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Batch size – the working volume of liquid in the tank. It can vary over time, for example while a tank is being filled or emptied. A ratio of liquid height to tank height above 0.2 is normally recommended. The orientation and geometry of the tank must also be known in order to determine the best type, number, size and position of impellers for the application. Operating data such as temperature and pressure must also be specified so that the machine selected maintains the critical process conditions. In many applications, liquid, solid and gas phases are all in contact with the continuous liquid. In such cases, the EuroScale level is determined for each category, and the most demanding mixing problem governs the equipment selection.
EuroScale Explained
When solving a mixing problem, the key is to achieve the desired result. However, this result is often difficult to express precisely or to link to a specific mixer. Before comparing mixer options, the required process response must be defined in order to determine the best mixing solution.
Liquid blending – Mixing intensity is related to the bulk velocity of the fluid in the batch. This is an average velocity used in flow calculations, because the actual velocity distribution in a batch mixing system is very complex.
The bulk velocity is calculated as follows:
Where:
 |
Vb (m/min) – bulk velocity of the fluid Q (m3/min) – volumetric flow rate of the impeller A (m2) – cross-sectional area of the tank |
Theoretical analysis, test results and extensive practical experience show that using mixing intensity, and therefore fluid velocity, as the scale-up variable gives a success rate of over 90% at production scale. The liquid blending process response is therefore based on the bulk velocity in the batch. The greater the differences in viscosity and density between the components, the higher the bulk velocity and the higher the EuroScale level required.
A scale from 1 to 10 was introduced as a simple, practical way for customers to specify the mixing intensity they need.
The volumetric pumping rate of an impeller at a given speed, divided by the total working volume in the tank (Q/V), expresses the mixing level as tank turnover. This simple criterion is widely used in industry for mixer sizing. It is the number of times the entire liquid content of the tank is circulated per unit of time. The mixing time, i.e. the time needed for the batch to reach complete uniformity in miscible liquid systems, can be derived from the turnover rate. The table below shows the relationship between bulk velocity and EuroScale level, and how mixing performance changes as the level increases.
Example based on:
- Tank diameter 1,500 mm, working volume 3 m³.
- Two-component liquid system.
- Concentration ratio of the low-viscosity to the high-viscosity component: 5:1.
- Mid-range SG and viscosity values used for each EuroScale level.
| EuroScale | Bulk velocity (m/min) | Process response | Turnover (min-1) | Mixing time (min) |
|---|---|---|---|---|
| 1 | 1.8 | Mild blending Very slow fluid movement, for applications needing minimal motion. Flat surface. Process objective: blending miscible components to uniformity. Limiting range: SG differences < 0.1; viscosity ratios < 100 |
Example for EuroScale 2: Average SG = 1.05 Average viscosity = 50 cP Q/V = 2.4 |
5 |
| 2 | 3.7 | |||
| 3 | 5.5 | Medium-intensity blending Moderate flow with typical medium viscosities. Suitable for the widest range of process industry applications. Rippling surface at low viscosities. Process objective: blending to uniformity. Limiting range: SG differences < 0.4; viscosity ratios < 7,500 |
Example for EuroScale 4: Average SG = 1.2 Average viscosity = 740 cP Q/V = 4.1 |
15 |
| 4 | 7.3 | |||
| 5 | 9.2 | |||
| 6 | 11.0 | High-intensity blending High flow for components that are difficult to blend. Rapid surface motion at low viscosities. Process objective: blending to complete uniformity. Limiting range: SG differences < 0.6; viscosity ratios < 50,000 |
Example for EuroScale 7: Average SG = 1.3 Average viscosity = 4,250 cP Q/V = 5.3 |
25 |
| 7 | 12.8 | |||
| 8 | 14.6 | |||
| 9 | 16.5 | Violent agitation Very high flow for the most demanding applications. Surging surface at low viscosities. Process objective: blending to uniformity. Limiting range: SG differences < 1.0; viscosity ratios < 100,000 |
Example for EuroScale 9: Batch volume = 3 m³ Average SG = 1.5 Average viscosity = 15,000 cP Q/V = 3.6 |
65 |
| 10 | 18.3 |
Table Key
Process objective:
The result to be achieved.
Complete uniformity:
The degree of uniformity we call “complete” is based on an empirical equation giving the time needed to mix the fluids to within 5% of the final concentration, i.e. more than 95% uniformity.
Limiting range:
Based on a multi-component liquid system whose SG differences and viscosity ratios fall within the limits of each level.
As the example above shows, as the EuroScale level rises, the tank turnover generally increases, because the impeller moves a larger quantity of fluid in the tank. However, the higher average viscosity reduces the impeller’s pumping efficiency, which ultimately gives a lower turnover for the 15,000 cP case.
Tank turnover therefore depends on the type of mixing action rather than on mixing intensity alone: low-speed mixing with a large impeller can produce a higher turnover than high-speed mixing with a small impeller. Low-speed mixing is increasingly important for high-viscosity fluids in laminar flow, which may be shear-sensitive or have complex rheology. A single batch system may need both: large-scale mixing to move the bulk fluid, and small-scale, high-shear mixing for homogenisation. Mixing time, which depends on turnover, therefore increases with viscosity, and longer mixing times are generally acceptable for high-viscosity products. The aim of choosing the EuroScale level is to keep the mixing time within a reasonable limit for maximum throughput, without selecting an impractical, oversized mixer. Ultimately, the practical experience of a mixing applications engineer across a wide range of processes, or pilot tests for a new process, provides the basis for equipment selection and scale-up.
Solids Suspension
The process response for solids suspension is relatively easy to quantify. It is defined by the suspension level and the distribution of solids in the batch. Most solids are suspended up to a distinct level, known as the cloud height, expressed as a percentage of the liquid height. Below this level the liquid is rich in solids; above it, only a few particles remain. The table below shows the relationship between cloud height and EuroScale level, with a description of mixing performance as the level increases. Tank turnover has been calculated for a set of example conditions as a point of comparison.
Example based on:
A 2,500 mm diameter tank with a working volume of 8.8 m³, containing 5% solids with an average particle size of 100 µm.
Liquid SG 1.0, solids SG 3.0, liquid viscosity 1 cP.
Impeller speed is the only variable, with an impeller diameter ratio D/T of 0.3.
| EuroScale | Suspension level | Cloud height (%) | Turnover (min-1) |
|---|---|---|---|
|
1 |
On the lower suspension
For applications requiring minimal suspension. Mainly used to keep solids moving and prevent them from settling on the tank bottom. |
41.6 |
2.2 |
|
2 |
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|
3 |
Lower Suspension
Suitable for the widest range of solids suspension applications: all solids are completely lifted off the tank bottom. |
59.4 |
3.1 |
|
4 |
|||
|
5 |
|||
|
6 |
Background uniformity: 80%
For applications where solids must be suspended to most of the batch height. |
80.2 |
4.2 |
|
7 |
|||
|
8 |
|||
|
9 |
Uniformity of the background
For applications where solids must be uniformly suspended throughout the batch volume. |
100 |
5.3 |
|
10 |
As the example above shows, as the EuroScale level rises, the impeller speed increases so that solids are suspended to a greater cloud height. This in turn increases tank turnover, because more fluid is moved in the tank.
Other tests show that:
- As the solids content and particle size increase, a higher impeller speed is needed to reach the required suspension level.
- As the batch volume increases, the impeller speed needed for the required suspension level stays the same, but the turnover rate is lower.
- As the liquid viscosity increases, the impeller speed increases to overcome the greater flow resistance. Turnover therefore increases, as a higher turnover is now needed to maintain the same suspension level.
Gas Dispersion
The process response for gas dispersion is relatively easy to quantify. It is based on the superficial gas velocity in the batch, calculated by dividing the volumetric gas flow rate by the cross-sectional area of the tank.
The process objective is generally mass transfer. The batch cycle time for complete dispersion can be controlled by producing the required bubble size, which governs the mass transfer rate. This determines the gas–liquid reaction time: slower reactions need finer dispersion, i.e. smaller bubbles.
Three simple categories describe the process response for gas dispersion:
- Low dispersion – The impeller is flooded: the gas rises through the impeller with little mixing.
- High dispersion – The gas is fully dispersed out to the tank wall.
- Uniform dispersion – The gas reaches the tank wall and also recirculates below the impeller.