foam remaining on the apex cone becomes maximum for the cone
angle of 70°. The region without the soap foam corresponds to the
upward flow and the region with soap foam corresponds to the
downward flow. Then it is considered that upward fluid and particles
of downward flow direction can be clearly separated with the apex
cone at an angle of 70°. The particles collected on the upper region of
the apex cone might be moved to the upward flow and not be
collected. The probability of these escaping particles will be a
minimum for the case of the apex angle of 70°. The fluid flow rates
into the dust box for various apex cone angles are important. The
maximum flow rate entering the dust box is obtained for the apex
cone angle of 60°.