Environmental Engineering Reference
In-Depth Information
of a gas component of a mixture of gases is independent
of the other gases. Therefore, the partial pressure of water
vapor in the atmosphere which is in equilibrium with water
is the saturation pressure given in Table 2.11. Similarly the
presence of air above water does not change the equilibrium
state of water (Fig. 2.35).
In nature, the water vapor in air is usually not in equilibrium
with adjacent bodies of water. This means that the partial pres-
sure of the water vapor in air, u v , is usually not the same as the
saturation pressure of the water vapor, u vo , at the correspond-
ing temperature. The water vapor in air at a given temperature
is therefore said to be undersaturated, saturated, or supersatu-
rated when the partial pressure of water vapor, u v , is less than,
equal to, or greater than the saturation water vapor pressure,
respectively. The saturated condition represents equilibrium
conditions between the water vapor and the water where evap-
oration and condensation take place at the same rate. On the
other hand, the undersaturated and supersaturated states of
water vapor are not at equilibrium conditions.
The supersaturated state indicates an excess of water vapor
which will eventually condense. In this case, the rate of
condensation exceeds the evaporation rate until the partial
pressure of the water vapor, u v , has been reduced to the
saturation vapor pressure, u vo . In the undersaturated state,
there is a lack of water vapor relative to the equilibrium
condition. Therefore, the rate of evaporation exceeds the rate
of condensation until the partial pressure of the water vapor,
u v , has reached the saturation water vapor pressure, u vo .
The partial pressure of the water vapor in air defines the
degree to which the air is saturated with water vapor at a
particular temperature. The degree of saturation with respect
to water vapor is referred to as the relative humidity, RH.
Figure 2.36
Visualization aid for understanding how air dissolves
in water.
The water lattice is relatively rigid and stable (Dorsey,
1940), and the density of water changes little as a conse-
quence of the presence of the dissolved air. An analogy
using a cylinder with a piston and a porous stone is useful
in visualizing the behavior of an air-water mixture. Consider
a cylinder with a porous stone at its base and a frictionless
piston at the top (Fig. 2.37). The porous stone has pores
equaling 2% of its total volume. The porous stone is used
to simulate the behavior of water. In this model, the cylinder
contains free air above the porous stone.
Let us suppose there is an initial pressure applied equally to
the free air and to the air in the porous stone in the cylinder. If
2.3.7 Air Dissolving in Water
Water molecules form a lattice structure with openings
referred to as “cages” that can be occupied by a gas
(Rodebush and Buswell, 1958), as illustrated in Fig. 2.36.
Air dissolves into the water and fills the “cages” which
have a volume of approximately 2% by total volume.
Initial
load
Frictionless
piston
Added
load
Initial
load
Volume change of
the free air
Frictionless
piston
Final volume of
free air
Initial volume
of free air
Volume change due to air
going into porous stone
Porous disk
Porous disk
Cylinder
Before
After
Figure 2.37
Piston and porous stone analogy for air dissolving in water.
 
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