Question Bank • QB_APE_003 NAT • 2 Marks

Section 6: Agricultural Process Engineering

Evaporation and Drying

Subtopic: Psychrometry – properties of air-water vapour mixture

Question Statement:

Moist air at total atmospheric pressure of $101.325\text{ kPa}$ has a partial pressure of water vapour of $2.35\text{ kPa}$. The humidity ratio (specific humidity) of this air in $\text{kg water/kg dry air}$ is ________ (round off to 4 decimal places).

Answer Key:

0.0148

Step-by-Step Solution & Explanation

The humidity ratio $W$ is given by: $W = 0.622 \times \frac{p_v}{P_t - p_v}$ Where: • $p_v = 2.35\text{ kPa}$ • $P_t = 101.325\text{ kPa}$ • $P_t - p_v = 101.325 - 2.35 = 98.975\text{ kPa}$ $W = 0.622 \times \frac{2.35}{98.975} = 0.622 \times 0.023743 = 0.014768\text{ kg/kg d.a.} \approx 0.0148$
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