Question

The constant rate period in drying refers to the stage when:

• A. Surface moisture is completely removed
• B. Internal moisture starts to migrate
• C. Drying rate remains constant
• D. The temperature of the material drops

Hint:

In the constant rate period, evaporation occurs at the surface and the drying rate stays steady — it's like drying a wet surface under steady heat.

Answer 1

The Correct Option is C

Drying of materials typically occurs in two major phases: the constant rate period and the falling rate period.
In the constant rate period, the surface of the material remains saturated with moisture, and water is continuously evaporated as fast as it can be supplied to the surface from within.
During this stage:
- The drying rate remains constant, hence the name.
- It resembles the evaporation of water from a free water surface.
- The temperature of the material remains close to the wet-bulb temperature.
- Heat supplied is used mainly for vaporizing the surface moisture.
This period continues until the surface moisture is no longer sufficient to maintain saturation, after which the process transitions into the falling rate period, where internal moisture migration dominates.
Incorrect options:
- (1) Surface moisture is not yet completely removed during this stage.
- (2) Internal moisture starts migrating primarily in the falling rate period.
- (4) The temperature of the material remains relatively constant in this stage.
Therefore, the defining characteristic of the constant rate period is: drying rate remains constant.