Was wenige Millimeter ausmachen können

From hobbock to IBC
- and the process tips over

An existing process is transferred from a small container to a larger one - for example, from a 30-liter hobbock to an IBC. The speed is often simply transferred.

What is overlooked:
The diameter of the impeller inevitably increases with the tank size.

Example:
Hobbock: Ø 170 mm
IBC: Ø 500 mm

And it is precisely this difference that fundamentally changes the physics of the process.

Same speed, completely different effect

The speed alone says nothing about how fast the agitator actually moves through the medium. The decisive factor is the speed at the outer edge of the agitator (agitator tip) - i.e. at the largest radius. If the same speed is maintained with a larger diameter, this means that the circumferential speed increases massively. The consequences in practice:

Increased shear forces in the medium
Increased air intake
Unnecessarily high energy consumption
Changed, often unstable mixing behavior

The process is therefore no longer comparable - despite the identical speed.

Same speed, completely different effect

The speed alone says nothing about how fast the agitator actually moves through the medium. The decisive factor is the speed at the outer edge of the agitator (agitator tip) - i.e. at the largest radius. If the same speed is maintained with a larger diameter, this means that the circumferential speed increases massively. The consequences in practice:

Increased shear forces in the medium
Increased air intake
Unnecessarily high energy consumption
Changed, often unstable mixing behavior

The process is therefore no longer comparable - despite the identical speed.

The decisive factor: circumferential speed

The physically relevant key figure for the design is the peripheral speed. It describes the linear speed of the agitator tip (outer radius) and results from

Peripheral speed = π × diameter × rotational speed

This clearly shows:
The diameter directly influences the speed - and changes the process more than the speed itself.

Practical example - correctly scaled

Initial situation:
Impeller Ø 170 mm
Speed 600 1/min
→ Peripheral speed: approx. 5.34 m/s

If this process is correctly transferred to an impeller with Ø 500 mm in the IBC, this results in a required speed of approx. 204 1/min.

Less than half the original speed - with comparable mixing behavior.

What this means for your application

Vorteile richtiger Skalierung bei Rührorganen

Conclusion - It all depends on the right key figure

A few millimeters more in diameter seem inconspicuous - but have an enormous influence on the process physics. If you only consider the rotational speed, you are making decisions based on an incomplete variable. If you take the circumferential speed into account, you understand the process.