Rotary dryer geometry: the typical length-to-diameter (L/D) ratio for most rotary dryers falls in which range?

Introduction / Context:
Rotary dryers are widely used for bulk solids (minerals, fertilizers, biomass). The length-to-diameter (L/D) ratio influences residence time, solids back-mixing, heat transfer, and footprint. A practical range is adopted in most industrial designs.

Given Data / Assumptions:

• Conventional single-pass rotary dryers with internal flights.
• Design targeted for typical residence times and reasonable shell flexure.
• Not considering specialized indirect or multi-pass units.

Concept / Approach:
An L/D between about 4 and 10 balances sufficient contact time with manageable structural deflection and drive torque. Too low L/D gives inadequate residence time and poor curtain formation; too high L/D increases shell deflection, support complexity, and capital cost without proportional performance gains.

Step-by-Step Solution:
Define needs: adequate residence time and solids curtain.Check structural constraints: shell bending and support spans.Standard vendor practice typically lands in L/D ≈ 4–10.Therefore, the correct range is 4 to 10.

Verification / Alternative check:
Vendor catalogs and design handbooks commonly quote L/D in this range for most throughputs and moisture levels, adjusting diameter for gas velocity and dust entrainment.

Why Other Options Are Wrong:
1–2: too short for effective drying with flights in most duties.10–20 or 20–30: possible but less common; increase mechanical challenges and cost.

Common Pitfalls:
Assuming longer is always better; neglecting gas velocity and carryover limits; overlooking material stickiness that may require lifter redesign rather than extreme L/D.

Final Answer:
4 to 10