Drying Theory and Dryers Questions and Answers

Answer: C

Correct answer: Raising the air velocity greatly increases the drying rate

This is the false statement. In the falling rate period the limiting resistance lies inside the solid, so raising the air velocity - which acts only on the external boundary layer - achieves very little.

The other three are true. Most of the drying time is usually spent in this period, because the rate keeps falling as the remaining moisture is more firmly held and has further to travel. The product temperature rises above the wet bulb value, since evaporative cooling no longer holds it there. And internal moisture movement is what controls the rate throughout.

Answer: D

Correct answer: Exhausting hotter and less saturated air

Exhausting hotter and less saturated air makes efficiency worse. It means more sensible heat is thrown away and less of the air's moisture-carrying capacity was used - the two largest avoidable losses in a hot air dryer, taken together.

The other three all improve efficiency. Recirculating part of the exhaust returns its sensible heat to the inlet; insulating the chamber cuts wall losses; and a heat exchanger recovers exhaust heat without returning the moisture with it. The design target is exhaust that leaves cool and near saturation.

Answer: A

Correct answer: The rate is controlled by diffusion within the solid

This is not true of the constant rate period - it describes the falling rate period. While a film of free water covers the surface, moisture reaches it from within as fast as it evaporates, so internal diffusion is not the limiting step.

The other three are all true of it: the surface is covered by free water, which is what defines the period; the surface sits at the wet bulb temperature, because evaporative cooling exactly balances convective heating; and air velocity strongly affects the rate, since the boundary layer is the controlling resistance.

Answer: B

Correct answer: Unwise, the spread of moisture growing with depth

A very deep bed is unwise. Air gives up heat and takes up moisture as it rises, so it reaches the upper layers cool, humid and nearly spent, while the lower layers continue to dry. The spread between top and bottom therefore widens more than proportionally with depth - a metre and a half of bed can leave a spread approaching ten moisture points.

The batch is larger but much of it is out of specification, and part is over-dried, which is a loss of saleable mass as well as of quality. Deeper beds dry less evenly, not more, and the objection applies to solids, not liquids.

Answer: C

Correct answer: Water leaves as vapour directly from ice

Freeze drying removes water by sublimation: the product is frozen and the pressure lowered below the triple point, so ice passes straight to vapour without ever becoming liquid. That is the defining feature and the source of every advantage the method has.

Because there is no liquid phase there is no capillary shrinkage, no migration of solutes to the surface and no thermal damage, so structure, colour and flavour are preserved better than by any other method. It runs at low temperature, is among the most expensive methods available, and is used for high-value products rather than bulk grain.

Answer: D

Correct answer: Raising the initial air temperature

Raising the initial air temperature would increase the risk. Case hardening arises when the surface dries far faster than moisture can arrive from within, and a hotter start widens exactly that mismatch.

The other three all reduce the risk by moderating the early surface drying rate: a lower starting temperature reduces the driving force, slightly higher initial humidity reduces the vapour pressure difference, and a lower initial velocity reduces the transfer coefficient at the surface. Once the interior has begun to give up moisture, conditions can safely be made more aggressive.

Answer: A

Correct answer: It removes a substantial quantity of moisture

This is the false statement. During tempering the grain rests without airflow, so there is nowhere for water to go and the average moisture content is essentially unchanged.

The other three are true. Tempering lets internal moisture gradients relax as water migrates from the wetter centre to the drier surface; that relaxation is what prevents the stresses that fissure kernels, so it protects head rice yield; and because no air is moved and no heat supplied, it costs time rather than energy. It also makes the following pass more effective, since the surface is wetter again.

Answer: B

Correct answer: The product's angle of repose

The angle of repose describes how a granular material heaps and flows. It matters for designing hoppers, conveyors and bin discharge, but it plays no part in the thermal sizing of a hot air dryer.

The other three are exactly what sizing rests on: the mass of water to be evaporated, the latent heat of vaporisation that fixes the energy each kilogram requires, and the available air flow with its condition, which determines how much moisture that air can carry away. Together they give the heat duty and the air flow the dryer must provide.

Answer: C

Correct answer: It depends on both the relative humidity and the temperature of the air

Equilibrium moisture content is not a single number for a material - it is a relationship, described by a sorption isotherm, between the moisture the product holds and the condition of the air surrounding it. It rises with relative humidity and, at a given relative humidity, generally falls somewhat as temperature rises.

It is therefore not a fixed property of the product, and it cannot be passed by drying for longer, since at equilibrium there is no driving force at all. It is also quite distinct from the critical moisture content, which marks the end of the constant rate period much higher up the curve.

Answer: D

Correct answer: Drying decays exponentially with time

In the falling rate period the moisture ratio follows an exponential decay, \( MR = \exp(-kt) \), so each further reduction takes progressively longer. With \( k = 0.25 \) per hour, four hours reaches a moisture ratio of \( 0.368 \) but reaching \( 0.10 \) takes \( 9.2 \) hours - more than twice as long again for a much smaller quantity of water.

The underlying reason is physical: the remaining moisture is more firmly held and has further to travel to the surface. The latent heat does not rise sharply, the product's specific heat in fact falls as it dries, and increasing the air flow does not help once the rate is internally controlled.