An aqueous slurry is filtered in a plate-and-frame filter, which operates at a constant $\Delta P$ of $100 \mathrm{psi}$. The filter consists of 20 frames, each of which have a projected area per side of $900 \mathrm{~cm}^{2}$. A total filtrate volume of $0.7 \mathrm{~m}^{3}$ is passed through the filter during a filtration time of $1200 \mathrm{~s}$, and the downtime for the filter is $900 \mathrm{~s}$. The resistance of the filter medium is negligible relative to that of the cake. You want to replace the plate-and-frame filter with a rotary drum filter with the same overall average capacity, using the same filter medium. The drum is $2.2 \mathrm{~m}$ in diameter and $1.5 \mathrm{~m}$ long and operates at $5 \mathrm{psi}$ vacuum with $25 %$ of the drum surface submerged in the slurry. At what speed (in rpm) should the drum be operated?