Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

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 Twb66.0°F and the f80 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.

Adiabatic Mixing of Airstreams

Many air-conditioning applications require the mixing of two airstreams.
This is particularly true for large buildings, most production and process
plants, and hospitals, which require that the conditioned air be mixed with a
certain fraction of fresh outside air before it is routed into the living space.
The mixing is accomplished by simply merging the two airstreams, as
shown in Fig. 14–29.
The heat transfer with the surroundings is usually small, and thus the mix-
ing processes can be assumed to be adiabatic. Mixing processes normally
involve no work interactions, and the changes in kinetic and potential ener-
gies, if any, are negligible. Then the mass and energy balances for the adia-
batic mixing of two airstreams reduce to

Mass of dry air: (14–21)

Mass of water vapor: (14–22)

Energy: m (14–23)

#

a 1 h 1 m

#

a 2 h 2 m

#

a 3 h 3

v 1 m#a 1 v 2 m#a 2 v 3 m#a 3

m

#

a 1 m

#

a 2 m

#

a 3

TminT 2 ¿66.0°F

T 2 70.4°F

Twb
constant

Chapter 14 | 735

2' 1

1

2

2'

AIR

Tmin T 2 95 °F

2

T 1 = 95°F

f = 20%

P = 14.7 psia

f^2

= 80%

f^1

= 20%

FIGURE 14–28

Schematic and psychrometric chart for

Example 14–7.