Filling of powders into bags – Challenges and Solutions.


Filling of powdery bulk materials into bags is often a technical challenge. The reason is the capability of the materials to capture air between the solid particles.

Reduction of the air into the material before filling the material into bags is one important measurement to reduce the amount of air in the filled bags. Due to technical and cost aspects, a total reduction of the air is often not feasible.

Thus a further reduction of the air during the bag filling process is desired. Bags with a certain de-aeration capacity allow de-aeration during and after filling.

Product properties: Good to know about them!

During the production of powders, the solid particles are often aerated with air. Due to the very small particle size and the particle shape of the powder, the air escapes very slowly.

If the bag filling is done directly after blending, most of the air which was added to the Powder during this process will be filled into the bags together with the Powder.

Figure 1. Decreasing of the filling level the bag due to de-aeration.

Since the air decreases the bulk density, relatively large bags have to be used to fill the powders in.  Because de-aeration is a function of time the air will escape after filling of the bags. Due to the de-aeration, the filling level in the bags will decrease (Figure 1) and the filled bags are fluffy, whereas compact bags are easier to handle and save logistic costs.

Consequently, the de-aeration of the powders before feeding into the bag filling systems is of huge interest to the powder production industry and its customers. The installation of pre-hoppers is a promising approach to answer to this interest.

Figure 2 Installation of a pre-hopper for de-aeration of powders before filling in bags

Figure 2 shows a common arrangement of bag filling machines in the industry (A).  The bag filling machine is installed directly downstream of a blender or huge silo. With this arrangement, all air which was added to the product, for blending or another process will be filled together with the product into the bags.

By installation of pre-hoppers (B) for de-aeration of powders a certain percentage of the air can escape from the product before filling into bags.

Figure 3 Basic working principle of pre-hoppers for de-aeration.

The basic working principle of pre-hoppers for de-aeration (Figure 3) is as follows:

  1. Filling of pre-hopper.
  2. Filling stop, de-aeration of material during a certain resting time and discharging into bag filling machine.
  3. Filling start and continuing of discharging into bag filling machine.

The filling level in the pre-hopper is controlled by level switch signals to start and stop feeding and discharging of the pre-hopper.

Bag Properties: How to utilize them!

Whereas a de-dusting filter is working due to under-pressure (with reference to atmospheric pressure) inside the system, an effective valve bag filling system for filling of powders is working due to overpressure inside the bags.

Figure 4 The inverted filter principle of valve bags.

Porous or perforated bag materials allow air to escape from a bag inside to outside due to the pressure difference between bag inside and outside, whereas the solid particles can’t pass the thin drains (Figure 4).

To maintain an overpressure inside the valve bags the connection of the bags to the filling systems has to be pressure-tight. The design of the relatively small valves of the bags allows the usage of filling spouts with inflatable sleeves at the filling systems. The filling systems have to be designed to create overpressure in the bags.

Figure 5 De-aeration during filling bags.

Figure 5 illustrates the main advantage of a porous or perforated valve bag compared with an open mouth bag.

  1. A high percentage of the air – captured in the bulk material – can escape during the filling process due to the overpressure in the bag.
  2. A further percentage of air will escape during a short filling brake after filling of 90 % of the target weight.
  3. Only a small certain percentage of air will remain in the bag after finishing the filling process. The amount of air depends on the properties of the bulk material and the properties of the bag.

Valve bag filling machines: What to know about them!

The HAVER M Series Filling Machines with Horizontal Impeller Filling System are designed to fill powdery bulk materials into valve bags, whereat the filling system is creating an overpressure inside the bags during filling.

The overpressure and the using of porous or perforated bag materials result in a filter effect, which allows air to escape from the bags, whereas the solid particles remain in the bags.

The design of the HAVER M Series Filling Machines considers modularity and scalability, in order to provide the most productive and economical solutions for different requirements. In case requirements change after a certain operation time filling capacities can be increased by installing additional machines. Manual operated systems could be automated by installing automatic bag placers. Ultrasonic Sealing systems could be installed later for automatic sealing of the bags.

Figure 6 Basic design.

According to Figure 6, the entire valve bag filling systems consists of:

  • Pre-hopper, to ensure first de-aeration of the bulk material before feeding into the filling machine.
  • Impeller system, to feed bulk material into the valve bags.
  • Impeller drive, variable speed as an option for filling of different kind of bulk materials.
  • Scissor valve, for dosing of the bulk material into the valve bags. Valve bags are weighed during filling.
  • Filling spout, to connect the valves of the valve bags with the filling system.
  • De-dusting system, to minimize dust emission.
Figure 7 The impeller system.

The impeller system (Figure 7) is the heart of the filling system. It feeds the product into the valve bag by creating an overpressure.

The modular design of the impeller system allows choosing

  • the impeller system material,
  • the impeller diameter,
  • the impeller bleeds number and design,
  • the impeller speed and
  • the filling channel diameter according to bulk materials and valve bags properties.
Figure 8 The scissor valve.

Under the most extreme conditions, independent of the bulk material type, the scissor gate is proven for dosing and complete closing of the filling channel.

The scissor gate guarantees an exact dosing of the filling flow rate during fine flow, which results in best weighing accuracy.

Three positions of the scissor gate are controlled by the HAVER MEC control system via a three-position pneumatic cylinder:

  1. Course flow, 100 % open.
  2. Fine flow, the opening can be adjusted manually according to the bulk material properties.
  3. Closed
Figure 9 The filling spout.

The filling spout (Figure 9) is the interface between the valve bag filling system and the valve bags.

Various designs are available to be chosen according to the bulk materials properties and the valve bags properties and dimensions.

Filling spouts with inflatable sleeves guarantee a material tight and overpressure tight connection of the valve bags with the filling system, which results in a dust emission free filling and compact bags.

Figure 10 The Dosing, Weighing and Filling Process.

To ensure the most efficient and economical filling process (Figure 10) the process parameters have to be adapted to the bulk material properties (e.g contained air) and valve bag properties (e.g. de-aeration capacity).

  1. High material flow and no filling interruption, for materials with less contained air and bags with high de-aeration capacity.
  2. Low material flow and two filling interruptions, for materials with much-contained air and bags with low de-aeration capacity.
Figure 11 The HAVER MEC Control System.

The dosing and weighing of the bulk material are controlled by the HAVER MEC Control System (Figure 11). The HAVER MEC Control Systems allows to program and to save 99 different sorts to fill a different kind of bulk materials with different filling weights into different bags.

Each sort comprises a set of parameter to adjust:

  • filling weight
  • material flow (kg/s) by impeller speed and sequence
  • course and the fine flow amount
  • sequence and duration of fluidization

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Courtesy: Guido Neu (Technical Expert)