Compressed Air Pipe Sizing Calculator

1. Purpose and Engineering Decision

A compressed-air pipe cannot be selected from FAD alone. The gas occupies a much smaller actual volume at line pressure than it does at the reference condition used for compressor capacity. The calculator must therefore normalize the flow basis, convert it to actual line volume at the intended operating pressure and temperature, then test candidate internal diameters for velocity and pressure loss.

The result is a preliminary engineering diameter, not a nominal trade size. The final product selection must use the actual internal diameter of the chosen pipe system, including schedule, wall thickness, liner or manufacturer geometry.

2. Required Calculation Sequence

3. Reference Flow to Actual Line Flow

Pipe velocity must use actual volumetric flow inside the pipe, not FAD or another reference-flow number.

Q_line = Q_ref x (P_ref,abs / P_line,abs) x (T_line / T_ref) x (Z_line / Z_ref)

If humidity correction is material, the source and line-state dry-air partial pressures should be used instead of total pressure.

4. Velocity Sizing Equation

v = Q_line / A

A = pi x D^2 / 4

D_min = sqrt[4 x Q_line / (pi x v_target)]

The target velocity is a design input rather than a universal fixed rule. A main header, long branch, short machine drop and transient service may justify different velocity limits. The tool should therefore avoid presenting one velocity as a mandatory global standard.

5. Single-Run Pressure-Loss Method

This keeps the logic explicit and allows the same engine to work with different pipe materials when actual internal diameter and roughness are known.

DeltaP = f x (L_eq / D) x (rho x v^2 / 2)

L_eq = L_straight + L_fittings

The equivalent fitting length may be entered directly.

6. Reynolds Number and Friction Factor

Re = rho x v x D / mu

Recommended friction-factor logic:

f = 0.25 / [log10(epsilon/(3.7D) + 5.74/Re^0.9)]^2

epsilon is absolute internal roughness. The calculator should either accept user-entered roughness or provide clearly labelled material presets that can be overridden. Actual product data should take priority over a generic material preset.

7. Air Density and Viscosity Inputs

For preliminary pressure-loss work, dry-air density may be calculated from absolute line pressure and temperature. A compressibility factor can be retained for specialist conditions; ordinary plant-air calculations may default to Z = 1.000 with that assumption shown.

rho = P_abs x M_air / (Z x R x T)

Dynamic viscosity changes mainly with temperature and only weakly with pressure at ordinary compressed-air conditions. The implementation can either calculate viscosity from a verified temperature correlation or allow a default engineering value with a visible assumption.

8. Input Model

9. Candidate Diameter Register

The website calculator should permit several candidate internal diameters to be tested at once. This is more useful than returning one theoretical diameter because commercial pipe systems are selected from discrete sizes.

10. Reference Test Case A - 15 m3/min FAD Main

Reference flow is 15 m3/min at 1.01325 bar(a) and 20 C. The line operates at 7.0 bar(g), 25 C, atmosphere 1.01325 bar(a). Check a 63 mm internal-diameter pipe over 100 m straight length plus 30 m equivalent fitting length. Use roughness 0.045 mm, Z = 1.000 and viscosity 1.85e-5 Pa.s.

The 63 mm candidate is therefore a plausible preliminary size only if the project's velocity and pressure-loss limits accept those values.

11. Reference Test Case B - Diameter Comparison

Using the same 15 m3/min FAD operating condition, compare several internal diameters by velocity alone.

This comparison shows why a nominal pipe label is not sufficient. The actual internal diameter strongly changes gas velocity, and pressure loss changes even more sharply because diameter also appears in the friction-length ratio and Reynolds number.

12. Reference Test Case C - Long Branch Screening

A branch carries 10 m3/min FAD at the same line pressure and temperature. Compare a 63 mm and 75 mm internal diameter over 200 m straight length plus 40 m equivalent fittings.

The example demonstrates why long branches should be checked on pressure loss as well as velocity.

13. Allowable Pressure-Loss Logic

The user enters the pressure-loss budget assigned to this run from the overall system pressure budget. The calculator then screens each candidate against that limit.

Pass_pressure = (DeltaP_calculated <= DeltaP_allowable)

Pass_velocity = (v_calculated <= v_limit)

Even then, the final design must consider downstream regulators, dryers, filters, hoses, couplings, valves and machine-entry losses.

14. Validation and Guardrails

15. Output Specification