Free Compressed Air

Overview

Engineering reference data for Free Compressed Air in gases and compressed air.

Key Formulas

Ideal Gas Law

$PV = nRT$

Pressure × Volume = moles × gas constant × temperature.

Boyle's Law

$P_1 V_1 = P_2 V_2$

At constant temperature.

Charles's Law

$\frac{V_1}{T_1} = \frac{V_2}{T_2}$

At constant pressure.

Variables

Symbol	Description	Unit
$P$	Pressure	Pa
$V$	Volume	m³
$T$	Temperature	K
$R$	Gas constant	8.314 J/(mol·K)

Compression Ratio in Practice

While the ideal compression ratio follows $CR = \frac{p_d}{p_s}$, real-world applications often require consideration of factors like valve losses and temperature rise. The relationship between gauge and absolute pressure is critical for accurate calculations: $p_{\text{abs}} = p_{\text{gauge}} + p_{\text{atm}}$ where $p_{\text{atm}}$ is typically ~1.013 bar or 14.7 psia at sea level.

Volume Flow Conversions

The relationship between free air volume flow ($q_F$) and compressed air volume flow ($q_C$) is bidirectional: $q_F = CR \times q_C$ $q_C = \frac{q_F}{CR}$

Key Implication: Compressing air reduces its volume dramatically. For example, air compressed to 7 bar gauge (~8 bar absolute) occupies roughly 1/8th of its original free air volume.

Unit Conversions

Common conversion factors for compressed air systems:

• Pressure: $1 \text{ psi} = 6.9 \text{ kPa} = 0.069 \text{ bar}$
• Flow: $1 \text{ ncfm (normal cubic foot per minute)} = 0.5 \text{ nl/s (normal liter per second)}$

Note: "Standard" (scfm) and "Normal" (nl/s) conditions vary by region. Ensure consistent reference conditions (temperature, pressure) when converting between units.

References

• Original Source