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

Then,

Therefore, this air-conditioning unit removes moisture and heat from the air

at rates of 0.131 kg/min and 511 kJ/min, respectively.

Evaporative Cooling

Conventional cooling systems operate on a refrigeration cycle, and they can

be used in any part of the world. But they have a high initial and operating

cost. In desert (hot and dry) climates, we can avoid the high cost of cooling

by using evaporative coolers, also known as swamp coolers.

Evaporative cooling is based on a simple principle: As water evaporates,

the latent heat of vaporization is absorbed from the water body and the sur-

rounding air. As a result, both the water and the air are cooled during the

process. This approach has been used for thousands of years to cool water.

A porous jug or pitcher filled with water is left in an open, shaded area.

A small amount of water leaks out through the porous holes, and the pitcher

“sweats.” In a dry environment, this water evaporates and cools the remain-

ing water in the pitcher (Fig. 14–26).

You have probably noticed that on a hot, dry day the air feels a lot cooler

when the yard is watered. This is because water absorbs heat from the air as

it evaporates. An evaporative cooler works on the same principle. The evap-

orative cooling process is shown schematically and on a psychrometric chart

in Fig. 14–27. Hot, dry air at state 1 enters the evaporative cooler, where it

is sprayed with liquid water. Part of the water evaporates during this process

by absorbing heat from the airstream. As a result, the temperature of the

airstream decreases and its humidity increases (state 2). In the limiting case,

the air leaves the evaporative cooler saturated at state 2. This is the lowest

temperature that can be achieved by this process.

The evaporative cooling process is essentially identical to the adiabatic satu-

ration process since the heat transfer between the airstream and the surround-

ings is usually negligible. Therefore, the evaporative cooling process follows a

line of constant wet-bulb temperature on the psychrometric chart. (Note that

this will not exactly be the case if the liquid water is supplied at a temperature

different from the exit temperature of the airstream.) Since the constant-wet-

bulb-temperature lines almost coincide with the constant-enthalpy lines, the

enthalpy of the airstream can also be assumed to remain constant. That is,

(14–19)

and

(14–20)

during an evaporative cooling process. This is a reasonably accurate approx-

imation, and it is commonly used in air-conditioning calculations.

h
constant

Twb
constant

511 kJ/min

Q

#

out^1 11.25 kg>min^231 85.4
39.3^2 kJ>kg^4 
^1 0.131 kg>min^21 58.8 kJ>kg^2

m

#

w^1 11.25 kg>min^21 0.0216
0.0100^2 0.131 kg/min

m

#

a

V

#

1

v 1



10 m^3 >min

0.889 m^3 >kg dry air

11.25 kg>min

734 | Thermodynamics

Water that

leaks out

Hot, dry

air

FIGURE 14–26

Water in a porous jug left in an open,
breezy area cools as a result of
evaporative cooling.

HOT,

DRY

AIR

2 1

1

2

2'

COOL,

MOIST

AIR

Liquid

water

Twb = const.

h = const.

~

~

FIGURE 14–27

Evaporative cooling.