A high speed disperser appears simple, but it is the machine most frequently mis-specified in a coatings or ink plant. Selection is usually made on motor power alone, which is the least useful of the four variables that actually determine performance: tip speed, disc diameter, vessel geometry and power sizing.
This guide sets out the calculation method for each variable. Tip speed is calculated from shaft speed and disc diameter and governs deagglomeration. Disc diameter is selected relative to vessel diameter and governs circulation. Power must be sized for peak viscosity during the batch, not for the average.
It also covers installation requirements, the practical sizing rules used for common batch sizes, and the specification mistakes that most often lead to underperformance. POLYC MACHINE supplies dispersers from laboratory units to production machines, specified by these calculations rather than by rule of thumb, drawing on 25 years of application experience.
Send your vessel dimensions and rheology data through the inquiry form, and POLYC MACHINE will specify disc diameter, tip speed range and drive power by calculation rather than by rule of thumb.
Key Takeaways
- Tip speed = pi x disc diameter x rpm / 60, and it governs deagglomeration
- Disc diameter must be selected relative to vessel diameter to drive circulation
- Power must be sized for peak viscosity during the batch, not the average
- Immersion depth must prevent a vortex reaching the disc at every stage
- Above roughly 30,000 to 50,000 cP a disperser is no longer the correct machine
Tip Speed: Formula and Target Range
Tip speed is the linear velocity at the outer edge of the dispersing disc. It is calculated as tip speed in metres per second equals pi multiplied by disc diameter in metres multiplied by rotational speed in revolutions per minute, divided by sixty. This is the parameter that determines the shear applied to the material.
For pigment dispersion, effective tip speed typically falls in the range of 18 to 24 metres per second depending on the formulation. Below that range deagglomeration is slow; above it the disc tends to incorporate air and generate heat without improving the result. The correct value should be established during development and then standardised.
The critical practical point is that tip speed, not rpm, is the meaningful variable. The same machine running at the same rpm with a different disc diameter produces a different tip speed and therefore a different result. Documenting tip speed rather than rpm makes validated settings transferable between machines.
Variable speed control is therefore worth specifying. It allows the wetting stage, the dispersion stage and the letdown or tinting stage to run at appropriate speeds, and it lets recipes store validated speeds per stage for repeatability.
Disc Diameter and Vessel Matching
Disc diameter should be selected as a proportion of vessel diameter rather than in absolute terms. A disc that is too small cannot drive circulation across the vessel floor, so material never reliably reaches the shear zone. A disc that is too large overloads the drive and raises viscosity locally.
A practical working range places the disc at a defined fraction of vessel diameter, chosen so the disc drives material down and outward across the floor and back up the walls. This toroidal flow is what makes dispersion uniform rather than localised around the disc.
Immersion depth must be sufficient that no vortex reaches the disc at any stage of the batch. A vortex that reaches the disc wastes energy on air and introduces foam, which then appears as surface defects in the finished product.
Because viscosity changes during the batch, the ideal immersion often differs between stages. Establishing it during commissioning and recording the setting makes the cycle reproducible rather than dependent on individual judgement.
Power Sizing Using the Rheology Curve
Power must be sized for the peak viscosity encountered during the batch. Viscosity typically peaks during or shortly after pigment addition, before the formulation is fully wetted and before any solvent or water adjustment. A drive sized for the average will slow or trip at exactly the point where throughput matters most.
The correct approach is to provide the full rheology curve through the cycle, measured at working temperature, and to size the drive for the worst case with margin. Where that data is unavailable, a conservative rating is preferable to a nominal one, because the cost of under-sizing is repeated slow batches and nuisance trips.
Power alone cannot compensate for poor circulation. If the disc-to-vessel ratio is wrong, a larger motor will not improve deagglomeration; it will simply draw more current. Geometry should be corrected before power is increased.
Where viscosity exceeds roughly 30,000 to 50,000 cP, a disperser is no longer the correct machine regardless of power. At that point a butterfly double shaft mixer or a planetary mixer should be specified instead.
Sizing by Batch Size and Installation Requirements
Batch size drives the broad machine class. Laboratory units handle roughly 0.5 to 20 litres for formulation work. Pilot and small production machines cover roughly 20 to 500 litres. Production machines cover roughly 500 to 5,000 litres and above, with platform-mounted configurations for the largest vessels.
Installation requirements are straightforward but frequently under-specified. The vessel must be stable and centred under the disc, with clamping that prevents vibration at speed. The lift must allow full travel for cleaning and vessel removal, and services including electrical supply and any cooling must be adequate.
Hydraulic lift is standard on production machines because it gives controlled positioning and safe working access. Where vessels are moved between stations, a floor-mounted machine with adequate lift height is usually more practical than a platform installation.
Where products change frequently, evaluate candidates on changeover provisions as seriously as on dispersion capability. Smooth shaft geometry, quick-release discs and good access reduce the time between products, which often determines effective weekly output.
Common Specification Mistakes
The first mistake is specifying on motor power alone. Power does not determine tip speed or circulation, so a powerful machine with poor geometry underperforms a correctly configured smaller one.
The second is transferring rpm settings between machines without recalculating tip speed. Because tip speed depends on disc diameter, identical rpm on different discs gives different results, and this is a common source of unexplained batch variation.
The third is neglecting the wetting stage. Adding pigment too quickly, or before the vehicle and dispersant are distributed, produces agglomerates that no amount of additional shear will remove. Extending the cycle in response only generates heat.
The fourth is specifying a disperser for a duty that requires a different machine. Once viscosity exceeds the circulation limit, or once a particle specification requires grinding, the correct response is a butterfly mixer, a planetary mixer or a bead mill rather than a larger disperser.
Technical Specifications
| Parameter | Specification |
|---|---|
| Tip speed formula | tip speed (m/s) = pi x disc diameter (m) x rpm / 60 |
| Target tip speed | 18 - 24 m/s for dispersion; lower for letdown and tinting |
| Disc sizing basis | proportion of vessel diameter, sufficient to drive floor-to-wall circulation |
| Immersion | deep enough that no vortex reaches the disc at any stage |
| Power sizing basis | peak viscosity from the full rheology curve at working temperature |
| Practical viscosity limit | approximately 30,000 - 50,000 cP |
| Capacity classes | laboratory 0.5 - 20 L; pilot 20 - 500 L; production 500 - 5,000 L and above |
Troubleshooting Guide
1. Dispersion slow despite a powerful motor
Cause: Disc-to-vessel ratio or immersion incorrect, so circulation is inadequate
Solution: Correct disc diameter and immersion to drive floor-to-wall circulation before increasing power
2. Different results on another machine at the same rpm
Cause: Tip speed differs because disc diameters differ
Solution: Calculate and set tip speed rather than rpm, so settings transfer between machines
3. Motor trips after pigment addition
Cause: Drive sized for average rather than peak viscosity, or disc oversized
Solution: Provide the full rheology curve for sizing and reduce disc diameter if the drive is overloaded
4. Foam forms and fineness stops improving
Cause: Vortex reaching the disc, or excessive speed for the stage
Solution: Lower speed, correct immersion, ensure adequate volume, and use stage-specific speeds
5. Results vary between operators
Cause: No documented cycle covering disc, immersion, speeds and addition
Solution: Document the validated cycle and train operators on the addition sequence and wetting time
Frequently Asked Questions
1. How do I calculate disperser tip speed?
Multiply pi by the disc diameter in metres and by the rotational speed in rpm, then divide by sixty. The result is tip speed in metres per second. For pigment dispersion the effective range is typically 18 to 24 m/s depending on formulation, and it should be set rather than assumed.
2. Why is tip speed more important than rpm?
Tip speed determines the shear applied to the material, while rpm alone does not, because tip speed also depends on disc diameter. The same rpm with a different disc gives a different result, so documenting tip speed makes validated settings portable between machines.
3. How do I choose disc diameter?
Select it as a proportion of vessel diameter so the disc drives material down and outward across the floor and back up the walls. Too small gives poor circulation; too large overloads the drive. Confirm against your actual range of batch volumes.
4. What power do I need?
Size for peak viscosity during the batch, not the average, since viscosity typically peaks shortly after pigment addition. Provide the full rheology curve measured at working temperature so the drive can be specified for the worst case with margin.
5. What viscosity limit applies to dispersers?
Practical operation typically extends to around 30,000 to 50,000 cP. Beyond that, circulation breaks down regardless of power and a butterfly double shaft mixer or planetary mixer is the correct machine rather than a larger disperser.
6. Should I specify variable speed?
Yes. It allows wetting, dispersion and letdown or tinting to run at appropriate speeds rather than forcing one compromise setting. Recipes can store validated speeds per stage, which improves repeatability and reduces air incorporation during finishing stages.
7. What installation provisions matter most?
Stable centred vessel with adequate clamping, full lift travel for cleaning and vessel removal, and sufficient electrical and any cooling services. Under-specified clamping causes vibration that limits the usable speed in practice.
Recommended POLYC MACHINE equipment
The following machines are commonly evaluated for the process described above. Final selection depends on formulation, target particle size, viscosity, capacity and contamination limits.
Related technical guides
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