Question

In pneumatic conveying and separation processes, the material is lifted only when compared to the terminal velocity, the air velocity is

• A. Lower
• B. Greater
• C. Equal to
• D. Much lower

Hint:

Vertical pneumatic conveying requires $v_{\text{air}} > v_t$, whereas winnowing/cleaning separates chaff from grain by setting $v_{t,\text{chaff}} < v_{\text{air}} < v_{t,\text{grain}}$.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Pneumatic conveying and air classification depend on the aerodynamic balance between upward drag force and downward gravitational force acting on a particle.

Step 2: Key Formula or Approach:
The terminal velocity ($v_t$) of a particle in an upward fluid stream occurs when the gravitational force equals the aerodynamic drag force:
\[F_g - F_b = F_d \implies v_{\text{rel}} = v_t\]

Step 3: Detailed Explanation:
If the air velocity $v_{\text{air}} < v_t$, the particle falls downward.
If $v_{\text{air}} = v_t$, the particle remains suspended in fluidized equilibrium.
When $v_{\text{air}} > v_t$, net upward aerodynamic drag exceeds gravity, lifting and conveying the particle vertically.

Step 4: Final Answer:
Hence, the air velocity must be greater than the terminal velocity for the material to be lifted.