GATE AG 2015 • Q52 NAT • 2 Marks

Section 3: Farm Power

Farm Power

Subtopic: Selection, operation, maintenance and repair of I.C. engines

Question Statement:

A ball mill of 1.8 m diameter is loaded with steel balls each having a diameter of 6 cm. The rotational speed of the ball is kept at 75% of the critical speed. The operational speed of the ball mill in rpm is _____.

Official Answer Key:

23 to 25

Step-by-Step Solution & Explanation

Critical speed of a ball mill: Nᴄ = 42.3/√(D−d) (D, d in metres, Nᴄ in rpm). Nᴄ = 42.3/√(1.8−0.06) = 42.3/√1.74 = 42.3/1.319 = 32.07 rpm. Operational speed = 0.75 × 32.07 = 24.05 rpm. This lies within the accepted range 23–25.

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