Which of the following statements regarding the zones and characteristics of deep-bed grain drying with upward airflow are CORRECT?
Options:
(A) The deep bed stratifies into three distinct zones: dried grain zone at the bottom, active drying zone in the middle, and wet (un-dried) grain zone at the top.
(B) The active drying zone moves upward through the bed in the direction of airflow as drying progresses.
(C) Grain in the upper wet zone may temporarily absorb moisture and re-wet if warm, humid exhaust air cools below its dew point.
(D) Air static pressure drop across the grain bed is strictly independent of the superficial air velocity.
Answer Key:
A, B, C
Step-by-Step Solution & Explanation
Analysis of deep-bed grain drying:
- Option A is CORRECT: As dry air enters at the bottom, it quickly saturates. Thus, bottom grain reaches equilibrium first (dried zone), middle grain is actively losing moisture (drying zone), and top grain remains at its initial moisture (wet zone).
- Option B is CORRECT: The drying front travels upward as lower layers reach equilibrium and can no longer absorb heat.
- Option C is CORRECT: If exhaust air picks up moisture and cools, its relative humidity approaches $100\%$, and condensation/sorption can cause temporary re-wetting of top layers.
- Option D is INCORRECT: Pressure drop across grain beds is strongly dependent on superficial air velocity, governed by Shedd's curves and Ergun's equation ($\Delta P / L = a V^2 / \ln(1 + b V)$).
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=================== Top (Air Exit) ===================
[ Zone 3: Wet / Undried Zone (T = Twb, M = M0) ]
-------------------------------------------------------
[ Zone 2: Active Drying Zone (Drying Front) ]
-------------------------------------------------------
[ Zone 1: Dried Zone (T = Tair, M = Me) ]
=================== Bottom (Air Inlet) ===============
^ ^ ^ ^ ^ ^
| | | | | | (Hot Dry Air Flow)
```