Moist Air Saturation

Overview

Engineering reference data for Moist Air Saturation in air psychrometrics.

Key Formulas

Humidity Ratio

$\omega = 0.622 \frac{P_v}{P_a}$

Mass of water vapor per mass of dry air.

Relative Humidity

$\phi = \frac{P_v}{P_{vs}} \times 100\%$

Ratio of actual to saturation vapor pressure.

Wet Bulb Temperature

$T_{wb} = T_{db} - \frac{P_{vs} - P_v}{\gamma}$

Temperature measured by wet-bulb thermometer.

Enthalpy of Moist Air

$h = c_p T + \omega h_g$

Sensible + latent heat per unit mass of dry air.

Variables

Additional Notes on Degree of Saturation

The degree of saturation ($\mu$) is distinct from relative humidity ($\phi$). While both compare the moisture content of air to its maximum capacity at a given temperature, they use different bases:

• Degree of Saturation ($\mu$): Ratio of humidity ratios ($x / x_s$).
• Relative Humidity ($\phi$): Ratio of partial pressures of water vapor ($p_w / p_{ws}$).

The second formula, $\mu = \phi (p_a - p_{ws}) / (p_a - p_w)$, is particularly useful when vapor pressures are known. It highlights that $\mu$ will always be greater than or equal to $\phi$ (for a given temperature and pressure) because the denominator $(p_a - p_w)$ is smaller than $(p_a - p_{ws})$.

This relationship is important in psychrometric calculations for processes involving changes in temperature or pressure, where the humidity ratio remains constant but the degree of saturation changes.

Source: engineeringtoolbox.com

References

• Original Source