SUPPRESSION SYSTEMS FOR FUGITIVE DUST CONTROL
An efficient dust suppression system is usually designed to both prevent dust from leaving the main product body and act quickly to return dust to the product if it becomes airborne.
There is no single solution for every fugitive dust problem; each situation brings with it its own particular set of conditions that need to be evaluated prior to resolving the matter. The vast majority of suppression problems are resolved by utilizing plain water as the medium, however in exceptional cases, the addition of chemicals to the water flow may be required.
Water is by far the most common medium used in suppression systems. It has the ability of ‘wetting’ extremely fine particles of dust while they are still in the body of the product increasing particle weight reducing its ability to become airborne.
This effect also applies to dust that has become airborne, particles encountering wetness increase in weight and drop back onto their source. The addition of water to individual dust particles also creates a ‘cohesive force’, causing each particle to adhere to adjacent particles, resulting in an increase in weight promoting them to fall back to the dust source.
Systems are often customized to suit the application and location. Raw water is usually drawn from a main supply or storage tank by a booster pump; this will aid the water’s passage through a series of specially designed filters, removing harmful solids prior to reaching the pressure pump and spray nozzles.
A pump (Figure 2) driven by a selected motor provides between 500 to 800 PSI of water pressure, which is then directed via suitably selected nozzles forming flat sprays over the product. The sprays are normally arranged to overlap in order to provide complete curtain coverage.
Although this form of suppression system can be designed to provide a temporary solution to mobile plant, it is usually more applicable to base the system as a permanent dust suppression source.
High pressure fog sprays are basically systems that provide water droplets that have been generated using water under extreme pressures along with highly engineered nozzles, and need to be effective while utilizing minimum volumes of water. The atomized droplets are normally less than 10 micron in diameter.
Water sprays with surfactants are utilized to make the water ‘wetter’. The accepted value of the surface tension of pure water is 72.6 dyn/cm; increasing the number of water droplets and reducing their individual size can reduce this figure. In effect, the water becomes ‘wetter’ and hence less water needs to be used while maintaining the desired result.
To a certain extent, increasing the water pressure and reducing the nozzle size will achieve this ‘wetter’ effect; however, the most common method is to add a suitable surfactant that can be gradually dosed in to the spray system. A minute amount of surfactant will reduce the surface tension from 72.6 to about 28 dyn/cm (Figure 5).
Installed directly into the clean water supply after the filters, the injector operates without either electricity or compressed air, using water pressure as the power source.
The water flow activates the injector, which takes up the required preset percentage of surfactant concentrate directly from the stock solution container. Inside the injector, the concentrate is efficiently mixed with the water and the water pressure forces the solution downstream.
The amount of concentrate will be directly proportional to the volume of water entering the injector, regardless of variations in flow pressure that may occur in the main line.
Water sprays with foams are created by maintaining the dosing method and utilizing a suitable surfactant, then adding pressurized air, ‘foam’ is produced, which can provide a blanketing effect, thus holding the product to the conveyor.
The foam has an excellent residual effect, enabling the number of spray points to be reduced. An established foam system can expand the surface area of a given volume of water 60 to 80 times, thus allowing for a much lower rate of moisture addition to the product.
Comparisons between dust suppression systems.
Percentage figures shown in the above diagram provide a graphic comparison with regard to the amount of moisture added to a process by the types of wet suppression already detailed.
What the above diagram does not indicate, is that as the water content of a system is reduced, the installation and capital costs of that system is increased. Usually this balance of water reduction and increasing costs far outweighs the costs incurred when excess water has to be removed from the product or the additional maintenance to such items as screens, etc.
Legislation is going to get tougher and plants will need to re-evaluate their current operating procedures with respect to maintaining and improving their existing fugitive dust control equipment.
Although this paper has advocated the effectiveness of utilizing water or a combination of water and chemicals as a medium to restrict dust from becoming airborne, we must conclude that the effectiveness is reliant only if the suppression method is specifically tailored to meet the immediate local conditions.
Financial benefits will be gained by greatly reducing the escape of product from the process flow and will provide a safe and more comfortable working environment for the plant personnel. Maintenance departments will need to alter their philosophy, no longer will the maintenance goal be to maintain production, and they will need to maintain, monitor and manage the resulting pollution.
By utilizing one of ACT’s Dust Suppression systems you will also decrease the amount of water needed to control dust on your plant and plant roads. (See our next blog on “Controlling field and road dust.)