Question Bank • QB_EXP_APE_002 NAT • 2 Marks

Section 6: Agricultural Process Engineering

Evaporation and Drying

Subtopic: Psychrometry – properties of air-water vapour mixture

Question Statement:

Moist air at a total barometric pressure of 101.325 kPa has a dry bulb temperature of 35°C and relative humidity of 50%. The saturation vapour pressure of water at 35°C is 5.628 kPa. The humidity ratio of the air in kg water vapour per kg dry air is ________ (rounded off to 3 decimal places).

Answer Key:

0.018

Step-by-Step Solution & Explanation

1. Relative humidity $\phi = \frac{p_v}{p_{vs}} = 0.50$. 2. Partial vapour pressure $p_v = 0.50 \times 5.628 = 2.814\text{ kPa}$. 3. Barometric pressure $P_b = 101.325\text{ kPa}$. 4. Humidity ratio formula: $W = 0.622 \times \frac{p_v}{P_b - p_v} = 0.622 \times \frac{2.814}{101.325 - 2.814} = 0.622 \times \frac{2.814}{98.511} = 0.01777\text{ kg/kg dry air} \approx 0.018$.
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