Table of Contents
An eductor is the cheapest circulation device in process engineering, but only when it is sized right. Undersize the motive flow and the tank stratifies: additive pools at the inlet, pH drifts between zones, settled solids build a layer no one sees until the batch fails. Oversize it and you paid for a pump and a pipe run that circulate more liquid than the process can use. The good news: eductor sizing is arithmetic you can do on a napkin, three numbers in, two results out, no software required.
This guide is that napkin math, organized as a calculator you can run on your own tank. The entire method fits in one line: pump flow × amplification ratio = circulation flow, and tank volume ÷ circulation flow = turnover time. Everything else, number of eductors, pressure, placement, refines those two results. Work through the three steps with your own numbers, and you will walk into the buying conversation with a spec instead of a guess.
The Three Numbers That Drive Every Eductor Spec
Every eductor sizing exercise, from a 200-liter day tank to a 200 m³ storage vessel, starts from the same three inputs. Every later calculation is just these three numbers rearranged.
Tank volume (V). The liquid volume that actually needs to be mixed, in liters or cubic meters. Not the nameplate capacity, the working volume present during normal operation. A 12 m³ tank that runs at 80 percent holds roughly 10 m³, and that 10 m³ is the number the eductor has to move.
Turnover target (N). How many times per hour the entire working volume should circulate through the mixing system. It is set by the duty, not by tank size: a gentle temperature equalization needs a couple of turnovers an hour, a critical cleaning bath may need twenty.
Circulation multiplier (the amplification ratio). How many liters of total circulation the eductor produces for every liter the pump supplies. Because the eductor drags surrounding liquid into its own discharge stream, one liter of pumped motive flow becomes four or five liters of circulation, the number that lets a small pump mix a large tank, and the one assumed wrong most often.
With those three inputs fixed, the calculation is two formulas:
- Required circulation flow = tank volume × turnovers per hour (Q = N × V).
- Pump flow needed = required circulation ÷ amplification ratio.
And when you are checking a pump you already own, the third formula answers the other direction:
- Turnover time = tank volume ÷ actual circulation flow.
That is the whole calculator. The rest of this guide is about choosing honest values for the three inputs, because most failed installations fail in the inputs, not the arithmetic.
Start With the Liquid Volume You Actually Mix
The tank volume in the calculation is the working volume at the normal operating level, not the volume the nameplate advertises. Three corrections matter more than any other sizing decision.
Use the operating level, not the brim. Measure the liquid depth during normal operation, below the fill point, above the pump suction cut-off, and multiply by the footprint. For a vertical cylinder: π × radius² × liquid depth. For a rectangular tank: length × width × liquid depth. Sizing to the brim oversizes the pump by the same percentage.
Account for level swings. Many tanks cycle between low and high operating levels. The eductor must do its job at the lowest level you actually run, because that is when the liquid is shallowest, the jet has the least room to develop, and the suction ports are closest to pulling air. Size for the worst case and the better days take care of themselves.
Do not forget liquid that is not there. If the tank holds a settled solids bed, a floating layer, or a heel that is never circulated, subtract it. The eductor mixes liquid, not sludge.
Set the Turnover Target by Duty, Not by Guess
The turnover target is the number most people invent. Industry practice, from tank mixing eductor bulletins and process mixing references, clusters into a repeatable pattern. General agitation and temperature equalization sit at 2–4 turnovers per hour; blending moves up to 4–8; solids suspension and chemical dosing run higher. Cleaning duties are the aggressive end: plating and rinsing baths run 10–20, critical cleaning can exceed 20.
| Duty | Turnovers per hour |
|---|---|
| General agitation / temperature equalization | 2–4 |
| Preventing stratification, gentle blending | 2–4 |
| Blending miscible liquids | 4–8 |
| Light solids suspension | 6–10 |
| Chemical dosing and dilution | 8–12 |
| Cleaning tanks (CIP support) | 10 or more |
| Plating and rinsing tanks | 10–20 |
| Heavily soiled or critical cleaning | up to 20+ |
The pattern behind the table: the faster the chemistry changes or the heavier the particles, the more turnovers the tank needs. A slow pH adjustment can get away with 2–3 turnovers an hour; a plating bath has minutes, and it pays for 10–20 to keep the metal distribution honest. When in doubt, use the middle of the range and note why you chose it. The turnover target should be justifiable in one sentence.
Turn Turnover Into Circulation, Then Back Out Pump Flow
With the working volume and the turnover target fixed, the required circulation flow is a straight multiplication.
Required circulation = tank volume × turnovers per hour.
A 10 m³ tank at 4 turnovers per hour needs 40 m³/h of circulation. A 50 m³ tank at the same 4 turnovers per hour needs 200 m³/h. The flow requirement grows linearly with the vessel, and so does the pump.
Then the amplification ratio converts that circulation requirement back into a pump duty:
Pump flow needed = required circulation ÷ amplification ratio.
Using a conservative 4:1 ratio, the 10 m³ tank needs 40 ÷ 4 = 10 m³/h of motive flow, about 167 L/min. The 50 m³ tank needs 200 ÷ 4 = 50 m³/h, about 833 L/min. The pump supplies only the motive stream; the eductor multiplies it into the circulation the process needs.
The same arithmetic runs in reverse when you are checking a pump you already own: multiply the pump flow by the ratio to get the circulation, then divide the tank volume by that circulation for the turnover time. That reverse direction is where the worked examples live. Most plants start from a pump they already have.
The Amplification Ratio: 4:1 to 5:1, and When to Use Which End
The amplification ratio is the heart of eductor sizing, a working range, not a fixed constant. Vendors quote their tank mixing eductors as mixing at 4:1 to 5:1; the standard guidance from the major eductor bulletins is to divide the required circulation by 5 to find the inlet flow. The actual ratio you get depends on motive pressure, body design, discharge back-pressure and liquid viscosity.
One terminology trap causes most of the confusion. Some catalogues quote an “entrainment ratio” that counts only the liquid drawn in through the suction ports; others quote a total ratio that counts the motive flow as well. A body with an entrainment ratio of 3:1 produces a total circulation of 4 liters per motive liter, a 4:1 total ratio. When the brief says “mixes at 5:1,” check which side of the pump it is counting, and size with the total ratio: liters of circulation per liter of pump flow.
The ratio moves with motive pressure in a repeatable pattern:
| Motive pressure at the eductor | Total circulation ratio (typical) |
|---|---|
| 1–2 bar | 2:1 to 3:1 |
| 2–3 bar | 3:1 to 4:1 |
| 3–4 bar | 4:1 to 6:1 |
Two rules of thumb follow. First, design on the low end of the band, 4:1 for a 3-bar feed, 3:1 if you cannot guarantee pressure at the eductor; the optimistic end is a bonus, not a design basis. Second, check the pressure at the eductor, not the pump flange. The ratio collapses quietly when line losses eat the head.
Viscosity drags the ratio down regardless of pressure: a thicker liquid resists being dragged into the jet, so induced flow falls and the total ratio shrinks. If your liquid is noticeably more viscous than water, oils, syrups, slurries, step the design ratio down half a point or more and check with the vendor. The same applies to discharge back-pressure: an eductor pushing into a tall tank works against the liquid head above it, and the ratio drops as the level rises. Size for the worst case and let better conditions be a bonus.
Worked Example: A 10 m³ Tank With a 100 L/min Pump
Now the calculator runs end to end on the numbers most people meet first: a 10 m³ tank and a modest pump already on site, delivering 100 L/min of motive flow at adequate pressure at the eductor, say 3 bar.
Step 1: pick the amplification ratio. At 3 bar, the middle of the working band gives about 4.5:1 total circulation: conservative enough to be safe, honest enough to be real.
Step 2: multiply pump flow by the ratio. 100 L/min × 4.5 = 450 L/min of circulation. In hourly terms, 450 L/min = 27,000 L/h = 27 m³/h. The pump still moves its original 100 L/min; the eductor supplies the other 350 L/min by entrainment.
Step 3: divide the tank volume by the circulation. 10,000 L ÷ 450 L/min = 22.2 minutes per turnover. Converted to turnovers per hour: 60 ÷ 22.2 = 2.7.
Check that result against the table. 2.7 turnovers per hour sits in the 2–4 band for general agitation and just under the 4–8 band for blending miscible liquids, correctly sized for a simple blend, but short of a solids suspension or dosing duty needing 6–10, and the arithmetic shows exactly how short before you spend any money.
The same example shows how sensitive the result is to the ratio assumption, which is why the low end of the band is the design basis. At 4:1, the same 100 L/min pump circulates 400 L/min, giving 25 minutes per turnover and 2.4 turnovers per hour. At 5:1 it circulates 500 L/min, giving 20 minutes and exactly 3.0. The honest middle, 4.5:1, 22 minutes, 2.7 turnovers per hour, is what a 3-bar system delivers on a normal day, and the spread from 2.4 to 3.0 is the difference between a system that meets spec and one that quietly doesn’t.
Worked Example: Sizing From a Turnover Target, Not a Pump
The reverse direction is the one to use when specifying a new system: start from the duty, set the turnover target, and let the pump fall out of the calculation.
Take a 50 m³ storage tank holding a blend that must stay uniform, blending miscible liquids, so 4 turnovers per hour from the table.
Step 1: required circulation. Q = N × V = 4 × 50 = 200 m³/h.
Step 2, pump flow. At a conservative 4:1 design ratio: 200 ÷ 4 = 50 m³/h of motive flow, about 833 L/min, one quarter of the circulation, which is exactly the point of the eductor.
Step 3: split across eductors. A single jet cannot reach a 50 m³ vessel evenly, so the flow splits. Four eductors on a header, each fed 12.5 m³/h (about 208 L/min) of motive flow, each circulate 50 m³/h for a total of 200 m³/h. If the tank is long and narrow, six eductors at 8.3 m³/h each cover the length better with the same total.
This is the sizing path the vendors use in their own examples: multiply the turnover rate by the tank volume, divide by the mixing ratio for the inlet flow, then split it across as many eductors as the tank geometry needs.
How Many Eductors, and Where They Go
The number of eductors follows the flow split, with geometry as the tiebreaker. As a starting layout, one eductor per 2–3 m of tank length is the commonly recommended spacing, arranged so the discharge streams reinforce each other. A 6 m long rectangular tank takes two or three bodies on the same header; a 12 m tank takes four or more. Multiple smaller eductors almost always beat one large one in a big tank, because the mixing is done by the jet’s reach, typically a few meters per body, not by the total flow alone.
Placement rules are short and unforgiving:
- Submerge the body well below the lowest operating level. The suction ports must draw liquid, not air. Under-submergence at low level is the classic cause of a system that mixes beautifully in the morning and gurgles at the end of a draining shift.
- Aim the discharge along the tank floor or across the long axis. A jet aimed at the far wall sweeps the floor and drags the whole volume; one aimed upward just churns the surface. Vendors typically recommend mounting close to the bottom, about 0.3 m up, with the discharge parallel or slightly downward.
- Keep the suction ports clear of the settled layer. If the tank accumulates solids, mount the bodies above the settled bed and let the aimed discharge keep it in motion.
The Pressure Check That Saves the Installation
Every formula in this guide assumes the motive flow arrives at the eductor with enough pressure to form a coherent jet. Most tank mixing eductors are specified for motive pressures in the 2–4 bar band, and the amplification ratio climbs with pressure inside that window, which makes the pressure check a sizing step, not a commissioning afterthought.
The check is simple: measure or calculate the pressure at the eductor, after the pipe run, after the strainer, after the valves, not at the pump discharge flange. A pump delivering 5 bar at its flange may present only 3 bar at an eductor 20 meters of pipe away, and the ratio quietly drops from the top of its band to the bottom. The worked examples above assume 3 bar at the body. If your installation delivers 2 bar, redo them at 3:1 and watch the pump flow requirement jump by a third.
The failure mode is silent: the system still runs, still makes a jet. It just circulates less than the calculation promised, and the tank stratifies slowly enough that nobody connects cause and effect until a batch fails. Budget the line losses before you buy, not after. If the pump is marginal on head, prefer a larger body that makes its ratio at lower pressure over a smaller one that needs 4 bar to work at all.
Turnover Time at a Glance: A Reference Table
Once the circulation flow is known, the turnover time is one division. The table below pre-computes it for the tank sizes and circulation rates that cover most plants, minutes per turnover, from tank volume ÷ circulation flow:
| Tank volume | 200 L/min circulation | 450 L/min | 800 L/min | 1,500 L/min |
|---|---|---|---|---|
| 5 m³ | 25 min | 11 min | 6 min | 3 min |
| 10 m³ | 50 min | 22 min | 13 min | 7 min |
| 25 m³ | 125 min | 56 min | 31 min | 17 min |
| 50 m³ | 250 min | 111 min | 63 min | 33 min |
Read it two ways. Horizontally: a 10 m³ tank that must turn over every 15 minutes needs between 450 and 800 L/min of circulation. Vertically: the 22-minute figure in the 10 m³ / 450 L/min cell is exactly the worked example above. The table is the formula with the arithmetic done.
Two unit traps live in this table. First, liters per minute versus cubic meters per hour: 450 L/min is 27 m³/h, a 60× difference, and mixing the two units is the single most common arithmetic error in eductor sizing. Second, minutes per turnover versus turnovers per hour: 22 minutes per turnover is 2.7 turnovers per hour, not 22 per hour. The reciprocal relationship is where most “the spec says it should be faster” arguments start.
Common Eductor Sizing Mistakes
Most failed eductor installations fail in one of a small set of predictable ways. Each has a tell and a fix.
- Sizing from the pump nameplate pressure instead of the pressure at the eductor. The ratio collapses with line losses and the system silently under-circulates. Fix: subtract pipe, fitting and strainer losses before choosing the ratio.
- Assuming the top of the ratio band. Designing at 5:1 when the installation delivers 2 bar is designing for a ratio the system cannot produce. Fix: design at 4:1 for a 3-bar feed, lower for anything less.
- Sizing to the nameplate volume instead of the working volume. The pump is 20–30 percent bigger than the duty needs. Fix: measure the operating level and recalculate.
- Using the wrong turnover target for the duty. A gentle blend designed at 20 turnovers per hour buys an oversized pump; a plating bath designed at 3 doesn’t mix at all. Fix: take the target from the duty table, and write down why you chose it.
- Mixing liters per minute with cubic meters per hour. A 60× unit slip that inflates or collapses every downstream number. Fix: convert everything to one unit before the first multiplication.
- Confusing minutes per turnover with turnovers per hour. 20 minutes per turnover is 3 per hour, and the two numbers get swapped in conversations, spec sheets and purchase orders. Fix: state the result both ways.
- One big eductor in a big tank. Total flow is right, coverage is not. The jet reaches a few meters, and the far corners stratify. Fix: split the motive flow across multiple bodies at 2–3 m spacing.
- Ignoring viscosity and back-pressure. The ratio falls as the liquid thickens and as the discharge works against head, so a water-based sizing comes up short on a syrup. Fix: step the design ratio down and confirm against the vendor’s curve at the actual conditions.
- Under-submergence. The body pulls air, entrainment collapses and the tank churns without mixing. Fix: mount below the lowest operating level, and check the worst case, not the best.
Every one of these shows up in the arithmetic before it shows up in the tank. Run the calculation with conservative inputs, and most of the list eliminates itself.
When the Calculator Does Not Apply
The turnover method assumes the liquid is fluid enough to be dragged into a jet and mixed by circulation. Three duties break that assumption.
High-viscosity liquids. Above roughly 50 cP, entrainment falls steeply, the amplification ratio shrinks toward 1:1, and the eductor stops being a multiplier. Helical ribbon agitators take over in that regime.
True high-shear work. An eductor blends and equalizes; it does not mill, disintegrate lumps, or emulsify to a controlled droplet size. If the duty is genuinely high shear, an eductor is the wrong tool regardless of the turnover math.
Fast-settling heavy solids. Keeping coarse, dense particles suspended needs turnover targets beyond what a reasonable eductor loop can deliver. Above about 10–12 turnovers per hour with heavy particles, consider mechanical agitation.
The calculator is honest about its limits: it sizes a circulation system, and circulation is the right answer for a large share of tank mixing, but not all of it.
Frequently Asked Questions
How do I calculate eductor circulation flow? Multiply the pump’s motive flow by the amplification ratio, the liters of total circulation per pumped liter, typically 4:1 to 5:1 at 3 bar or better. A 100 L/min pump at 4.5:1 circulates 450 L/min.
How do I calculate tank turnover time? Divide the working tank volume by the circulation flow. A 10 m³ tank with 450 L/min of circulation turns over in 10,000 ÷ 450 = 22 minutes, about 2.7 times per hour.
How many turnovers per hour does a tank need? It depends on the duty: 2–4 for general agitation and temperature equalization, 4–8 for blending, 6–12 for solids suspension and chemical dosing, and 10–20+ for cleaning and plating baths.
Why is my eductor circulating less than the calculation says? Check, in order: pressure at the eductor after line losses, submergence (is it pulling air?), viscosity and discharge back-pressure, and the ratio assumption itself. All four drag the ratio down from the catalogue value.
Run the Numbers Before You Buy
The entire eductor sizing calculator is three inputs and two divisions: working volume, turnover target, amplification ratio, then pump flow = circulation ÷ ratio, and turnover time = volume ÷ circulation. Run it once with conservative inputs and the spec writes itself. The worked examples are the sanity checks: a 10 m³ tank with a 100 L/min pump at 3 bar gives about 450 L/min of circulation and a 22-minute turnover; a 50 m³ blend tank at 4 turnovers per hour needs 200 m³/h of circulation, a 50 m³/h motive pump, and four eductors on a header.
For the theory behind the Venturi effect and entrainment, the what is an eductor guide goes deeper into how the device converts motive momentum into bulk circulation. If you are working with a small pump that looks too weak for the tank, the tank mixing with a small pump guide shows how induction ratio and placement let a modest pump turn over a whole vessel. And when you are ready to spec hardware, the eductor nozzle range covers the body sizes and entrainment options for tank mixing and transfer duty.
If you would like the numbers checked against your actual tank, send the working volume, the duty, and the pump flow and pressure you can deliver at the tank, the engineering desk will run the eductor sizing and confirm the body count and motive flow before you spend anything.
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Send your flow, pressure, fluid and target coverage. We come back with nozzle options and figures, not a catalogue number.
Written by
Ray ChanIndustrial spray nozzle specialist. I size tank cleaning, atomizing, flat-fan and spiral nozzles against real duty conditions, flow, pressure, fluid and target, rather than catalogue numbers. Every guide here comes from actual sizing work.