Drying Theory and Dryers Questions and Answers

Answer: C

The heat needed is \( 500 \times 2300 = 1.15 \times 10^6 \) kJ per hour, and each kilogram of air gives up \( 95 \) kJ, so \( 1.15\times10^6/95 = 12100 \) kg. Air flow rather than chamber volume usually limits a spray dryer's capacity.

Answer: D

With the same specific air flow the drying front travels at the same speed, so half the depth takes about half the time. The uniformity also improves, since the top layer sees air that has passed through less material.

Answer: A

Correct answer: 3.09 MJ per kg of water

The heat supplied per hour is the fuel rate times its calorific value, and the specific energy consumption divides that by the water evaporated:

\[ Q = 42 \times 14 = 588\ \mathrm{MJ/h} \]

\[ SEC = \frac{588}{190} = 3.09\ \mathrm{MJ\ per\ kg\ of\ water} \]

Against the latent heat of \( 2.3 \) MJ/kg this corresponds to an efficiency of about 74 per cent, which is good for a direct-fired dryer. The distractor \( 2.30 \) is the latent heat itself - the theoretical floor, not the actual consumption.

Answer: B

Correct answer: 28 degrees

The wet bulb depression is simply the difference between the two readings:

\[ \Delta T = 60 - 32 = 28\ \mathrm{degrees} \]

A large depression means rapid evaporation from the wet wick, which means the air is far from saturation and has high drying potential. A depression of zero would mean saturated air, in which no drying could occur at all. The dry and wet bulb pair is what fixes every other psychrometric property on the chart.

Answer: C

The second is correct and it shows why the first fails: the resistance is internal, so faster air over the surface buys very little. Velocity matters greatly during the constant rate period instead, where the resistance is external.

Answer: D

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second explains the first. Heating air without adding moisture leaves the actual vapour pressure unchanged, but the saturation vapour pressure rises steeply with temperature - roughly doubling every ten degrees or so in the range of interest.

Since relative humidity is \( p_v / p_{sat} \), a larger denominator with an unchanged numerator gives a lower ratio. This is exactly why dryers heat their air: heating creates drying potential without adding a single gram of water, and the humidity ratio is unchanged until evaporation begins.

Answer: A

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second is the reason for the first. Equilibrium moisture content is by definition the value at which the product's vapour pressure matches that of the surrounding air, so no net transfer occurs in either direction.

It follows that the exhaust air condition sets a floor on how dry the product can become: once the product has reached equilibrium with that air, running the dryer longer accomplishes nothing. Drying further requires drier air - which in practice means heating it, since heating lowers the relative humidity and hence the equilibrium moisture.

Answer: B

Correct answer: Statement I is incorrect but Statement II is correct

Statement I is incorrect. Drying as fast as possible from the start is often damaging, not desirable.

Statement II is correct and explains why: rapid surface drying can form an impermeable skin - case hardening - that traps moisture within the product, so drying appears complete while the centre is still wet and the product may spoil in store. In grain the same steep gradient produces internal stresses that fissure the kernel and cut head rice yield. The practical answer is gentler initial drying, or intermittent drying with tempering periods between passes.

Answer: C

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second explains the first. During the constant rate period the surface is covered by free water, and evaporation from it removes latent heat at exactly the rate convection supplies sensible heat from the air.

That balance is the definition of the wet bulb temperature, so the surface sits there and the product stays cool however hot the drying air. It is why heat-sensitive materials tolerate high inlet temperatures while they are still wet, and why they become vulnerable once the falling rate period begins and the surface temperature climbs towards the air temperature.

Answer: D

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second explains the first. Air passing through the lower layers gives up sensible heat and takes up moisture, so it emerges cooler and more humid - with its drying potential partly spent.

By the time it reaches the upper layers of a deep bed it may be close to saturation and able to remove very little, so those layers dry slowly while the bottom is over-dried. The depth is therefore limited by how much drying potential the entering air carries, which is why practical designs reverse the airflow, recirculate the grain, or simply keep the bed shallow.