A gravity settling chamber consists of a horizontal rectangular duct $6 \mathrm{~m}$ long, $3.6 \mathrm{~m}$ wide, $3 \mathrm{~m}$ high. The duct is used to trap sulfuric acid mist droplets entrained in an air stream. The droplets settle out as the air passes through the duct and may be assumed to behave as rigid spheres. If the air stream has a flow rate of $6.5 \mathrm{~m}^{3} / \mathrm{s}$, what is the diameter of the largest particle that will not be trapped by the duct? (For the Acid: $ho=1.75 \mathrm{~g} / \mathrm{cm}^{3}, \mu=3 \mathrm{cP}$; For Air: $ho=0.01 \mathrm{~g} / \mathrm{cm}^{3}, \mu=0.02 \mathrm{cP}$ ).