Environmental Engineering Reference
In-Depth Information
Table 3.12
Saturated Vapour Pressure
p
of Water and the Dew-point
Temperature
ϑ
dew
at 70 per cent Relative Air Humidity as a Function of the
Ambient Air Temperature
ϑ
A
ϑ
A
in °C
10
12
14
16
18
20
22
24
26
28
30
p
in kPa
1.23
1.40
1.60
1.82
2.07
2.34
2.65
2.99
3.37
3.79
4.25
ϑ
dew
in °C
4.8
6.7
8.6
10.5
12.5
14.4
16.3
18.2
20.1
22.0
23.9
Table 3.12 shows the saturated vapour pressures and the dew-point
temperatures calculated using equations (3.51) and (3.52) for various
temperatures.
With the vaporizing mass flow
m
•
V
(eg kg of water/hour) and the heat of
vaporization
h
V
= 2.257 kJ/kg of water, the vaporization losses are found
from:
(3.53)
However, empirical equations are often used. When calculating the
vaporization losses using the wind speed
v
Wind
at height 0.3 m and using the
saturated vapour pressure
p
, the water temperature
ϑ
W
, the ambient air
temperature
ϑ
A
, the relative humidity
ϕ
and the swimming pool surface area
A
W
, we get (Hahne and Kübler, 1994):
(3.54)
Neglecting the transmission losses, the total losses from a swimming pool with
a surface area of
A
W
=20m
2
, a wind speed
v
Wind
= 1 m/s, ambient air
temperature
ϑ
A
= 20°C, water temperature
ϑ
W
= 24°C and relative humidity
ϕ
= 0.7 become:
A large amount of energy would thus be needed to compensate for the losses.
However, solar radiation onto the pool surface produces gains that reduce the
losses significantly.
With solar irradiance
E
, water surface area
A
W
and absorptance
α
abs
, the
solar radiation gains
Q
•
sol
are estimated as:
(3.55)
The absorptance
α
abs
of pools with white tiles is about 0.8, with light blue tiles
0.9 and with dark blue tiles over 0.95. The absorptance also increases with the
depth of the water. A solar irradiance of
E
= 319 W/m
2
at an absorptance of
0.9 already compensates for the losses of the 20 m
2
swimming pool from the
example above.
