Location of Fog Systems
The installation of fog systems is a little unusual in that fog systems are designed to treat the air around the material, rather than the material itself. Therefore, the application point for the fog mist is generally near the end of the transfer point. This placement allows the material to settle and any pick-ups for active or passive dust-collection systems to see dust-laden air without risk of blinding the filtration media with the moistened particles.
Fog-generation nozzles are installed to cover the full width of the conveyor’s skirted area. It is recommended that the height of the transfer point skirtboard be at least 600 millimeters (24 inches) to allow the output cone of the nozzles to reach optimum coverage and fill the enclosure. The nozzle spray pattern should be designed so that airborne materials pass through the curtain of fog without putting spray directly onto the main body of material. The spray is directed above the materials, rather than at the materials.
The spray pattern from fog nozzles should not be directed onto any surface, and the nozzles should be shielded from being struck by the bulk material.
Pluses and Minuses of Fog Systems
Fog systems provide effective dust control combined with economical installation and operating costs. System operating costs are low when compared to conventional dust-collection systems.
A well-designed fogging system provides control of dust at the point of application without the need for chemical additives. This is especially important for processes such as the conveying of wood chips destined for papermaking. Many mills are concerned about the application of any chemical that might negatively affect the pulp or degrade the quality of finished paper. As fog systems add water without any additives, they protect the integrity of the process.
With fog systems, total moisture addition to the bulk material can be as low as 0.1 percent to .05 percent. This makes fog suppression systems attractive in industries such as cement and lime production that cannot tolerate excess moisture.
Because of the small orifice size of the nozzles, potable (drinking-quality) water is typically required for fog suppression systems. Filtration to remove suspended materials from the water supply is normally necessary. Nozzles can plug if the water supply is contaminated or if the water treatment system is not serviced at required intervals. Preparations such as drains and heat-traced plumbing should be provided for plants in cold-weather environments.
Another consideration prior to choosing a fogging system is the air volume and air velocity at the open area surrounding the transfer point or chute. Fog systems using single-fluid nozzles (those that do not require compressed air) tend to be more compatible with engineered systems that control the air movement through a transfer point. These systems should not be used in “open area” applications. For truly effective performance, fog dust-suppression systems require a tight enclosure around the transfer point to minimize turbulent, high-velocity air movement through the system. Since droplets are small, both fog and dust could be carried out of the treatment area onto surrounding equipment by high-velocity air leaving the chute.
Another potential drawback of fogging application is that this form of dust treatment is application point specific. Dust control is achieved only at the point of application; there is little or no residual, or carry-over, benefit. Although one system can often control more than one transfer point, several fogging devices may be required for a complex conveyor system with multiple transfer points. The capital expenditure may preclude fogging if the conveyor system is too extensive.
See Also: Fog Suppression to Control Belt Conveyor Dust Part 1