Steam Pressure Blending in Cogeneration Plants

INTRODUCTION

Combined Heat & Power (CHP) plants often juggle Multiple Steam Levels: a High-Pressure (HP) boiler supply, Low-Pressure (LP) turbine exhaust, and a Medium-Pressure (MP) header serving process users or district heating.

The classic fix for an MP shortfall is to Throttle HP Steam through a valve—simple, but wasteful. A smarter approach is Steam Pressure Blending with a Jetomat—a controllable Steam Jet Ejector / Steam Thermocompressor that Entrains LP Turbine Exhaust and boosts it with a small portion of HP steam to make stable MP steam.

You convert what would be condenser load into useful heat, Saving Energy and cutting emissions—without installing a separate small boiler.

WHAT IS STEAM PRESSURE BLENDING WITH a JETOMAT?

Steam pressure blending uses a controllable Thermo Vapour Recompressor to combine two sources:

• Motive: HP steam from the boiler (via a shaped Nozzle).

• Suction: LP turbine exhaust or flash steam otherwise headed to the condenser.

Inside the ejector, the motive jet accelerates through the Nozzle, creating a vacuum that Entrains LP vapor in the Mixing Chamber. The Diffuser then reconverts velocity to pressure, delivering Mixed Steam at the MP setpoint. That’s the Steam Jet Ejector Working Principle—a compact Steam Compressor with no rotating parts.

Why it beats throttling: instead of dumping HP pressure across a valve, you do useful work—recycling LP vapor into the MP header.

COMPONENTS OF THE BLENDING LOOP

Controllable Motive Nozzle (with Actuator)

• Pneumatic (fast, spring fail-safe) or electric (fieldbus-friendly) actuator adjusts throat area to modulate motive mass flow.

• Typical stable turndown: 3:1–5:1.

Suction Take-off

• From a backpressure turbine exhaust manifold, LP extraction, or a flash tank ahead of the condenser (typically 0–2 bar(g), 100–120 °C).

• Short, insulated piping; good drainage to avoid slugging.

Mixing Chamber + Diffuser and Nozzle

• Sets achievable entrainment ratio ( \omega=\dot m_s/\dot m_m ) and compression ratio ( \pi= p_{\text{mixed}}/p_{\text{suction}} ).

• Proper Nozzle Design prevents stall; the Diffuser recovers pressure to the MP header (e.g., 3–6 bar(g)).

MP Users (Heat Sinks)

• Process reboilers, evaporators, dryers, or district heating exchangers.

• A Steam Separator downstream is optional; most users accept saturated (slightly wet) steam.

Controls & Interlocks

• Primary PID on Mixed Pressure (MP header).

• Turbine-side permissives: maintain minimum turbine exhaust pressure/flow to protect power output and stability.

THERMODYNAMICS IN PLAIN LANGUAGE

Two levers describe ejector behavior:

• Entrainment Ratio Ω = m_suction / m_motive — how much LP vapor is lifted per kg of motive HP steam.

• Compression Ratio π= Motive Pres / Suction Pres — how high you boost suction pressure.

For single-stage steam ejectors, ( π approx 1.3–2.0 ) is typical.
That fits common CHP blends like LP 1.5 bar(g) → MP ~3–4 bar(g) with motive at 20–45 bar (g).

EXAMPLE

• Motive (HP) header: 40 bar(g) saturated

• Suction (LP) source: 1.5 bar(g) backpressure turbine exhaust (≈ 127 °C)

• Target mixed (MP) header: 4.0 bar(g) (≈ 152 °C)

• Selected design point: ( Ω = 0.9 )

MASS BALANCE

• Want to reclaim m_suction = 8 t/h of LP exhaust that used to head to the condenser.

• Motive required m_motive = m_suction ~ 8/0.9 = 8.9 t/h.

• Mixed to MP m_discharge = m_motive + m_suction ~ 16.9 t/h @ 4 bar(g).

FUEL / STEAM IMPLICATION

• Each 1 kg of LP vapor recovered displaces ≈1 kg of fresh MP steam.

• Here, 8 t/h less boiler steam is needed for MP service (the motive contribution is not “extra” load—it replaces throttled HP and avoids condenser duty on the LP side).

ELECTRICAL & THERMAL BALANCE

• Blending reduces condenser load, potentially improving overall plant heat rate.

• Ensure the turbine heat balance is respected: keep a minimum LP flow through the turbine path to maintain stable power output and exhaust temperature.

CONTROL STRATEGY

1.     Primary Control Variable: MP header pressure (or a downstream temperature proxy).

2.     Modulating Element: Jetomat Nozzle Spindle (via actuator).

3.     Feed-forward: Bias controller output with MP demand (process/district load) and turbine exhaust pressure.

4.     Protection Logic:

• LP suction low-limit: if LP pressure drops, the controller trims motive to prevent over-pulling the turbine.

• Motive low-pressure permissive: pause modulation if HP header sags to protect other Heating Systems.
  

5.     Response: With a modern actuator, setpoint steps settle in <2–5 s, holding ±0.05 bar on the MP header in typical installations.

WHY EJECTOR BLENDING BEATS PRV THROTTLING

PRACTICAL INTIGRATION IN A CHP

Piping & Layout

• Keep the LP suction run short and insulated; slope to avoid condensate pockets.

• Non-return on motive; slow-opening motive valve to avoid water hammer.

• Add a compact Steam Separator upstream only if motive line is wet.

Sizing Inputs

• Motive P/T, LP P/T & available kg/h (by season/load), target MP setpoint, allowable DP to users.

• Pick feasible ( \omega ) and ( \pi ); confirm with Thermocompressor Design maps (nozzle/diffuser geometry).

Operating Envelope

• Design for 3:1–5:1 turndown so night/weekend district loads and shoulder-season operation remain stable.

• Set a minimum turbine exhaust constraint to protect power output when MP demand is low.

KPIs to Trend

• Boiler firing (fuel), MP make-up steam, condenser heat rejection, MP header stability, and Steam and Condensate return rate.

• Expect immediate reductions in condenser load and throttled-steam use.

BENEFITS

Fuel & CO₂ reduction

• LP vapor recovery directly cuts boiler steam for MP; typical CHP sites report 5–15% steam savings on the MP demand served, with several-percent plant-wide energy recovery when multiple blends run.

Heat-rate improvement

• Less condenser duty; better utilization of turbine exhaust heat.

Capacity & flexibility

• Serve new MP loads (process/district heating) without a new small boiler.

• Fast modulation keeps MP steady during rapid demand changes.

Reliability & maintenance

• Ejector has no rotating parts; routine attention is limited to the actuator/positioner.

• Fewer throttling losses and hot-end valve erosion.

System simplicity

• One compact device replaces PRV throttling plus vent management—cleaner heat transfer solutions across the site.

CONCLUSION

Steam pressure blending with a Jetomat Steam Jet Thermocompressor lets CHP plants recycle LP turbine exhaust into a useful MP header using a controlled sip of HP steam.

You avoid PRV waste, trim condenser load, and stabilize distribution—translating directly to Fuel Savings and lower emissions.

HOW TO PROCEED

• Quantify LP exhaust (kg/h) by season and note MP shortfalls.

• Collect motive/LP/MP pressures and allowable DP to users.

• Request a Thermocompressor Design (entrainment ratio, compression ratio, Diffuser and Nozzle geometry, actuator selection).

• Pilot one blend loop; trend boiler firing, condenser duty, MP stability, and CO₂ factors for 2–4 weeks.

• Scale to additional headers for a site-wide Heat Recovery System, integrating with your existing Steam Equipment and Heating Systems.