The pressure in a gas container is due to the individual molecules striking
the wall of the container and exerting a force on it. This force is proportional
to the average velocity of the molecules and the number of molecules per
unit volume of the container (i.e., molar density). Therefore, the pressure
exerted by a gas is a strong function of the density and the temperature of the
gas. For a gas mixture, the pressure measured by a sensor such as a trans-
ducer is the sum of the pressures exerted by the individual gas species, called
the partial pressure. It can be shown (see Chap. 13) that the partial pressure
of a gas in a mixture is proportional to the number of moles (or the mole
fraction) of that gas.
Atmospheric air can be viewed as a mixture of dry air (air with zero mois-
ture content) and water vapor (also referred to as moisture), and the atmo-
spheric pressure is the sum of the pressure of dry air Paand the pressure of
water vapor, called the vapor pressurePv(Fig. 3–61). That is,
(3–28)(Note that in some applications, the phrase “vapor pressure” is used to indi-
cate saturation pressure.) The vapor pressure constitutes a small fraction
(usually under 3 percent) of the atmospheric pressure since air is mostly
nitrogen and oxygen, and the water molecules constitute a small fraction
(usually under 3 percent) of the total molecules in the air. However, the
amount of water vapor in the air has a major impact on thermal comfort and
many processes such as drying.
Air can hold a certain amount of moisture only, and the ratio of the actual
amount of moisture in the air at a given temperature to the maximum amount
air can hold at that temperature is called the relative humidityf. The relative
humidity ranges from 0 for dry air to 100 percent for saturated air(air that
cannot hold any more moisture). The vapor pressure of saturated air at a given
temperature is equal to the saturation pressure of water at that temperature.
For example, the vapor pressure of saturated air at 25°C is 3.17 kPa.
The amount of moisture in the air is completely specified by the tempera-
ture and the relative humidity, and the vapor pressure is related to relative
humidity fby
(3–29)where Psat @ Tis the saturation pressure of water at the specified temperature.
For example, the vapor pressure of air at 25°C and 60 percent relative
humidity is
The desirable range of relative humidity for thermal comfort is 40 to 60 percent.
Note that the amount of moisture air can hold is proportional to the satura-
tion pressure, which increases with temperature. Therefore, air can hold
more moisture at higher temperatures. Dropping the temperature of moist air
reduces its moisture capacity and may result in the condensation of some of
the moisture in the air as suspended water droplets (fog) or as a liquid film
on cold surfaces (dew). So it is no surprise that fog and dew are common
occurrences at humid locations especially in the early morning hours when
PvfPsat @ 25°C0.6 1 3.17 kPa 2 1.90 kPaPvfPsat @ TPatmPaPvChapter 3 | 149TOPIC OF SPECIAL INTEREST* Vapor Pressure and Phase Equilibrium
*This section can be skipped without a loss in continuity.
AirWater
vaporPatm = Pa + PvFIGURE 3–61
Atmospheric pressure is the sum of
the dry air pressure Paand the vapor
pressure Pv.