Water Vapor-and Other Gases
words, the saturation vapor pressure at dew point temperature is equal to
the ambient vapor pressure:
es(Td) = ea.
(3.13)
This can also be determined from Fig. 3.2 by following horizontally from
the ambient vapor pressure, and reading the temperature at the intersection
of the horizontal line and the h, = 1 line. It can be obtained more
precisely from Table A.3, and by inverting Eq. (3.8):
where the constants are the same as in Eq. (3.8).
Another important moisture variable is wet bulb temperature, Tw. To
find the wet bulb temperature, determine the temperature drop which
could be achieved by adiabatic evaporation of water into air (adiabatic
meaning "without heat exchange"). Air is cooled by evaporating water
into it, but the evaporation of water into the air raises its vapor pressure.
Since the change in heat content of the air due to changing its temperature
must equal the latent heat of evaporation for the water evaporated into
the air, we can write:
where h is the latent heat of vaporization of water (44 Wmol) and c,
is the specific heat of air (29.3 J mol-' K-I). Equation (3.15) is most
often written in terms of vapor pressure and used for determining vapor
pressure from wet bulb and dry bulb temperatures:
Here, y = c,/h is called the thermodynamic psychrometer constant. It
has a value of 6.66 x
C-' with a slight temperature dependence
(O.Ol%/C) due to the temperature dependence of A.
A psychrometer is an instrument consisting of two thermometers.
One thermometer measures the air temperature. The second thermometer,
whose "bulb" is covered with a wet cotton wick, measures the wet bulb
temperature, Tw . Equation (3.16) is used to determine the vapor pressure
of the air from these measurements. Clearly, a real psychrometer is not an
adiabatic system since both heat and water vapor are exchanged with the
surrounding air, and the thermometers absorb and emit radiation. Later in
the book the tools needed to analyze a real psychrometer are developed,
but the result of such an analysis yields an equation like Eq. (3.16) with
an apparent psychrometer constant y *. For an adequately ventilated psychrometer with a good wick and radiation shield the value of the apparent
psychrometer constant is close to the thermodynamic constant, but poorly
designed or ventilated psychrometers can have much lower constants.
Equation (3.16) defines the family of straight, diagonal lines shown in
Fig. 3.2. The wet bulb temperatures are labeled along the h, = 1 line.
words, the saturation vapor pressure at dew point temperature is equal to
the ambient vapor pressure:
es(Td) = ea.
(3.13)
This can also be determined from Fig. 3.2 by following horizontally from
the ambient vapor pressure, and reading the temperature at the intersection
of the horizontal line and the h, = 1 line. It can be obtained more
precisely from Table A.3, and by inverting Eq. (3.8):
where the constants are the same as in Eq. (3.8).
Another important moisture variable is wet bulb temperature, Tw. To
find the wet bulb temperature, determine the temperature drop which
could be achieved by adiabatic evaporation of water into air (adiabatic
meaning "without heat exchange"). Air is cooled by evaporating water
into it, but the evaporation of water into the air raises its vapor pressure.
Since the change in heat content of the air due to changing its temperature
must equal the latent heat of evaporation for the water evaporated into
the air, we can write:
where h is the latent heat of vaporization of water (44 Wmol) and c,
is the specific heat of air (29.3 J mol-' K-I). Equation (3.15) is most
often written in terms of vapor pressure and used for determining vapor
pressure from wet bulb and dry bulb temperatures:
Here, y = c,/h is called the thermodynamic psychrometer constant. It
has a value of 6.66 x
C-' with a slight temperature dependence
(O.Ol%/C) due to the temperature dependence of A.
A psychrometer is an instrument consisting of two thermometers.
One thermometer measures the air temperature. The second thermometer,
whose "bulb" is covered with a wet cotton wick, measures the wet bulb
temperature, Tw . Equation (3.16) is used to determine the vapor pressure
of the air from these measurements. Clearly, a real psychrometer is not an
adiabatic system since both heat and water vapor are exchanged with the
surrounding air, and the thermometers absorb and emit radiation. Later in
the book the tools needed to analyze a real psychrometer are developed,
but the result of such an analysis yields an equation like Eq. (3.16) with
an apparent psychrometer constant y *. For an adequately ventilated psychrometer with a good wick and radiation shield the value of the apparent
psychrometer constant is close to the thermodynamic constant, but poorly
designed or ventilated psychrometers can have much lower constants.
Equation (3.16) defines the family of straight, diagonal lines shown in
Fig. 3.2. The wet bulb temperatures are labeled along the h, = 1 line.