Subtopic: Functional requirements, principles of working, construction and operation of manual, animal, tractor and renewable energy operated equipment for tillage, sowing, planting, fertilizer application, inter-cultivation, spraying, mowing, chaff cutting, harvesting and threshing
Question Statement:
In the cleaning shoe of a combine harvester, aerodynamic separation of chaff and short straw from sound grain kernels is accomplished by maintaining the cleaning air blast velocity between:
Options:
(A) $0.5\text{ and } 1.0\text{ m/s}$
(B) The terminal velocity of chaff ($1.5\text{--}2.5\text{ m/s}$) and the terminal velocity of grain ($8.0\text{--}10.0\text{ m/s}$)
(C) Strictly above the terminal velocity of the heaviest grain kernels (gt; 12\text{ m/s}$)
(D) $0.1\text{ and } 0.3\text{ m/s}$
Answer Key:
B
Step-by-Step Solution & Explanation
- **Aerodynamic Principle of Cleaning Shoe:**
- Chaff and straw particles have low density and high projected area, with terminal velocities of $v_{t,\text{chaff}} \approx 1.5\text{--}2.5\text{ m/s}$.
- Clean wheat grain has a higher terminal velocity of $v_{t,\text{grain}} \approx 8.0\text{--}10.0\text{ m/s}$.
- By maintaining air blast velocity in the intermediate range ($4.0\text{--}6.0\text{ m/s}$), chaff is levitated and blown out of the rear, while heavier grains fall through the chaffer and sieve.