How is the required mixer size calculated?

The sizing is based on batch volume, bulk density, desired filling level and container type. If the batch is specified as mass, it is first converted to volume using the bulk density of the product.

The required vessel volume is calculated based on the batch volume and the selected fill factor. The calculated volume is then matched with a standard vessel within the permitted filling range.

Finally, the container size and the estimated total load are checked against the volume and weight capacity of the mixer. The result is an initial sizing of a compatible mixer/container combination.

What does bulk density mean for the sizing?

The bulk density is the ratio of a powder's mass to the volume occupied by the powder, typically expressed in kg/L or g/cm³. Numerically, the two units are equivalent: 1 g/cm³ = 1 kg/L.

When sizing based on batch mass, the bulk density is used to calculate the batch volume:

Batch volume [L] = batch mass [kg] / bulk density [kg/L]

A 100 kg batch with a bulk density of 0.5 kg/L therefore takes up about 200 L, whereas the same mass at 0.8 kg/L takes up about 125 L.

The bulk density used should be representative of the actual condition of the product. Aeration, compaction, particle size distribution and handling can alter the measured bulk density and thereby affect the design basis.

What filling level should a tumble mixer have?

The degree of filling is the ratio between the product volume and the nominal volume of the container:

Filling ratio [%] = product volume / container volume × 100

With tumble blending, sufficient free volume is necessary to create the desired material movement during rotation. Too high a fill level can restrict the movement of the powder, while a very low fill level can alter the blending mechanism and process progression.

This calculator uses 33–66 % as the design range for tumble blending. It is a sizing range and should not be interpreted as a universal optimal fill factor.

The optimal value depends, among other things, on container geometry, the flow properties of the powder, cohesion, density differences between the components, and the requirements for blend homogeneity.

How is the container size chosen?

The theoretically required vessel volume is calculated as:

Vessel volume [L] = batch volume [L] / desired filling level

For example, a batch of 100 L at a desired filling level of 50 % gives a theoretical vessel volume of 200 L.

Since the calculated volume does not necessarily correspond to an available standard size, the calculator compares the result with relevant standard containers. A standard size is only accepted if the resulting actual filling level still falls within the defined operating range for the selected mixer type.

The choice of container should also be assessed in relation to geometry, handling, cleaning, product discharge and compatibility with the mixer and other process equipment.

What is the difference between a tumble mixer and an agitated mixer?

A tumble mixer blends the product by rotating the container itself. The powder contents are repeatedly moved as a result of the container's rotation and geometry, and the blending takes place without a traditional mechanical agitator inside the product.

This typically provides a relatively gentle process with low mechanical impact, but the mixing efficiency depends, among other things, on the vessel geometry, rotation ratio, degree of filling, and the properties of the powder.

A mixer with paddle attachment using a mechanical agitator, such as a stirrer, which moves the product inside a stationary or otherwise configured container. This allows for a different energy input and material movement than with tumble blending.

The choice between the principles should therefore be based on, among other things, powder flow properties, cohesion, particle characteristics, component ratios, risk of segregation, desired mixing time and requirements for product integrity - not solely on the batch size.

When does the calculation require review?

The calculator performs a initial capacity-based design. It does not in itself document that a given configuration is suitable for the specific process.

A review should be carried out when the application involves conditions that cannot be adequately described through batch volume, bulk density, degree of fill and standard capacities.

This applies, for example, to:

• drift close to the mixer's mass or volume limits
• cohesive, poorly flowing or strongly aerated powders
• significant differences in particle size or density with a risk of segregation
• Requirements for ATEX or other explosion protection
• containment or handling of potent/hazardous products
• special requirements for materials, surfaces or hygienic design
• CIP/WIP or other specific cleaning requirements
• NIR/PAT, valves, mounting arms or other optional equipment
• special containers or non-standard batch sizes
• integration with upstream/downstream process equipment
• critical requirements for mixture homogeneity or documented process performance

In these cases, you should product data and process requirements are assessed together, and how relevant testing with the actual product may be before the final mixer and vessel configuration is determined.