Subtopic: Psychrometry – properties of air-water vapour mixture
Question Statement:
Which of the following psychrometric principles and thermodynamic relationships are CORRECT?
Options:
(A) During sensible heating of air without moisture addition, the dry-bulb temperature increases while the humidity ratio and dew-point temperature remain constant.
(B) In an adiabatic evaporative cooling process, the dry-bulb temperature decreases while the thermodynamic wet-bulb temperature remains nearly constant.
(C) At 100% relative humidity (saturation), dry-bulb temperature, wet-bulb temperature, and dew-point temperature are all identical.
(D) The specific volume of moist air decreases as its dry-bulb temperature increases at constant barometric pressure.
Answer Key:
A, B, C
Step-by-Step Solution & Explanation
Analysis of psychrometric properties:
- Option A is CORRECT: Sensible heating follows a horizontal path moving rightward on the psychrometric chart ($W = \text{constant}$); because vapor pressure is unchanged, dew point is also constant.
- Option B is CORRECT: Adiabatic humidification follows lines of constant wet-bulb temperature / constant enthalpy.
- Option C is CORRECT: At saturation ($\phi = 1.0$), vapor pressure equals saturation vapor pressure ($p_v = p_{vs}$), meaning $T_{db} = T_{wb} = T_{dp}$.
- Option D is INCORRECT: By the ideal gas equation ($v = \frac{R_a T}{P - p_v}$), specific volume is directly proportional to absolute temperature; heating air causes volumetric expansion, increasing specific volume.