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

on this principle is called a sling psychrometerand is shown in Fig. 14–13.
Usually a dry-bulb thermometer is also mounted on the frame of this device
so that both the wet- and dry-bulb temperatures can be read simultaneously.
Advances in electronics made it possible to measure humidity directly in a
fast and reliable way. It appears that sling psychrometers and wet-wicked ther-
mometers are about to become things of the past. Today, hand-held electronic
humidity measurement devices based on the capacitance change in a thin poly-
mer film as it absorbs water vapor are capable of sensing and digitally display-
ing the relative humidity within 1 percent accuracy in a matter of seconds.
In general, the adiabatic saturation temperature and the wet-bulb tempera-
ture are not the same. However, for air–water vapor mixtures at atmospheric
pressure, the wet-bulb temperature happens to be approximately equal to the
adiabatic saturation temperature. Therefore, the wet-bulb temperature Twb
can be used in Eq. 14–14 in place of T 2 to determine the specific humidity
of air.

Chapter 14 | 725

Dry-bulb

thermometer

Wet-bulb

thermometer

wick

Wet-bulb

thermometer

FIGURE 14–13

Sling psychrometer.

EXAMPLE 14–3 The Specific and Relative Humidity of Air

The dry- and the wet-bulb temperatures of atmospheric air at 1 atm (101.325

kPa) pressure are measured with a sling psychrometer and determined to be

25 and 15°C, respectively. Determine (a) the specific humidity, (b) the rela-

tive humidity, and (c) the enthalpy of the air.

Solution Dry- and wet-bulb temperatures are given. The specific humidity,

relative humidity, and enthalpy are to be determined.

Properties The saturation pressure of water is 1.7057 kPa at 15°C, and

3.1698 kPa at 25°C (Table A–4). The constant-pressure specific heat of air

at room temperature is cp1.005 kJ/kg · K (Table A–2a).

Analysis (a) The specific humidity v 1 is determined from Eq. 14–14,

where T 2 is the wet-bulb temperature and v 2 is

Thus,

(b) The relative humidity f 1 is determined from Eq. 14–11ato be

f 1 

v 1 P 2

1 0.622v 12 Pg 1



1 0.00653 21 101.325 kPa 2

1 0.6220.00653 21 3.1698 kPa 2

0.332 or 33.2%

0.00653 kg H 2 O/kg dry air

v 1 

1 1.005 kJ>kg#°C 23115 
 252 °C 4  1 0.01065 21 2465.4 kJ>kg 2

1 2546.5
62.982 2 kJ>kg

0.01065 kg H 2 O>kg dry air

v 2 

0.622Pg 2

P 2 
Pg 2



1 0.622 21 1.7057 kPa 2

1 101.325
1.7057 2 kPa

v 1 

cp 1 T 2 
T 12 v 2 hfg 2

hg 1 
hf 2