Describe the typical drying rate curve and explain its phases.

The drying curve has three phases: Initial period - material heats up to wet-bulb temperature; Constant rate period - surface stays wet, rate limited by external heat/mass transfer, moisture content decreases linearly; Falling rate period - begins at critical moisture content when surface dries, rate becomes internally limited (diffusion through solid), divided into first falling rate (unsaturated surface) and second falling rate (receding evaporation front).