What are the three stages of drying?

In drying theory, especially in industrial and food science, the drying process is typically described as having three distinct stages. The first stage is the initial or constant rate period, also called the warm-up period. During this stage, the surface of the material is saturated with free water, and evaporation occurs at a relatively constant rate as heat is applied. The material's temperature rises to the wet bulb temperature and moisture is removed quickly from the surface. The second stage is the first falling rate period, also called the unsaturated surface drying period. Here, the surface is no longer fully saturated, dry patches begin to appear, and the drying rate starts to decrease because moisture must travel from inside the material to the surface before it can evaporate. The material begins to heat up more. The third stage is the second falling rate period, where the surface is completely dry and drying is controlled entirely by internal moisture diffusion. Water trapped deep inside the material moves slowly outward, so drying becomes much slower and requires more energy, and the material temperature approaches the drying air temperature. This three-stage model applies to lumber, food, laundry, and building materials. In laundry context, you might simplify it as warming, rapid evaporation, and slow final drying where trapped moisture leaves.