Water Vapor and Other Gases
C, = 0.611 kPa/lOl kPa = 0.006. The other mole fractions are 0.012,
0.023, and 0.042 mol/mol or 6, 12,23, and 42 mmol/mol.
There are a couple of points from the example that are worth noting. Vapor pressures and mole fractions are used in many calculations
throughout this book, so it would be a good idea to find some way of
remembering approximate values for these quantities at saturation. Note
that the vapor pressure approximately doubles for each 10" C temperature
increase. Exactly doubling would give 0.6, 1.2, 2.4, and 4.8 kPa for the
four temperatures. All but the last one are within a few percent of the actual value, and it is only a little over ten percent high. If you can remember
the vapor pressure at zero, you can therefore estimate saturation vapor
pressures at higher temperatures. The other point to note is that conversion
to mole fraction at sea level involves division by a number close to 100.
The mole fraction, expressed as a percent, is therefore nearly the same
as the vapor pressure. This is also the fraction of a saturated atmosphere
made up of water vapor at the indicated temperature. The mole fraction,
expressed in mmol/mol is just the vapor pressure in kPa multiplied by 10
(and is equal to the vapor pressure in millibars).
3.3 Condition of Partial Saturation
In nature, air is seldom saturated, so we need to know more than just the
temperature to specify its moisture condition. Partial saturation can be
expressed in terms of ambient vapor pressure or mole fraction, relative
humidity, vapor deficit, dew point temperature, or wet bulb temperature.
Ambient vapor pressure is simply the vapor pressure that exists in the air,
as opposed to saturation vapor pressure, which is the maximum possible
vapor pressure for the temperature of the air. Relative humidity is the ratio
of ambient vapor pressure to saturation vapor pressure at air temperature:
Relative humidity is sometimes multiplied by 100 to express it as apercent
rather than a fraction, but it is always expressed as a fraction in this book.
The relationship between saturation vapor pressure and ambient vapor
pressure at various humidities is shown in Fig. 3.2. The curved lines,
labeled on the right, show vapor pressures at humidity increments of 0.1
for temperatures from 0 to 40° C.
The vapor deficit is the difference in vapor pressure or mole fraction
between saturated and ambient air:
D = es(T,) - e, = es(T,)(l - h,)
(3.12)
where the second relation follows from Eq. (3.1 1). The vapor deficit at any
temperature and relative humidity is the difference, in Fig. 3.2, between
the saturation line (h, = 1) at T, and the line for the ambient relative
humidity.
The dew point temperature is the temperature at which air, when cooled
without changing its water content or pressure, just saturates. In other
C, = 0.611 kPa/lOl kPa = 0.006. The other mole fractions are 0.012,
0.023, and 0.042 mol/mol or 6, 12,23, and 42 mmol/mol.
There are a couple of points from the example that are worth noting. Vapor pressures and mole fractions are used in many calculations
throughout this book, so it would be a good idea to find some way of
remembering approximate values for these quantities at saturation. Note
that the vapor pressure approximately doubles for each 10" C temperature
increase. Exactly doubling would give 0.6, 1.2, 2.4, and 4.8 kPa for the
four temperatures. All but the last one are within a few percent of the actual value, and it is only a little over ten percent high. If you can remember
the vapor pressure at zero, you can therefore estimate saturation vapor
pressures at higher temperatures. The other point to note is that conversion
to mole fraction at sea level involves division by a number close to 100.
The mole fraction, expressed as a percent, is therefore nearly the same
as the vapor pressure. This is also the fraction of a saturated atmosphere
made up of water vapor at the indicated temperature. The mole fraction,
expressed in mmol/mol is just the vapor pressure in kPa multiplied by 10
(and is equal to the vapor pressure in millibars).
3.3 Condition of Partial Saturation
In nature, air is seldom saturated, so we need to know more than just the
temperature to specify its moisture condition. Partial saturation can be
expressed in terms of ambient vapor pressure or mole fraction, relative
humidity, vapor deficit, dew point temperature, or wet bulb temperature.
Ambient vapor pressure is simply the vapor pressure that exists in the air,
as opposed to saturation vapor pressure, which is the maximum possible
vapor pressure for the temperature of the air. Relative humidity is the ratio
of ambient vapor pressure to saturation vapor pressure at air temperature:
Relative humidity is sometimes multiplied by 100 to express it as apercent
rather than a fraction, but it is always expressed as a fraction in this book.
The relationship between saturation vapor pressure and ambient vapor
pressure at various humidities is shown in Fig. 3.2. The curved lines,
labeled on the right, show vapor pressures at humidity increments of 0.1
for temperatures from 0 to 40° C.
The vapor deficit is the difference in vapor pressure or mole fraction
between saturated and ambient air:
D = es(T,) - e, = es(T,)(l - h,)
(3.12)
where the second relation follows from Eq. (3.1 1). The vapor deficit at any
temperature and relative humidity is the difference, in Fig. 3.2, between
the saturation line (h, = 1) at T, and the line for the ambient relative
humidity.
The dew point temperature is the temperature at which air, when cooled
without changing its water content or pressure, just saturates. In other