Academic Research
Research literature has long recognized that small droplet size is the key to dust suppression system. The following explanation comes from work at the Colorado School of Mines:
When water droplets that are sprayed to control coal dust are too large, the dust particles flow around the droplets, and thus are not absorbed-but water droplets too small simply evaporated. So CSM is looking into theories governing formation of clouds to improve existing water spray technology.
Experience gained over the years with water sprays has established the following facts: (1) For a given spray nozzle, the collection efficiency for small dust particles increases as the pressure increases, and (2) at a given pressure, the efficiency increases as the nozzle design is changed so as to produce smaller droplets. The conclusion is clear-cut; the smaller droplets are more effective in knocking small dust particles out of the air. The reason for this is not hard to see.
Consider a water droplet about to impinge on a dust particle, or what is aerodynamically equivalent, a dust particle about to impinge on a water droplet, as shown in the drawing. If the droplet diameter is much greater than the dust particle, the dust particle simply follows the airstream lines around the droplet, and little or no contact occurs. In fact, it is difficult to impact micron-size particles on anything, which is why inertial separators do not work well at these sizes.
If, on the other hand, the water droplet is of a size that is comparable to that of the dust particle, contact occurs as the dust particle tries to follow the streamlines. Thus the probability of impaction increases as the size of the water spray droplets decreases.