A particulate control device has an efficiency which is a function of particle size, as follows: eff(dp) = 1 - exp(- St ) , where St is the Stokes number, and ρp = 1.5 g cm-3, uo = 10 m/s, and L = 1.0 mm. Calculate the collection efficiency for this device, for a mono-disperse aerosol, with dp = 5.0 μm. How many such devices would have to be placed in series to achieve a collection efficiency greater than or equal to 99.0 %?