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CHAPTER 1

PSYCHROMETRICS
Composition of Dry and Moist Air ............................................
U.S. Standard Atmosphere .........................................................
Thermodynamic Properties of Moist Air ...................................
Thermodynamic Properties of Water at Saturation ...................
Humidity Parameters.................................................................
Perfect Gas Relationships for Dry and
Moist Air.................................................................................

1.1
1.1
1.2
1.2
1.2

Thermodynamic Wet-Bulb and Dew-Point Temperature ............ 1.9
Numerical Calculation of Moist Air Properties ......................... 1.9
Psychrometric Charts............................................................... 1.10
Typical Air-Conditioning Processes ......................................... 1.12
Transport Properties of Moist Air ............................................ 1.15
Symbols .................................................................................... 1.15

1.8

Licensed for single user. © 2009 ASHRAE, Inc.

P

SYCHROMETRICS usesthermodynamic properties to analyze conditions and processes involving moist air. This chapter
discusses perfect gas relations and their use in common heating,
cooling, and humidity control problems. Formulas developed by
Herrmann et al. (2009) may be used where greater precision is
required.
Hyland and Wexler (1983a, 1983b), Nelson and Sauer (2002),
and Herrmann et al. (2009) developedformulas for thermodynamic
properties of moist air and water modeled as real gases. However,
perfect gas relations can be substituted in most air-conditioning
problems. Kuehn et al. (1998) showed that errors are less than 0.7%
in calculating humidity ratio, enthalpy, and specific volume of saturated air at standard atmospheric pressure for a temperature range
of −50 to 50°C. Furthermore, theseerrors decrease with decreasing
pressure.

flat interface surface between moist air and the condensed phase.
Saturation conditions change when the interface radius is very small
(e.g., with ultrafine water droplets). The relative molecular mass of
water is 18.015 268 on the carbon-12 scale. The gas constant for
water vapor is
Rw = 8314.472/18.015 268 = 461.524 J/(kgw ·K)

U.S. STANDARDATMOSPHERE
The temperature and barometric pressure of atmospheric air vary
considerably with altitude as well as with local geographic and
weather conditions. The standard atmosphere gives a standard of
reference for estimating properties at various altitudes. At sea level,
standard temperature is 15°C; standard barometric pressure is
101.325 kPa. Temperature is assumed to decrease linearly withincreasing altitude throughout the troposphere (lower atmosphere),
and to be constant in the lower reaches of the stratosphere. The
lower atmosphere is assumed to consist of dry air that behaves as a
perfect gas. Gravity is also assumed constant at the standard value,
9.806 65 m/s2. Table 1 summarizes property data for altitudes to
10 000 m.
Pressure values in Table 1 may be calculated fromCOMPOSITION OF DRY AND MOIST AIR
Atmospheric air contains many gaseous components as well as
water vapor and miscellaneous contaminants (e.g., smoke, pollen,
and gaseous pollutants not normally present in free air far from pollution sources).
Dry air is atmospheric air with all water vapor and contaminants
removed. Its composition is relatively constant, but small variations
in theamounts of individual components occur with time, geographic location, and altitude. Harrison (1965) lists the approximate
percentage composition of dry air by volume as: nitrogen, 78.084;
oxygen, 20.9476; argon, 0.934; neon, 0.001818; helium, 0.000524;
methane, 0.00015; sulfur dioxide, 0 to 0.0001; hydrogen, 0.00005;
and minor components such as krypton, xenon, and ozone, 0.0002.
Harrison...
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