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 helps to reduce clogging.
Your specific operating conditions will ultimately determine which nozzle style and spray pattern should be used. Figure provides an overview of the options and some guidelines for use 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 0° to 175°. The angle you need will be determined by the spray pattern, the number of nozzles used and the nozzle placement.
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
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.
Nozzle 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.