Calculator Inputs
Formula Used
The calculator uses an isothermal ideal gas relation with Darcy pipe friction and fitting loss coefficients.
Gas constant: R = 8314.462618 / MW
Resistance term: M = fL / D + ΣK
Pressure square relation: P1² - P2² = G² R T M
Mass flow: m = G A
Reynolds number: Re = G D / μ
Equivalent fitting length: Le = ΣK D / f
Outlet Mach estimate: Mach = Vout / sqrt(kRT)
How to Use This Calculator
- Select the solve mode for mass flow or outlet pressure.
- Choose absolute pressure or gauge pressure.
- Enter gas temperature, molecular weight, viscosity, and heat capacity ratio.
- Enter pipe inside diameter, length, roughness, and optional Darcy factor.
- Add fitting quantities and K values for valves, elbows, tees, and reducers.
- Press Calculate to see results above the form.
- Use CSV or PDF buttons to download the same calculated result.
Example Data Table
| Item | Example Value | Unit |
|---|---|---|
| Gas | Air | MW 28.97 |
| Inlet pressure | 700 | kPa absolute |
| Outlet pressure | 500 | kPa absolute |
| Temperature | 20 | °C |
| Pipe inside diameter | 50 | mm |
| Straight length | 100 | m |
| Four elbows | K = 0.9 each | Total K 3.6 |
| One check valve | K = 2.0 | Total K 2.0 |
Understanding isothermal gas flow
Isothermal compressible flow assumes the gas stays at one temperature. This happens when a long pipe exchanges heat with its surroundings. The method is useful for plant air, fuel gas, vapor lines, and instrument gas headers. It gives a practical pressure loss estimate without a full thermal model.
Why fittings matter
Fittings disturb the stream. Elbows, valves, tees, reducers, and exits add local losses. These losses are entered as K values. The calculator converts them into the same pressure square relation used for pipe friction. This makes the result clearer for layout review. A short pipe with many valves can lose more pressure than a long straight run.
Design approach
The tool can solve two common tasks. First, it can estimate mass flow from inlet and outlet pressures. Second, it can predict outlet pressure from a known mass flow. Absolute pressure is required because gas density changes with pressure. Gauge pressure can be used, but the calculator adds atmospheric pressure internally when selected.
Friction and gas properties
Darcy friction may be entered directly. It may also be estimated from roughness, diameter, viscosity, and Reynolds number. Laminar flow uses the simple sixty four over Reynolds relation. Turbulent flow uses an explicit rough pipe estimate. Molecular weight sets the specific gas constant. Temperature sets gas density and sound speed.
Interpreting results
Check the Reynolds number first. Then review friction factor, total K, equivalent length, and pressure drop. The outlet Mach estimate warns when velocity becomes high. It is not a full choking model, but it highlights risk. High Mach values mean noise, vibration, and larger errors.
Good practice
Use clean units and realistic K values. Enter pipe inside diameter, not nominal size. Use absolute inlet data from operating cases. Compare several fitting layouts. Export the CSV or PDF for calculation records. Always confirm final designs with the governing code and project standards.
Limits to remember
The model treats flow as steady, one dimensional, and isothermal. It ignores elevation change, heat imbalance, and real gas compressibility corrections. It works best at moderate pressure ratios and moderate Mach values. For critical services, compare results with validated software, vendor data, and measured plant performance before purchase or installation approval final release.
FAQs
What is isothermal compressible flow?
It is gas flow where temperature stays constant while pressure and density change. The model is useful for long pipes that exchange heat with surroundings.
Why must pressure be absolute?
Gas density depends on absolute pressure. Gauge pressure can be entered, but it must be converted before using the ideal gas equation.
What does fitting K mean?
K is a local loss coefficient. It represents extra resistance caused by elbows, valves, tees, reducers, entrances, exits, and similar components.
Can I enter my own friction factor?
Yes. Enter a positive Darcy friction factor to override the automatic estimate. Enter zero to estimate it from roughness and Reynolds number.
Does this calculator check choking?
It estimates outlet Mach number and sonic pressure risk. It is a warning method, not a full critical flow design model.
Which pipe diameter should I enter?
Enter actual inside diameter. Nominal pipe size can cause errors because wall schedule changes the flow area and velocity.
What gas molecular weight should I use?
Use the mixture molecular weight for the operating gas. For dry air, 28.97 is commonly used as the numeric value.
When should I use specialist software?
Use specialist software for high pressure ratios, real gas effects, multiphase flow, critical services, code reports, and vendor guarantees.