Rotary dryer design: the typical height of lifting flights is what fraction of the drum inside diameter d?

Introduction / Context:
Lifting (or showering) flights inside rotary dryers pick up solids and cascade them through the gas stream, greatly enhancing heat and mass transfer. Flight geometry, especially flight height relative to drum diameter, affects curtain density, residence time, and risk of choking or poor showering.

Given Data / Assumptions:

• Cocurrent or countercurrent rotary dryer handling granular solids.
• Typical industrial operating speeds and standard flight layouts.
• Focus on widely used rule-of-thumb proportions.

Concept / Approach:
Practical experience indicates flights around 8–12% of drum diameter produce a good solids curtain without excessive holdup or blockage. Too small flights yield thin curtains and poor contact; excessively large flights can cause avalanching, erratic showering, and higher drive loads.

Step-by-Step Solution:

Verification / Alternative check:
Design handbooks and vendor application notes provide comparable ranges, with fine-tuning by test runs.

Why Other Options Are Wrong:

• Very small flights underperform in mixing/contacting.
• Very large flights lead to instabilities, high torque, and poor showering quality.

Common Pitfalls:
Ignoring material stickiness or shrinkage; not adjusting flight angle along the dryer length; overlooking influence of gas velocity on curtain stability.

Final Answer:
0.08 to 0.12 d