NeqSim’s Ejector mixes a high-pressure motive stream with a lower-pressure suction stream and flashes the combined stream to a specified discharge pressure. Use it for screening and process integration, then qualify the selected geometry and operating envelope against vendor data and accountable engineering review.

Model and input contract

The current implementation performs one quasi-one-dimensional energy, momentum, and diffuser-recovery calculation. Although ejector literature distinguishes constant-pressure mixing (CPM) and constant-area mixing (CAM), the public Ejector API does not expose a CPM/CAM model selector. Do not describe a run as a selected CPM or CAM calculation.

Use these input conventions:

The motive and suction fluids must use compatible component definitions and mixing rules. The discharge pressure is a required operating target. A manually supplied mixing pressure is limited to the suction pressure by the implementation.

Executable gas-ejector example

The following complete Java 8 program is compiled and executed by the documentation test. Run with assertions enabled, for example java -ea EjectorProcessExample.

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import neqsim.process.equipment.ejector.Ejector;
import neqsim.process.equipment.stream.Stream;
import neqsim.process.equipment.stream.StreamInterface;
import neqsim.process.mechanicaldesign.ejector.EjectorMechanicalDesign;
import neqsim.process.processmodel.ProcessSystem;
import neqsim.thermo.system.SystemInterface;
import neqsim.thermo.system.SystemSrkEos;

public final class EjectorProcessExample {
  private static final Logger logger = LogManager.getLogger(EjectorProcessExample.class);

  private EjectorProcessExample() {}

  public static void main(String[] args) {
    SystemInterface motiveFluid = new SystemSrkEos(293.15, 10.0);
    motiveFluid.addComponent("methane", 1.0);
    motiveFluid.createDatabase(true);
    motiveFluid.setMixingRule(2);

    Stream motive = new Stream("motive gas", motiveFluid);
    motive.setFlowRate(0.50, "kg/sec");

    SystemInterface suctionFluid = new SystemSrkEos(293.15, 1.0);
    suctionFluid.addComponent("methane", 1.0);
    suctionFluid.createDatabase(true);
    suctionFluid.setMixingRule(2);

    Stream suction = new Stream("suction gas", suctionFluid);
    suction.setFlowRate(0.15, "kg/sec");

    Ejector ejector = new Ejector("gas booster", motive, suction);
    ejector.setDischargePressure(3.0);
    ejector.setEfficiencyIsentropic(0.75);
    ejector.setSuctionNozzleEfficiency(0.90);
    ejector.setMixingEfficiency(0.85);
    ejector.setDiffuserEfficiency(0.80);

    ProcessSystem process = new ProcessSystem();
    process.add(motive);
    process.add(suction);
    process.add(ejector);
    process.run();

    StreamInterface discharge = ejector.getMixedStream();
    EjectorMechanicalDesign design = ejector.getMechanicalDesign();
    double entrainmentRatio = ejector.getEntrainmentRatio();
    double compressionRatio = ejector.getCompressionRatio();
    double expansionRatio = ejector.getExpansionRatio();

    assert Math.abs(discharge.getPressure("bara") - 3.0) < 1.0e-4;
    assert Math.abs(ejector.getMassBalance("kg/sec")) < 1.0e-6;
    assert Math.abs(entrainmentRatio - 0.30) < 1.0e-10;
    assert Math.abs(compressionRatio - 3.0) < 1.0e-2;
    assert Math.abs(expansionRatio - 10.0 / 3.0) < 1.0e-2;
    assert design.getMotiveNozzleThroatArea() > 0.0;
    assert design.getMixingChamberArea() > 0.0;
    assert design.getDiffuserOutletArea() > 0.0;

    logger.info(
        "Pd={} bara, ER={}, CR={}, expansion ratio={}",
        discharge.getPressure("bara"),
        entrainmentRatio,
        compressionRatio,
        expansionRatio);
  }
}

The example uses prescribed inlet mass flows. Therefore the entrainment ratio is the identity

\[ER=\frac{\dot m_{suction}}{\dot m_{motive}}\]

and is not solved from ejector geometry or a vendor capacity curve.

Results and mechanical design

After run(), inspect:

Quantity Method Interpretation
Discharge stream getMixedStream() Combined stream at the specified discharge pressure
Entrainment ratio getEntrainmentRatio() Prescribed suction mass flow divided by prescribed motive mass flow
Compression ratio getCompressionRatio() Discharge absolute pressure divided by suction absolute pressure
Expansion ratio getExpansionRatio() Motive absolute pressure divided by discharge absolute pressure
Critical back pressure getCriticalBackPressure() Internal thermodynamic screening estimate in bara
Area ratio getAreaRatio() Calculated mixing area divided by motive-nozzle throat area
Mach diagnostics getMotiveNozzleMach(), getSuctionMach(), getMixingMach() Calculated local velocity divided by estimated speed of sound
Breakdown flag isInBreakdown() True when the specified discharge exceeds the calculated critical back pressure

getMechanicalDesign() exposes calculated areas, velocities, characteristic lengths, and volumes. These values are preliminary process-model sizing outputs. They are not a fabrication drawing, materials assessment, code check, nozzle-stress analysis, or vendor guarantee.

Performance-curve semantics

generatePerformanceCurve(minPd, maxPd, points) reruns the same ejector object at a series of discharge pressures. Each returned row is [dischargePressureBara, entrainmentRatio, compressionRatio].

Because the inlet stream flow rates remain fixed in the default flow-input mode, the reported entrainment ratio also remains fixed unless the caller changes an inlet flow. In that mode the method does not solve the suction capacity available at each back pressure and must not be presented as a predictive vendor performance map. In fixed-geometry mode (previous section) the entrainment ratio is calculated at every point. Failed pressure points are logged and omitted, so callers must check the returned row count. The original discharge pressure is restored and the ejector is rerun before the method returns; do not operate the same mutable ejector concurrently.

Fixed-geometry (off-design) mode

The default mode above takes both flow rates as inputs. When the nozzle and mixing section are already built, call setFixedGeometry(throatAreaM2, mixingAreaM2) (or setFixedGeometryAreaRatio(throatAreaM2, areaRatio)) to run the ejector off-design. Only the pressure, temperature and composition of the motive and suction streams are then used; the flows are results:

  1. the motive flow is the choked flow of the nozzle throat at the motive state;
  2. the entrained (suction vapour) flow follows from the one-dimensional constant-pressure mixing model with the nozzle, suction, mixing, and diffuser efficiencies of the Ejector (Keenan et al. 1950, ESDU 86030), maximised over the mixing pressure;
  3. the discharge pressure that the mixed stream can recover follows from the momentum and energy balances. For a back pressure above that critical value the entrainment falls (Regime.SUBCRITICAL) and finally collapses (Regime.BREAKDOWN, isInBreakdown() is true).

getOperatingPoint() returns an EjectorOperatingPoint with motive demand, entrainment capacity and ratio, compression and expansion ratios, mixing pressure, critical back pressure, getBreakdownMargin() (critical back pressure divided by discharge pressure minus one) and warnings. getOperatingWindowReport() adds the supply utilisation of the suction stream and the margin to a maximum discharge pressure set with setMaxDischargePressureLimit. generatePerformanceCurve returns a variable entrainment ratio in this mode, and generateFixedGeometryCurve and generateSuctionPressureCurve return full operating points without changing the streams. autoSizeFixedGeometry(designSuctionFlow, margin) sizes the area ratio for a critical back pressure of at least (1 + margin) times the discharge pressure. Only the vapour phase of a two-phase stream is used, and the suction flow of the stream is ignored unless setLimitEntrainmentToSuctionSupply(true) is set. The model is a screening calculation and is not a replacement for vendor performance curves.

The following complete Java 8 program is compiled and executed by the documentation test.

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import neqsim.process.equipment.ejector.Ejector;
import neqsim.process.equipment.ejector.EjectorOperatingPoint;
import neqsim.process.equipment.stream.Stream;
import neqsim.process.processmodel.ProcessSystem;
import neqsim.thermo.system.SystemInterface;
import neqsim.thermo.system.SystemSrkEos;

public final class FixedGeometryEjectorExample {
  private static final Logger logger = LogManager.getLogger(FixedGeometryEjectorExample.class);

  private FixedGeometryEjectorExample() {}

  public static void main(String[] args) {
    SystemInterface motiveFluid = new SystemSrkEos(353.15, 60.0);
    motiveFluid.addComponent("methane", 1.0);
    motiveFluid.createDatabase(true);
    motiveFluid.setMixingRule(2);
    Stream motive = new Stream("motive gas", motiveFluid);
    motive.setFlowRate(1.0, "kg/sec");

    SystemInterface suctionFluid = new SystemSrkEos(303.15, 2.0);
    suctionFluid.addComponent("methane", 0.8);
    suctionFluid.addComponent("ethane", 0.2);
    suctionFluid.createDatabase(true);
    suctionFluid.setMixingRule(2);
    Stream suction = new Stream("suction gas", suctionFluid);
    suction.setFlowRate(1.0, "kg/sec");

    Ejector ejector = new Ejector("fixed ejector", motive, suction);
    ejector.setDischargePressure(3.0);
    ejector.setFixedGeometryAreaRatio(1.0e-4, 15.0);

    ProcessSystem process = new ProcessSystem();
    process.add(motive);
    process.add(suction);
    process.add(ejector);
    process.run();

    EjectorOperatingPoint point = ejector.getOperatingPoint();
    assert point.getMotiveFlow() > 0.0;
    assert point.getSuctionFlow() > 0.0;
    assert point.getRegime() == EjectorOperatingPoint.Regime.CRITICAL;
    assert ejector.getBreakdownMargin() > 0.0;
    assert Math.abs(ejector.getMassBalance("kg/sec")) < 1.0e-6;
    logger.info("entrainment ratio {}, critical back pressure {} bara", point.getEntrainmentRatio(),
        point.getCriticalBackPressure());

    ejector.setDischargePressure(40.0);
    process.run();
    assert ejector.isInBreakdown();
    assert ejector.getEntrainmentCapacity() == 0.0;
  }
}

Engineering limits