Subtopic: Mechanics and energy requirement in size reduction of agriculture produce
Question Statement:
A ball mill of 1.8 m diameter is charged with balls each having diameter of 40 mm for grinding solid material. The rotational speed of the balls is 80% of the critical speed. The operating speed of rotation in revolution per minute is
Options:
(A) 18
(B) 22
(C) 26
(D) 30
Official Answer Key:
C
Step-by-Step Solution & Explanation
Critical speed of a ball mill (rpm): Nc = 42.3/√(D−d) (D, d in metres, effective mill diameter reduced by ball diameter). Nc = 42.3/√(1.8−0.04) = 42.3/√1.76 = 42.3/1.327 = 31.88 rpm. Operating speed = 0.80 × Nc = 0.80 × 31.88 = 25.5 rpm ≈ 26 rpm, matching option (C).
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