Drying Theory and Dryers Questions and Answers

Answer: A

Expressing it against dry air gives a quantity that does not change when the air is heated or cooled without adding or removing moisture. That constancy is what makes the psychrometric chart so useful in following a drying process.

Answer: B

Correct answer: Actual vapour pressure over saturation pressure

Relative humidity is the ratio of the actual partial pressure of water vapour in the air to the saturation vapour pressure at the same temperature, usually expressed as a percentage:

\[ \phi = \frac{p_v}{p_{sat}} \times 100 \]

It therefore measures how near the air is to saturation, which is what governs its capacity to take up more moisture. The mass of vapour per unit mass of dry air is the humidity ratio, a different quantity; the temperature at which condensation begins is the dew point; and the difference between dry and wet bulb readings is the wet bulb depression.

Answer: C

Correct answer: The steady temperature of a wetted thermometer

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

Because the evaporation rate depends on how far the air is from saturation, the wet bulb reading together with the dry bulb fixes the air's humidity - which is the whole basis of psychrometry. The dew point is a different temperature, reached by cooling rather than by evaporation.

Answer: D

Correct answer: The temperature at which saturation occurs on cooling

The dew point is the temperature to which moist air must be cooled, at constant pressure and constant moisture content, for it to become saturated and begin to condense. At that temperature the actual vapour pressure equals the saturation vapour pressure.

It is not the wet bulb temperature, which is reached by evaporative cooling and lies between the dew point and the dry bulb; the two coincide only when the air is already saturated. Nor is it the adiabatic saturation temperature, a related but distinct quantity, and drying does not stop at the dew point.

Answer: A

Correct answer: The moisture in balance with the air

The equilibrium moisture content is the value a product reaches when it is left long enough in air of a given temperature and relative humidity: at that point it neither gains nor loses moisture, because its vapour pressure equals that of the air.

It sets the floor for any drying operation - no dryer can take a product below the equilibrium value for its exhaust air, however long it runs. It varies with the air's relative humidity and with the material, and it is a property of the product, not of the air alone.

Answer: B

Correct answer: The moisture above the equilibrium value

Free moisture is the moisture present in excess of the equilibrium value for the surrounding air - the part that can actually be removed by drying in those conditions:

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

The distinction matters because drying rate curves are plotted against free moisture, not total moisture: the rate falls to zero when the free moisture reaches zero, whatever the total may be. Total moisture includes the equilibrium fraction, and bound moisture is that held so firmly in the structure that it exerts a lowered vapour pressure.

Answer: C

Correct answer: Free water covers the surface throughout

The constant rate period lasts as long as a continuous film of free water covers the surface. Evaporation then behaves as it would from a free water surface, so the rate depends only on the external conditions - air temperature, humidity and velocity - and stays constant while moisture from within replenishes the surface as fast as it is lost.

The product temperature stays at the wet bulb temperature rather than rising, and internal moisture movement does not yet limit the process; that becomes controlling in the falling rate period, once the surface can no longer be kept wet.

Answer: D

Correct answer: The end of the constant rate period

The critical moisture content is the moisture at which the surface can no longer be kept continuously wet, so the constant rate period ends and the falling rate period begins. Below it, control passes from the external air conditions to the movement of moisture within the solid.

Its value depends on the material, its thickness and the drying rate itself: drying harder pushes the critical moisture higher, because the surface dries out sooner. It is not the equilibrium moisture, which is the floor reached at the far end of drying, nor a storage criterion.

Answer: A

Correct answer: Movement of moisture within the solid

In the falling rate period the surface is no longer continuously wet, so the rate is set by how quickly moisture can reach the surface from inside - by liquid diffusion, capillary flow or vapour movement through the pores. The rate falls as the remaining moisture has further to travel and is more firmly held.

This is why raising the air velocity, which is effective during the constant rate period, gives little benefit here: the bottleneck is inside the solid, not in the boundary layer. Reducing the particle or layer thickness helps far more, since it shortens the internal path.

Answer: B

Correct answer: A dry skin trapping moisture inside

Case hardening occurs when the surface dries far faster than moisture can move out from the interior. The outer layer shrinks and its structure closes up, forming a skin of low permeability that traps moisture within the product.

The consequences are practical: drying appears to finish while the centre is still wet, and the product may later spoil or go mouldy in store as that moisture redistributes. The remedy is to dry more gently at the start - lower air temperature, higher humidity, or intermittent drying with rest periods that let internal gradients relax.