A centrifuge with a radius of $2 \mathrm{ft}$ and a length of $1 \mathrm{ft}$ has an overflow weir located $1 \mathrm{ft}$ from the centerline. If particles with $S G=2.5$ and diameters of $10 \mu \mathrm{m}$ and less are to be removed from an aqueous suspension at a flow rate of $100 \mathrm{gpm}$, what should the operating speed of the centrifuge be (in rpm)?