Volumetric Screw Feeders

Volumetric screw feeders are the most economical solution for feeding bulk solids at a controlled rate. Unlike gravimetric systems, flow control is based on regulating the rotational speed of the feeding element – a screw with a centre shaft or a shaftless spiral. Thanks to their simple construction without a weighing system, these units offer a lower capital investment whilst still delivering feeding repeatability of ±2–3% of setpoint after calibration.

The DSL V family performs equally well as a stand-alone continuous feeding system or as part of a batch dosing system for portion preparation. Importantly, every DSL V volumetric feeder can be upgraded in the future to a full gravimetric (loss-in-weight) system by adding a strain gauge weighing platform and controller – without replacing the feeder itself.

The correct selection of the feeding element has a direct impact on feeding stability and repeatability. Depending on the bulk material properties, we use two screw types, which can be freely interchanged to extend the feeding range.

A volumetric feeder is the optimal solution for applications where the material has a relatively constant bulk density and accuracy requirements are not as stringent as for multi-component recipes. It is also the most common choice as a material infeed to a weigh hopper, mixer or pneumatic conveying system.

Conversely, where accuracy below ±0.5% is required, or where full mass flow traceability or process validation (GMP) is needed, we recommend a DSL G gravimetric solution. It is worth noting, however, that every DSL V feeder can be upgraded to a gravimetric version at any time.

Parameter	Volumetric (DSL V)	Gravimetric (DSL G)
Measurement principle	Constant volume per unit time	Continuous loss-in-weight (LIW)
Typical accuracy	±2–4% after calibration	<±0.5% of setpoint
Feedback loop	None (open-loop)	PID controller (closed-loop)
Density variation compensation	Requires manual recalibration	Automatic, in real time
Capital cost	Lower – simpler construction	Higher (weighing system + controller)
Upgrade path	Can be upgraded to gravimetric	Full system from the outset
Optimal application	Homogeneous materials, vessel infeed, conveying	Multi-component recipes, high-value ingredients, audits

In a volumetric feeder, bulk material is discharged from the hopper into the feeding chamber, where the feeding element (screw or spiral) conveys it at a controlled rotational speed. The volumetric feed rate depends directly on the rotational speed, geometry and diameter of the feeding element. Meanwhile, the horizontal agitator driven by a separate motor ensures uniform screw fill, which is particularly important when handling cohesive and poorly flowing materials.

DSL V feeders find wide application in numerous process configurations, including use as:

• material infeed to a weigh hopper or mixer
• component of a pneumatic conveying system (feeding rotary airlock valves)
• feeder on a packaging line
• raw material infeed beneath a FIBC (bulk bag) discharge station
• stand-alone feeding unit in simpler continuous applications

What Is the Difference Between a Volumetric and a Gravimetric Feeder?

A volumetric feeder meters a constant volume of material per unit time by regulating screw speed. It has no feedback loop – it assumes a constant bulk density. A gravimetric (loss-in-weight) feeder, by contrast, measures the actual mass loss and adjusts the screw speed in real time. As a result, the gravimetric system is more accurate (<±0.5% vs ±2–3%), whilst the volumetric feeder is simpler and has a lower capital cost.

Can the DSL V Feeder Be Upgraded to a Gravimetric Version?

Yes – this is one of the key advantages of the DSL family. The DSL V volumetric feeder is fully compatible with the weighing system and controller used in the DSL G gravimetric version. The upgrade involves adding strain gauge load cells and a PID controller without replacing the feeder itself. This allows you to start with the more economical volumetric solution and transition to a full loss-in-weight system as needs evolve.

What Is the Accuracy of a Volumetric Feeder?

The maximum feeding error after calibration is ±2–3% of setpoint. In practice, accuracy depends on the consistency of the material’s bulk density – the more homogeneous the product, the better the repeatability. It is worth noting, however, that a volumetric feeder does not automatically compensate for density variations, so for materials with variable properties, periodic recalibration or an upgrade to a gravimetric system is recommended.

What Are the Most Common Uses of a Volumetric Feeder?

The most common applications include material infeed to a weigh hopper or mixer, feeding pneumatic conveying systems, dosing on packaging lines and raw material infeed beneath FIBC (bulk bag) discharge stations. Additionally, the DSL V feeder performs well as a stand-alone feeding unit in simpler continuous applications where precise mass flow control is not required.

Is the Feeder Suitable for Explosive Atmospheres?

Yes – we offer DSL V feeders in an intrinsically safe design compliant with the ATEX Directive. All electrical components are certified for use in dust explosion hazard zones, and the construction ensures continuity of equipotential bonding for proper dissipation of electrostatic charges.