Powder Dosing
The right technologies for dosing your powder
Flexible dosing with precision and high quality
Screw feeders are the most widely used equipment for powder dosing. In addition, weigh belt feeders and vibratory feeders can be utilized for powder dosing. MESPO can help identify the solution that best supports your specific powder dosing requirements.
Loss-in-Weight powder dosing
A gravimetric powder feeder is based on a volumetric feeder that is either suspended from load cells or mounted on a weigh frame. Dosing control is achieved by continuously comparing the actual loss in weight against the desired setpoint. A controller constantly regulates the screw motor speed based on these measurements.
The accuracy of a gravimetric dosing system typically ranges from 0.2 to 0.5%. To achieve high accuracy, several factors come into play, and the tare weight of the equipment must be kept as low as possible. This ensures that the load cell capacity is utilized optimally for weighing the product rather than the equipment. Another critical parameter is the number of refills, which should be minimized. This is because the feeder operates volumetrically during refill cycles and consequently cannot measure what is being discharged.
Example:
Calculation
- Maximum dosing capacity: 600 kg/h
- Bulk density: 0.5 kg/l
- Maximum dosing capacity in l/h = 600 / 0.5 = 1200 l/h
- Number of refills per hour: 15
- Refill capacity: 15 x maximum dosing capacity per hour
Result
- Refill volume = 1200 / 15 = 80 l per refill
- Hopper size = 80 / 60 × 100 = 133 l; the closest standard hopper is then selected
Refill capacity = 1200 l * 15 = 18,000 l/h
These two factors are contradictory. A low tare weight dictates that the equipment cannot be arbitrarily large and heavy, whereas a low refill frequency requires a large hopper.
Gain-in-Weight powder dosing
Gain-in-Weight dosing systems are configured with one or more volumetric powder feeders discharging into a hopper or bin suspended from load cells or positioned on a weigh platform. To maximize weighing accuracy, the weighing system calculates when to stop the screw feeder relative to the target setpoint. This accounts for material in flight (from the moment the product exits the feeder until it registers in the weighing system). This function is referred to as "material in flight compensation." To eliminate dosing errors caused by material in flight, a shut-off valve is often installed at the outlet of the powder feeder.
A Gain-in-Weight dosing accuracy is typically in the range of 0.5 – 0.75%. The feeder is frequently operated with a two-speed cycle: coarse dosing at high screw speed followed by fine dosing (dribble feed) at low screw speed.
Advantages of Gain-in-Weight:
- A simpler weighing system can be utilized
- More cost-effective solution than Loss-in-Weight, as it requires only a single weighing system
- Manual refill systems can be integrated
Disadvantages of Gain-in-Weight:
- Only one component can be dosed at a time, resulting in longer cycle times compared to a Loss-in-Weight system.
- When batching very small micro-ingredients, accuracy may be reduced because the weighing system must be scaled to handle the total batch weight. Consequently, it may be necessary to implement two separate weigh hoppers: one for micro-ingredients and one for major ingredients.
Gain-in-Weight technology is typically used in batching systems where ingredients are weighed into a recipe. This can take place directly in a weigh hopper, mixer, or intermediate bulk container (IBC).
Loss-in-Weight / Gain-in-Weight
It is also possible to combine the two dosing principles: Loss-in-Weight and Gain-in-Weight.
This is achieved, for instance, by placing a weigh hopper or container on load cells alongside a series of volumetric feeders that dose the major ingredients. These additions are registered by the weighing system of the container or weigh hopper. Subsequently, minor ingredients can be fed via one or more Loss-in-Weight feeders. Finally, the weighing system of the container/hopper acts as a gross checkweigher for the total batched weight.
Combining these two technologies leverages the advantages of both systems.
Design and integration
Common to both Loss-in-Weight and Gain-in-Weight systems is the requirement to account for the following factors during engineering and integration:
- Differential pressure across equipment – which can disrupt weighing accuracy
- Venting of the weigh hopper
- Ambient and structural vibrations
- Air drafts through the production facility, which can influence the weighing system
Screw selection
Screw configuration depends entirely on the material being handled. For example, when dosing raisins, an open spiral screw is typically selected. Conversely, a free-flowing powder requires a screw design capable of positive shut-off to prevent flushing. In such cases, a screw with a very tight pitch or a twin-screw feeder is typically specified.
FAQ
Volumetric powder dosing
Volumetric powder dosing is governed by screw diameter, screw profile, and rotational screw speed. A volumetric feeder can be operated in continuous feeding or batch feeding mode. In batch mode, volumetric dosing is time-controlled.
Volumetric screw feeders deliver a precise quantity of material based on volume. They are well-suited for applications where constant volume is sufficient to maintain process consistency. Volumetric dosing is often utilized where material bulk density remains stable. Volumetric feeders are engineered to handle a broad range of bulk materials, including powders, pellets, flakes, and granules. The feeder operates by conveying a fixed volume of material per unit of time, ensuring process consistency. Volumetric feeding is ideal for applications where absolute high precision is not critical, but a steady, reliable material flow is required.
Volumetric powder feeding is thus commonly applied in duties with moderate accuracy requirements, typically within approximately 3–4%. Feeder capacity is calibrated by running the unit for a set time across various screw speeds. The measured values are plotted to generate a calibration curve correlating feed rate with rotational speed. This allows the operator to easily adjust feeder speed whenever output requirements change.
As shown in the example to the right, the discharge rate is not linear across the entire speed range. Furthermore, this calibration method does not account for the reduction in screw fill level that occurs as the supply hopper empties.
Advantages of volumetric powder dosing:
- Simplicity and Cost Efficiency: Volumetric feeders are generally simpler and more cost-effective than their gravimetric counterparts, as they do not require dynamic weighing instrumentation.
- Robustness: These feeders are recognized for their durability and ability to operate continuously in demanding environments without loss of precision.
- Flexibility: They can be adapted to handle various material types and throughput rates, making them versatile across a wide range of industrial applications.
- Technology and customization: Various configurations and sizes are available to meet specific customer requirements. Volumetric feeders can be integrated into existing production lines with minimal system modifications.
Gravimetric dosing
Gravimetric screw feeders measure and meter products based on weight rather than volume. This makes them ideal for processes where high dosing accuracy is critical, as they compensate for variations in bulk density. Gravimetric screw feeders are frequently utilized in complex processes such as continuous blending or in operations requiring tight material flow control to ensure optimal end-product quality.
Gravimetric dosing systems operate by continuously weighing the material as it is conveyed. This enables real-time adjustment of the feed rate to ensure the specified mass flow per unit of time is consistently delivered to the process. This is particularly advantageous in applications where material bulk density is subject to fluctuation.
Gravimetric dosing is ideal for applications demanding high accuracy, such as pharmaceutical manufacturing, chemical processing, food production, or other industries where even minor dosing deviations can compromise product quality.
Advantages of gravimetric dosing:
- High Precision: Gravimetric feeders ensure extremely accurate dosing, which is essential for rigorous quality control processes.
- Flexibility: They can handle a wide variety of bulk solids, including powders, granules, and flakes, while adjusting to variations in material properties.
- Quality Control: Precise control of the material flow ensures that critical production specifications are consistently met.
- Technology and Integration: Gravimetric feeders feature advanced load cell technology and integrated control systems that enable continuous monitoring and dynamic adjustment of the material flow. This facilitates seamless integration into existing process control systems.
Types of Gravimetric Feeders
- Batch Feeders: Utilized in processes where exact quantities of material must be discharged in discrete batches.
- Continuous Feeders: Engineered for processes requiring a constant mass flow with high accuracy.