A circulating screw mixing tank uses a rotating helical screw to move liquid through a confined side chamber and back through the tank. Hiscox figure 1430 shows the screw beside a partition and sloping return guide.
Circulating Screw Propeller and Mixing Tank Interactive Calculator
Explore estimated pumped flow, turnover, tip speed and Reynolds number. Enter a pumping number appropriate to your screw and operating conditions; the illustration explains the circulation path rather than predicting detailed fluid motion.
Equation Used
- Pumping number supplied by the user for the relevant screw geometry and operating regime.
- Fixed density 1000 kg/m³; viscosity converted from cP to Pa·s.
- Working tank volume is used for nominal Q/V turnover.
- Shaft rotation is displayed 20 times slower than the entered rpm. Flow arrows show direction only.
- Tank, return guide and screw pitch are illustrative, not fabrication dimensions.
Circulation is an estimate from the pumping number you supply. The calculator assumes density 1000 kg/m³ and Newtonian viscosity. A fixed pumping number does not automatically account for viscosity, draft-tube losses or this particular screw geometry. Tank turnover is not a validated blend time.
A helical screw and a return chamber
Hiscox figure 1430 illustrates a long screw rotating in one side of a mixing tank. A partition separates the screw region from the main liquid volume, and a sloping member guides the returning circulation.
The cutaway animation shows one chosen operating direction: liquid rises through the screw chamber, crosses into the main tank and returns below the partition. The helical flight is attached to a central shaft. A belt pulley at the top drives that shaft.
A continuous helical flight runs along the screw shaft inside the side chamber. Flow arrows describe the circulation route; they are not particle trajectories from a fluid simulation. Pitch, clearances and tank shape are illustrative.
Circulating and mixing liquids
The historical description lists laundries, soap crutching and oil refining among applications of circulating screw mixing tanks. The common purpose is moving liquid repeatedly through a driven circulation region.
Actual mixing performance depends on liquid properties, tank internals, screw geometry, loading and operating conditions. The calculator provides a coefficient-based comparison, not an equipment selection or validated blend time.
What the calculator estimates
The pumping-number definition gives Q = Nq N D³. Use shaft speed N in revolutions per second and outside diameter D in metres to obtain Q in m³/s. The displayed circulation multiplies by 60,000 to obtain L/min.
Nominal turnover is Q/V using consistent volume units. Tip speed is πDN. Reynolds number is ρND²/μ, using the fixed density 1000 kg/m³ and converting viscosity from cP to Pa·s.
Nq is specific to geometry and operating conditions. Holding it fixed while varying viscosity keeps Q fixed in this simple model; only Reynolds number changes. That is an assumption, not a claim that real circulation is independent of viscosity.
Default comparison
At 400 rpm, 300 mm screw diameter and Nq = 0.7, estimated circulation is 7,560 L/min. A 5,000 L working volume gives 1.512 nominal turnovers per minute. Tip speed is about 6.28 m/s.
At 1 cP and the assumed density, Reynolds number is 600,000. Increasing viscosity to 1,000 cP reduces it to 600. The fixed-coefficient flow estimate remains 7,560 L/min, but that coefficient may no longer apply.
Doubling diameter at constant N and Nq multiplies estimated Q by eight and tip speed by two. Doubling volume halves turnover without changing Q. These are scaling relationships, not proof of equivalent mixing quality.
Limits of a pumping-number estimate
A large circulation estimate does not establish homogeneity, suspension quality, heat transfer or motor power. The return path and local recirculation can affect how much of the tank is effectively mixed.
For a real screw, choose Nq from relevant tests or supplier data across the applicable operating regime. Motor selection, mechanical clearances and permitted operating conditions require the actual screw, tank and fluid specifications.
Circulating-screw questions
Why does viscosity change Reynolds number but not estimated flow?
The simple flow equation uses the entered Nq as a fixed coefficient. Viscosity affects Reynolds number directly; its effect on flow must be represented through an appropriate Nq or a more detailed model.
Is tank turnover the same as blend time?
No. Q/V is a nominal circulation rate. The time needed to reach a specified uniformity depends on the flow pattern and process.
Are the arrows calculated fluid trajectories?
No. They explain the chosen circulation direction. The animation is a mechanical cutaway, not CFD.
Does the animation run at actual shaft speed?
It runs 20 times slower so the screw flight remains visible. Numerical results use the full entered rpm.
Is the default pumping number verified for this historic screw?
No. It is an adjustable assumption. Use relevant measured or supplier data for a real design.
References & Further Reading
- Gardner D. Hiscox, Mechanical Movements, Powers, Devices and Appliances, printed page 343, figure 1430: circulating screw propeller and mixing tank.
- Post Mixing Optimization and Solutions, Pumping, definition Q = Nq N D³ and geometry-specific flow number.
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