Modern Engineering Thermodynamics

12.6 The Sling Psychrometer 421
Then, Eq. (12.30) becomes
Adiabatic saturator inlet humidity ratio
ω 1 = c paðT 2 − T 1 Þ + ω 3 ðh fg2 Þ
h g1 − h f 2
(12.31)
Thus, by simply measuring the inlet temperature T 1 and the outlet adiabatic saturation temperature T 2 = T 3 ,we
can calculate the inlet humidity ratio of the air–water vapor mixture, ω 1 ,directlyfromEq.(12.31).However,
an adiabatic saturator must be extremely long to obtain 100% relative humidity at the outlet. This difficulty is
overcome by the sling psychrometer discussed next.
12.6 THE SLING PSYCHROMETER
Figure 12.3 illustrates a simple device for determining air humidity, called a sling psychrometer. Itcontainstwo
thermometers, one of which is covered with a wick saturated with ambient temperature liquid water. These two
thermometers are called dry bulb and wet bulb. When the sling psychrometer is spun rapidly in the air, the evaporation
of the water from the wick causes the wet bulb thermometer to read lower than the dry bulb thermometer.
After the psychrometer has been spun long enough for the thermometers to reach equilibrium
temperatures, the unit is stopped and the two thermometers are quickly read. A psychrometric chart (or table) is
then used to convert the dry bulb temperature T DB and the wet bulb temperature T WB into humidity information.
The wet bulb temperature is approximately equal to the adiabatic saturation temperature, so T WB ≈ T 2 = T 3
in Eq. (12.31).
Figure 12.4 illustrates the major characteristics of a psychrometric chart. Larger charts of professional engineering
quality can be found in Charts D.5 and D.6 of Thermodynamic Tables to accompany Modern Engineering
Wet bulb thermometer
0 10 20
30 40 50 60 70 80 90 100
0 10 20
30 40 50 60 70 80 90 100
Water soaked
(wet) wick
Dry bulb
thermometer
Swivel
Handle
FIGURE 12.3
A sling psychrometer.
Water vapor partial pressure p w
p w
Saturation curve
T WB = T DP
T WB
φ = 100%
State
point
φ = constant
ω
T WB = constant
Humidity ratio ω
Dry bulb temperature
T DB
FIGURE 12.4
The elements of a psychrometric chart. The intersection of the dry bulb and wet bulb constant temperature lines determine the state of
the water vapor in the system, from which T DP , p w , ϕ, and ω can then be found.