Dry Fog Dust Suppression System

What is Dry Fog?

Dry fog is a mist that where the droplets are so small that mist does not feel wet to touch.

The use of Dry Fog for dust suppression is based on the concept of agglomeration. Dry Fog technology uses compressed air and water to create micron sized droplets. When like-sized droplets and dust particles collide, agglomeration occurs which results in larger particles that fall back into the process by gravity because they are too heavy to remain suspended in the air.

To achieve agglomeration at the dust source, equal amounts of like sized particles of water droplets and dust particles must exist and they must be contained in an enclosed area.

At the treatment point, fog droplets are used to blanket the dust source and prevent the dust particles from escaping.

Advantages

Highly efficient and effective at reducing fugitive dust and suppressing respirable dust
Low water use
No chemicals
Minimises wetting and overall moisture content of the material
Returns useable material to the product stream
Ease of installation
Low maintenance
Flexible system configuration

The Technology

Dry Fog nozzles use a combination of focused compressed air and water and an external resonator to create an ultrasonic shock on the water particles to break them into a fine fog. They are commonly referred to as ultrasonic air atomising nozzles. The technology is illustrated in the following diagram.

DSI Dry Fog Nozzle

The inlet water and air pressures for air atomising nozzles are relatively low (1 bar water and 5 bar air). These pressures can be easily adjusted to change the water droplet size and allow the fog to be 'drier' or 'wetter' depending on the material, application or other factors.

It is all about droplet size

Droplet size is a key factor in the ability to suppress small (eg. less than 10 micron) dust particles. The table below lists the typical average droplet size for different fogging / misting technologies. Ultrasonic Dry Fog nozzles are significantly smaller than any other technology.

Technology	Average Droplet Size
Ultrasonic Dry Fog Nozzles	4 to 10 micron
Other Air Atomising Nozzles	40 to 60 micron
High Pressure (>80 bar) Fog	30 to 60 micron
Low Pressure (<15 bar) Misting including Fog Cannons	100 to 200 micron

The reason droplet size is so important is that airborne dust particles join with water droplets of the same size much more efficiently than different sized water droplets. This is due to the air stream that occurs at the surface of the dust particle and water droplet. The larger the particle/droplet the thicker the air stream and the further it protrudes from the surface. When there is a mismatch in particle/droplet sizes it is more difficult for the smaller particle/droplet to push through the air stream of the larger particle. The following image demonstrates this.

To effectively suppress PM10 dust particles you require water droplets smaller than 10 micron.

Maximising the total water surface area also improves dust suppression performance because there is more surface for the dust particles to impact. To maximise the total surface area you need to minimise the droplet size. Similarly the likelihood of collision is related to the number of droplets - the higher the number of droplets the higher the chance of a dust particle impacting a water droplet. Again reducing the droplet size dramatically increases the number of droplets.

The following table shows the number of droplets and their surface area for one litre of water.

Droplet Size	Droplet Number	Droplet Surface Area
10 micron	1.9 trillion	600 m2
30 micron	71 billion	200 m2
100 micron	1.9 billion	60 m2
300 micron	71 million	20 m2
1 mm	1.9 million	6 m2

To put this into an easy to understand image, 1 litre of 100 micron droplets has the surface area of a cricket pitch, and 10 micron droplets have the surface area of the whole cricket ground. And there are 1000 times as many 10 micron droplets in 1 litre of water than there 100 micron droplets.

The combination of more efficient dust suppression due to similar sized droplets and particles, larger surface area and much larger number of droplets makes Dry Fog exceptionally efficient at suppressing airborne dust.

Applications of Dry Fog

Dry Fog is a versatile dust suppression solution with applications across a wide range of industries. It is particularly effective at suppressing dust at conveyor transfer points, where bulk material movement often generates significant dust emissions. Other common applications include trippers, reclaimers, crushers, screens, ROM bins, truck dumps, railcar loading and unloading, ship loaders and unloaders, and ash silo discharge chutes.

One of the key advantages of dry fogging is its ability to suppress dust while minimizing the moisture content added to bulk materials. This assists in preventing clogging of equipment due to sticky material.

The following illustrate some dry fog applications.

ROM Bins / Dump Hoppers

For ROM Bins / Dump Hoppers it is important to prefill the bins prior to dumping so the displaced air that normally is full of dust is prefilled with fog.

Conveyors & Transfer Points

The following images show that the transfer points are filled with fog so any air that escapes (eg travels with the conveyed material) is full of fog suppressing the dust.

Dry Fog conveyor and transfer point dust control at CS Energy

Conveyor transfer point dust control using Dry Fog

Ship loading and unloading

For ship loading the fog is delivered around the discharge chute and the movement of the material carries the fog with it suppressing any dust. For ship unloading the hold is filled with fog to capture the dust generated when the grab picks up the material.

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Truck & Train Unloading

Why DSI Dry Fog?

DSI's low flow, ultrasonic nozzles create fog droplets 1-10μm in size which easily agglomerate to dust particles as small as PM2.5 and PM10. These ultra-fine fog droplets attach to similar size dust particles and drop them back into the process. As the particles agglomerate they also join and suppress the larger dust particles as well. When compared to alternative technologies, DSI Dry Fog's ultra-fine atomization can agglomerate to the widest range of particle sizes due to its small droplet size.

Wet Earth uses the DSI Dry Fog System because it is a robust, proven solution that has been used since 1997 in over 50 countries.

The DSI nozzles produce the finest droplets and have large throw distances compared to similar solutions from other vendors. The DSI nozzle enclosures provide robust protection to the nozzles and their complete solution simplifies installation. DSI nozzles are manufactured in the US and solutions are manufactured in a combination of the US and Australia.

The solution design utilises the experience of DSI's engineers based in the US (who have designed thousands of systems) which ensure you get the best possible performance for your system.

Wet Earth DSI Dry Fog installations in Australia include:

Whitehaven Daunia
CS Energy Callide Power Station
Jellingbah Coal
Ora Banda Mining
Peabody Wambo
Fulton Hogan Eastern Creek

Other DSI Dry Fog installations in Australia include:

BHP Port Hedland Operations
Citic Pacific Sino Iron
FMG Cloudbreak and Christmas Creek
Newcrest Cadia And Telfer Operations
Newmont Boddington Gold
First Quantum Ravensthorpe
Glencore George Fisher, Mt Isa, Hail Creek and Townsville Port

What about High Pressure Misting ?

High pressure misting is another approach to dust suppression, but it differs significantly from dry fog in both technology and effectiveness. High pressure misting systems use water at pressures typically greater than 80 bar and small orifice nozzles to generate larger droplets (usually in the 30 to 60 micron range). While these larger droplets can suppress visible dust, they are less effective at capturing the fine dust particles especially those smaller than 10 microns that pose the greatest health risks and are most likely to become airborne.

In contrast, dry fog systems use compressed air and water to create smaller sized droplets (3 to 6 micron) that are much better suited for binding with fine dust particles. This allows dry fog to achieve superior dust suppression, particularly for respirable dust that can travel offsite or be inhaled by workers. Additionally, dry fog systems require less water than high pressure misting systems to achieve the same dust suppression performance due to the larger surface area of the smaller droplets.

It is important for airborne dust suppression for water droplet sizes to match dust particle sizes. A good discussion of droplet size dust suppression performance and particle size travel can be found on our Dust Control Theory page.

By choosing dry fogging over high pressure misting, industries benefit from improved dust control, reduced water usage, and lower operational costs, all while ensuring compliance with stringent health and safety regulations.

The DSI Dry Fog System

DSI Dry Fog systems allow for flexible configuration with pre-assembled modular components. The type of application, type of bulk material creating dust, and operating environments are considered when customizing your system.

Housed in protective enclosures, we supply complete systems comprised of electrical controls, filtration, flow regulation, supply lines and nozzle mounts. If required, we can supply all necessary ancillary items including air compressors, air receiver stations, pumps, and self-cleaning secondary filtration systems.

Key system components are described in more detail below.

DSI Dry Fog Nozzles

The DSI Dry Fog nozzle is the key element of the system converting the water into a dust suppressing fog. The nozzles are stainless steel and manufactured to exacting standards to ensure their performance. There are other ultrasonic air atomising nozzles available on the market, but most droplet sizes are well above the 1 to 10 micron size droplets that the DSI nozzles produce.

The following table provides an example of the droplet size distribution for the DSN-3 nozzle.

DSI Water Droplet size

The following table lists the specifications for the range of DSI Dry Fog nozzles.

DSI Dry Fog nozzle range

Universal Fogger Assembly

Ultrasonic air atomising nozzles are prone to damage due to the thin wire that supports the resonator. The Universal Fogger Assembly addresses this with the use of a robust enclosure for the nozzle as shown below. The enclosure also combines air and water lines into a single flexible hose from the supply enclosure.

DSI Dry Fog Nozzle assembly

Fogger Mounts

It is important to be able to install the nozzles so that they can be positioned and angled for optimum coverage. There are a range of Fogger Mounts that provide robust methods to position the Foggers so they are secure and easy to access.

Ball and Socket Mount

The Ball and Socket Mount allows the Fogger to be fitted from the outside of transfer points and angled to provide best dust suppression performance.

DSI Dry Fog Ball & Socket Mount

Angle Bracket Mount

The Angle Bracket Mount allows the Fogger to be connected to most surfaces and allow the Fogger to be angled to the direction which provides best performance.

DSI Dry Fog Angle Mount

Fogging Manifolds

The Fogging Manifolds provide a multi-directional stainless-steel housing for the fog nozzles which are then connected through one conduit to the Multi-Function Modules. The Fogging Manifolds have the following features:

Heavy duty steel casing
Easily mounted
Factory fitted internal manifolding, single connection to Multi-Function Modules for easy installation
Unidirectional nozzle mounts allow adjustment of the fog direction to where it is needed

DSI Dry Fog Manifold

Fogging Solution Modules

There are a range of modules that can be included in a DSI Dry Fog solution which depend on the project size and layout.

The key functions of these modules are:

Filtration - typically multiple levels with self-cleaning function
Pressure control of water and air - to ensure the ideal performance of the atomising nozzles.
Electronic control - interface to other site systems
Distribution of the water and air to each nozzle

If you have a particular dust control application you would like to discuss or would like more information then please call us on +61 2 6062 3300 or send an email to sales@wetearth.com.au.