1kg of moist air of RH 70% at 21°C is cooled at constant pressure of 1 bar to 5°C. The vapour pressure at 21°C and 5°C are 0.025 bar and 0.0087 bar. The percentage of water vapour that condenses into water, at 5°C is
- A. 66%
- B. 85%
- C. 51%
- D. 17%
Answer: A. 66%
Explanation: Specific humidity $\omega = 0.622 P_v / (P - P_v)$. Initial $P_{v1} = 0.7 \times 0.025 = 0.0175$. Final (saturated) $P_{v2} = 0.0087$. Calculate $\omega_1$ and $\omega_2$. Percentage condensed = $(\omega_1 - \omega_2)/\omega_1$.