Question

Dry air is fed into a tray dryer. The percentage relative humidity of the air leaving the dryer is 60% at 70°C and 101.35 kPa. If saturated vapour pressure of water at 70°C is 31.2 kPa, the humidity of the air leaving the dryer in kg water per kg dry air (round off to 3 decimal places) will be \(\underline{\hspace{2cm}}\).

Hint:

To calculate the humidity, use the ratio of the vapour pressure and dry air pressure, considering relative humidity.

Answer 1

Correct Answer: 0.135

The humidity of air is given by: \[ \text{Humidity} = \frac{P_{\text{vapour}}}{P_{\text{dry}}} = \frac{P_{\text{sat}} \cdot \phi}{P_{\text{dry}} - P_{\text{sat}} \cdot \phi}. \] Given:
- \( P_{\text{sat}} = 31.2 \, \text{kPa} \),
- \( \phi = 0.60 \) (Relative humidity),
- \( P_{\text{dry}} = 101.35 \, \text{kPa} \).
After calculations, the humidity of the air is: \[ \text{Humidity} \approx 0.135 \, \text{kg water per kg dry air}. \] Thus, the humidity is \( 0.135 \, \text{kg/kg} \).