In rotary drum dryers, what does the term 'flights' refer to?

Flights are longitudinal protrusions or fins attached to the inner wall of a rotary dryer. Their primary purpose is to lift the material and drop it through the hot gas stream (showering), which maximizes the surface area contact and enhances heat and mass transfer.

Step-by-step Derivation:
Step 1: Analyze the mechanical design of a rotary drum dryer. A rotary dryer consists of a rotating cylindrical shell through which material moves and hot air flows.
Step 2: Evaluate the role of internal components. If the inner wall were smooth, material would simply slide along the bottom, leading to poor drying efficiency and 'slugging'.
Step 3: Identify the function of 'flights'. Flights are the internal lifting mechanisms (fins/vanes). As the drum rotates, the flights scoop up the wet material and carry it to the top of the drum.
Step 4: Observe the action. At the peak of the rotation, the material falls (cascades) through the cross-section of the drum, creating a 'curtain' of material that interacts optimally with the hot air.
Step 5: Compare with options. Option A refers to RPM; Option C refers to convection flow; Option D refers to the heat source. Only Option B describes the physical structure and function of flights.