Discussion
An operator doubles the fan speed on a grain dryer late in a batch and sees almost no improvement in drying rate.
Why?
Answer: C
Correct answer: The bed is in the falling rate period
Late in a batch the grain is in the falling rate period, where the rate is limited by how quickly moisture can move from inside each kernel to its surface. The air's job is only to carry away what arrives there, and doubling the fan speed does nothing to speed up internal diffusion.
Air velocity is effective earlier, during the constant rate period, when the surface is continuously wet and the boundary layer is the resistance. Neither a failed heater nor saturated air fits - both would have shown up earlier - and true equilibrium would stop drying altogether rather than merely slow it.
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