Drying Theory and Dryers Questions and Answers

Answer: A

Correct answer: A spray dryer

A spray dryer atomises the liquid extract into hot air, and because the droplets are tiny they dry in seconds directly to a free-flowing powder - a single step from liquid feed to finished product. The very short residence time, combined with evaporative cooling that holds each droplet near the wet bulb temperature until it is nearly dry, is what makes it suitable for heat-sensitive material despite high inlet air temperatures.

Deep bed and tunnel dryers handle solids, not liquids. A drum dryer can dry a liquid but produces flakes that must then be milled, so it is not a single step to powder.

Answer: B

Correct answer: Only part of the drying potential is used

Exhaust air that is still hot and still well short of saturation has capacity to take up moisture that was never used. The air passed through too quickly, or contacted too little product, so both its sensible heat and its remaining drying potential leave with it.

This is the main efficiency loss in hot air drying, and it points to recirculating part of the exhaust, increasing the residence time or deepening the bed. It does not indicate an undersized heater - the air reached 60 degrees as intended - and the product cannot be at equilibrium with air this far from saturation.

Answer: C

Correct answer: Dry in stages with tempering between

Removing ten points of moisture in one continuous pass creates a steep gradient between the surface and the centre of each kernel. The differential shrinkage sets up internal stresses that fissure the grain, and those fissures break during milling - which is exactly what a fall in head rice yield reports.

Drying in stages with tempering between passes lets the gradients relax before they can crack the kernel, and it is the standard remedy. Raising the temperature or the air flow would dry the surface faster still and make the problem worse, and a lower final moisture would compound it.

Answer: D

Correct answer: critical

The critical moisture content is where the surface can no longer be kept continuously wet, so the constant rate period ends and the falling rate period begins. Control passes at that point from the external air conditions to moisture movement within the solid.

It is not a fixed property of the material: drying harder raises it, because the surface dries out sooner, and a thicker piece raises it too. Equilibrium moisture is the floor at the far end of drying, free moisture is the removable excess above that floor, and bound moisture is the fraction held at a reduced vapour pressure.

Answer: A

Correct answer: free

Free moisture is the moisture present above the equilibrium value for the air in question, and it is the only part that air can actually remove:

\[ X_{free} = X - X_{eq} \]

Drying rate curves are plotted against free moisture for precisely this reason - the rate falls to zero as the free moisture does. Note the qualifier "for the air in question": change the air's humidity and the equilibrium value moves, so the same product has a different free moisture content. Bound moisture is that held at a vapour pressure below pure water's.

Answer: B

Correct answer: wet bulb

The wet bulb temperature is the steady reading of a thermometer whose bulb carries a wet wick with air moving over it. Evaporation from the wick removes latent heat until it exactly balances the sensible heat arriving by convection, and the reading settles there.

Because the balance point depends on how far the air is from saturation, the wet bulb reading with the dry bulb fixes every other psychrometric property. It also matters in drying because a wet product surface sits at this same temperature throughout the constant rate period, which is what keeps heat-sensitive material cool while it is still wet.

Answer: C

Correct answer: case hardening

Case hardening is the formation of a dry, impermeable skin when the surface dries far faster than moisture can arrive from the interior. The outer layer shrinks and its structure closes, sealing the remaining moisture inside.

Drying then appears to finish while the centre is still wet, and the product may spoil in store as that moisture redistributes. The remedy is a gentler start - lower temperature, slightly higher humidity, lower velocity - or intermittent drying with rest periods. Caking is particles sticking together, browning a chemical reaction, and scorching thermal damage.

Answer: D

Correct answer: tempering

Tempering is a rest period between drying passes, with the grain held without airflow so that moisture can migrate from the wetter centre of each kernel towards the drier surface. The kernel becomes more uniform, so the next pass can remove water from a wetter surface without over-drying it.

The benefit is quality: relaxed gradients mean fewer fissures and a higher head rice yield after milling. Tempering costs time but almost no energy, and it does not change the average moisture content - nothing is removed while the grain rests.

Answer: A

Correct answer: 1.6 kg per kg dry matter

The critical moisture is where the constant rate period ends. Reading down the table, the rate holds at \( 6.0 \) through \( 2.40 \) and \( 1.60 \), then falls to \( 4.1 \) at \( 0.90 \). The transition therefore occurs somewhere between \( 1.60 \) and \( 0.90 \), and of the values offered, \( 1.6 \) is the last point still on the constant rate plateau and so the closest estimate.

The value \( 2.4 \) is still well within the constant rate period, and \( 0.9 \) and \( 0.3 \) are already in the falling rate period, past the transition rather than at it.

Answer: B

Correct answer: The exhaust leaves well short of saturation

Compare the exhaust with saturation at the same temperature: the humidity ratio is \( 0.021 \) against \( 0.058 \) at saturation, a relative humidity of about 38 per cent. The air leaves with well over half its moisture-carrying capacity unused, and at 44 degrees it is still warm.

The dryer has clearly removed moisture - the humidity ratio rose from \( 0.012 \) to \( 0.021 \) - and the heater added none, holding \( 0.012 \) while raising the temperature to 70 degrees and dropping the relative humidity to 6 per cent. The scope for improvement lies in using more of that remaining capacity.