The term drying refers generally to the removal of moisture from a
substance. It is one of the oldest, most commonly used and most energy
consuming unit operation in the process industries.
The hot air drying technique is the most frequently used technique to
produce dehydrated vegetables and fruits, in which heat is transferred
from the hot air to the product by convection, and evaporated water
is transported to the air also by convection.
Air that has a low moisture concentration is blown through the material
bed so that it absorbs water molecules from the product away. In some
dryers the air flow is also what is used to heat the material being
dried.
Hot air driers are classified as parallel flow, counter flow, direct
flow, or cross flow, pending on the direction in the product moves in
direction to the direction of flow of the heated air.
Hot-air drying is efficient at removing free water as long as
it’s present as continuous liquid phase (high water content)
preventing thermal deterioration of the products; this removal is
governed by Fick’s Law. The water elimination generates
shrinkage decreasing the internal water diffusivity and
increasing the thermal conductivity of the product and thus the
internal heat transfer.
Hot air drying
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