Compressed-Air Pipe Pressure Drop
The shop-air question the compressed-air bench never answered: how much pressure a run loses.
Example
You enter
- Airflow (scfm) 100
- Pipe inside diameter (in, actual ID) 1.049
- Run length incl. fitting equiv. (ft) 100
- Line pressure (psig) 100
- Air temperature (°F) 68
- Pipe roughness (ft) 0.00015
You get
- Density (lb/ft³) 0.586718
- Actual (CFM) 12.816
- Velocity (fps) 35.5897
- Reynolds 150079
- Friction factor 0.023779
- Pressure drop 2.18164
Details, formula, and sources
Built from the ideal gas law and Darcy-Weisbach on this catalog's own Colebrook solver, not an empirical air constant. The trap it exists to show: standard cubic feet are a MASS measure, so 100 scfm at 100 psig is only 12.8 ACTUAL cfm - sizing off scfm directly overstates velocity nearly 8x. 100 scfm through 1-in pipe over 100 ft drops 2.18 psi; flags the 10%-of-line rule of thumb.
rho = P_abs x 144 / (R_air x T_R) with R_air = 53.35 ft-lbf/lb-R; Q_actual = scfm x (14.7/P_abs) x (T_R/527.67); Re = rho V D / mu; f from Colebrook-White; dP = f (L/D) rho V^2 / (2 gc) / 144.
Darcy-Weisbach with the Colebrook-White friction factor and the ideal gas law, by name; NO empirical compressed-air constant or sizing table is used.
All relations are public engineering formulas; the Colebrook solver is the same one this catalog already uses for duct and pipe friction.
Estimate. AHJ and licensed professional govern.
Field names used by the API: scfm, pipe_id_in, length_ft, line_pressure_psig, air_temp_f, roughness_ft, density_lb_ft3, actual_cfm, velocity_fps, reynolds, friction_factor, pressure_drop_psi
- No empirical constant friction from the repo's verified Colebrook solver, density and volume from the ideal gas lawDarcy-Weisbach / Colebrook-White
- Viscosity derivation 1.81e-5 Pa-s at 68 F converted with the exact 0.67197 lb/(ft-s) per Pa-s factorstandard air property
- Inlet density used for the whole run; conservative, since density rises as pressure dropsstated simplification