Drying Theory and Dryers Questions and Answers

Answer: C

The one that is not a type of solar dryer is the Forced conduction-type solar dryer.

Solar dryers are classified by how air moves through them: natural convection (air rises by buoyancy) and forced convection (a fan moves the air). Designs include cabinet dryers, low height solar tunnel dryers and walk-in-type solar tunnel dryers, which are large polyhouse-like dryers that a person can walk into.

Air cannot be 'forced' by conduction. Conduction is heat transfer through solids, so 'forced conduction-type' is a mixed-up term and is the odd one out.

Answer: D

The air speed is 0.5-5.0 m/s.

In a tray or cabinet dryer, food is spread on trays stacked in an insulated cabinet, and heated air is blown across or through the trays by a fan, with ducts and baffles distributing it evenly. Food engineering texts give the air velocity in such dryers as about 0.5 to 5 m/s, fast enough to carry away moisture without blowing the product off the trays.

0.1 to 0.4 m/s would be too slow to remove moisture effectively, and 8 to 9 m/s is the range of fluidizing or pneumatic dryers, not trays.

Answer: D

The ratio is the Thermal efficiency of the dryer.

Thermal efficiency compares the heat actually used to evaporate the moisture (the latent heat of evaporation of the water removed) with the total heat energy of the fuel charged. It shows how much of the fuel's energy goes into useful drying.

The heat utilization factor is the ratio of the temperature drop of the air in the dryer to the temperature rise from heating, a measure of how well the hot air is used. Coefficient of performance belongs to refrigerators and heat pumps, and 'effective heat utilization energy' is not a standard term.

Answer: C

The jet velocity is 10-100 m/s.

Impingement dryers blow high-velocity jets of hot air from nozzles or slots directly onto the product surface. The jets break up the boundary layer of still air at the surface, giving very high heat and mass transfer rates. To do that, jet velocities typically run from about 10 m/s to as high as 100 m/s, with air at 100 to 350 degrees C.

2 to 7 m/s is the velocity of ordinary through-flow and cross-flow dryers, too low to produce the impingement effect.