1) What “Dry Air Equivalent” really means
Eductors and ejectors move momentum, not volume.
For gases, performance depends on gas density, which is set by:
• Molecular weight
• Pressure
• Temperature
We rate gas ejectors in SCFM of dry air at standard conditions (14.7 psia, 60°F, MW = 28.97).
So, every real gas load must be converted to “dry air equivalent SCFM” before using ejector curves.
2) The physics
Entrainment and pressure recovery depend on the ratio of motive flow to suction flow and on suction pressure .
For gases, that ratio depends on mass flow, not ACFM or SCFM.
Dry-air equivalent is simply a way to express mass flow as if the gas were air.
3) The Dry Air Equivalent conversion formula
If you have any gas (or vapor mixture), convert it to dry-air-equivalent SCFM:
Where:
- MW = molecular weight
- 28.97 = molecular weight of dry air
- Temperatures in absolute (°R)
This correction is the gas-phase equivalent of the specific-gravity corrections for liquids .
4) Why molecular weight matters
From ideal-gas physics:
Heavier gases have higher density → harder to entrain → require larger ejector.
Examples:
| Gas | MW | DAE Factor |
|---|---|---|
| Hydrogen | 2 | 0.26 |
| Methane | 16 | 0.74 |
| Air | 28.97 | 1.00 |
| CO₂ | 44 | 1.23 |
| Propane | 44 | 1.23 |
| Chlorine | 70.9 | 1.56 |
So:
• 10 SCFM of chlorine behaves like 15.6 SCFM of air
• 10 SCFM of hydrogen behaves like 2.6 SCFM of air
5) Temperature correction
Note that temperature affects vapor pressure and flashing, and must be corrected using NPSH .
For gases, temperature affects density, so it affects entrainment the same way.
Hot gas = lighter = easier to pull
Cold gas = heavier = harder to pull
That is why the √(Tstd/Tgas) term is used.
6) How this plugs into ejector sizing
Desired suction flow ÷ Tabulated suction flow = Sizing Factor
For gas ejectors:
- Convert real gas to DAE SCFM
- Look up suction capacity on our ejector curve (which is in SCFM of air)
- Compute sizing factor
- Pick ejector
This is the gas-phase equivalent of correcting liquids to water before using tables.
7) What about steam-motive ejectors?
Steam ejectors always assume:
• Suction gas is “air-like” unless corrected
• Steam flow is based on mass, not volume
If you pull something heavier than air:
• Steam consumption goes up
• Stage count may increase
DAE tells the steam-ejector vendor exactly how much harder the job is.
8) What about mixtures (air + solvent vapor)?
Use average molecular weight:
Then apply the same DAE formula.
Example:
50% air (29) + 50% hexane vapor (86):
MW = (0.5 × 29) + (0.5 × 86) = 57.5
DAE factor = √(57.5 / 28.97) = 1.41
So 100 SCFM of this vapor behaves like 141 SCFM of air.
“This formula…corrects them to a set of standard conditions…so performance can be predicted for different fluids.”
That is exactly what Dry Air Equivalent is doing — it converts any gas back to the standard reference gas (air) so the ejector curves are valid.