A water jet eductor uses a motive nozzle to entrain a secondary liquid through a suction chamber, mix the streams and discharge through a diffuser. This calculator estimates water flow from an entered pressure difference and nozzle diameter, with an assumed entrainment ratio.
Ejector or Jet Pump Interactive Calculator
Trace motive and suction water through an eductor. Pressure drop and nozzle diameter determine motive flow; throat pressure and entrainment ratio are entered assumptions.
Equation Used
- Equal-density incompressible water streams.
- Assumed discharge coefficient 0.95.
- Throat pressure and entrainment ratio are inputs, not predictions.
- Illustrative body geometry and flow markers.
Water-flow comparison with assumed throat pressure and entrainment ratio; not a pump selection.
Follow the open flow path
The axial cutaway shows the motive inlet on the left, a converging nozzle, the top suction port, a parallel mixing throat and the widening diffuser on the right. The flow passages remain open; there is no piston, impeller or valve cycling inside this basic device.
Blue and green markers identify the two incoming streams. Their motion is a direction guide, not a computed particle trajectory or turbulence field. The separate nozzle detail changes with the diameter input; the remaining body proportions are illustrative.
Water-driven transfer and mixing
Water eductors are used for liquid transfer and mixing when a pressurized motive supply is available. Different constructions serve different fluids and duties. This page models equal-density, incompressible water streams, not steam, compressed air or a gas vacuum ejector.
The pressure, diameter and entrainment inputs allow conditional flow comparisons. Neither the drawing nor the sliders choose an eductor size or establish a manufacturer performance point.
The calculator’s flow model
Use absolute pressures in kPa. The positive nozzle pressure difference is ΔP=1000(Pm−Pt) in pascals. With water density ρ=1000 kg/m³ and assumed discharge coefficient Cd=0.95, mean exit speed is v=Cd√(2ΔP/ρ).
For nozzle diameter d in metres, A=πd²/4. Motive flow Qm=Av; suction flow Qs=ER×Qm; and total flow Qt=Qm+Qs. Multiply cubic metres per second by 1000 for litres per second.
The coefficient is an assumption, not a calibration. Throat pressure and entrainment ratio are independent entered operating assumptions. The calculator does not derive them from the nozzle/throat area ratio, downstream pressure or suction piping.
Default water-flow comparison
At 400 kPa absolute motive pressure, 50 kPa assumed throat pressure and a 10 mm nozzle, the model gives about 25.13 m/s mean exit speed and 1.97 L/s motive flow. Entering ER=1.5 gives about 2.96 L/s suction flow and 4.94 L/s combined discharge.
At fixed pressure difference, doubling nozzle diameter multiplies area and all three flows by four without changing this model’s mean exit speed. Changing ER changes suction and total flow; it does not change the prescribed nozzle pressure difference or motive flow.
What the model does not establish
An actual eductor couples motive conditions, suction conditions, mixing geometry and discharge backpressure. The entered throat pressure and entrainment ratio may not be jointly attainable for a particular device. Consult its performance data.
No suction-lift limit, discharge pressure, efficiency, cavitation margin, solids capacity or temperature rating is calculated. Nozzle spacing, diffuser geometry and erosion resistance must be assessed for the actual device and fluid conditions.
Jet-pump calculator questions
Why does changing diameter not change the displayed velocity?
The pressure difference and coefficient set velocity in this model. Diameter changes area and therefore flow.
Does the entrainment slider predict what a real pump will draw?
No. It is an assumed ratio used to calculate a conditional suction flow. It must come from suitable operating data for a real selection.
Are the dots actual fluid particles?
No. They indicate direction and stream identity only.
Can this calculate steam ejectors?
No. Its fixed water density and incompressible equation do not model steam expansion or condensation.
Construction reference
Schutte & Koerting, Water Jet Eductors, Bulletin 2M, pages 1–2: nozzle, suction chamber, mixing throat and diffuser arrangement, with sectioned Fig. 264/266 examples. The animation is a generic schematic, not a dimensioned copy or a manufacturer performance prediction.
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