Water Vapor Saturation Pressure Air

Overview

Engineering reference data for Water Vapor Saturation Pressure Air in thermodynamics.

Key Formulas

First Law

$\Delta U = Q - W$

Energy is conserved — heat added minus work done.

Ideal Gas Law

$PV = nRT$

Relates pressure, volume, and temperature of an ideal gas.

Heat Transfer

$Q = mc\Delta T$

Sensible heat transfer.

Carnot Efficiency

$\eta = 1 - T_C/T_H$

Maximum efficiency between two temperatures.

Variables

Symbol	Description	Unit
$U$	Internal energy	J
$Q$	Heat	J
$W$	Work	J
$P$	Pressure	Pa
$V$	Volume	m³
$T$	Temperature	K

Important Note

The equations on this page are for pure water vapor, not moist air. Water vapor is almost always present in the surrounding air, and its saturation properties are critical for HVAC, meteorology, and process engineering calculations.

Water Vapor Saturation Pressure

The maximum saturation pressure of water vapor in moist air varies with the temperature of the air-vapor mixture:

$p_{ws} = \frac{e^{(77.3450 + 0.0057T - 7235/T)}}{T^{8.2}}$

where:

• $p_{ws}$ = water vapor saturation pressure (Pa)
• $e$ = Euler's number (≈ 2.718)
• $T$ = dry bulb temperature of the moist air (K)

Water Vapor Density

The density of water vapor can be expressed as:

$\rho_w = \frac{0.0022 \, p_w}{T}$

where:

• $p_w$ = partial pressure of water vapor (Pa)
• $\rho_w$ = density of water vapor (kg/m³)
• $T$ = absolute dry bulb temperature (K)

Saturation Pressure and Density Reference Table

Example Calculation

Calculate the saturation pressure of water vapor at 25 °C:

Step 1: Convert temperature to Kelvin:

$T = 25\,°C + 273 = 298\,K$

Step 2: Apply Equation (1):

$p_{ws} = \frac{e^{(77.3450 + 0.0057 \times 298 - 7235/298)}}{298^{8.2}} = 3130 \text{ Pa}$

Unit Conversion Reference

From	To	Multiply by
1 lb/ft³	kg/m³	16.018
1 kg/m³	lb/ft³	0.0624

Interactive Charts

References

• Original Source