Discussion
Arrange these dryers in order of typical residence time, from shortest to longest:
1. Fluidised bed dryer
2. Spray dryer
3. Freeze dryer
4. Deep bed grain dryer
Select the correct sequence.
Answer: A
Correct answer: 2, 1, 4, 3
Residence time follows the scale of the material and the mechanism. A spray dryer (2) is the fastest, drying atomised droplets in seconds because their surface-to-mass ratio is enormous. A fluidised bed dryer (1) takes minutes, every particle being fully exposed to the air.
A deep bed grain dryer (4) takes hours, since the air must work its way through a static bed and moisture must diffuse out of whole kernels. A freeze dryer (3) is slowest of all, often a day or more, because sublimation under vacuum is inherently slow and heat must be supplied without melting the ice.
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