Ejector Design

Chapter from the Miscellaneous Design Guide.

EJECTOR DESIGN & OPERATION

Ejector Fundamentals

Steam jet ejectors use high-pressure motive steam to entrain and compress low-pressure gas/vapor. They are the primary vacuum-generating device in refinery vacuum distillation units.

Operating Principle:
• Motive steam expands through a converging-diverging nozzle → supersonic velocity
• Low pressure at nozzle exit entrains suction gas
• Mixed stream enters diffuser → kinetic energy converts to pressure
• Discharge at intermediate pressure (or to next stage)

Staging:
• Single-stage: compression ratio up to 6:1 (discharge/suction pressure)
• Two-stage: compression ratio up to 30:1 (with intercondenser)
• Three-stage: compression ratio up to 200:1 (typical vacuum tower)
• Each stage: ejector + intercondenser (to remove condensable vapors)

Performance Characteristics:
• No moving parts — highly reliable
• Performance is FIXED by geometry — no control except on/off
• Motive steam pressure must be stable (±5% affects capacity significantly)
• Backpressure sensitive — must stay within design range

Ejector System Troubleshooting

Symptom	Possible Cause	Diagnostic	Fix
Loss of vacuum (gradual)	Fouled intercondenser, CW temp rise	Check CW temp, condenser ΔP	Clean condenser, increase CW
Loss of vacuum (sudden)	Ejector broken (nozzle damage, steam loss)	Check motive steam P, inspect ejector	Repair/replace nozzle, restore steam
High steam consumption	Worn nozzle (enlarged throat)	Measure nozzle throat ID	Replace nozzle
Surging/hunting	Operating near break point	Check suction pressure trend	Increase motive steam P or reduce load
Condensate backup	Barometric leg issue, level control	Check barometric seal, hotwell level	Clear drain, fix level control
CW in process	Intercondenser tube leak	Sample hotwell condensate for HC	Plug/replace leaking tubes

Source: Miscellaneous_Design_Guide_v1.xlsx · sheet “Ejector Design”

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