Tank turnover time and true mixing time are related but not interchangeable. Turnover time is the working volume divided by a defined circulation rate. Mixing time is the measured time required for a process variable—such as concentration or temperature—to reach an acceptance band throughout the tank. Eductor selection can start with turnover, but it should be released against a process-specific mixing test.
Three flow values must be kept separate
| Term | Meaning | Do not confuse it with |
|---|---|---|
| Motive flow | Liquid delivered by the external pump through the eductor | Total internal circulation |
| Induced flow | Surrounding tank liquid entrained by the motive jet | A universal fixed multiple |
| Total circulation | Motive plus induced flow for the defined model and condition | Measured blend completion |
Use model-specific performance data at the pressure actually available at the eductor. Manufacturer product data demonstrates that circulation rate changes with pressure, so one ratio should not be applied to every model or operating point.
How to calculate nominal turnover time
For working volume V and total useful circulation Qc,total:
Tturnover = V ÷ Qc,total
If V is in m³ and Qc,total is in m³/h, the result is hours. Multiply by 60 for minutes. Qc,total should be the sum of useful model-specific circulation from all operating eductors, adjusted when branch pressures differ.
Example formula for a cylindrical tank
For an upright cylindrical vessel, V = πD²H/4 at working liquid depth H. Use actual process volume rather than nominal vessel capacity.
Nominal turnovers per hour
Turnovers per hour = Qc,total ÷ V. This is a comparison metric, not proof of blend quality. It is most useful when all projects use the same definition and measurement boundary.
Why actual mixing time can be longer—or shorter
- Injection location: adding concentrate directly into a strong circulation path can shorten distribution compared with adding it to a weak zone.
- Density difference: buoyancy or sinking can reinforce or oppose the eductor flow.
- Viscosity: higher viscosity changes entrainment, jet spread and bulk circulation.
- Tank proportions: a tall narrow tank and a shallow wide tank can share volume but require different layouts.
- Internals: coils and baffles divide the flow field.
- Measurement criterion: reaching ±10% at one location is not the same as reaching a tighter band at multiple elevations.
- Chemistry: dissolution or reaction time may control even after physical circulation is established.
For solids, suspension or off-bottom motion may be the acceptance condition; a turnover value alone is insufficient.
How to measure mixing time
- Define the acceptance band before the test.
- Select a safe tracer or process variable, such as conductivity or temperature, that represents the real mixing task.
- Choose measurement points at different elevations and expected slow zones.
- Record initial conditions, tank level, pump data, valve positions and eductor configuration.
- Introduce the change at a documented location and time.
- Record every point until all locations remain inside the acceptance band for the required period.
- Repeat under the worst relevant level, viscosity or temperature condition.
Do not infer complete mixing from a surface vortex or from one sensor near the return. Instrument accuracy and response time should be appropriate for the acceptance band.
Hypothetical turnover and test example
The values illustrate the calculation and are not a promised result.
- Working volume: 40 m³
- Verified total circulation from all eductors at operating pressure: 160 m³/h
- Nominal turnover time: 40 ÷ 160 = 0.25 h = 15 min
- Turnovers per hour: 4
A conductivity test might still show that the most distant upper point reaches the project acceptance band after 22 minutes. In that case, 15 minutes remains the nominal hydraulic turnover, while 22 minutes is the measured mixing time for that test condition. Both values should be recorded with their definitions.
How to improve a slow result
Check pressure and flow at the header, blocked passages, branch imbalance, suction short-circuiting, jet direction and slow zones before increasing pump size. Repositioning a unit or correcting a loaded strainer may solve the actual cause.
Use the sizing worksheet, placement guide and troubleshooting guide together.
Send Jeltecn the tank volume, acceptance criterion and measured operating point for a review.
Technical reference
For model-specific circulation data and the dependence on operating pressure, see Spraying Systems Co. 46550 Tank Mixing Eductor product data.