Rate of Drying: When
a wet non-porous granular solid material is placed in a tray and dried with a
carrier gas like air under constant drying conditions of velocity, temperature,
humidity, and pressure of air at the inlet, the experimental data can be
plotted as drying rate vs. free moisture content or time, Fig.1. The obtained
typical drying rate curve is divided into a constant rate period (AB), a first
falling rate period (BC), and a second falling rate period (CD). The free
moisture content at the end of the constant rate period (B) is known as CMC.
During the drying process, the moisture content is plotted based on the entire
material. In general, at the start of the drying process, there is an unsteady
state of a warming-up period during which wet solid keep changing till the
beginning of the constant rate period.
Fig.1: Rate of drying
The drying rate curves
are not smooth and continuous which indicates that the drying process involves
a single mechanism throughout. In drying calculations, the water content in the
wet solid is usually expressed on a dry weight basis, i.e. Kg of water/Kg of
dry solid.