5.35: Untitled Page 97
- Page ID
- 18230
Chapter 5
In order to determine the absolute humidity, we use Eq. 5‐35 in the more precise form given by Eq. 5‐36
mass of water
mass of water/volume
humidity =
(3)
mass of dry air
mass of dry air/volume
and this leads to an expression for the humidity given by
MW
y
MW
y
H2O
H O
H O
H O
H O
2
2
2
2
humidity
MW
y
MW (1 y
)
air
air
air
air
H O
2
(4)
18.05 g water/mol 0.095
0 066
.
g water / g dry air
28.85 g dry air/mol (1 0.095)
Once again we have used the subscript “air” as a convenient substitute for dry air and we will continue to make use of this simplification throughout our study of humidification processes. Assuming that water vapor and air behave as ideal gases at atmospheric pressure, we use the ideal gas law given by Eq. 5‐3 to compute the total concentration of the mixture. The concentration of the gas mixture is the total number of moles of air and water per unit volume of the mixture. This can be expressed as n
p
101 , 300 Pa
3
c
35 mol/m
(5)
3
V
RT
m Pa
8 314
.
348 16
.
K
mol K
This result gives the total number of moles of gas per unit volume of mixture. In order to determine the molar flow rates of water and dry air, we carry out the following calculations to obtain
M
c
Q y
c Q
H O
H O
H O
2
2
2
(6a)
3
3
0 . 095 35 mol/m 100 m /min 332.5 mol water/min M
c Q y c Q
air
air
air
(6b)
(1 y
) c Q 3 , 167 . 5 mol dry air /min
H2O
Two‐Phase Systems & Equilibrium Stages 175
Table 5‐2. Vapor Pressure of Water as a Function of Temperature T, C
Vapor Pressure, mm Hg
T, F
Vapor Pressure, in Hg
0
4.579
32
0.180
5
6.543
40
0.248
10
9.209
50
0.363
15
12.788
60
0.522
20
17.535
70
0.739
25
23.756
80
1.032
30
31.824
90
1.422
35
42.175
100
1.932
40
55.324
110
2.596
45
71.88
120
3.446
50
92.51
130
4.525
55
118.04
140
5.881
60
149.38
150
7.569
65
187.54
160
9.652
70
233.7
170
12.199
75
289.1
180
15.291
80
355.1
190
19.014
85
433.6
200
23.467
90
525.76
212
29.922
95
633.90
220
34.992
100
760.00
230
42.308
105
906.07
240
50.837
110
1,074.56
250
60.725
115
1,267.98
260
72.134
120
1,489.14
270
85.225
125
1,740.93
280
100.18
130
2,026.16
290
117.19
135
2,347.26
300
136.44
5.4.2 Modified mole fraction
In general, the most useful measures of concentration are the molar concentration c and the species mass density . Associated with these A
A
concentrations are the mole fraction defined by Eq. 5‐1f and the mass fraction defined by Eq. 5‐1c. Sometimes it is convenient to use a modified mole fraction or mole ratio which is based on all the species except one. If we identify that one species as species N, we express the modified mole fraction as