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

Hint:

Think about what happens to air as it passes over wet grain and gains moisture during drying.

Answer 1

The Correct Option is C

Step 1: Adiabatic drying works on evaporative cooling. Hot air is passed over wet grain, and the sensible heat of the air is used to evaporate moisture from the grain. No outside heat is added or removed, so the process moves along a nearly constant wet bulb temperature line on the psychrometric chart.

Step 2: Because sensible heat converts to latent heat of evaporation, the dry bulb temperature (DBT) of the air falls as it dries the grain.

Step 3: At the same time, the moisture removed from the grain enters the air stream. This raises the humidity ratio (mass of water vapour carried per kg of dry air). So the humidity ratio always increases in this process, it never falls.

Step 4: With DBT falling and moisture content rising together, the air keeps moving closer to its saturation point. Relative humidity, which measures how close the air is to saturation, therefore also increases.

Step 5: The water vapour pressure in the air depends directly on how much moisture the air is carrying. Since the humidity ratio is increasing, the partial pressure of water vapour in the air rises as well.

Step 6: Checking the options: option 1 wrongly shows vapour pressure falling, option 2 wrongly shows humidity ratio falling, and option 4 wrongly shows all three falling. Only option 3, where all three quantities rise together, matches the physics of adiabatic drying.

So the correct answer is option 3.