Moisture Content

Moisture Content

Moisture Content

Critical moisture content (CMC): CMC is the average MC when the constant rate period ends and the first falling rate period starts.

CMC is the point where the surface water is no longer replaced at a rate fast enough to maintain a continuous film (saturation condition) i.e. surface of the material is without water but there is a lot of water inside the material.

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 CMC.

Since the CMC is the average moisture through the material, its value depends on—

1. the rate of drying
2. the thickness of the material and
3. the factors influencing moisture movement within the solid.

Equilibrium moisture content (EMC): If a hygroscopic material is maintained in contact with air at constant temperature and humidity, the material will gain or loss moisture until equilibrium is reached, the material will attain a definite moisture content (MC). This moisture is termed the EMC of the specified conditions.

So, EMC may be defined as the MC of a solid in equilibrium with an atmosphere of a given relativity at this humidity.

Importance of EMC:

1. EMC of a solid is particularly important in drying because it represents the limiting MC for given conditions of humidity and temperature.
2. If the material is dried to a MC less than it normally possesses in equilibrium with atmospheric air, it will return to its equilibrium value on storage unless special precautions are taken.