The more you know about dust and the various techniques to control it, the greater your chances are of specifying a system that will get the job done at the lowest possible cost. This article reviews how to devise a strategy for dealing with dust.
Wet spray systems can offer significant advantages over ventilation/exhaust systems or structural enclosures. They are:
- Highly effective.
- More economical to operate.
- Quickly installed; offer straightforward operation.
- More durable.
- Reliable; offer consistent performance with routine maintenance.
Prevention or Suppression
Wet dust control systems use spray nozzles to apply humidity, water, and/or chemicals to:
- The dust source to prevent the dust from becoming airborne.
- Airborne dust particles to suppress (capture or knock down) dust and minimise the distance it travels.
Requirements for dust prevention are very different than those for dust suppression even though both involve applying moisture. It’s important to understand the differences to ensure optimal performance.
To prevent dust, the following factors must be considered: Materials respond to moisture differently. When applying moisture to ore, one gallon per tonne usually provides adequate wetting. However, coal repels water and requires the use of more moisture and, in some cases, chemical additives to increase absorption. Too little moisture results in airborne dust. Too much moisture may compromise the integrity of the material, cause costly production problems/equipment damage and create dangerous sludge.
Most dust particles created during breakage are not released into the air. The dust stays attached to the material and adequate wetting is required to ensure it stays attached (partially processed minerals and coal may be more sensitive to moisture than unprocessed material).
If the material being sprayed is stationary (storage pile), drop size and spray angle are critical. If the material is moving (conveyor) drop size and drop velocity are top concerns. When dust is airborne and needs to be suppressed, it is important to match liquid drop size to the particle size of the dust. Drops that are larger than the dust particle avoid collision with the dust.
When drops are smaller than the dust particle, they may collide, but the drops evaporate too quickly and release the captured particles. The greatest chance for suppression occurs when the diameter of the drop size and dust particle are comparable. Dust suppression is most effective in areas where there is little air turbulence.
Specifying the proper spray nozzle
Once you have a plan for where and how to control dust, the next step is to choose spray nozzles that will deliver the desired performance. Consider these factors:
Drop size refers to the size of the individual drops that comprise a nozzle’s spray pattern. Each spray pattern provides a range of liquid drop sizes. Many factors can affect drop size such as liquid properties, nozzle capacity, spray pressure and spray angle.
Air atomizing nozzles produce the smallest drop sizes followed by hydraulic fine spray, hollow cone, flat fan, and full cone nozzles.
For dust suppression, drops between 20 and 200 µm are typically required as airborne dust particles are usually in this size range. To produce this very small drop size, a higher degree of atomisation is required. Atomisation is achieved by pumping water through nozzles at high pressure or by using a combination of compressed air and water pumped at lower pressure to produce very small drops or fog.
If compressed air is available and economically feasible, air atomising nozzles are generally a better choice. They produce smaller drops and have larger flow passages than hydraulic fine spray nozzles, which helps to reduce clogging.
Your specific operating conditions will ultimately determine which nozzle style and spray pattern should be used. The table below provides an overview but be sure to check published performance data to verify flow rates and drop size at the operating pressures you’ll be using.
Spray angles range from 0O to 175O. The angle you need will be determined by the spray pattern, number of nozzles used and nozzle placement.
Operating pressure and flow rate will be determined by how much moisture you need to apply.
- Increasing pressure decreases drop size.
- High pressure sprays are better suited for enclosed areas.
- Nozzles operating at higher pressures should be placed close to the dust source to minimise the amount of air set in motion along the spray path.
- Keep nozzles out of the range of equipment or falling debris that could cause damage.
- Be sure nozzles are accessible for maintenance.
- Water hardness increases the surface tension of water and may increase the amount of water needed for adequate wetting.
- Be aware of the quality of the water source and provide filtration if necessary. Particulate in the system can clog nozzles and increase maintenance and wear.
To ensure effective, reliable dust control, consider automated system operation rather than manual operation. The system cost is typically offset quickly through savings on chemicals, water, electricity, labour and better performance as proper wetting is ensured.
Spray nozzles are designed for long-lasting, trouble-free performance; however, they will wear over time. Inspect nozzles often and replace when needed.
- Plugging/clogging: Use water clarification devices and strainers.
- Corrosion: Specify nozzles in the appropriate materials for the solutions being sprayed.
- Scale build-up: Control hardness level of the water and use chemical additives.
- Caking: Clean nozzles before build-up interferes with performance. Soak nozzles in water to loosen debris before cleaning.
- Changes in spray patterns, spray angles, and distribution.
- Changes in flow rate and pressure.
- Accidental damage: Cleaning tools should be significantly softer than the nozzle material – use a toothbrush or toothpick, never metal objects.
Ensuring successful dust defeat
Following these strategies can help you specify a wet dust control system; however, it is always wise to get expert advice. Leading spray technology manufacturers typically offer complimentary evaluations, proposals and more detailed information than supply house catalogues. Investing time with these companies is recommended.