Rotary dryers (countercurrent): the typical slope (fall) provided is approximately what range in metre per metre of length?

Introduction / Context:
Rotary dryers rely on slight inclination to promote axial movement of solids while the drum rotates. The slope must be large enough to convey material yet small enough to provide adequate residence time for heat and mass transfer. In countercurrent operation, gas and solids flow in opposite directions, and fine control of slope is essential to achieve target outlet moisture and temperature profiles.

Given Data / Assumptions:

• Countercurrent rotary dryer handling granular or friable solids.
• Standard rotational speeds and typical flighting arrangement.
• Industrial practice ranges for slope expressed as metre per metre (m/m).

Concept / Approach:
Experience shows that only a small geometric incline is required. Typical designs use slopes up to a few centimeters per meter of length, which translates to about 0 to 0.08 m/m. Larger slopes accelerate solids excessively, reducing residence time and risking insufficient drying or wide residence-time distributions. Very small or zero slope may be used for very fine or sticky solids where holdup must be increased.

Step-by-Step Solution:

Verification / Alternative check:
Vendor design charts and scale-up rules corroborate small inclinations; adjustments are complemented by changes in rpm and flighting.

Why Other Options Are Wrong:

• 0–0.8 m/m and larger values are unrealistically steep and would dump solids too quickly.
• Even higher ranges further shorten residence time and cause poor control.

Common Pitfalls:
Ignoring particle size effects; failing to adjust rpm/flights along with slope; not accounting for shrinkage or stickiness as moisture falls.

Final Answer:
0 to 0.08