The Eductor Principle: How a Small Jet Moves Five Times Its Own Volume

An eductor looks like a pipe fitting with a jet inside, costs less than the pump that feeds it, and quietly does a job that would otherwise need a motor, a gearbox and a shaft seal: it moves several times its own pumped volume. The principle is the same one a carburetor uses and the same one a perfume atomizer uses: a fast fluid stream creates a local low-pressure zone that drags other fluid along with it. Manufacturers publish the same headline number for liquid eductors: they entrain three to five times the flow that is pumped through them, and the combined discharge stream circulates four to six times the pumped flow. That one ratio is why a small pump with an eductor can mix a tank that a pump ten times larger, running in a plain loop, would barely keep uniform.

This guide is the full treatment of the eductor: the physics that makes it work, the published flow-amplification figures you can size from, the worked arithmetic, and the selection and placement decisions that separate an installation that stirs a whole tank from one that makes a noisy jet in the corner. It is written to be read before you open a catalog, because the catalog numbers only make sense once the principle is clear.

What Is an Eductor?

An eductor, also called a jet pump, an ejector or a venturi pump, is a static fluid-handling device with no moving parts. A pressurized liquid, called the motive flow, enters through a converging nozzle and accelerates to high velocity. That jet passes through a suction chamber and a narrow throat, then through a diverging section called the diffuser before it leaves the body. Because the fast jet creates a low-pressure zone in the suction chamber, surrounding liquid is drawn in through side ports, mixes with the motive stream, and the combined flow discharges from the body as a single high-energy plume.

Nothing rotates inside an eductor. There is no impeller, no shaft, no seal, no motor mounted on the vessel. The only penetration into the tank is the motive line. That single fact drives most of the engineering value: no seal to leak on a hazardous service, no gearbox to maintain, no shear zone from a spinning impeller, and a device that can be made from almost any machinable material because it is just carefully shaped holes.

The parts have settled names in every vendor catalog, and it is worth fixing the vocabulary now because the sizing math below uses it:

Motive flow: the pumped liquid that enters at pressure and supplies the energy. In tank mixing, this is usually liquid recirculated from the tank itself.
Suction chamber: the zone where the accelerating jet drops in pressure and pulls in surrounding liquid.
Induced flow: the tank liquid drawn in through the side ports by that low-pressure zone.
Discharge flow: motive plus induced, the total stream that leaves the body. This is the flow that does the mixing.
Induction ratio (also called entrainment ratio): induced flow divided by motive flow. An induction ratio of 4:1 means the eductor moves four liters of tank liquid for every liter the pump supplies.
Throat: the narrow section between suction chamber and diffuser where the jet and induced flow mix.
Diffuser: the diverging section that converts some of the jet’s velocity back into pressure so the discharge stream can push deep into the tank instead of stalling at the body.

The Physics: Bernoulli and the Venturi Effect

The eductor is the industrial application of a principle Giovanni Battista Venturi described in 1797 and Daniel Bernoulli formalized even earlier in his Hydrodynamica: in a flowing fluid, pressure and velocity trade against each other. Bernoulli’s equation states that along a streamline, the sum of pressure energy, kinetic energy and elevation stays roughly constant. Squeeze a flowing liquid through a narrower passage and it must speed up to conserve mass; speed it up and its pressure must fall to conserve energy. That is the entire engine of the eductor.

The sequence inside the body, step by step:

Converging nozzle. The motive liquid arrives at moderate pressure and low velocity. The nozzle narrows, the liquid accelerates, and its pressure drops sharply. A jet leaves the nozzle tip at high velocity.
Suction chamber. The fast jet emerges into a chamber connected to the tank through side ports. The low pressure at the jet surface, lower than the tank liquid at the ports, is what draws the surrounding liquid in. It is the same mechanism that pulls fuel into a carburetor venturi.
Throat mixing. The motive jet and the induced flow mix in the throat. Momentum transfers from the fast core to the slow induced liquid, so the combined stream slows down and its pressure begins to recover.
Diffuser. The diverging section converts remaining velocity back into pressure. A well-designed diffuser recovers enough pressure that the discharge stream leaves the body as a coherent, energetic plume that can push tens of meters across a tank instead of dribbling out of the body.

Step 4 is the one people skip when they imagine an eductor is “just a nozzle.” A plain nozzle converts pressure to velocity and leaves it there: the jet dissipates a short distance from the tip. The eductor adds the diffuser, which recovers a large share of that velocity into pressure so the mixed stream keeps pushing after it leaves the body. Published effective flow fields for tank-mixing eductors run from about 1 m to over 10 m of reach depending on size and pressure. The plume, not the body, is what does the mixing.

Two details keep the picture honest. First, the process is not free energy: Bernoulli’s equation is a conservation statement, and the eductor trades the pump’s pressure for entrainment. The mixing and the diffuser recovery are not perfectly efficient, so you lose some motive head: the price of moving several times the pumped volume. Second, liquids are effectively incompressible, which is why the eductor works so cleanly for them: the whole device is a momentum exchange between two streams of nearly identical density. Gas eductors and steam ejectors work on the same geometry but with compressible flow, which changes the design rules; this guide covers liquid eductors unless stated otherwise.

How Much Flow Does an Eductor Amplify?

This is the number everyone asks first, and it is the one with the most solid published backing. Liquid tank-mixing eductors from the major manufacturers quote the same band:

Entrained flow: three to five times the pumped flow, depending on body size, design and motive pressure. Spraying Systems’ tank-mixing eductor bulletins state that eductors “can entrain up to five times the amount of pumped solution” and give the entrained stream as three to five times the inlet flow; their mini tank-mixing eductors quote the same three-to-five range.
Circulation rate: four to six times the pumped flow, because the discharge stream is the motive flow plus the entrained flow. A spray-tech industry bulletin phrases it as the eductor circulating “four to five gallons of solution for each gallon pumped.”
Compact and mini bodies: the same three-to-five entrainment band, at smaller flows. Mini tank-mixing eductors take inlet flows up to roughly 12 L/min and are aimed at plating, rinsing and small-process tanks.

The practical takeaway is that a 4:1 to 5:1 induction ratio is a defensible default for sizing liquid tank-mixing eductors. The same ratio the industry’s own sizing method uses (see the worked example below, which divides required circulation by five). What varies with design is how much pressure it takes to hold that ratio, and how far the ratio falls as back-pressure rises.

Motive pressure is the lever. The published operating band for tank-mixing eductors is roughly 0.5–4 bar (about 7–60 psi). Within that band the induction ratio climbs with pressure, though with diminishing returns. Doubling the pressure does not double the entrainment. The relationship is not linear, and the honest way to state it is as a range:

Motive pressure at the eductor	Typical induction ratio (published band)	Circulation per liter pumped
1–2 bar	2:1 to 3:1	3–4 L
2–3 bar	3:1 to 4:1	4–5 L
3–4 bar	4:1 to 5:1	5–6 L

Ranges are compiled from manufacturer-published performance data (e.g., Spraying Systems Bulletin 635A/635B, spray-tech TE-series literature). Confirm against the vendor’s curve at your actual pressure. The ratio falls with back-pressure and viscosity.

Why does the ratio saturate? Because the suction side is limited by what the tank liquid can supply. The induced flow has to be dragged into the jet against its own inertia and viscosity, and there is only so much momentum in the motive stream to do the dragging. Push more pressure in and the jet gets faster, but the entrainment gain per unit of pressure shrinks, which is exactly what the published curves show, and why sizing an eductor is a matter of picking a pressure window, not cranking pressure to the maximum.

Worked Example: Sizing an Eductor From Turnover

The industry-standard sizing method, published in the same bulletins, is disarmingly simple: decide how many tank volumes you want circulated per hour, multiply by tank volume to get the required circulation, then divide by five, the nominal 5:1 ratio, to get the motive flow the pump must deliver.

Worked in metric: a 20 m³ tank needs four turnovers per hour. Required circulation is 20 × 4 = 80 m³/h. Divide by five: the motive flow needed is 16 m³/h. If the pump can deliver 16 m³/h at 3–4 bar at the eductor, one body, or two smaller bodies fed by the same header, will hold roughly a 4:1 to 5:1 ratio and circulate the tank four times per hour.

The same arithmetic in the units of a small plant: a pump moving 2,000 L/h through a well-fed eductor at a 4:1 ratio produces about 8,000 L/h of discharge flow. A 10 m³ tank therefore sees roughly 8,000 liters of circulation per hour through one eductor: a complete turnover every 75 minutes, with no additional pipe, no strainer, no motor. Add a second eductor on the same header and circulation doubles to 16,000 L/h; add a third and the same 10 m³ tank turns over every 25 minutes while the pump still moves its original 2,000 L/h. That multiplication is the entire business case: the pump only has to feed the motive stream, and the eductor supplies the muscle.

Turnover targets are published too, and they are a useful reality check on the math. Plating, rinsing and similar surface-treatment tanks are typically run at 10–25 tank volumes per hour; blending and dissolution duties run hotter, in the 20–120 volumes per hour band, depending on how fast the additive must homogenize. A tank mixed at 4 volumes per hour is on the gentle end: fine for equalization and slow chemistry, underpowered for fast blending, which is why the target must be set from the duty before any catalog is opened.

If you want the same arithmetic as a repeatable worksheet, the eductor sizing calculator walks through tank volume, turnover target and the 4:1 to 5:1 multiplier step by step, with a turnover-time reference table for common duties.

Eductor vs Pump vs Agitator vs Spray Nozzle

The eductor occupies a specific niche, and the fastest way to misuse it is to confuse it with the three devices it is often compared to:

Criterion	Eductor loop	Plain pump loop	Mechanical agitator	Spray nozzle
Moving parts in the tank	None	None	Impeller + shaft + seal	None
Flow multiplied	3–5× motive flow	1× pump flow	n/a (impeller-driven)	1× pump flow
Best at	Circulating, blending, lifting	Transferring over distance	High-shear dispersion	Coating, cleaning, cooling surfaces
Tank penetration	Motive line only	Pipe in/out	Shaft seal	Pipe in
Maintenance	Throat wear, occasional	Seals, motor	Seals, bearings, gearbox	Orifice wear
Shear on product	Low, localized	Low	High near impeller	High at atomization

The contrast that matters most for this site’s readers is with a pump and with a spray nozzle. A pump loop moves exactly what the pump moves, one liter per liter, and the pump must overcome the friction of pushing that volume through pipe and back. An eductor loop multiplies the pumped flow by the induction ratio, which is why a small pump can do a big tank’s job at all. Against a spray nozzle, the distinction is duty: a spray nozzle is built to put liquid onto a surface or into a gas, atomizing, coating, quenching, scrubbing, while an eductor is built to move and blend liquid inside a vessel. They share a jet, and that is where the similarity ends. An agitator, finally, wins where you genuinely need high shear, milling, emulsifying to a fixed droplet size, disintegrating lumps, and loses everywhere else on capital cost, maintenance and the shaft-seal penetration it forces through the vessel wall.

Where Eductors Earn Their Keep

Tank mixing and blending: the home duty. The discharge plume carries concentrate from the injection point across the whole volume instead of letting it pool under the inlet, which is why eductors are the standard answer for homogenizing slurries, emulsions and additive doses.
Plating and surface-treatment lines: compact eductors mounted in the tank keep solution uniform at the 10–25 turnovers per hour that plating and rinsing baths need, without the rack interference a mechanical agitator brings.
pH and chemistry equalization: pull reagent into circulation so the tank does not develop acid or alkali pockets. An eductor aimed along the floor erases the dead zones where pH probes read lies.
Suspending solids and sweeping floors: the aimed discharge keeps settled solids in motion and can sweep debris or sludge toward a filter intake, which is a published application of tank-mixing eductors.
Jet pumping and sump duty: an eductor can lift liquid from a pit or sump several meters using a motive stream, with nothing submerged but the body. Published lift figures for liquid-jet eductors run roughly 3–6 m depending on motive pressure.
Vacuum generation: liquid-driven eductors pull vacuum in the low-single-digit kPa range (roughly 3–6 kPa absolute in typical published figures), enough for filtration, degassing and priming where a mechanical vacuum pump is overkill or where the process fluid must never touch a vane or oil.
Liquid transfer and dilution: entrain one liquid into another without a second pump: dosing a concentrate into a carrier stream is the same physics as tank mixing, done inline.
Aeration and oxidation: draw air in with the liquid stream for aerobic treatment; some tank-mixing eductors are built specifically to induce air as well as liquid.
Sealed and hazardous vessels: no shaft penetration and no dynamic seal, which matters for flammable, toxic or sterile services. The motive line is the only penetration.

The common thread is circulation: the eductor is a tool for moving liquid around a vessel or from one place to another, not for breaking material apart. If the duty is genuinely high shear, an eductor is the wrong tool, and buying a bigger pump to fake it is the wrong tool too.

Selection: The Numbers That Matter

Four numbers fix an eductor specification, and they come from the duty, not the catalog:

Tank volume and required turnover per hour: these set the required circulation, and the 5:1 rule converts it to motive flow.
Motive pressure available at the eductor: measured at the body after line losses, not at the pump discharge flange. A pump making 5 bar at its own flange may present only 3 bar at an eductor 20 m of pipe away, and the induction ratio quietly drops. Published operating bands for liquid eductors are roughly 2–6 bar, with tank-mixing bodies typically quoted at 0.5–4 bar.
Liquid properties: viscosity and solids content. Thicker liquids resist being dragged into the jet, so induction falls; the flow-through chamber design used in modern tank-mixing eductors is explicitly meant to pass particulates with minimal clogging, but a heavy solids layer still changes the sizing.
Discharge head and back-pressure: the height of liquid above the body and any downstream resistance. The induction ratio falls as back-pressure rises; published sizing guidance shows eductor efficiency dropping steadily as the discharge head grows, which is why the body is sized for the worst case, not the best day.

Two geometric parameters inside the body set its performance, and they are worth knowing even if you never specify them by name. The nozzle-to-throat area ratio, roughly 2–4 in typical liquid eductors, fixes where the operating point sits on the entrainment curve; the practical entrainment band of 1:2 to 1:5 for liquid service comes out of that geometry. The diffuser half-angle, about 5–7° in well-designed bodies, is the compromise between pressure recovery and length: too steep and the flow separates from the wall and recovery collapses, too shallow and the body gets long for no benefit. Both are design constants, chosen by the manufacturer, but they explain why two eductors of the same pipe size can entrain different amounts at the same pressure.

There is also a suction-side check that engineers new to eductors miss: NPSH applies to the induced flow as well as the motive flow. The suction chamber runs at reduced pressure, that is the point, so a liquid near its vapor pressure will flash at the ports instead of being drawn in smoothly, and the eductor will cavitate and lose entrainment. If the tank liquid is hot or volatile, check the vapor-pressure margin before sizing the body.

Placement and Installation

The single biggest performance factor on a tank is not which eductor you buy but where you put it. The published placement guidance is consistent across manufacturers:

Submerge the body well below the lowest operating level. An eductor must draw tank liquid, not air; mounted too high it pulls a vortex and loses entrainment. Level swings are the classic cause of a system that mixes on Monday and gurgles on Friday.
Aim the discharge along the floor or across the long axis. A discharge aimed at the far wall sweeps the floor and pushes the plume across the whole volume; one aimed upward just churns the surface. The plume does the mixing, so point it where the mixing needs to happen.
Space the bodies so the plumes reinforce. Common published guidance starts at roughly 300 mm between bodies and scales with tank length: one eductor per 2–3 m of tank length on a header, all discharge streams pointing the same way so they add rather than cancel.
Keep the suction ports clear of the settled bed. A body sitting in sludge pulls sludge; mount above the settled layer and aim the plume to keep it in motion.
Mind the plume reach. A published rule of thumb puts a tank-mixing eductor’s plume reach at roughly 4 m per 1 bar of pressure drop across the body. A 2 bar drop reaches about 8 m, which tells you how many bodies a long tank needs in one line.

Troubleshooting an Eductor Loop

Symptom	Likely cause	Check	Fix
No suction at the ports	Motive pressure below minimum	Pressure at the eductor, not the pump	Raise pump output or shorten the line
Vortex and gurgling	Under-submergence at low level	Operating level vs body position	Lower the body or the level alarm
Weak turnover	Induction ratio collapsed	Back-pressure and viscosity	Re-size for worst case; add a body
Body clogged	Solids in the motive line	Strainer and throat	Clean the throat; add or clean a strainer
One body dead in a header	Air lock or a valve part-closed	Valves and venting	Purge the header; open the valve
Mixing pattern gone	Discharge re-aimed wrong after repair	Aim of the bodies	Re-aim along the floor or long axis

Nearly all eductor failures are system problems, pressure that was never measured at the body, a body that draws air at low level, a line that was never purged, rather than failures of the device itself. That is the maintenance story in one sentence: an eductor with no moving parts fails slowly, and what kills the installation is almost always upstream.

Frequently Asked Questions

What is an eductor used for? Moving and blending liquid: tank mixing, pH and temperature equalization, suspending solids, jet pumping from sumps, liquid transfer, aeration and vacuum generation. It is a circulation tool, not a high-shear mixer.

How does an eductor work without moving parts? A high-velocity motive jet creates a low-pressure zone (the Venturi effect, from Bernoulli’s equation) that draws surrounding liquid in through side ports. The combined stream passes through a diffuser that recovers pressure, so the discharge plume keeps pushing across the tank.

How much flow does an eductor amplify? Published figures for liquid eductors: entrained flow is three to five times the pumped flow, and total circulation is four to six times the pumped flow. The standard sizing default is a 5:1 ratio.

Can an eductor mix my tank with the pump I already have? Usually yes, if the pump delivers enough pressure at the eductor. Most tank-mixing bodies need roughly 1–4 bar at the body. Size from the pressure at the eductor after line losses, not the pump nameplate.

What is the difference between an eductor and an ejector? Largely terminology. Both are jet pumps on the Venturi principle. In practice “eductor” usually describes liquid-driven liquid service (mixing, pumping), while “ejector” often implies gas or steam motive service, but vendors use the words interchangeably.

Does an eductor shear or damage the product? Much less than an agitator. The jet is intense only in a small zone; the rest of the circulation is gentle drag. For shear-sensitive products an eductor loop is usually the safer choice.

Why is my eductor not pulling suction? Motive pressure below the minimum at the body, under-submergence pulling air, or a clogged throat. Measure pressure at the eductor and check the liquid level above the suction ports.

Installation Checklist

Motive pressure measured at the eductor, not at the pump
Body submerged below the worst-case operating level
Discharge aimed along the floor or the long axis
Spacing at one body per 2–3 m of tank length, plumes reinforcing
Suction ports clear of the settled solids layer
Induction ratio confirmed at actual pressure, viscosity and level
Header purged and vented before start-up
Turnover time logged at commissioning as the baseline

If you are weighing an eductor against a bigger pump or an agitator, send us the tank volume, the pump you have, and the duty, blend, dilute, equalize or suspend, and the engineering desk will size the motive flow and submergence from your numbers. The eductor product range covers the bodies themselves, and for the placement-and-sizing companion to this piece, see the small-pump tank mixing guide, which walks the induction-ratio table and the placement rules in operating detail.