Moist Air Daltons Law

Overview

Engineering reference data for Moist Air Daltons Law 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

Density Formulation of Dalton's Law

The relationship between partial pressure, density, and temperature for moist air can be expressed using the specific gas constants for dry air (Ra = 286.9 J/(kg·K)) and water vapor (Rw = 455 J/(kg·K)).

The partial pressure of dry air is given by $p_a = \rho_a R_a T$, and for water vapor by $p_w = \rho_w R_w T$. Substituting these into Dalton's Law yields:

$p = \rho_a \cdot 286.9 \cdot T + \rho_w \cdot 455 \cdot T$

where:

• $\rho_a$ is the density of dry air (kg/m³)
• $\rho_w$ is the density of water vapor (kg/m³)
• $T$ is the absolute temperature (K)

References

• Original Source