Experimental studies were conducted on the classification of flyash pa
rticles by use of a modified cyclone with movable 50% cut size on a be
nch scale. The 50% cut size of cyclone changed from 1 to 10 mu m by th
e change of apex cone height and blow-up flow rate. When the height of
the apex cone was higher at a constant blow-up flow rate, the 50% cut
size increased. The reason for this was due to the fact that a decrea
se in the effective cross-sectional area near the inlet of the dust bo
x gave rise to the increased average upward fluid velocity. In the cas
e of blow-down, two kinds of different-sized fine particles were obtai
ned simultaneously. When the blow-down flow rate was 5%, collection ef
ficiencies of 4 and 28% were obtained with mean particle diameters 2 a
nd 6 mu m respectively. When both the blow-down method and a movable g
uide plate at the cyclone inlet were used, particles of average diamet
er of 1.5 mu m were collected. Although the cyclone diameter was 400 m
m, elaborate classification of flyash particles was achievable.