1) Start with the 3 pressures that define any gas-exhaust eductor
The operating principle is the same for gas service: a high-pressure motive stream accelerates through a nozzle, entrains the suction fluid, and the mixture discharges at an intermediate/outlet pressure.
For gas exhausting you must define:
- Suction condition (vacuum level)
- Suction pressure, Psuction (psia) (or inHg vacuum converted to psia)
- Suction temperature, Tsuction (°F/°R)
- Gas composition / MW (air? solvent vapor? corrosive?)
- Discharge / outlet condition
- Backpressure, Pout (psia) (to atmosphere, to flare header, to scrubber, etc.)
- Include downstream friction losses (the catalog stresses including outlet friction; same idea here).
- Motive supply condition (what powers the eductor)
- Motive type: steam, air, nitrogen, water, etc.
- Motive pressure at the nozzle inlet (flowing, not just gauge on the header)
- Motive temperature (esp. steam quality/superheat)
2) Define the required gas removal rate at suction conditions
People often specify “SCFM” at standard conditions, but the eductor “sees” the actual volumetric flow at suction pressure and temperature.
If you’re given SCFM, convert to ACFM at suction (ideal gas approximation):
Use:
Why this matters: at deeper vacuum (lower psia), the same mass flow becomes a much larger ACFM that the eductor must ingest.
3) Choose the eductor “family” that matches gas exhausting
Note that the nozzle design differs for compressible vs. incompressible motive fluids: gas motives use converging-diverging nozzles to benefit from compressibility.
For exhausting gases, the common choices are:
- Steam jet ejector (steam motive): best for high vacuum, dirty/condensable vapors, simple/no moving parts.
- Air/N₂ jet ejector (gas motive): convenient if you have compressed gas; typically less efficient than steam for deep vacuum.
- Liquid jet eductor (water motive): great when you can tolerate wet suction or you’re pulling through a scrubber; vacuum level is usually limited vs. steam jets.
4) Translate “vacuum requirement” into an equivalent suction severity
Note the importance of “lift”/NPSH thinking which emphasizes that reduced pressure in the suction chamber can cause flashing if the liquid’s vapor pressure is high.
For gas exhausting, the equivalent caution is:
- Condensation (steam motive into cold vapors)
- Choking/sonic flow limits at the suction nozzle
- Two-phase mixtures if vapors condense or liquid carryover occurs
Use this as a design check:
- If the suction stream can condense at the mixing temperature, plan for knockout/drainage and expect performance shift.
5) Account for outlet/backpressure and recovery
Discharge pressure depends on the motive-to-suction ratio and suction pressure.
For gas exhausting, this is critical:
- Ejectors are very sensitive to backpressure.
- If your discharge goes to a header that can rise, size for worst-case Pout (and add margin).
Rule of thumb practice:
- Don’t undersize the discharge line; keep it large/short—your catalog makes the same point for outlet piping.
6) Use turndown strategy if gas flow varies
We suggest using multiple eductors in parallel when you need more than ~35% turndown.
For gas exhausting, that approach is often the cleanest way to handle:
- Startup peaks vs steady operation
- Batch venting
- Variable leak rates
Typical approach:
- 2×50% or 3×33% ejectors with isolation valves
- Stage/sequence them as demand changes
7) Practical sizing workflow:
A) Suction (gas to be exhausted)
- Gas: __________________ (air / N₂ / solvent vapor / mix)
- Molecular weight (MW): ________
- Suction pressure: ________ psia (or ____ inHg vac)
- Suction temperature: ________ °F
- Required flow: ________ SCFM @ standard or ________ ACFM @ suction
- Is it wet/condensable? Yes / No
- Corrosive/toxic? Yes / No
B) Discharge
- Discharge destination: atmosphere / scrubber / flare / header
- Discharge (back) pressure: ________ psia (include worst case)
- Discharge temperature limit? ________
- Allowable noise? Yes / No (gas jets can be loud)
C) Motive
- Motive fluid: steam / air / N₂ / water
- Motive pressure at nozzle (flowing): ________ psig
- Motive temperature/quality: ________
- Motive supply limit (max flow): ________
D) Mechanical / installation
- Materials: CS / SS / alloy / lined
- Connections: suction ___, motive ___, discharge ___
- Any solids/particulates? Yes / No
- Need check valve or isolation? Yes / No
50+ years of eductor experience allows Northeast Controls to provide you with –
- Ejector performance curve (suction flow vs suction pressure) at your Pm and Pout
- Motive consumption at the duty point
- Recommended nozzle size and body size
- If deep vacuum: confirm if single stage is adequate or if multi-stage is required
8) A quick “sanity check” example (numbers illustrate the process)
Say you need 200 SCFM of air at 10 psia suction, 80°F, discharging to atmosphere (14.7 psia).
Convert to ACFM at suction:
So the eductor must ingest ~305 ACFM at suction conditions, not “200”.
9) Common mistakes (what to watch)
- Using SCFM without converting to suction ACFM (undersizes the ejector).
- Ignoring discharge header backpressure swings.
- Long/small discharge piping (kills performance)—the catalog warns to include friction losses and keep outlet piping properly sized.
- Expecting big turndown from one nozzle (use parallel units per the catalog’s turndown guidance).
- Condensation/two-phase effects not considered.