A sodium hydroxide spray in air at 30oC is to be collected in a gravity settler. The unit is 30 ft wide, 15 ft high, and 40 ft long. The volumetric flow rate of the air is 30 ft3/s. The specific gravity of the mist droplets is 1.21. Given: the density of water is 62.4 lb/ft3 and the density of air is 0.0728 lb/ft3. The viscosity of air is 1.245×10-5 lb/ft.s. Assume that all mist droplets are spherical in shape. Determine the smallest mist droplet size (in μm) that will be entirely collected by the settler.