Case Study: Dry Fog Dust Suppression at a Bulk Material Transfer Point
A dry fog system cut visible dust at a bulk transfer point without wetting the product. Sizing notes, nozzle counts and published ranges inside.
When a transfer point makes dust, the plume is visible before the people are. The chute drops dry aggregate onto a moving belt, the impact shatters the pile, and fines lift off the belt. Published references on transfer point dust control put respirable dust concentrations several times above the occupational exposure limit during live loading. The same references cap product moisture gain from fog suppression below 0.5 percent, which is the whole point of dry fog. This case is about a bulk handling operation caught between dust control and a dry product, and the dry fog system that settled both.
Case Snapshot
The reference configuration below is the generic shape of the installation. It is the starting point the sizing was built from. Every value is a realistic working figure for a bulk transfer point of this class.
| Item | Value |
|---|---|
| Industry | Bulk materials handling, aggregates and minerals (anonymized) |
| Duty | Dust suppression at a conveyor transfer point |
| Material handled | Dry, moisture-sensitive aggregate and mineral fines |
| Original setup | Wet sprinkler suppression over the transfer zone |
| System chosen | Dry fog air-atomizing nozzles inside the enclosed transfer point |
| Operating point | Water at 1 to 3 bar, air at 5 to 7 bar at the manifold, droplets 1 to 10 µm VMD |
| Material requirement | 316L or polymer air-atomizing heads with hardened tips for abrasive fines |
| Compliance | Occupational dust exposure limits for respirable dust |
The transfer point class matters more than the exact tonnage. A chute dropping dry material onto a belt inside an enclosure is the most common dust source in bulk handling. The logic below transfers to most crushers, screens, and belt transfer duties. The operating point was chosen from the air and water supplies on site, not from the catalogue. It sits inside the reference envelope for the dry fog nozzle class.
The Challenge
The problem was visible from the moment the belt started. Fines lifted off the impact zone and rode the air currents out of the transfer point. On dry days the plume hung over the walkway, and operators worked in it. Published references on transfer point dust describe this pattern exactly: the material stream, the impact, and the induced air that carries fines away.
Wet sprinklers had been tried first. They knocked the dust down and wet the product at the same time. The moisture spec rejected the product when the gain climbed, and the wet fines caked on the belt and the idlers. A sprinkler system that fails the product test is not a dust control system at all.
Respirable dust is the part that reaches the lungs, and it is the part that matters for compliance. Published guidance on occupational dust exposure covers respirable fractions at transfer points. The limit is measured in milligrams per cubic meter of air. The operation could not demonstrate compliance while the plume was live.
Dust did not stop at the walkway. It settled on bearings, idlers, and the drive house, and it loaded the baghouse. Published maintenance references for bulk handling link airborne fines to premature bearing wear and filter loading. Housekeeping cost shift hours, because the walkway had to be hosed before it could be walked.
The operation sat at the hard end of the published context. Transfer points are the highest dust source in many bulk plants. Respirable fractions can dominate the load when the material is dry and the drop is high. Any fix had to cut the respirable fraction without adding moisture to the product.
The Solution
The fix was dry fog. Dry fog is a cloud of droplets small enough to stay airborne for seconds, sized to match the dust particles themselves. Published references on dry fog dust suppression define the droplet range at roughly 1 to 10 µm. That is the same size class as the respirable fines at a transfer point. When a droplet and a dust particle of similar size collide, they stick, agglomerate, and fall back into the material stream.
Positioning decided the outcome. The nozzles were arranged around the enclosed transfer point in two bands. The first band sits above the impact zone, where the dust is generated. The second band sits at the discharge curtain, where the induced air would otherwise carry fines out. Published installation guidance for dry fog calls for coverage of the whole enclosure mouth, not a single nozzle aimed at the plume.
The system draws water and compressed air through a control panel that meters both. Water runs at 1 to 3 bar and air at 5 to 7 bar at the manifold, inside the reference envelope for the nozzle class. Each nozzle atomizes a fraction of a liter of water per minute into the air stream. The result is a fine cloud that wets the dust and nothing else.
Wet suppression was rejected on the moisture test. Sprinklers flow liters per minute and soak the product surface, which fails a moisture spec in minutes. Dry fog meters a fraction of a liter per minute per nozzle, and the droplets evaporate or fall with the dust. Published references put the moisture gain from dry fog below 0.5 percent, which keeps the product inside spec.
The Engineering Behind the Choice
The air-to-water ratio is what makes fog out of water. Published nozzle data for air-atomizing heads gives air-to-water mass ratios of roughly 5:1 to 10:1 for fine atomization. At that ratio the air stream tears the water into droplets in the 1 to 10 µm class. A ratio too low produces wet spray, and a ratio too high wastes compressed air.
Collision physics explains why the size match matters. Capture efficiency between a droplet and a particle peaks when the two are similar in size. A large droplet sails past a small particle, because the particle follows the air around the droplet instead of hitting it. Matching the droplet to the dust puts the collision in the efficient zone, which is why dry fog works where coarse spray does not.
Air consumption is the running cost to size. Published data for the small air-atomizing class puts air demand at roughly 2 to 10 Nm³/h per nozzle at 6 bar. The figure depends on the water flow. Compressor sizing starts from the nozzle count and the manifold pressure, with a margin for the duty cycle. The air must be dry and filtered, because moisture and oil in the line end up in the cloud.
Water use is where the comparison is won. A wet sprinkler at a transfer point flows liters per minute, and several of them cover one chute. A dry fog nozzle meters a fraction of a liter per minute. Published references on dust suppression put the water saving at 50 to 90 percent, which is the number that usually sells the retrofit.
The compressor was sized from the nozzle count, not from the water line. Total air demand is the sum of the nozzle air flows at the manifold pressure, plus a margin for line losses and future rows. The plant’s existing compressor absorbed the load once the transfer point count was fixed.
The enclosure does half the work. Rubber curtains close the mouth of the transfer point so the induced air stays inside. The fog cloud travels with the dust inside the enclosure, and the agglomerated material falls back onto the belt. Published guidance for transfer point dust control treats the enclosure and the fog as one system.
Nozzle count follows a rule of thumb. One dry fog nozzle covers roughly 1 to 1.5 m of transfer point width. A second row sits at the discharge end, where the curtain closes. A single-chute transfer point typically lands at 8 to 12 nozzles before the trial, and the trial settles the final spacing.
Freeze protection is a winter detail that matters. The water line to the manifold is heat-traced or drained at shutdown, because a frozen line starves the fog exactly when dust is worst. Compressed air lines rarely freeze, because dry air carries little moisture. The design review covers the coldest month before the system ships.
The Results
No site audit numbers appear in this table. The figures come from published references on dry fog dust suppression at transfer points. They are the range this class of system delivers when it replaces wet sprinklers on the same duty.
| Metric | Typical published result | Why |
|---|---|---|
| Visible dust reduction | 80 to 95 percent at transfer points | Droplets matched to the particle size collide and agglomerate, so fines fall instead of lifting |
| Water use | 50 to 90 percent lower than wet suppression | Fog nozzles meter a fraction of a liter per minute where sprinklers flow liters per minute |
| Product moisture gain | Below 0.5 percent | Droplet mass is tiny and the air stream keeps the product surface dry |
| Filter and maintenance load | Reduced | Fines that agglomerate and fall do not reach the baghouse or settle on bearings |
These are published ranges, not promises for a specific site. The actual result depends on the material, the drop height, and the enclosure, which is why the sizing step comes before the purchase. For this operation, the change put the walkway back in service and brought the respirable dust picture inside the exposure limit. That is the outcome compliance cared about most.
The operation used the published ranges as the acceptance band for the trial. Visible dust, moisture gain, water use, and filter load were compared against the old sprinkler baseline before the change was made permanent. That is the honest way to run a retrofit. It is also the reason the numbers above are framed as ranges rather than as a single promise.
Why This Case Matters
The droplet-size logic transfers. Start with the dust particle size, then the droplet size, then the air supply. That order rules out most of the catalogue before flow is discussed. The dust suppression nozzle guide walks through the same steps for any transfer point, and it ends with a shortlist instead of a guess.
If your transfer point makes dust, check the enclosure first. Most dust escapes are containment failures, not nozzle failures. The enclosure is too open, or the curtains are missing, or the induced air has no place to go. The wide-angle coverage notes in the wide angle nozzle guide for dust suppression pair with this case directly.
The dust suppression application page shows where this duty sits in the wider range. BoreJet ships samples for fog pattern testing before production, and a pattern test on the actual enclosure beats any catalogue guess.
The moisture story transfers too. Any dry product that cannot tolerate wetting is a candidate for the same sizing path. The constraint that killed the sprinklers is the constraint that dry fog is built for. This case shows that the droplet size is the design decision, not the pump.
What to send for a sizing, in one list:
- Transfer point width and drop height, because the nozzle count comes from the width.
- Material type, moisture content, and fines fraction, because that decides the droplet size class.
- Enclosure and curtain details, because containment decides the nozzle positions.
- Available compressed air pressure and flow, because the air supply sizes the system.
- Water pressure and quality at the manifold, because the feed must be steady and filtered.
- Freeze exposure and the shutdown routine, because the water line needs protection.
- The dust measurement or the occupational exposure limit in force, because the acceptance band comes from compliance.
Each item changes the answer, so a complete list returns a sized configuration in one round.
Send the duty conditions through the contact page. BoreJet comes back with the nozzle model, the count, and the air consumption, sized to your transfer point rather than guessed from a catalogue. The product range for this duty lives on the air atomizing page, and the sizing path above is the same one that produced this result.
Your Duty May Differ
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