Dry fog vs water spray: which problem is being solved?

Two different dust problems

Dust causes trouble in two broad ways. Some dust is already airborne: fine particles thrown up at a crusher discharge, a conveyor transfer chute or a hopper, which drift out of the enclosure and settle on equipment and people. Other dust is still on the ground or in the material: the surface of a stockpile, a haul road or a loaded truck, waiting for wind or traffic to lift it.

Conventional water spray mainly tackles the second problem by wetting material and surfaces so that particles stick together and stay put. Dry fog mainly tackles the first by putting a cloud of very fine droplets where the airborne dust is, so that particles collide with droplets, agglomerate and fall out of the air. Asking which problem is being solved is the most useful first step.

How water spray and sprinkler systems work

Spray and sprinkler systems use hydraulic nozzles — flat fan, full cone or hollow cone, for example — fed by pumped water. Droplets are comparatively large and are aimed at the material bed, the stockpile surface or the road. Wetting raises the moisture content at the surface and binds fines, which reduces the dust that can be lifted later.

This approach suits open areas, stockpiles, haul roads, and material that can tolerate added moisture. It is simple and works with water alone, but the water ends up in the product. Larger droplets are also less effective at capturing very fine dust that is already in the air, because small particles tend to follow the air stream around a large droplet rather than hit it.

How dry fog works

Dry fog systems generate a dense fog of very fine droplets, usually with two-fluid nozzles that use compressed air to atomise water, sometimes by other atomising methods. The aim is to match droplet size more closely to the airborne dust so that collisions are more likely. Because the droplets are small and the fog is confined to the dust source, the total water added to the material is low compared with wetting.

Dry fog works best where the dust can be contained: an enclosed transfer chute, a crusher feed or discharge with skirting and curtains, a hopper with a hood. In open, windy areas the fog is blown away before it can act. It needs compressed air as well as water, and fine nozzles need clean, filtered water.

Which approach fits?

Consider dry fog when the dust is airborne at an enclosed or enclosable point, and the material or the downstream process has tight moisture limits — cement, some minerals and fuels are common examples. Consider water spray or sprinklers when the problem is surface dust over a wide area, the material can accept water, and compressed air is not practical. Fog cannons sit between the two for large open areas, and wind fences reduce the wind that lifts dust in the first place.

Many sites use more than one method: dry fog at transfer points, sprinklers on stockpiles and wheel washing at the gate. Good enclosure of chutes and transfer points improves the result of any method, and should be checked before spray hardware is specified.

What to establish before choosing

Before any design, confirm the dust source and when it occurs, the material and its acceptable moisture, throughput and belt speed, how enclosed the point is, ambient conditions including wind and freezing risk, and the water, compressed air and power available. Photos and a layout drawing help greatly. No method should be specified on a dust-reduction promise without measuring the conditions on site.