Step 1: Understanding the Concept:
Pneumatic conveying is a process of transporting bulk materials (like grains, powders) through a pipe using a gas flow (usually air). For a particle to be lifted and transported vertically by the air stream, the upward force exerted by the air must overcome the downward force of gravity on the particle. This concept is directly related to the terminal velocity of the particle.
Step 2: Detailed Explanation:
1. When a particle is in a moving fluid (like air), it experiences an upward drag force. This drag force increases with the relative velocity between the particle and the fluid.
2. The particle also experiences a downward gravitational force (its weight).
3. Terminal velocity is the constant speed that a freely falling object eventually reaches when the resistance of the medium (like air) through which it is falling equals the force of gravity. At terminal velocity, the net force on the object is zero, and its acceleration is zero.
4. For pneumatic conveying, the situation is reversed. The particle is stationary, and the air is moving upwards. The material will be lifted (begin to accelerate upwards) only when the upward drag force exerted by the air is greater than the downward gravitational force.
5. The air velocity at which the drag force exactly balances the particle's weight is, by definition, the particle's terminal velocity.
6. Therefore, to lift and convey the material, the air velocity must be greater than the terminal velocity of the particles.
Let's analyze the other options:
- (A) Drag coefficient: This is a dimensionless number that relates the drag force to the fluid density, velocity, and area. It is a property used to calculate drag, not a velocity itself.
- (B) Bulk density: This is the mass of the material per unit volume, including the space between particles. It's a property of the material, not a velocity.
- (C) Angle of repose: This is the steepest angle at which a pile of granular material remains stable. It relates to friction between particles, not their behavior in an air stream.
Step 3: Final Answer:
The material is lifted only when the air velocity is greater than its terminal velocity. Therefore, option (D) is correct.