Making the best choice for your operations can get complicated since there are dozens of factors to consider. 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. In the sections that follow, we’ll delve into dust and review how to go about devising a strategy for dealing with it.
Why Water Sprays Are Usually the Way to Go
In most operations, wet spray systems offer significant advantages over ventilation/exhaust systems or structural enclosures. By comparison, wet spray systems are:
• Highly effective
• More economical to operate
• Quickly installed and offer straightforward operation
• More durable
• Reliable and offer consistent performance with routine maintenance
Depending on the type, volume, and location of the dust, wet systems are sometimes used in conjunction with other technologies. Whether used solo or in tandem with other systems, there are many variables to consider when specifying a water spray system. The first step is to assess your operations and determine the appropriate strategies for dealing with dust.
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 or Airborne dust particles to suppress or capture the dust and minimize the distance it travels
The system requirements for dust prevention are very different than the requirements for dust suppression even though both are applying moisture. It’s important to understand the differences between the two to ensure optimal performance.
When the goal is to prevent dust, the following factors must be considered to ensure a positive result
• Materials respond to moisture differently. For example, when applying moisture to ore, one gallon per ton usually provides adequate wetting. On the other hand, coal repels water and will require the use of more moisture and chemical additives to increase absorption.
Precision application of moisture is essential. Too little moisture results in airborne dust. Too much moisture may compromise the integrity of the material, cause costly production problems and equipment damage and create dangerous sludge – a maintenance nightmare.
• 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. Keep in mind that partially processed minerals and coal may be more sensitive to moisture than unprocessed material.
• If the material being sprayed is stationary, as on a storage pile, drop size and spray angle are critical. If the material is moving, as on a conveyor, drop size and drop velocity are the top concerns.
When dust is airborne and needs to be suppressed (also referred to as captured or knocked down), 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.
The Secret to Success: Specifying the Proper Spray Nozzle
Once you’ve assessed your operation and have a basic plan for where and how you need to control dust, the next step is to choose spray nozzles that will deliver the desired performance. Here’s an overview of the factors to consider when evaluating nozzles.
• 3Drop size
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 atomization is required. Atomization 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 atomizing nozzles are generally a better choice. They produce smaller drops and have larger flow passages than hydraulic fine spray nozzles which help to reduce clogging.
• 3Spray pattern
Your specific operating conditions will ultimately determine which nozzle style and spray pattern should be used.
• 3Spray angle
Spray angles range from 0° to 175°. The angle you need will be determined by the spray pattern, the number of nozzles used and the nozzle placement.
• 3Operating pressure
Operating pressure and flow rate will be determined by how much moisture you need to apply. Keep these simple rules in mind:
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 minimize the amount of air set in motion along the spray path
• 3Surface wetting
To increase surface wetting, use nozzles that produce a large number of small drops and decrease the contact angle of the spray on the material. Impact can also increase surface wetting and this can be achieved by increasing operating pressure. Keep in mind that drops normally travel through turbulent air before they hit the material. Friction drag of air reduces the impact velocity as the water travels away from the nozzle orifice.
• 3Nozzle placement at transfer points
Nozzles being used for dust prevention should be placed as close to the beginning of the transfer point as possible. The force of the moving material helps the water penetrate the material as it moves through the transfer point.
Nozzles in airborne dust suppression systems treat the air around the material and are generally placed at the end of transfer points so the material load can settle. Nozzles are positioned so they are spraying above the material and not on it. See Figure 5.
• 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
System Control Options, Optimizing Performance System Control Options, Optimizing Performance
Don’t Overlook System Control
To ensure effective, reliable dust control, consider automated system operation rather than relying on manual operation. The system cost is typically offset quickly through savings on chemicals, water, electricity, labor and better performance as proper wetting is ensured.
Turnkey spray systems monitor operating conditions using sensors as needed to detect material motion, chemical/water usage, temperature, humidity, conveyor speed and more. The sensors send data to the spray controller.
Preventing Problems and Optimizing Performance
Turnkey spray systems monitor and automatically adjust system components based on changes in operating conditions. Once you’ve specified your wet dust control system, the next steps are installation, operation, and maintenance.
Here are more than a dozen tips that can help ensure long-term, trouble-free performance.
In operations using feed chutes, keep water pressure below 60 psi (4.1 bar) to avoid pressurization and forcing dust from the enclosure
Using more nozzles at lower flow rates and positioning them closer to the material is usually more effective than using fewer sprays at higher flow rates
• Use flexible plastic strips around areas with water sprays for containment and inadvertent wetting of non-target areas
• Keep conveyor belts clean. Use a water wash system to spray and scrape build-up from belts and spray the bottom of return belts to reduce dust from a dry belt
• Use water instead of brooms to clean plant floors
If you experience any of these problems, chances are you are applying more moisture than needed:
•Material is sticking to screen cloth/conveyors
• Sludge accumulation in chutes and areas around transfer points
• Belt slippage
• Reduce flow rate
• Use fewer nozzles
• Check nozzles for wear. Capacity will increase as nozzle orifices wear. If nozzles are worn, replace them
• Add a spray controller to regulate moisture application
If fugitive dust persists, possible solutions include:
• Increase flow rate
• Increase the number of nozzles used
• Adjust nozzle placement to ensure sprays are reaching the target area
• Consider enclosures to protect nozzles from air/wind or use nozzles that produce larger drops if sprays are drifting off target
• For airborne dust suppression, determine dust particle size and ensure nozzle drop size is comparable
• Inspect nozzles for clogging
Optimizing Performance Optimizing Performance
Spray nozzles are designed for long-lasting, trouble-free performance, however, like all precision components, spray nozzles do wear over time. Spray performance can suffer and costs can rise. The best strategy is to inspect nozzles on a regular basis and replace them when needed.