Controllable vs. Fixed Ejectors: Why Modulation Matters
INTRODUCTION
Paper machines don’t run at one perfect point all day. Basis weight changes, sheet speeds change, seasons change—and your headers move with them. A traditional Fixed-Nozzle steam ejector (fixed Steam Thermocompressor) is tuned for a single design point; off that point it can lose suction or over-compress, forcing bypasses and wasting steam. A Jetomat controllable ejector solves this with a Variable-area Motive Nozzle driven by an actuator: it trims the jet to the actual load so mixed pressure stays put and suction holds—even as conditions swing. Think of it like Globe Valve vs. Control Valve: fixed nozzle ≈ globe (one position), Variable Nozzle ≈ True Control Valve.
WHAT IS A CONTROLLABLE VS. A FIXED EJECTOR?
Fixed-Nozzle Ejector
Single geometry: Nozzle → Mixing Chamber → Diffuser.
Narrow control window (~2–5% of design).
Works best at constant demand; needs higher motive pressure (≥6–7 barg) to stay on map.
Controllable Jetomat Ejector
Same hydrodynamics, but the Nozzle throat area (A_t) is modulated via a spindle and actuator (pneumatic or electric).
Wide effective range; control from ~10–15% up to 70–80% of capacity is typical.
Holds discharge/mixed pressure constant under changing Motive or Suction conditions, and can operate even with Low Motive Pressure (1–3 barg) in many services.
Core physics: Venturi/Bernoulli. Accelerating the motive jet drops static pressure (vacuum) to entrain suction vapor; the Diffuser reconverts velocity to pressure—classic Steam Jet Ejector Working Principle for compressible media.
COMPONENTS
Variable-area Motive Nozzle
Actuator + positioner move the spindle to change (A_t), thereby tuning motive mass flow (\dot m_m), jet Mach number and entrainment.
Pneumatic (fast, spring fail-safe) or electric (fieldbus-friendly) options.
Mixing Chamber
The high-velocity jet entrains low-pressure vapor (Flash Steam) from returns/dryers; momentum exchange happens here.
Diffuser
Diffuser and Nozzle pairing sets feasible compression and turndown; pressure is recovered to your setpoint.
Controls
PID on mixed-steam pressure (or temperature proxy) drives the actuator; stability across the operating map is the whole point of modulation.
These parts are custom-sized per application to cover your min/normal/max conditions rather than a single point.
WHY MODULATION MATTERS
Two non-negotiables describe ejector behavior:
Entrainment ratio ( \omega = \dot m_s / \dot m_m ) (suction/motive)
Compression ratio ( \pi = p_{\text{mixed}}/p_{\text{suction}} )
A Fixed Nozzle locks you to one ((\omega, \pi)). When header pressure drops or suction rises, you slide off the map → Loss of Suction or Over-Compression.
A Variable Nozzle shifts (A_t) to re-center operation—keeping suction and mixed pressure stable across 10–15% up to 70–80% load range (vs. 2–5% for fixed). That’s why controllable units fit fluctuating demand or fluctuating boiler supply, not just steady duty.
PAPER-MACHINE EXAMPLES
A) Day–night production swings on a dryer group
Target mixed pressure: 3.5 barg; Motive Header: 9–11 barg across the day.
Suction (flash) flow: 1.0–2.2 t/h as sheet speed/GSM changes.
Fixed-nozzle: sized for 1.6 t/h.
At 1.0 t/h, the device over-compresses; you see hot spots, controller fights, vents crack.
At 2.2 t/h, suction collapses; traps lift, moisture profile drifts.
Controllable Jetomat:
Closes the nozzle at low load to avoid over-compression; opens at high load to preserve entrainment.
Mixed pressure holds within a tight band; sheet temperature variation shrinks, enabling speed increases and fewer breaks. (Typical mill reports show 20–30% steam reduction potential vs. throttling when recovery stays engaged off-design.)
B) Seasonal header variation (winter 11 barg, summer 9 barg)
Fixed-Nozzle needs ≥6–7 barg motive margin; in summer afternoons suction can fall out.
Controllable unit opens the nozzle at lower motive pressure to hold setpoint, so the Flash Steam Recovery System keeps running—no afternoon plumes, no PRV work-arounds.
SIDE-BY-SIDE: FIXED VS. CONTROLLABLE
BENEFITS
Energy & steam economy
Holding suction through swings keeps Steam Jet Ejectors in compression mode (not bypass) so you genuinely Reuse Flash Steam instead of venting it. Plants targeting dynamic operation see double-digit Saving Energy, year after year.
Quality & Throughput
Stable mixed pressure → thinner Steam Condensate films on cans → stronger Heat Transfer Solutions and flatter temperature profiles. That improves moisture control and reduces warp/wrinkles on lightweight grades.
Reliability
Variable-nozzle units avoid the off-design “dead zones” where fixed units chatter or lose suction. Baelz notes many replacements of failed fixed Thermocompressors with the variable-area design for this reason.
Lifecycle Cost
While a controllable unit includes actuator/positioner, the slides show the 10-year operating cost is lower because fuel/steam savings dominate—especially where demand varies.
PRACTICAL SELECTION
Is Energy Efficiency a Priority (steam cost high)? If yes → favor controllable.
Is Demand or Suction Pressure Variable? If yes → Variable Nozzle; if truly constant → fixed can suffice.
Required Turndown? If >10–20%, choose Variable; fixed is comfortable only in ~2–5% band.
Bonus: You can pair the controllable nozzle with either pneumatic actuation (fast, spring fail-safe) or electric (digital integration)—pick based on loop speed, environment, and utilities. (See our actuator guide for details.)
TRYING IT BACK TO KEYWORDS
A Jetomat’s nozzle design and Diffuser apply the Thermocompressor Working Principle to keep Steam and Condensate in circuit: the device acts as a compact Vapor/Steam Compressor, turning Flash Steam into useful mixed steam for your Heating Systems—a built-in Heat Recovery System. This often allows trap banks to shrink (simpler Condensate and Flash Steam Recovery System) and reduces need for separate Steam Separator hardware at the user.
CONCLUSION
Fixed ejectors are fine at one point; modern paper mills need performance Across Many Points. A controllable Jetomat tracks your process—basis weight changes, header dips, seasonal swings—so suction and mixed pressure remain on spec, energy is reused (not vented), and runnability improves.
What to do now
Map min/nominal/max for motive pressure, suction flow/pressure, and target mixed pressure on each dryer group.
Flag where fixed units are bypassed or unstable (loss of suction, over-compression).
Request a Thermocompressor Design check (entrainment map sized to 10–15% → 70–80% control range; motive as low as 1–3 barg if applicable).
Pilot a controllable unit; track mixed-pressure stability (±0.05 bar), steam/ton, and moisture profile over 2–4 weeks.
Standardize across groups to lock in 20–30% steam savings potential where variability is high.