# Mechanical Operations Questions and Answers – Critical Speed

This set of Mechanical Operations Multiple Choice Questions & Answers (MCQs) focuses on “Critical Speed”.

1. If a small diameter column is used with deep bed through solids, this is called _____
a) Sledging
b) Slugging
c) Skipping
d) Centering

Explanation: Travel of successive bubbles up through the column separated by slug cause the phenomenon of slugging.

2. What does the below equation represent?
ub = 0.7√(gDb )
a) Bed velocity
b) Bubble velocity
c) Bank velocity
d) Bishop velocity

Explanation: The following empirical relation for follows for bubble velocity in the fluidized bed, this equation holds well if Db taken as equivalent diameter.

3. At what temperature fluidized bed operates?
a) 1075 to 1225 K
b) 555 to 4555 K
c) 100 to 500 K
d) 500 to 1500 K

Explanation: Fluidized bed furnace operates at a temperature of range of 1075 to 1225 K but most close to 1175 K to be precise.

4. What are the velocities of pneumatic conveying?
a) 45 to 60 m/s
b) 50 to 45 m/s
c) 15 to 30 m/s
d) 1 to 10 m/s

Explanation: The suspending fluid in a pneumatic conveyer is a gas, usually air flowing at velocities between 15 and 30 m/s in pipes from 50 to 400 mm in diameter.

5. What does the below equation represent?

Sanfoundry Certification Contest of the Month is Live. 100+ Subjects. Participate Now! a) Final velocity
b) Approach velocity
c) Carry velocity
d) Critical velocity

Explanation: When the mass ratio of solid to gas (r) is usually less than 5, for such suspensions the critical velocity may be estimated from the empirical relation.

6. What does the below equation represent? b) Drag force
c) Final force
d) Diagonal force

Explanation: The drag force upon a particle is given by the above relation, where C is the drag coefficient which is dimensionless.

7. What is the unit of dimensional constant?
a) lb. ft. / lb. force sec
b) lb. ft. / lb. force sec2
c) lb. ft. / lb. force sec-1
d) lb. ft2. / lb. force sec

Explanation: The value of dimensional constant is 32.17 and its units as in FPS system is given by lb. ft. / lb. force sec2.

8. What does the below equation represent? a) Settling Velocity
b) Terminal settling velocity
c) Final settling velocity
d) Medium velocity

Explanation: The above relation represents the terminal settling velocity, where g is acceleration due to gravity and mp is the mass of particle.

9. Mixing of liquids and suspension of solids may be accomplished by ______
a) Dryer
b) Sparger
c) Clamper
d) Bolder

Explanation: Mixing of liquids and suspension of solids may be accomplished by bubbling with an inert gas introduced uniformly at the bottom of the tank.

10. When _______ occurs below a critical power input the gas bubbles are not affected laterally but move upward.
a) Weeping
b) Flooding
c) Mixing
d) Entrainment

Explanation: Below a critical power input the gas bubbles are not affected laterally but move upward with their natural buoyancy, this condition is called gas flooding of the impeller.

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