Subtopic: Fluidization of granular solids – pneumatic, bucket, screw and belt conveying
Question Statement:
In pneumatic conveying of grains in a vertical duct, the minimum air velocity required to initiate upward transport of grains must be:
Options:
(A) Greater than the terminal falling velocity of the grain kernels
(B) Equal to half the terminal falling velocity
(C) Independent of grain particle size and density
(D) Equal to the critical acoustic velocity
Answer Key:
A
Step-by-Step Solution & Explanation
In vertical pneumatic transport, the superficial air velocity must exceed the terminal free-falling velocity ($V_t$) of the individual grain particles so that the upward drag force exceeds the gravitational body force, providing net upward acceleration ($V_{transport} = V_{air} - V_t$).