14-62
14-105 Two airstreams are mixed steadily. The specific humidity, the relative humidity, the dry-bulb temperature, and the
volume flow rate of the mixture are to be determined.
Assumptions 1 Steady operating conditions exist 2 Dry air and water vapor are ideal gases. 3 The kinetic and potential
energy changes are negligible. 4 The mixing section is adiabatic.
Properties Properties of each inlet stream are determined from the psychrometric chart (Fig. A-31) to be
h1  62.7 kJ/kg dry air
1  0.0119 kg H 2 O/kg dry air
v 1  0.882 m 3 /kg dry air
1
and
32C
40%
20 m3/min
h2  31.9 kJ/kg dry air
P = 1 atm
AIR
 2  0.0079 kg H 2 O/kg dry air
v 2  0.819 m 3 /kg dry air
Analysis The mass flow rate of dry air in each stream
is
m a1 
V1
20 m 3 / min

 22.7 kg/min
v 1 0.882 m 3 / kg dry air
m a 2 
V2
25 m 3 / min

 30.5 kg/min
v 2 0.819 m 3 / kg dry air
2
3
3
3
T3
25 m3/min
12C
90%
From the conservation of mass,
 a3  m
 a1  m
 a 2  ( 22.7  30.5) kg / min  53.2 kg / min
m
The specific humidity and the enthalpy of the mixture can be determined from Eqs. 14-24, which are obtained by
combining the conservation of mass and energy equations for the adiabatic mixing of two streams:
 a1  2   3 h2  h3
m


 a 2  3   1 h3  h1
m
22.7 0.0079   3 319
.  h3


30.5  3  0.0119 h3  62.7
which yields,
 3  0.0096 kg H 2O / kg dry air
h3  45.0 kJ / kg dry air
These two properties fix the state of the mixture. Other properties of the mixture are determined from the psychrometric
chart:
T3  20.6C
3  63.4%
v 3  0.845 m 3 /kg dry air
Finally, the volume flow rate of the mixture is determined from
V3  m a 3v 3  (53.2 kg/min)(0.845 m 3 / kg)  45.0 m 3 /min
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