Environmental Engineering Reference
In-Depth Information
The August-Roche-Magnus equation for the calculation
of dew point can be written as
The Bosen (1958) equation can be rearranged such that
the relative humidity is computed when the dew-point tem-
perature and the dry bulb temperature are known:
b
aT/ (b
ln
h
r
100
+
T )
+
t
d
=
aT/ (b
ln
h
r
100
(2.30)
100
t
d
−
8
0
.
1
T
+
112
a
−
+
T )
+
h
r
=
(2.33)
112
+
0
.
9
T
where:
where:
a
=
17.271 and
237.7
◦
C.
b
=
dry bulb temperature,
◦
C,
T
=
Rearranging Eq. 2.30 and solving for relative humidity
h
r
give
dew-point temperature,
◦
C, and
t
d
=
100 exp
ab
t
d
−
T
h
r
=
relative humidity, %.
T )
t
d
+
b
h
r
=
(2.31)
(b
+
Table 2.8 contains calculated relative humidity values for
typical values for dry bulb temperature and dew-point temper-
ature. The results show that the proposed simplified equation
provides reasonable estimates of the relative humidity as long
as the computed values are higher than 50%. The August-
Roche-Magnus equation and the Bosen equation result in sim-
ilar values for the computed relative humidity throughout
the entire range of temperatures. Either the August-Roche-
Magnus equation or the Bosen equation can be used to compute
relative humidity from dew-point temperature measurements.
where:
h
r
=
relative humidity, %,
dry bulb temperature,
◦
C,
T
=
dew-point temperature,
◦
C
,
and
t
d
=
=
a
,
b
constants with values equal to those shown above.
The Bosen (1958) equation was also developed for the
approximation of dew-point temperature when relative
humidity and dry bulb temperature are known. The Bosen
(1958) equation for the calculation of dew point is as follows:
2.3.3.5 Thermal Properties of Air
The specific heat of air,
C
a
, is a function of temperature
and typical values are presented in Table 2.9. The thermal
conductivity of air,
λ
a
,
is also a function of temperature and
typical values for
λ
a
are presented in Table 2.9.
0
.
9
T ) h
0
.
125
r
t
d
=
(a
+
−
a
+
0
.
1
T
(2.32)
where:
112 when temperature is measured,
◦
C.
=
a
Table 2.8 Comparisons of Computed Relative Humidity Values
Relative Humidity,
h
r
Dry Bulb
Dew-Point
Simplified
August-Roche-
Bosen
Temperature (
◦
C)
Temperature (
◦
C)
Equation
Magnus Equation
Equation
25.0
24.5
97.5
97.06
97.06
25.0
24.0
95.0
94.20
94.20
25.0
22.0
85.0
83.48
83.49
25.0
20.0
75.0
73.84
73.85
25.0
18.0
65.0
65.19
65.21
25.0
16.0
55.0
57.44
57.46
25.0
12.0
35.0
44.33
44.34
25.0
10.0
25.0
38.82
38.83
25.0
6.0
5.0
29.57
29.57
20.0
19.0
95.0
93.98
94.01
20.0
16.0
80.0
77.79
77.88
20.0
12.0
60.0
60.03
60.16
20.0
10.0
50.0
52.56
52.71
20.0
6.0
30.0
40.04
40.19
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