US Patent 7628831 Cyclonic separating apparatus

A cyclonic separating apparatus includes a chamber for separating dirt and dust from an airflow, an inlet to the chamber and a shroud. The shroud includes a wall having a multiplicity of through-holes forming an outlet from the chamber, each of the through-holes having substantially rectangular cross-sections with width to height ratios in the range of 1.5:1 to 1:1.5. The rectangular cross section of the through-holes maximizes the available through-hole area in the shroud, produces a low pressure drop across the shroud and reduces the amount of material required for manufacture. Further, this ratio allows the cross-sectional area of the through-holes to be dimensioned to prevent larger particles of dirt and dust from passing through the through-holes in the shroud, while still providing the required structural integrity.