Discussion
Assertion (A): Raising air velocity does little to shorten the falling rate period.
Reason (R): In that period the rate is controlled by moisture movement within the solid.
Select the correct answer from the options below.
Answer: C
Correct answer: Both A and R are true, and R is the correct explanation of A
Both statements are true and the reason explains the assertion. In the falling rate period the surface is no longer continuously wet, and the rate is set by how quickly moisture can travel from within the solid to the surface - by diffusion, capillary flow or vapour movement.
Air velocity acts only on the boundary layer outside the solid, which is no longer the limiting resistance, so raising it achieves little. What does help is shortening the internal path: thinner slices, smaller particles, or a longer time. This is the practical reason most of the drying time is spent in this period.
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