × sqrt(γ / (ZRT)) × (2 / (γ + 1))^((γ + 1) / (2(γ - 1)))
Subsonic mass flow: ṁ = Cd × A × P × sqrt((2γ / ((γ - 1)ZRT)) × [r^(2/γ) - r^((γ + 1)/γ)])
where r = Pambient / Ptank
Process assumptions:
This approach is practical for engineering estimates, screening studies, nozzle sizing checks, and comparing ideal versus corrected real-gas behavior.
Blowdown time is the estimated duration required for a pressurized tank to vent down from its starting pressure to a selected final pressure, or to ambient pressure if the target is lower than ambient.
Use the ideal option when gas behavior is close to ideal over the full pressure range, or when you want a quick first-pass estimate. It is often reasonable for air and nitrogen at moderate conditions.
The real option applies a constant compressibility factor, Z. This changes calculated density, stored mass, and mass flow. It gives a better approximation when gas non-ideality matters but a full property package is unavailable.
Compressible-flow equations require absolute pressure, not gauge pressure. Absolute pressure measures from perfect vacuum, so it correctly represents density, energy state, and the downstream-to-upstream ratio used for choked-flow checks.
Choked flow occurs when the pressure ratio is low enough that the exit condition reaches the critical limit. Beyond that point, reducing downstream pressure further does not increase mass flow the same way.
During rapid discharge, gas leaves the vessel faster than heat can enter from the surroundings. Internal energy falls, so the remaining gas cools. That cooling lowers pressure and often lengthens blowdown compared with an isothermal case.
A vented tank cannot depressurize below the surrounding outlet pressure without an assisting device. If you enter a target below ambient, the calculator stops at ambient because passive blowdown cannot continue past that condition.
It is best for engineering estimates. Accuracy depends on the constant Z assumption, chosen γ and R values, nozzle characterization, and whether the tank is truly well mixed. Critical designs should be checked with higher-fidelity thermodynamic tools.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.