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Moist Air - Water Vapor and Saturation Pressure

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Water vapor is almost always present in the surrounding air.

Note! - the equations below is for pure water vapor - not moist air .

Saturation Pressure of Water Vapor

The maximum saturation pressure of the water vapor in moist air varies with the temperature of the air vapor mixture and can be expressed as:

p ws = e (77.3450 + 0.0057 T - 7235 / T) / T 8.2 (1)

where

p ws = water vapor saturation pressure (Pa)

e = the constant 2.718.......

T = dry bulb temperature of the moist air (K)

Density of Water Vapor

The density of water vapor can be expressed as:

ρ w = 0.0022 p w / T (2)

where

p w = partial pressure water vapor (Pa, N/m2)

ρ w = density water vapor (kg/m 3 )

T = absolute dry bulb temperature (K)

Saturation pressure and density of water vapor for common temperatures

Moist Air - Saturation Pressure and Density vs. Temperature
TemperatureSaturation PressureDensity
( oC)( o F)(Pa)(mmHg)(psia)(inHg)(kg/m 3 )10 -3
(lb/ft 3 )
0 32 603 4.6 0.09 0.18 0.005 0.30
10 50 1212 9.2 0.18 0.36 0.009 0.59
20 68 2310 17.4 0.33 0.68 0.017 1.08
30 86 4195 31.7 0.61 1.24 0.030 1.90
40 104 7297 55.1 1.06 2.15 0.051 3.20
50 122 12210 92.2 1.8 3.60 0.083 5.19
60 140 19724 149 2.9 5.82 0.13 8.13
70 158 30866 233 4.5 9.11 0.20 12.3
80 176 46925 354 6.8 13.8 0.29 18.2
90 194 69485 525 10.1 20.5 0.42 26.3
100 212 100446 758 14.6 29.6 0.59 36.9
120 248 196849 1486 28.6 58.1 1.10 68.7
140 284 358137 2704 51.9 105.7 1.91 119
160 320 611728 4619 88.7 180.5 3.11 194
180 356 990022 7475 144 292.1 4.80 300
200 392 1529627 11549 222 451.2 7.11 444
  • 1 lb/ft 3 = 16.018 kg/m 3
  • 1 kg/m 3 = 0.0624 lb/ft 3

Example - Saturation Pressure Water Vapor

The Saturation pressure of water vapor in moist air at dry bulb temperature 25 oC can be calculated:

First, convertion from °C to K:

( 25 °C) + 273 = 298 (K)

Then Eq. (1) is used:

p ws = e (77.3450 + 0.0057 (298 K) - 7235 / (298 K) ) / 298 [K] 8.2

= 3130 (Pa)

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