Standard air volume flow - SCFM - can be converted to actual air volume flow - ACFM - with the expanded air ratio equation
EAR = (30 / P) ((T + 460) / 520) (1)
EAR = ACFM / SCFM = expanded air ratio - ratio between actual air volume flow - ACFM - and standard air volume flow - SCFM
P = absolute vacuum pressure (inches Hg) where 29.92 inches Hg is normal atmospheric pressure and 0 inches Hg is absolute vacuum
T = temperature (oF)
Actual air volume flow can be calculated by multiplying standard air volume with the expanded air ratio.
Expanded air ratio for temperatures 60oF (15.5oC) are indicated in the diagram below:
The diagram above can be used to calculate air flow in SI-units where
- actual air flow - al/s
- standard air flow - sl/s
Example - Vacuum Expansion of Air
From the table above the expansion ratio of air from atmospheric pressure to 50% vacuum is 2.
The relation between vacuum in % and other pressure units are indicated in
Air, LNG, LPG and other common gas properties, pipeline capacities, sizing of relief valves.
Vacuum is defined as air pressure below atmospheric pressure.
Vacuum pumps - calculate flow rate and required evacuation time.
Measure air volume flow in vacuum.
Converting between vacuum units like % vacuum - mm Mercury - psi - torr - micron - kPa.
Air velocity diagrams for vacuum pipe lines.
Calculate pressure drops in vacuum pipe lines.
Online calculator, figures and tables giving the boiling temperatures of water in varying vacuum, SI and Imperial units.