Eductor Tank Mixing

Eductor Tank Mixing

TL;DR

Tank eductors use the Venturi principle to entrain surrounding liquid into a high-velocity motive stream, increasing total tank circulation by several times. Proper design depends on the target turnover rate (turnovers/h), entrainment ratio (ER), pump flow and pressure, and nozzle placement. Typical goals: 3–5 turnovers/h for homogenization, 5–10 turnovers/h for solids suspension.

What is a Tank Eductor? Working Principle

A tank eductor consists of a nozzle and a converging–diverging body. Pressurized liquid (motive flow) exits the nozzle at high velocity, creating a low-pressure zone that entrains surrounding liquid. The combined flow is discharged into the tank.

• Entrainment ratio (ER): entrained flow / motive flow (typical: 3–6)

• Total outlet flow:

  Qout=(1+ER) QmQ_{\text{out}} = (1 + ER)\, Q_mQout​=(1+ER)Qm​

  where QmQ_mQm​ = motive flow (pump flow into eductor).

This means eductors provide multiple times the circulation of the pump flow without mechanical agitators.

When to Use Tank Eductors?

• Homogenization: Mixing of miscible liquids (e.g., glycol–water blends)

• Solids suspension: Preventing settling of crystals, pigments, or minerals

• Temperature equalization: Eliminating hot/cold zones in heated tanks

• Dead zone elimination: Sweeping corners and bottom areas of the tank

• CIP support: Assisting clean-in-place operations by improving circulation

Design Targets (Velocity & Turnover Rates)

• Turnovers per hour (N): number of times tank volume is recirculated per hour

  • Homogenization: N = 3–5/h

  • Solids suspension: N = 5–10/h

• Local velocity targets:

  • Bottom sweeping: 0.3–0.6 m/s

  • Heavy solids: 0.6–1.0 m/s

• Jet reach: Depending on eductor size/pressure, effective reach is typically 3–7 m in water at ~2 bar.

Step-by-Step Sizing Method

1) Define the Process Goal

• Homogenization, solids suspension, or temperature control?

• Select target turnover rate N.

2) Required Circulation

Qreq=N×VQ_{\text{req}} = N \times VQreq​=N×V

• $V$: Tank volume (m³)

• QreqQ_{\text{req}}Qreq​: Required total circulation (m³/h)

3) Eductor Flow & Quantity

• Assume entrainment ratio ER (e.g., 4)

• For one eductor: Qout=(1+ER) QmQ_{\text{out}} = (1+ER)\,Q_mQout​=(1+ER)Qm​

• Number of eductors: n=Qreq/Qoutn = Q_{\text{req}} / Q_{\text{out}}n=Qreq​/Qout​

• Pump flow: Qpump=n×QmQ_{\text{pump}} = n \times Q_mQpump​=n×Qm​

• Pump pressure: must provide 1.5–3 bar ∆P at eductor inlet (plus piping losses)

4) Nozzle Exit Velocity Check

• Area: A=πd2/4A = \pi d^2 / 4A=πd2/4

• Velocity: v=Qout/Av = Q_{\text{out}} / Av=Qout​/A

• Typical nozzle velocities: 15–30 m/s are acceptable; confirm with supplier charts.

Example Calculation

• Tank volume: 10 m³

• Target: 5 turnovers/h → Qreq=50 m3/hQ_{\text{req}} = 50\,\text{m}^3/hQreq​=50m3/h

• ER = 4, motive flow per eductor: 5 m³/h

• One eductor output: Qout=(1+4)×5=25 m3/hQ_{\text{out}} = (1+4)\times 5 = 25\,\text{m}^3/hQout​=(1+4)×5=25m3/h

• Required eductors: $50/25=2$

• Pump flow: 2×5=10 m3/h2 \times 5 = 10\,\text{m}^3/h2×5=10m3/h at ~2 bar (plus losses)

Velocity check (20 mm nozzle):

• Area = 0.000314 m²

• Qout=0.006944 m3/sQ_{\text{out}} = 0.006944\,\text{m}^3/sQout​=0.006944m3/s

• Velocity = 22.1 m/s → acceptable.

Placement Guidelines

• Height: 1–1.5 nozzle diameters above the tank bottom

• Angle: parallel or slightly upward (10–30°) for sweeping action

• Wall sweeping: direct jets along tank walls to eliminate dead zones

• Cross-pattern: arrange 2–4 eductors opposite each other for circulation loops

• Avoid short-circuiting near inlets/outlets

• Use multiple smaller eductors instead of one large nozzle for even distribution

Pump & Piping Considerations

• Ensure pump delivers required flow and ∆P

• Piping velocity ≤ 2–3 m/s to minimize loss and noise

• Individual isolation valves for each eductor for balance and maintenance

• Install filters/strainers to prevent clogging

Materials & Durability

• PP / Glass-filled PP: general service, low to medium temp

• PVDF: aggressive acids/solvents

• 316L SS: high temp, hygienic or food-grade

• PTFE linings/seals: for highly corrosive duty

Eductor vs. Mechanical Agitator

Common Mistakes to Avoid

• Undersized turnover rate → slow mixing → increase number of eductors

• Wrong angle/position → dead zones → reorient jets for wall/bottom sweeping

• Excessive pipe velocity → noise and losses → increase line diameter

• No filtration → clogging → add strainers and plan cleaning cycles

Industrial Applications

• Chemical & Petrochemical: neutralization, solvent blending, polymer solutions

• Food & Beverage: syrup mixing, CIP assistance

• Water & Wastewater: chemical dosing, sludge suspension

• Mining & Pigments: slurry agitation, settling prevention

• Heat management: thermal equalization in storage tanks

Operation & Maintenance

• Regular inspection of nozzles, seals, clamps

• Cleaning/back-flush in solids-laden service

• Performance checks: sample concentrations and temperature profiles

FAQs

How do I choose entrainment ratio? From vendor data; typical 3–6.
How many eductors do I need? Use n=Qreq/Qoutn = Q_{\text{req}}/Q_{\text{out}}n=Qreq​/Qout​; usually 2–6 per medium tank.
What if viscosity increases? Jet reach and ER drop; consider more eductors or hybrid with mechanical mixing.