Air Velocity Formula

The free-air flow rate is first scaled to line pressure using the ratio of atmospheric pressure to absolute line pressure. The result is divided by each pipe's internal area. The factor 144 converts square feet to square inches, and the factor 60 converts minutes to seconds.

\begin{equation}\label{eq:air-velocity} v_i = Q \cdot \frac{P_{atm}}{P_{line} + P_{atm}} \cdot \frac{k_{144}}{A_i} \cdot \frac{1}{k_{60}} \end{equation}

Symbols

\(v_i\)	Compressed air velocity for nominal pipe size i \([\unit{ \foot\per\second}]\)
\(Q\)	Volumetric free-air flow rate \([\unit{ \standardCubicFeetPerMinute}]\)
\(P_{atm}\)	Atmospheric absolute pressure \([\unit{ \psia}]\)
\(P_{line}\)	Pipe gauge pressure \([\unit{ \psig}]\)
\(k_{144}\)	Square inches per square foot (144) \([\unit{ \inch\squared\per\squareFoot}]\)
\(A_i\)	Internal area for nominal pipe size i \([\unit{ \inch\squared}]\)
\(k_{60}\)	Seconds per minute (60) \([\unit{ \second\per\minute}]\)

Note

The internal areas used by this calculation are in square inches. This matches the formula's 144 in2/ft2 conversion and the implementation table in Compressed Air Pipe Data.