1) Define the 3 pressures that control ejector performance
Eductors work by converting motive pressure to velocity, entraining suction fluid, then recovering some pressure in the diffuser. The recovered discharge pressure depends on (a) motive-to-suction ratio and (b) how much suction pressure (vacuum) is being pulled.
You must define:
A) Suction condition (vacuum requirement)
- Target suction pressure Ps (psia) or vacuum in inHg (convert to psia)
- Suction temperature Ts
- Gas composition / MW (air vs solvent vapor vs mixed gases)
- Required removal rate (SCFM or lb/hr)
B) Discharge / backpressure
- Destination: atmosphere, scrubber, flare header, condenser, etc.
- Backpressure Pd (psia), including worst case
- Include downstream friction losses (critical). The catalog stresses including friction losses and calculating them using combined flow (motive + suction).
C) Motive supply
- Motive fluid: steam or compressed gas
- Motive pressure Pm (use flowing pressure at ejector inlet)
- Motive temperature (steam quality/superheat matters; gas temperature matters)
2) Convert the required gas load into “what the ejector actually sees”
Most specs are given as SCFM (standard). Ejector capacity is tied to mass flow and suction density, so always convert to an equivalent ACFM at suction (ideal gas):
Why this matters: at deeper vacuum (lower (P_s)), the same SCFM turns into much larger ACFM, which drives ejector size and/or staging.
3) Apply the catalog’s sizing workflow (adapted to gas exhausting)
Your catalog’s sizing method is essentially:
- Determine suction requirement
- Determine outlet requirement including friction losses (use combined flow) and keep outlet piping ≥ outlet connection
- Select motive pressure closest to or lower than available motive pressure
- Use a tabulated suction flow and compute a Sizing Factor (Desired/Tabulated), then pick a unit meeting/exceeding it
- Calculate motive consumption based on the selected factor
For gas exhausting (steam or gas motive), what changes?
- You typically won’t use the liquid suction tables (those are for liquid suction performance).
- Instead, you request the same “tabulated” values from the manufacturer as gas ejector performance curves:
- suction capacity vs suction pressure (at your Pd and Pm)
- motive consumption (steam lb/hr or gas SCFM) at that point
So you’re still doing Desired / Tabulated logic — the “table” is just the vendor’s gas curve rather than the liquid tables.
4) Decide whether you need a single stage or multiple stag
A single-stage ejector is often fine for:
- venting to atmosphere with modest vacuum
- exhausting at/near atmospheric suction
For deep vacuum, you usually need:
- multi-stage steam jet ejectors (often with intercondensers for steam systems)
- or a hybrid (steam ejector + mechanical pump)
Practical rule: the deeper the vacuum and the higher the discharge backpressure, the more likely you need staging.
5) Steam motive sizing guide (what to specify / what to expect)
Inputs you must provide
- Steam pressure at ejector inlet (psig) and condition (saturated vs superheated)
- Suction pressure (psia) / vacuum target
- Discharge/backpressure Pd (psia)
- Gas composition + temperature
- Required gas removal rate (SCFM or lb/hr)
What you should request back
- Required ejector stage count and stage sizes
- Steam consumption (lb/hr) at duty point
- Cooling water and condenser duties (if intercondensers are used)
- Discharge temperature and material recommendations
Install note (from catalog logic): treat discharge piping as a performance limiter—include friction and size the discharge line appropriately for the combined flow.
6) Compressed gas motive sizing guide (air/N₂) (what to specify / what to expect)
Inputs you must provide
- Motive gas type and flowing pressure at inlet
- Motive temperature
- Suction pressure / vacuum target
- Discharge/backpressure
- Required gas removal rate and composition
What you should request back
- Motive gas consumption (SCFM)
- Ejector size/model and suction capacity curve
- Required noise control (gas jets can be loud)
- Whether operation is near “choked” limits (vendor will confirm)
7) Turndown and control
If your gas load varies a lot, a single nozzle won’t give broad turndown. The catalog guidance: if you need >35% turndown, use two or more eductors in parallel to cover the range.
Common approach:
- 2×50% or 3×33% ejectors with isolation valves
- Stage them on/off as demand changes
8) Any RFQ to us would need to include:
Gas Ejector (Steam or Gas Motive) Sizing Request
- Service: Exhausting gas/vapor from __________
- Gas composition / MW: __________
- Suction pressure target: ____ psia (or ____ inHg vacuum)
- Suction temperature: ____ °F
- Required removal rate: ____ SCFM (standard conditions: ____°F, ____ psia)
- Discharge/backpressure: ____ psia (includes friction losses; destination: __________)
- Motive: Steam / Air / N₂
- Motive pressure at inlet (flowing): ____ psig
- Motive temperature/condition: ____ (steam quality/superheat if steam)
- Materials: CS / 304 / 316 / alloy / lined
- Any condensables/particulates/corrosives?: __________
- Requested output: stage count, ejector size(s), suction capacity curve, motive consumption, recommended piping sizes, and any condenser duties (steam systems)