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Evaporative cooling pronlem solution manual
Course: fliud mechanics (2222222)
16 Documents
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University: جامعة كفر الشيخ
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EXAMPLE 14–7 Evaporative Cooling of Air by a Swamp Cooler
Air enters an evaporative (or swamp) cooler at 14.7 psi, 95°F, and 20 percent
relative humidity, and it exits at 80 percent relative humidity. Determine
(a) the exit temperature of the air and (b) the lowest temperature to which
the air can be cooled by this evaporative cooler.
Solution Air is cooled steadily by an evaporative cooler. The temperature
of discharged air and the lowest temperature to which the air can be cooled
are to be determined.
Analysis The schematic of the evaporative cooler and the psychrometric
chart of the process are shown in Fig. 14–28.
(a) If we assume the liquid water is supplied at a temperature not much dif-
ferent from the exit temperature of the airstream, the evaporative cooling
process follows a line of constant wet-bulb temperature on the psychrometric
chart. That is,
The wet-bulb temperature at 95°F and 20 percent relative humidity is deter-
mined from the psychrometric chart to be 66.0°F. The intersection point of
the Twb ⫽66.0°F and the f⫽80 percent lines is the exit state of the air.
The temperature at this point is the exit temperature of the air, and it is
determined from the psychrometric chart to be
(b) In the limiting case, air leaves the evaporative cooler saturated (f⫽100
percent), and the exit state of the air in this case is the state where the Twb
⫽66.0°F line intersects the saturation line. For saturated air, the dry- and
the wet-bulb temperatures are identical. Therefore, the lowest temperature to
which air can be cooled is the wet-bulb temperature, which is
Discussion Note that the temperature of air drops by as much as 30°F in
this case by evaporative cooling.
Tmin ⫽T2¿⫽66.0°F
T2⫽70.4°F
Twb ⬵constant
2' 1
1
2
2'
AIR
Tmin T295°F
2
T1 = 95°F
f = 20%
P = 14.7 psia
f
2
= 80%
f1 = 20%
FIGURE 14–28
Schematic and psychrometric chart for
Example 14–7.
