Mechanical Operations

121. For a non-spherical particle, the sphericity

  1. Is defined as the ratio of surface area of a sphere having the same volume as the particle to the actual surface area of the particle
  2. Has the dimension of length
  3. Is always less than 1
  4. Is the ratio of volume of a sphere having the same surface area as the particle to the actual volume of the particle

Correct answer: (A)
Is defined as the ratio of surface area of a sphere having the same volume as the particle to the actual surface area of the particle

122. For a sphere falling in the constant drag co-efficient regime, its terminal velocity depends on its diameter (D) as

  1. d
  2. √d
  3. D2
  4. 1/d

Correct answer: (C)
D2

123. For a turbine agitated and baffled tank, operating at low Reynold's number (based on impeller diameter), the power number (Np) varies with NRe as

  1. Np ∝ NRe
  2. Np ∝ √NRe
  3. Np → constant
  4. Np ∝ 1/NRe

Correct answer: (D)
Np ∝ 1/NRe

124. For achieving maximum capacity of the ball mill, the ball charge should be equal to about _________ percent of the ball mill volume.

  1. 10
  2. 25
  3. 50
  4. 75

Correct answer: (C)
50

125. For benefication of iron ore, the most commonly used method is

  1. Flocculation
  2. Froth floatation
  3. Jigging & tabling
  4. None of these

Correct answer: (C)
Jigging & tabling

126. For classification of potable (drinking) water, we use a _________ filter.

  1. Gravity sand
  2. Plate and frame
  3. Vacuum leaf
  4. Rotary vacuum

Correct answer: (A)
Gravity sand

127. For coarse reduction of hard solids, use

  1. Impact
  2. Attrition
  3. Compression
  4. Cutting

Correct answer: (C)
Compression

128. For crushing of solids, the Rittinger's law states that the work required for crushing is proportional to

  1. The new surface created
  2. The size reduction ratio
  3. The change in volume due to crushing
  4. None of these

Correct answer: (A)
The new surface created

129. For efficient grinding, ball mills must be operated

  1. At a speed less than the critical speed
  2. At a speed more than the critical speed
  3. At a speed equal to the critical speed
  4. With minimum possible small balls

Correct answer: (A)
At a speed less than the critical speed

130. For grinding of cereals, grains, spices, pigments, saw dust, cork etc., the most extensively used size reduction equipment is a

  1. Buhrstone mill
  2. Ball mill
  3. Crushing rolls
  4. Hammer mill

Correct answer: (A)
Buhrstone mill

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