A 200 cm diameter ball mill uses10 cm balls for dry grinding, then20 cm balls for wet grinding. Find change in operating speed in rpm, to two decimals. Use π=3.14,g=9.81 m/s².
Critical speed is $N_c=60\sqrt{2g/(D-d)}/(2\pi)$. Diameters2,.1,.2 m give about 30.70 and 31.55 rpm. Under the intended empirical operating fractions65% critical for dry and 75% for wet, speeds are19.95 and 23.66 rpm, an increase of **3.70 rpm**. These operating fractions are not explicitly supplied;critical-speed change alone is only about .84 rpm, so the empirical convention is necessary.
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