Subtopic: Psychrometry – properties of air-water vapour mixture
Question Statement:
During an adiabatic saturation (evaporative cooling) process of unsaturated moist air, which of the following parameters remain CONSTANT or change as specified?
Options:
(A) The thermodynamic wet-bulb temperature remains practically constant
(B) The dry-bulb temperature decreases as sensible heat is converted into latent heat
(C) The humidity ratio (moisture content) of the air increases
(D) The relative humidity of the air decreases
Answer Key:
A, B, C
Step-by-Step Solution & Explanation
In an adiabatic evaporative cooling process:
1. Sensible heat from the unsaturated air evaporates liquid water, converting sensible heat into latent heat along a constant wet-bulb / enthalpy line ($T_{wb} \approx \text{constant}$). (Statement A True)
2. Because sensible heat is extracted from the air, the dry-bulb temperature $T_{db}$ decreases. (Statement B True)
3. Evaporated water vapor joins the air stream, increasing humidity ratio $w$. (Statement C True)
4. As air cools and picks up moisture towards the saturation line, relative humidity ($RH$) increases towards $100\%$, never decreases. (Statement D False)