Why flour is not as easy to dose as it seems?
Product flow, screw filling and dosing accuracy in flour milling

At first glance, flour may seem relatively straightforward to handle. It is dry, fine and seemingly homogeneous. In a production process, however, it proves to be a demanding bulk material. Variations in particle size, composition, moisture content, compaction and bulk density affect how a particular flour flows from a silo or hopper into a feeder.

This is why dosing flour in a flour mill involves more than a screw and a drive. Consistent and reproducible dosing starts with understanding the behaviour of the product.

Different flour streams, one product specification
During the milling process, a grain kernel is reduced in size and separated step by step. This creates different fractions and flour streams. Depending on the process and the desired end product, these streams can be stored separately and later recombined in defined proportions. Ingredients may also be added to give a flour the desired properties or composition.

Dosing therefore plays a role at several points in the production process. This may involve relatively large product flows, but also much smaller quantities that need to be accurately added to a blend. This is where an important technical question arises: how do you ensure that the quantity the dosing system is supposed to deliver actually reaches the process accurately and reproducibly?

Flour does not always flow the same way
A key complication is that flour does not always form a consistent powder flow. Even a relatively small change in product properties can affect flow behaviour. Fine powders can be cohesive, compact or behave differently after being stored in a silo or hopper for a longer period. Differences between raw materials and production batches can also have an impact.

This becomes particularly noticeable at the discharge of a silo or feed hopper. The product may, for example, fail to flow down sufficiently, compact above an outlet or form a bridge. A complete blockage is not required to disrupt the dosing process. Fluctuating product supply to the dosing screw alone can already cause variations in the quantity being dosed. A dosing screw can only dose reproducibly if it is also filled reproducibly.

A kilogram is always a kilogram, but a litre is not always the same
This difference in product behaviour is also important when choosing between volumetric and gravimetric dosing. In volumetric dosing, a dosing screw conveys a certain product volume per unit of time at a given screw speed. If screw filling and the bulk density of the flour remain sufficiently constant, stable dosing can be achieved. But if compaction means that the same litre of flour suddenly contains more mass, or if fluctuating product supply means that the screw is no longer filled consistently, the screw speed may remain exactly the same while the number of kilograms per hour changes. This creates an important distinction:

One kilogram of flour is always one kilogram, but one litre of flour does not necessarily represent the same mass under all conditions.

When the actual mass rather than the volume determines the recipe or product specification, gravimetric dosing can therefore offer clear advantages.

Loss-in-weight: dosing based on mass
In a loss-in-weight system, abbreviated as LIW, the complete feeder, including its feed hopper, is continuously weighed. During dosing, the total weight of the system decreases. The feeder controller measures the rate of weight loss and compares it with the target feed rate, for example in kilograms per hour. If necessary, the feeder output is automatically corrected by adjusting the screw speed.

The feeder controller therefore does not merely rely on the theoretical volume displaced by the screw, but monitors the actual mass being discharged from the feeder. This makes it possible to automatically correct, within certain limits, for variations in factors such as bulk density or screw filling.

The screw still needs to be fed consistently
Even accurate gravimetric control starts with a reliable product supply. If flour does not flow sufficiently from the feed hopper to the dosing screw, the control system cannot compensate for this indefinitely. Bulk material flow within the feeder should therefore be as uniform as possible.

Kubota Brabender Technologie’s single- and twin-screw feeders use, among other solutions, an agitator for this purpose. It helps prevent bridging and promotes uniform screw filling along the length of the trough.

The agitator therefore has an important function in the dosing process. It is not simply intended to keep the product moving, but above all to keep the filling of the dosing screw as consistent as possible.

Why not all dosing screws are the same
The geometry of the dosing screw itself is the next factor to consider. There is no universal dosing screw suitable for every type of flour or powder. The correct screw profile depends on the flow behaviour of the bulk material, the required capacity, the dosing range and the required accuracy.

Depending on the product properties, single-spiral, single-blade or twin-concave screws can be used. Spiral and blade screws are also available with a progressive pitch.

The choice of feeder should therefore be based on questions such as:

• How easily does the product flow?
• Is the powder cohesive or highly free-flowing?
• Does it tend to compact?
• How uniformly is the screw filled from the feed hopper?
• How is the feeder refilled from the upstream system?
• What minimum and maximum feed rates are required?
• What turndown ratio is required?
• What dosing accuracy does the process require?

The selection of a feeder therefore starts not with the equipment itself, but with the bulk material.

Activating the product without an internal agitator
Another way to achieve a consistent product supply to the dosing screw is the FlexWall® principle from Kubota Brabender Technologie. This system uses a flexible polyurethane trough. External massage paddles act on the flexible trough wall. This movement activates the product and feeds it towards the dosing screw.

The material is therefore kept moving without requiring an internal agitator inside the trough. This principle can be particularly useful for powders whose flow needs active support. The product-contact parts can also be made relatively easy to access for cleaning.

The trapezoidal shape of the FlexWall® also allows several feeders to be arranged compactly around a central feed point. This can be useful when different product streams or ingredients need to be combined in a subsequent process step.

What happens during refilling?
Gravimetric loss-in-weight dosing presents another interesting control challenge: the feeder has to be refilled periodically.

During dosing in gravimetric mode, the system measures the loss in weight. During dosing in refill mode, the opposite happens: the weight increases rapidly over a short period. At that moment, the control system can no longer determine the current product flow from the loss in weight in the same way. The feeder controller therefore temporarily switches to semi-volumetric control during this short refill phase. After refilling, the weight signal stabilises again and gravimetric mode resumes control of the process.

Short and controlled refill cycles are therefore important. Kubota Brabender Technologie’s LIW systems also use a self-learning control system that analyses dosing behaviour during normal operation and uses this information during the semi-volumetric refill phase. This seemingly small part of the process illustrates how much control technology is ultimately involved in maintaining consistent dosing performance.

Hygienic Design for a dry process
Because flour is processed for human consumption, the hygienic design of dosing equipment is also important. Hygienic Design is not only about the materials from which the equipment is made. The construction itself also determines how easily product residues can be removed.

Examples include smooth surfaces, rounded transitions, as few hard-to-reach areas as possible, and product-contact parts that are easy to access or dismantle. This is particularly relevant in a dry process environment. Flour left behind in difficult-to-reach areas can accumulate and complicate product changeovers or cleaning. At the same time, the introduction of moisture should be avoided wherever possible in dry processes.

The feeder systems mentioned are therefore also available in versions designed in accordance with relevant Hygienic Design principles and requirements, including EHEDG and FDA requirements.

The dosing solution follows the product
Reliable flour dosing is ultimately not determined by one specific feeder or one particular type of screw. It is the combination of factors that matters:

product properties, flow behaviour, capacity, turndown ratio, dosing accuracy, cleanability and the position of the feeder within the overall process.

A free-flowing main product stream at a relatively high feed rate requires a different approach from a fine, cohesive powder that needs to be added in a much smaller quantity.

Selecting a dosing system should therefore start with evaluating the actual bulk material and process conditions. Based on this information, the most suitable dosing principle, product activation method, screw profile and control strategy can then be determined for the application.

From product properties to the right dosing solution
SPS | Solids Process Solutions supports manufacturers in selecting and integrating dosing technology for dry bulk materials. Together with Kubota Brabender Technologie, we consider not only the required capacity, but also the behaviour of the product and the role of the feeder within the overall production process.

Because reliable dosing starts not with the feeder, but with the product that has to pass through it.