Discussion
Consider the following statements about the constant rate period:
1. The surface is covered by a film of free water.
2. The surface temperature is close to the wet bulb temperature.
3. The rate is controlled by diffusion within the solid.
Which of the statements given above is or are correct?
Answer: B
Correct answer: 1 and 2 only
Statement 1 is correct: the constant rate period lasts exactly as long as a film of free water covers the surface. Statement 2 is correct: evaporation removes latent heat as fast as convection supplies sensible heat, so the surface sits at the wet bulb temperature.
Statement 3 is false and describes the falling rate period. While the surface stays wet, moisture from within reaches it as fast as it evaporates, so internal diffusion is not limiting; control lies entirely with the external air conditions. Diffusion takes over only once the critical moisture is passed.
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