Chapter 14 | 737

Analysis We take the mixing sectionof the streams as the system. The

schematic of the system and the psychrometric chart of the process are

shown in Fig. 14–30. We note that this is a steady-flow mixing process.

The mass flow rates of dry air in each stream are

From the mass balance of dry air,

The specific humidity and the enthalpy of the mixture can be determined

from Eq. 14–24,

which yield

These two properties fix the state of the mixture. Other properties of the mix-

ture are determined from the psychrometric chart:

Finally, the volume flow rate of the mixture is determined from

Discussion Notice that the volume flow rate of the mixture is approximately

equal to the sum of the volume flow rates of the two incoming streams. This

is typical in air-conditioning applications.

Wet Cooling Towers

Power plants, large air-conditioning systems, and some industries generate
large quantities of waste heat that is often rejected to cooling water from
nearby lakes or rivers. In some cases, however, the cooling water supply is
limited or thermal pollution is a serious concern. In such cases, the waste
heat must be rejected to the atmosphere, with cooling water recirculating
and serving as a transport medium for heat transfer between the source and
the sink (the atmosphere). One way of achieving this is through the use of
wet cooling towers.
A wet cooling toweris essentially a semienclosed evaporative cooler. An
induced-draft counterflow wet cooling tower is shown schematically in

V

#

3 m

#

a 3 v 3 ^1 83 kg>min^21 0.844 m

(^3) >kg 2 70.1 m 3
/min
v 3 0.844 m^3 >kg dry air
f 3 89%
T 3 19.0°C
h 3 50.1 kJ>kg dry air
v 3 0.0122 kg H 2 O/kg dry air
60.5
22.5

0.0182
v 3
v 3
0.010

79.0
h 3
h 3
39.4
m#a 1
m

a 2

v 2
v 3
v 3
v 1

h 2
h 3
h 3
h 1
m

a 3 m

a 1 m

a 2 ^1 60.522.5^2 kg>min83 kg>min
m

a 2 
V

2
v 2

20 m^3 >min
0.889 m^3 >kg dry air
22.5 kg>min
m#a 1 
V

1
v 1

50 m^3 >min
0.826 m^3 >kg dry air
60.5 kg>min
·
·
T 2 = 32°C
f 2 = 60%
V 2 = 20 m^3 /min
Saturated air
T 1 = 14°C
V 1 = 50 m^3 /min
2
1
Mixing^3
section
P = 1 atm
V 3
v 3
f 3
T 3
3 2
1
14 °C32°C
f^1
= 100% f
2 = 60%
·
FIGURE 14–30
Schematic and psychrometric chart for
Example 14–8.