Drying Theory and Dryers Questions and Answers

Answer: C

Correct answer: Deep bed

A bulk commodity of modest value has to be dried cheaply, and the deep bed dryer is the only one in the table with low capital cost. Its long residence time and moderate product damage are acceptable when the material is grain worth a few units per tonne and the alternative is a machine costing many times as much.

Freeze drying is very high capital cost and takes days, spray drying is high cost and suits liquids, and a fluidised bed at moderate cost still needs far more air and power. Dryer selection is as much an economic decision as a technical one.

Answer: D

Correct answer: Sensible heat in the exhaust air

The exhaust sensible heat, at 98 MJ per hour and 17 per cent, is the largest recoverable item. It is heat that did no drying at all and simply left with the air, so recirculating part of the exhaust or fitting a heat exchanger can return much of it.

The latent heat at 74 per cent is the largest item overall but is irreducible - it is the energy the water must absorb to become vapour. Heating the product at 6 per cent and wall losses at 3 per cent are real but small, and insulation can only address the last of them.

Answer: A

Correct answer: Moisture spread grows rapidly with depth

The spread column tells the story directly: \( 1.6 \) points at \( 0.4 \) m, but \( 9.8 \) points at \( 1.6 \) m - a sixfold increase for a fourfold increase in depth, so the growth is more than proportional. The cause is that air gives up heat and takes up moisture as it rises, arriving at the upper layers nearly spent.

The bottom in fact dries most, falling from \( 13.0 \) to \( 11.2 \) per cent as the bed deepens and the drying time lengthens, while the top is left wetter and wetter. This is precisely why deep beds are limited in depth or the grain is recirculated.

Answer: B

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. Heating raises the saturation vapour pressure steeply - roughly doubling every ten degrees in this range - while adding no water, so the humidity ratio is unchanged and the relative humidity falls sharply.

That drop in relative humidity is the drying potential. This is why hot air drying begins at the heater: it creates the vapour pressure difference between product and air that drives evaporation, without any of the water having to be supplied or removed beforehand.

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.

Answer: D

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. Equilibrium moisture is defined by the product's vapour pressure equalling that of the air, so at that point there is no driving force left and no net transfer in either direction.

It follows that the exhaust air condition sets an absolute floor: running the dryer longer accomplishes nothing once the product has come into balance with it. Drying further requires air of lower relative humidity, which in practice means heating it more - which lowers the equilibrium moisture and reopens the driving force.

Answer: A

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 practice. Rapid initial drying can seal the surface - case hardening - trapping moisture within, and in grain the same steep gradient fissures the kernel.

So a gentle start keeps the internal gradient shallow while the product is wettest and most vulnerable. Later, once the surface can no longer be kept wet and the rate is limited by internal diffusion, there is much less risk in raising the temperature, and doing so shortens what would otherwise be a very long falling rate period.

Answer: B

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. Air passing up through the bed gives up sensible heat to evaporate water and takes up that water as vapour, so it reaches the upper layers cooler, more humid and with much of its drying potential spent.

The lower layers therefore dry fast and may be over-dried, while the upper layers dry slowly and are left wet - the wide spread of final moisture. The effect worsens more than proportionally with depth, which is why practical designs limit the bed, reverse the airflow, or recirculate the grain between passes.

Answer: C

Correct answer: Both A and R are true, but R is not the correct explanation of A

Both statements are true. Freeze drying is indeed reserved for high-value products, and sublimation under vacuum does preserve structure and flavour better than any other drying method, because no liquid phase ever exists in the product and so there is no capillary shrinkage, no solute migration and no heat damage.

But the reason does not explain the assertion. Freeze drying is restricted to high-value products because of its cost - very high capital outlay, deep vacuum, and cycles running to a day or more. Superior quality is the reason one would want to freeze dry; cost is the reason one usually cannot.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both statements are true and the reason gives the mechanism. During a drying pass the surface dries much faster than the centre, and it is the resulting steep gradient - with the outer layer shrinking against a wetter core - that generates the stresses which fissure the kernel.

Tempering interrupts that chain before it completes: resting without airflow lets moisture migrate outward until the kernel is more uniform, so the stress never reaches the fissuring threshold. Fewer fissures mean fewer kernels breaking in the mill, which is exactly what head rice yield measures.