Outdoor airborne dust used to have a simple solution: require workers to wear an N95 mask or respirator. Easy, right? However, that doesn’t take into consideration several factors such as proper use of the mask, correct and consistent upkeep, and monitoring usage. Not that masks are a bad solution – they’re called Personal Protective Equipment (PPE) for a reason - but as a dust control solution, they just don’t address the presence of airborne particulate emissions as a serious regulatory violation.
When regulators from the Occupational Safety and Health Administration (OSHA) attach a Personal Dust Monitor to a worker, they are measuring exposure, regardless of whether a mask is worn or not. Generally, inspectors are looking for Respirable Crystalline Silica (RCS) particulate matter (PM) smaller than 10 microns (µm) in size at less than 50 micrograms (µg) in weight over an 8-hour time-weighted average (TWA), i.e., a single shift. Once measured, inspectors usually order violators to address air quality, regardless of personal protective equipment.
This is why preventing dust emissions at the source is more desirable than passive protection such as masks. More and more, regulators are identifying atomized mist as their preferred method for airborne dust control.
THE CONSEQUENCES OF DUST
RCS is invisible to the naked eye, but is the cause of silicosis and pneumoconiosis, the main chronic lung diseases found in miners, construction workers, and other bulk handling industry employees. The substance has also been linked to kidney disease, acute pneumonia, lung cancer and even serious mental decline. Research published in 2019 in the Life Sciences Journal concluded that the exposure of healthy subjects to ambient PM affects the brain, causing inflammation and memory loss.
An OSHA violation can be accompanied by fines and -- in some extreme cases – operational closures until a viable solution is implemented. Reported cases of dust related health issues tied to workplace operations could be followed by increased scrutiny, higher health insurance premiums or even lack of coverage.
Once airborne dust passes the site line, the Environmental Protection Agency (EPA) takes over, and serious permit violations can dramatically affect operations. Violations can also impact the availability and cost of future permits, depending on operators’ ability to demonstrate that they have taken serious steps to address particulate emissions, such as the addition of a mist cannon to their equipment portfolio.
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CONTROLLING AIRBORNE DUST
Currently, the only viable method for controlling outdoor dust emissions is using water. To effectively control airborne particulates, water droplets must be approximately equal in size to the particles. One reason is that the droplets must be small enough to travel with particles on the same ambient currents and collide with them. If droplets are too large, then the “slipstream effect” comes into play, and those collisions are less frequent. Small droplet size is one of the primary reasons why atomized mist is preferred by bulk handlers over hoses or sprinklers.
Hoses typically create water droplets 200 - 10,000 µm in size as compared to atomized mist, which ranges from 50 – 200 µm. Larger than 200 µm, droplets are unable to linger in the air or collide with dust particles. Due to their mass, they fall and form air currents around them. Like wind traveling around an airplane wing giving it lift, large droplets cause the same slipstream effect. When the tiny particles encounter the larger droplets, they get caught in the air draft around the drops, which diverts them away rather than being absorbed. However, when a particulate collides with an atomized droplet, their combined mass drives both to the ground.
CONTROLLING DUST AND RUNOFF
Having an employee with a hose standing by a storage pile and wetting it down seems counterintuitive if they are exposed to dust during the process. Moreover, spraying often occurs in active workfaces with large machinery, for example, demolishing a structure, digging or disrupting surfaces, or offloading tons of material. This exposes workers to moving equipment where the chance of an accident is greatly increased.
Runoff is another concern for dust suppression. The water volume from large hoses can be as much as 500 gallons per minute (gpm), the runoff from which can create soft muddy ground, standing water hazards, and may carry regulated substances. Site permits for bulk handling commonly have a provision limiting runoff to local environmental standards. In some cases, rules can be quite stringent, limiting runoff both in volume and content. This is particularly true for ports, power plants and other facilities located near waterways.
Atomized mist cannons are automated machines that are placed, turned on, and left alone. The largest sized cannon will use a maximum of 39 gpm to cover an area of up to 5 football fields when using the 359º oscillator. In addition to safely providing airborne dust control, cannons also reduce the labor costs of operation over that of hoses, with significantly less runoff.
HOW DOES A MISTING CANNON WORK?
Industrial mist cannons (aka, “fog cannons” or “dust cannons”) employ a cone-shaped barrel with atomizing nozzles in the front and a powerful fan in the back, protected by a mesh safety grate. They can be direct-wired or powered by a generator set. Water is usually supplied by a 1.5 in. (38.10 mm) hose with a cam-and-groove quick disconnect (sizes may vary depending on the size of the equipment and the available water sources). Water is run through an in-line 30 mesh 595 micron filter system to remove impurities that can clog nozzles, then to a booster pump to increase pressure – which is why the cannon only initially requires 10PSI (0.69 BAR) of constant water pressure to operate. The flow is pumped to a brass or stainless steel manifold with as many as 38 nucleating nozzles, which fracture the water into a fine atomized mist.
The powerful rear fan directs air through the cannon ring, propelling the mist up to 100 m (328 ft) and covering as much as 280,000 square feet with a single machine using full oscillation and 0-50º vertical adjustment.
WHAT DUST CONTROL SOLUTION IS RIGHT FOR MY SITE?
To improve the safety on any site, operators must first understand the application and the source of the dust. These seem like no brainers, but working with coal, shale, and other hydrophobic materials may require a surfactant water additive and a dosing pump. A hydrophobic substance will repel water, causing it to bead and roll off, but the surfactant causes the water to spread across the substance. This can not only benefit the airborne capture of particulates but also cause clumping in stockpiles which can further suppress emissions from wind or disruption by equipment.
For safety, take into account:
- Mobility – Many operators need to adjust machines for a myriad of reasons: task switching, varying wind direction, different sites, etc. Consider the needs of the application and choose a model mounted on a wheeled cart or skid where appropriate.
- Versatility – Some cannons have varying spray options for differing wind conditions or access to high-reach places.
- Proximity to power sources – Mounted on a mobile carriage, some cannons come with diesel gensets that power them through an entire shift and beyond.
- Source of dust – Some outdoor/indoor storage areas are static and have many sources of dust. Disruption from front loading or reclaiming causes dust. Loading from a stacker conveyor or tripper conveyor are also sources. Whether inside or outside, tower-mounted cannons cover a wide area and reach high above stockpiles. Specially designed misting heads address localized conveyor dust.
- Wind – Wind can be a concern for atomized mist, but bulk handlers have found that positioning machines upwind allows the mist to travel with dust particles. During windy conditions, both mobility and versatility are essential qualities that should be considered.
If the application is frequently exposed to winds, such as a port or the demolition of a tall building, some pressurized options should be considered. Although fan driven atomized mist has a long reach, it can be affected by high winds. To extend to tall areas – such as high-reach cranes performing demolition or emptying the hold of a tall cargo ship -- consider pressurized atomized dust control options like the DustBoss® Surge®.
SELF-CONTAINED VERSATILITY
The DustBoss Fusion is a self-contained dust control solution that's mounted on a mobile roadworthy trailer and can be customized to the needs of the customer. All models of the DustBoss cannon family have a Fusion version. The most popular model, the DustBoss® DB-60 Fusion™, combines the mobility of the regular DB-60 on a road-worthy carriage with the power of a 45 kW Kohler genset with Tier IV diesel engine featuring up to 100 gallon fuel capacity (~48 hours runtime).
Demolition contractors and quarry operators have found Fusions convenient for large sites and rural settings where access to power can be a challenge. With such a long run time, refueling and maintenance on the genset is easy and infrequent.
MOBILE MIST CANNONS
Clean air and a compliant site require machines that can be properly positioned to tackle dust at the point of emission. The family of DustBoss® DB-30, DB-60, DB-100, and DB-Surge® includes the workhorses of the DustBoss line. Known for their durability and reliability, most units remain in operation more than a decade after production, even in punishing industrial environments. Pulled on a mobile carriage by pickup truck or moved by forklift on a skid, the units are simply placed, turned on, and the machine takes care of the rest.
Ports and demolition contractors have benefited greatly from being able to move and place the cannon to provide dust control within minutes, with little disruption or labor. If operators find they need to adjust the settings or oscillation, the units are accompanied by a heavy-duty remote that can be activated from a safe distance on the ground or from the cab of loading equipment.
GET ABOVE STORAGE PILES
Air quality around storage piles is not just about workplace safety, but also about protecting nearby communities. There is no dry bulk handling sector, from steel to limestone, that doesn’t need to sequester a product into an area for either transport or waste management. These piles can be several stories tall and emit a tremendous volume of dust if not properly handled. The use of foams and covers for surface suppression can be a challenge due to the height and constant disruption as the material is stored, churned, and transported to make room for more.
Some materials like slag -- which is super-heated -- or coal, due to its churn volume – are generally stored outside. Managers of outdoor storage piles have found that tower-mounted DustBoss mist cannons can reach to the top of piles, offering both surface suppression and airborne particulate control. The mist settles lightly on the surface of the piles creating a damp outer shell (surfactants dosed by an optional dosing pump or unit like the DB-30 Injektor™ are recommended for hydrophobic substances like coal).
Although large indoor storage facilities enclose emissions, they can be notoriously dusty workplaces, requiring respirator masks and other PPE for staff to move through and perform maintenance. Units placed high above the piles on wall mounted platforms or structural beams allow constant control.
DUST CONTROL AT EMISSION POINTS
Both indoor and outdoor storage points commonly have belt conveyors feeding them. The point at which the material is released from the belt is called the “discharge point.” Depending on the length of the drop and the impact of the material, this freefall is where cargo separates and tremendous volumes of dust are emitted. A DustBoss DB-Ring creates a curtain of mist around the material stream, which follows cargo from the discharge point to the impact point at the top of the storage pile, controlling dust emissions.
Conveyor transfer points can have serious dust issues. The DustBoss Mini was designed for emission points where a constant throughput of material consistently creates dust. These are directional misting heads that, when pointed at a specific spot, prevent dust from leaving the area.
SAFETY IN HIGH HEAT WORK ENVIRONMENTS
Safety isn’t just about dust. There are also high heat environments that require industrial air circulation and engineered cooling mist to keep workers comfortable. One example is die casting operations, where DustBoss units have been employed with or without water to improve morale and create a more sustainable environment.
The difference between an air cannon and a large shallow misting fan like the ones commonly seen on the sideline of football games, for example, is that the concentrated air in a cannon is delivered in a cone rather than a general area immediately in front of the fan. The cone allows the cannon to cover a considerably larger area, provide superior control over air circulation and dictate the direction toward ventilation points.
DUST AND SAFETY IN THE WORKPLACE
Most employers will agree that the health and welfare of their employees is paramount. No matter the size of the site, a dust management plan can be implemented using a wide variety of mist cannons and accessories. Our dust control experts can help create a management plan within a reasonable budget geared toward compliance, health, and safety of workers.
Contact us for a free assessment of your site safety.
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