In many manufacturing environments, High Speed Dispersers (HSDs) are often viewed simply as high-speed mixers. There is a common assumption that “the higher the RPM, the better the mixing results.” However, based on RST experience in industrial mixing and troubleshooting HSD applications, this way of thinking does not always lead to the right solution.

The reason is that dispersion is different from mixing or blending:

• Mixing/blending aims to make the materials in the tank uniform.
• Dispersion aims to wet solid particles (wetting), break up agglomerates (deagglomeration), and distribute the particles into the liquid phase. A stabilization step usually follows so the particles do not clump together again.

Because the challenge is more complex, simply raising blade speed does not automatically improve results. Based on our experience with industrial mixing equipment and HSD troubleshooting, when a customer says "our HSD already runs at high RPM, but the result is still not what we expected," we do not immediately recommend raising the RPM. We check the following four things.

1. Tip Speed: RPM Is Not the Only Parameter

Tip speed is the linear speed at the tip of the rotating blade, measured in m/s. Two HSDs running at the same RPM can produce different tip speeds if their blade diameters differ.

HSD Tip Speed Formula

Tip Speed (m/s) = (π × D × RPM) / 60

D = blade diameter in meters and RPM = revolutions per minute.

Calculation Example

A blade with a diameter of 450 mm (0.45 m) at 1,000 RPM:

Tip speed = (3.1416 × 0.45 × 1,000) / 60 ≈ 23.6 m/s

Another machine at the same RPM but with a larger blade will produce a higher tip speed. That is why the right question is "What tip speed are we achieving?" and not only "What is the RPM of this machine?"

RPM Table for a Target Tip Speed of about 22 m/s

This table shows that a larger blade does not need a high RPM to reach the same tip speed.

What Tip Speed Is Commonly Used?

For pigment dispersion in paint, ink, and coatings, mixer manufacturer literature generally cites a range of about 20–25 m/s (roughly 4,000–5,000 ft/min). A range of 22–25 m/s is often used as a starting point for evaluation. The optimum value still depends on the formulation, viscosity, pigment concentration, blade design, tank geometry, and process conditions. No single tip speed value guarantees the best result for every formulation. 

Is a Higher Tip Speed Always Better?

No. Excess mechanical energy can cause:

• higher power consumption,
• a rise in product temperature,
• excessive turbulence,
• air entrainment (trapped air and foam),
• an unstable vortex,
• higher mechanical load.

The goal is an operating condition that delivers the specified dispersion quality while keeping energy use, temperature, and equipment load under control.

2. Blade-to-Tank Diameter Ratio: One System, Not Two Components

Blade diameter defines the working area of the impeller. Tank diameter, liquid height, and vessel geometry define the material circulation pattern.

• Tank too large relative to the blade: circulation does not effectively reach the whole area, leaving zones of material that barely move.
• Blade too large for the tank: excessive turbulence, air entrainment, and higher power demand.

Starting rule of thumb: a blade-to-tank diameter ratio of about 1:3 (tank : blade ≈ 3:1) is often used for certain HSD applications. This is a starting guideline, not a universal design standard.

The right ratio depends on:

• disperser blade design,
• product viscosity,
• solids concentration,
• tank geometry and liquid height,
• the required circulation pattern,
• the process objective.

3. Blade Position and Flow Pattern Inside the Tank

Blade position affects how material circulates. In a suitable HSD configuration, the blade draws material into the high-shear zone and then redistributes it throughout the tank. This pattern often forms a doughnut-shaped flow (doughnut effect). The flow pattern is also influenced by impeller geometry, tank dimensions, liquid height, viscosity, solids concentration, shaft position, and baffles or other vessel features.

What Happens When Circulation Is Ineffective?

• Material does not circulate optimally.
• Material accumulates in certain areas (dead zones).
• Material does not enter the high-shear zone.
• Process time gets longer and results are hard to keep consistent.

4. Blade Clearance: Distance from the Tank Bottom

Blade clearance is the distance between the disperser blade and the tank bottom. This dimension affects flow in the lower part of the vessel.

Chain of effects: poor circulation → dead zones → longer process time → higher energy consumption.

As a starting reference, mixer manufacturer guidance often cites a distance of about one blade diameter from the tank bottom (roughly 0.5–1.5× the blade diameter). The final value depends on:

• blade diameter and design,
• tank geometry,
• product viscosity and density,
• solids concentration,
• liquid height and process requirements.

Consider blade clearance from the design stage, then verify it through trials on the actual product.

What Happens If These 4 Parameters Are Ignored?

These impacts do not always occur, but they are possible:

1. Longer production time because extra processing is needed to reach the target fineness.
2. Higher energy consumption per batch due to longer run time or excess mechanical energy.
3. Higher operating cost from longer cycles and greater energy use.
4. Higher mechanical load, contributing to wear on the shaft, bearings, coupling, motor, and gearbox.

Need Help Selecting or Evaluating an HSD?

Planning to buy a new High Speed Disperser, upgrade an existing machine, or dealing with dispersion problems? For a custom HSD application review, these are the points to discuss:

• the material to be processed and the batch size,
• product viscosity and the required dispersion quality,
• blade type and tank geometry,
• motor and gearbox configuration,
• operating conditions.

The goal is a mixing system that fits your formulation, process, and production needs.

PT Rho Sigma Trijaya (RST)
Industrial Mixing Solutions | Custom Mixer & Stainless Steel Tank Fabrication
WA: +62 878-8073-3615
Email: sales1@rhosigmatrijaya.co.id