Step 1: Understanding the Concept:
A cyclone separator uses centrifugal force to separate solid particles from a gas stream. The fluid enters tangentially, creating a vortex.
Step 2: Detailed Explanation:
1. Mechanism: Large and dense particles have more inertia. The centrifugal force throws them against the walls of the separator, where they lose velocity and fall to the collection hopper.
2. Fine Particles: Very fine particles (usually $< 5$ \(\mu\)m) have very little mass. The centrifugal force acting on them is often less than the drag force of the air stream. Therefore, these particles stay suspended in the air and exit through the top outlet rather than being separated.
Step 3: Final Answer:
As particle size decreases, the efficiency of a cyclone separator drops significantly, making it least efficient for very fine particles.