Periods

Period Drying rate curve

The periods of drying (Drying rate curve): Periods taken to dry the material on the dryer is called periods of drying.

When a wet solid is first placed in a drying oven, it begins to absorb heat and increases in temperature. At the same time, the moisture begins to evaporate and thus tends to cool the drying solid. After a period of initial adjustment, the rate of heating and cooling become equal and the temperature of the drying material stabilize.

Ø      AB portion → warming up period: When a solid is placed in a dryer, it absorbs heat and increases in temperature until stabilization.

Ø      BC portion → Constant rate period: At point-B, the temperature is stabilized and remains constant as long as there is a film of moisture at the surface of the drying solid.

Between points B & C, the moisture evaporating from the surface is replaced by water diffusing from the interior of the solid at a rate equal to the rate of evaporation. The rate of drying is constant and the time BC is referred to as the constant rate period.

Ø      CD portion → First falling rate period: At point C, the surface water is no longer replaced at a rate fast enough to maintain a continuous film. Dry spots begin to appear and the rate of drying begins to fall off. The moisture content at which this occurs is referred to as the critical moisture content (CMC).

Between point C & D, the number and area of the dry spots continue to grow and the rate of drying falls steadily. The time CD is referred to as the first falling rate period.

Ø      DE portion → second falling rate period: At point D, the film of surface moisture is completely evaporated and point-D is referred to as the second critical point.

Between points D & E, the drying rate falls even more rapidly than the first falling rate and the time DE is called second falling rate period.

At point-E, the drying rate is almost equal to zero and the equilibrium moisture content period begins and the solid is in equilibrium with its surroundings i.e. its temperature and MC remains constant. Continued drying after this period is a waste of time and energy.