Water Vapor

Overview

Engineering reference data for Water Vapor 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

Symbol	Description	Unit
$\omega$	Humidity ratio	kg/kg
$\phi$	Relative humidity	%
$P_v$	Vapor pressure	Pa
$P_{vs}$	Saturation vapor pressure	Pa
$T_{db}$	Dry bulb temperature	°C
$T_{wb}$	Wet bulb temperature	°C

Water Vapor Specific Heat Data

Important Notes

• Isobaric (Cₚ) vs. Isochoric (Cᵥ): The data provided is for isobaric specific heat (Cₚ), applicable to systems at constant pressure. Isochoric specific heat (Cᵥ) is used for constant-volume closed systems.
• Temperature Dependence: Specific heat varies significantly with temperature, increasing approximately 80% from 175 K to 6000 K.
• High-Temperature Behavior: At temperatures above 1500 K, dissociation of water vapor becomes appreciable, making pressure a significant variable. The provided values assume undissociated states.
• Engineering Applications: For accurate mass and volume flow calculations in heated or cooled systems, specific heat values should be corrected according to the temperature using this table.

Interactive Charts

References

• Original Source