Question

In the adiabatic drying process, if there is a decrease in the value of dry bulb temperature, then

• A. The values of humidity ratio, relative humidity increases and water vapour pressure decreases
• B. The values of humidity ratio, relative humidity decreases and water vapour pressure increases
• C. The values of humidity ratio, relative humidity and water vapour pressure increases
• D. The values of humidity ratio, relative humidity and water vapour pressure decreases

Hint:

Adiabatic drying follows a constant wet bulb temperature line; a fall in dry bulb temperature means the air is approaching saturation.

Answer 1

The Correct Option is C

Picture a psychrometric chart. Adiabatic drying moves the air state along a line of constant wet bulb temperature, a line that runs from the dry bulb axis down toward the saturation curve. A drop in dry bulb temperature means the point has slid further along this line, closer to saturation.
Every quantity that measures how full of moisture the air is grows as it nears saturation: the humidity ratio $W$ rises because the grain keeps releasing moisture into the air, the relative humidity rises because the air is nearer its saturation limit at the new lower temperature, and the vapour pressure $p_w$ rises because it depends only on how much water vapour is present, which has gone up.
None of these three can fall while the air keeps absorbing moisture from the grain, which rules out options 1, 2 and 4.
\[\boxed{\text{Humidity ratio, relative humidity and water vapour pressure all increase}}\]