Preventing dust formation: Clean bagging of powders and granulates

When filling a bag, the air contained therein is displaced. This is the primary cause of dust release. The extent of the emission depends on how this air can escape in the area of the filling nozzle and bag valve.

Turbulence, filling speed, and bag geometry act as amplifiers, not as independent causes.

During filling, air is displaced along the filling nozzle and the bag valve. This leads to a backflow toward the plant environment. This return air causes high local flow velocities and turbulence. Fine particles with low settling velocities follow this flow with almost no inertia and are discharged from the bag.

This effect is amplified under certain process conditions – in particular by:

• turbulent flows in the area of the filling nozzle,
• high filling speeds with local pressure peaks,
• uneven bag unfolding and uncontrolled air flow,
• decreasing particle size with low settling velocity.

Dust formation is therefore not a random event, but reproducible and predictable.

Conventional extraction systems only come into play downstream: they remove dust from the ambient air, but do not prevent its formation during the filling process.

Typically, dust emissions are concentrated at a few recurring process points. For example, a central leak point is located at the transition between the filling nozzle and the bag valve. At this point, displaced air can escape at high speed, carrying fine particles with it.

Another critical moment occurs when removing the bag after filling. When the bag is detached from the filling nozzle, there is often a brief release of pressure. This can cause dust still present in the valve area to be released.

Leaky machine housings or inadequately encapsulated system components also contribute to continuous dust emissions.

The physical release of dust during the bagging process has consequences. It is the common cause of safety risks, health hazards, and economic losses. The finer and more energy-rich the dust, the more critical its effects.

Fine combustible dust can be explosive if five conditions are met simultaneously:

• Combustible dust
• Oxygen
• Ignition source
• Sufficient dust concentration
• Turbulence

In bagging systems, dust release and turbulence are process-related and not uncommon. Even short-term emissions can lead to an explosive atmosphere locally.

For this reason, different ATEX zones are often defined in powder bagging plants. For example, product handling areas inside containers or pipes may be classified as Zone 20, while Zone 21 is often found around the filling nozzle.

Other areas of the plant, such as filling chambers or filter systems, can be classified as Zone 22.

From a technical point of view, it is most effective to prevent dust clouds from forming in the first place, rather than detecting or controlling them retrospectively.

When dealing with powders that are hazardous to health, it is not only the efficiency of individual filters that is crucial, but also the architecture of the entire bagging system. OEL limits can only be reliably adhered to in the long term if the product flow takes place in a largely closed system and air movements are specifically controlled.

This is less about capturing dust retrospectively and more about preventing it within the process. Open transitions or selective extraction systems quickly reach their limits when dealing with substances with very low limits.

Dust is not waste, but a lost product. Every fine particle carried away with the exhaust air leads to direct material loss and at the same time increases the load on filters and exhaust air systems.

In continuous filling processes, even small amounts of dust add up to measurable annual losses over long periods of operation. It is not the visible dust discharge that is decisive, but the continuous removal of the finest particles via exhaust air and filter systems.

From a process engineering perspective, the question is therefore not how dust can be extracted as efficiently as possible, but how its formation can be prevented during the bagging process.

Dust-free bagging does not require higher extraction power, but rather a fundamentally different process logic: air movements are not accepted as an unavoidable side effect, but are actively controlled and integrated into the process. The only effective approach is one that prevents dust formation directly at the source.

The VeloVac technology does not rely on removing dust that has already been generated, but rather on the controlled removal of the air displaced during filling before it can transport dust.

At the heart of the system is a closed vacuum chamber in which a defined vacuum is created during filling. In practice, VeloVac typically operates in a vacuum range of approximately 400 to 600 mbar. The air displaced when the bulk material is introduced is specifically removed from inside the bag.

The vacuum is parameterized specifically for each product, taking into account the properties of the bulk material, its compaction, and the bag combination used. The vacuum control system can be used to influence the filling speed and throughput while simultaneously controlling the mechanical load on the bag.

From a process engineering perspective, this results in several effects:

• No backflow along the filling nozzle and bag valve
• Reduced turbulent flows in the inlet area
• No dust released into the environment, as the process takes place in a closed vacuum chamber

The main difference to conventional extraction technology is that VeloVac does not work as a downstream dust collection system. The aim is controlled filling without any dust being released.

A significant proportion of dust emissions do not occur during filling itself, but immediately before. The causes of this are defective or incorrectly opened valve bags.

However, the ValvoDetect system detects such errors before filling. During the feeding of the empty valve bag, optical sensors check whether the valve is correctly opened. Defective bags are automatically detected and ejected from the process.

Compressed air nozzles and pitot tube sensors also ensure that the bag is positioned exactly before sealing. This prevents incomplete sealing and thus both product losses and dust emissions due to leaky valves.

This means that defective bags are detected at two critical points in the process: during bag feeding and before sealing.

After filling, securely closing the bag valve is another critical process phase. Dust in the valve area can impair the tightness of mechanical or thermal closure methods.

In contrast, ultrasonic sealing with ValvoSeal uses high-frequency mechanical vibrations to weld the bag material locally. Since the welding process takes place directly in the material composite, a stable connection can be established even if there are dust particles in the valve area.

This prevents typical defects such as torn or incomplete seams and eliminates the risk of subsequent dust emissions through leaky valve areas.

The cost-effectiveness of a bagging system is not measured solely by the investment costs or nominal output. The decisive factors are the running costs and risks over the entire life cycle of the system. Low-dust bagging processes reduce a number of indirect cost factors.

Dust emissions necessitate regular cleaning of the machine, peripheral equipment, and the surrounding area. In addition to the time required, this results in costs due to downtime, personnel commitment, and cleaning and disposal expenses. If dust emissions are prevented at the process stage, these costs are reduced structurally.

The plant technology itself is also affected by dust. Filters, sensors, and moving parts are subjected to greater stress and wear out more quickly. Low-dust processes, on the other hand, extend maintenance intervals and increase plant availability.

Dust-free bags and pallets reduce complaints along the supply chain. Contaminated packaging can disrupt logistical processes and lead to goods being rejected by customers.

Our practical experience confirms that early detection of defective bags measurably reduces the costs of complaints and cleaning, among other things. Technically clean bagging not only reduces dust emissions, but also improves safety, product quality, and the cost-effectiveness of the entire packaging process.

Dust emissions during the bagging of powders and granulates are not an unavoidable by-product. They are the result of uncontrolled air displacement during the filling process. Those who only capture dust after it has been generated are merely treating the symptoms. Those who control the air flow in the process itself, on the other hand, eliminate the cause.

Technologies such as vacuum chamber filling, optical bag detection, and ultrasonic sealing show that dust-free operation, process reliability, and cost-effectiveness are not mutually exclusive, but rather mutually dependent. Less dust means less risk of explosion, less product loss, less cleaning effort, and cleaner packaging along the entire supply chain.

The crucial question for plant operators is therefore not how efficient their extraction system is, but whether their bagging process still generates dust at all.