Drying Theory and Dryers Questions and Answers

Answer: A

Correct answer: A is false but R is true

The assertion is false, and the reason is why. Air leaving hot and unsaturated indicates poor efficiency, not good: it carries away sensible heat that did no drying and, as the reason states, unused capacity to take up moisture.

Good efficiency shows in the opposite signature - exhaust that is as cool and as close to saturation as the process allows, because that means the heat and the drying potential were both spent on the product. The remedies follow directly: recirculate part of the exhaust, lengthen the residence time, or recover the heat.

Answer: B

Correct answer: 1 and 2 only

Statement 1 is correct: the constant rate period lasts exactly as long as a film of free water covers the surface. Statement 2 is correct: evaporation removes latent heat as fast as convection supplies sensible heat, so the surface sits at the wet bulb temperature.

Statement 3 is false and describes the falling rate period. While the surface stays wet, moisture from within reaches it as fast as it evaporates, so internal diffusion is not limiting; control lies entirely with the external air conditions. Diffusion takes over only once the critical moisture is passed.

Answer: C

Correct answer: 1 and 2 only

Statement 1 is correct: heating adds no water, so the humidity ratio is unchanged. Statement 2 is correct: relative humidity is \( p_v/p_{sat} \), and heating raises \( p_{sat} \) steeply while \( p_v \) stays put, so the ratio falls.

Statement 3 is false. The dew point depends only on the actual vapour pressure, which is fixed by the humidity ratio - so heating at constant humidity ratio leaves the dew point exactly where it was. This is a useful check: heating changes relative humidity dramatically and dew point not at all.

Answer: D

Correct answer: 1 and 2 only

Statement 1 is correct: a spray dryer atomises a liquid or slurry feed into hot air. Statement 2 is correct: a fluidised bed suspends free-flowing granular solids in an upward air stream.

Statement 3 is false. A drum dryer spreads a thin film of liquid or slurry on a heated rotating drum and scrapes off the dried sheet; it has no way to handle whole grains, which need a through-flow air dryer. The feed state - liquid or solid - is what determines the whole architecture of a dryer.

Answer: A

Correct answer: 1 and 2 only

Statement 1 is correct: free moisture is the moisture above the equilibrium value, the removable fraction. Statement 2 is correct: bound moisture is held chemically or in fine capillaries and cell walls, so it exerts a vapour pressure below that of pure water and is correspondingly harder to remove.

Statement 3 is false. Critical moisture is not a fixed property of the material - it depends on how hard the drying is and on the thickness of the piece. Drying faster raises it, because the surface dries out sooner relative to the supply of moisture from within.

Answer: B

Correct answer: A-2, B-1, C-4, D-3

Each term takes its standard definition. Humidity ratio (A) is the mass of vapour per unit mass of dry air (2). Relative humidity (B) is the actual vapour pressure over the saturation pressure at the same temperature (1).

Dew point (C) is the temperature at which saturation occurs on cooling at constant moisture content (4), and wet bulb temperature (D) is the steady reading of a wetted thermometer in moving air (3). The distractors mainly exchange the first two, which is the pairing to check: a mass ratio against a pressure ratio.

Answer: C

Correct answer: A-2, B-1, C-4, D-3

Each dryer is matched to the feed it can handle. A spray dryer (A) takes liquids and slurries (2), atomising them into hot air. A fluidised bed dryer (B) takes free-flowing granular solids (1) that can be suspended in an upward air stream.

A deep bed dryer (C) takes bulk grain at modest cost (4), its low capital cost matching the value of the commodity. A freeze dryer (D) takes high-value heat-sensitive products (3), whose quality justifies its expense. The list runs, in effect, from cheapest feed handling to most expensive quality preservation.

Answer: D

Correct answer: A-2, B-1, C-4, D-3

Each moisture term is matched to its meaning. Free moisture (A) is that above the equilibrium value (2) and therefore removable. Bound moisture (B) is held at a vapour pressure below that of pure water (1), which is what makes it hard to remove.

Critical moisture (C) is where the constant rate period ends (4) and control passes to internal diffusion. Equilibrium moisture (D) is that in balance with the surrounding air (3), the floor no dryer can pass with that air. Note the relation: free moisture is measured from the equilibrium value upward, so the two definitions are complementary.

Answer: A

Correct answer: A-2, B-1, C-4, D-3

Each problem is matched to the remedy that addresses its cause. Case hardening (A) is caused by too rapid initial surface drying, so the answer is to start gently and raise the temperature later (2). A wide moisture spread in a deep bed (B) comes from air being spent as it rises, so recirculating the grain between passes (1) evens out the exposure.

Fissured rice kernels (C) come from steep internal gradients, so tempering between passes (4) lets them relax. Low drying efficiency (D) is dominated by sensible heat lost in the exhaust, so recirculating part of the exhaust air (3) recovers it.

Answer: B

Correct answer: Equilibrium moisture content

Equilibrium moisture content is a property of the product, not of the air - the moisture a particular material holds when in balance with air of a given condition. Two materials in the same air settle at different values, and each needs its own sorption isotherm.

The other three describe moist air alone: humidity ratio is its mass of vapour per unit mass of dry air, dew point the temperature at which it would saturate on cooling, and wet bulb temperature the steady reading of a wetted thermometer in it. Keeping air properties and product properties apart is the basic discipline of drying calculations.