A steam jet ejector (also called a steam jet vacuum system or SJVS) uses high-velocity steam — the 'motive fluid' — to entrain and compress a suction gas load, raising it from vacuum conditions to a higher discharge pressure. Unlike mechanical vacuum pumps, ejectors have no moving parts, are inherently simple in construction, and can handle hot, corrosive, or condensable vapours that would destroy rotary equipment.
The Working Principle: Four Zones
- Zone 1 — Nozzle: High-pressure motive steam (typically 5–12 bar gauge) expands through a convergent-divergent (de Laval) nozzle, converting its pressure energy into kinetic energy. The steam exits at supersonic velocity (Mach 2–5), simultaneously creating a low-pressure zone at the nozzle exit.
- Zone 2 — Suction Chamber: The low pressure created by the supersonic steam jet entrains the suction gas — the vapour load from the process being evacuated. The suction vapour and motive steam mix in the suction chamber.
- Zone 3 — Diffuser: The mixed stream enters a convergent-divergent diffuser. In the convergent section, velocity decreases and pressure rises (ram compression). A normal shock wave forms within the diffuser, further raising pressure. The divergent section continues the pressure recovery.
- Zone 4 — Discharge: The mixed stream discharges at intermediate pressure — higher than suction, lower than motive steam pressure. In a multi-stage system, this discharge feeds the suction of the next stage.
Single-Stage vs Multi-Stage Systems
A single ejector stage can achieve a compression ratio (discharge pressure / suction pressure) of approximately 4:1 to 8:1. For deep vacuum applications — below 100 mbar absolute — multiple stages in series are required, with intercondensers between stages to remove condensable vapours (mainly water) and reduce the volumetric load on subsequent stages.
| Configuration | Suction Pressure Achievable | Typical Application |
|---|---|---|
| Single stage | 150–300 mbar abs | Light vacuum, vacuum distillation columns operating at moderate vacuum |
| Two stage with intercondenser | 25–50 mbar abs | Vacuum distillation, deaeration, evaporators |
| Three stage with 2 intercondensers | 3–10 mbar abs | Vacuum crystallisers, solvent recovery, pharmaceutical dryers |
| Four or five stage | 0.1–3 mbar abs | Molecular distillation, freeze drying, high-vacuum reactors |
| Combination ejector + liquid ring | 1–50 mbar abs | Hybrid systems — ejectors for deep vacuum, LRVP as backing pump |
The Role of Intercondensers
Between each ejector stage, an intercondenser (typically a shell-and-tube or surface condenser) condenses the water vapour from the previous stage's discharge. This is critical for two reasons:
1. Reduced volumetric load: Water vapour at 25 mbar absolute has a specific volume roughly 50 times greater than water vapour at 1 bar absolute. Condensing the water between stages dramatically reduces the gas volume entering the next stage, allowing each subsequent ejector to be much smaller.
2. Reduced steam consumption: Without intercondensers, each stage must handle the full steam load of all previous stages. Intercondensers reduce total motive steam consumption by 40–60% compared to a system without interstage condensation.
The cooling water requirement for intercondensers is the system's main utility cost. For a three-stage system serving a 100 kPa suction load, cooling water consumption may be 20–30× the mass flow of the suction gas load.
Sizing Parameters: What to Provide for an Ejector Enquiry
- Suction pressure (the vacuum level to be maintained, in mbar absolute or mmHg absolute)
- Suction gas composition and flow rate — what is being evacuated? Air? Steam? Organic vapour? Mixture? State mass flow rate (kg/hr) or volumetric flow (m³/hr at suction conditions)
- Suction gas temperature
- Discharge pressure — atmospheric? Or into a downstream vessel at some pressure?
- Motive steam pressure and quality (saturated, superheated — degrees of superheat if known)
- Available cooling water inlet temperature (critical — determines intercondenser duty and system staging)
- Available cooling water outlet temperature allowed
- Whether the system is required to handle air-in-leakage (and estimated leakage rate)
Steam Jet Ejector vs Liquid Ring Vacuum Pump: When to Choose Each
For most chemical plant and pharmaceutical vacuum applications where steam is available and the suction gas contains condensable vapours, steam jet ejectors offer the best combination of reliability, deep vacuum capability, and low maintenance cost. Where electricity cost is the primary concern and cooling water is limited, liquid ring vacuum pumps or hybrid systems are preferred.
| Criterion | Steam Jet Ejector | Liquid Ring Vacuum Pump (LRVP) |
|---|---|---|
| Moving parts | None — passive device | Rotating impeller and shaft seal |
| Suction gas temperature | Any — even superheated vapours | Limited to service liquid temperature + ~15°C |
| Handling corrosive vapours | Excellent — choose corrosion-resistant materials (SS, titanium) | Fair — impeller material must resist corrosion |
| Handling condensable vapours | Excellent — vapours condense in intercondensers | Good — vapours condense in service liquid |
| Deep vacuum (< 5 mbar) | Yes — with sufficient stages | Difficult; single-stage LRVP limited to ~30–50 mbar |
| Motive energy | Steam (high consumption — steam at 10 bar has high cost) | Electricity (motor-driven; typically more efficient) |
| Capital cost | Low — no mechanical components | Higher — mechanical construction, shaft seal, motor |
| Maintenance | Very low — no wearing parts; corrosion is main failure mode | Regular — seal replacement, bearing maintenance, impeller inspection |
| Startup | Immediate — no warm-up required | Immediate |
| Noise | High — supersonic steam jet creates significant noise; requires silencer | Low — relatively quiet |
| Cooling water requirement | High — for intercondensers | Moderate — for service liquid cooling |
RELATED EQUIPMENT FROM OHM ENGINEERS