Subtopic: Concentration and drying of liquid foods – evaporators, tray, drum and spray dryers
Question Statement:
A single drum dryer of diameter $D = 0.80\text{ m}$ operates at a rotational speed of $5.0\text{ rpm}$. If the active drying film covers an arc of $270^\circ$ ($3/4$ of the drum circumference) before being scraped off by the doctor blade, the drying contact time of the product on the drum surface in seconds is ________ (answer in integer).
Answer Key:
9
Step-by-Step Solution & Explanation
1. Time for one complete revolution ($360^\circ$):
$t_{rev} = \frac{60\text{ s}}{5.0\text{ rpm}} = 12.0\text{ seconds}$
2. Active residence time over $270^\circ$ arc:
$t = \frac{270^\circ}{360^\circ} \times t_{rev} = 0.75 \times 12.0\text{ s} = 9.0\text{ seconds}$