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Modern Engineering Thermodynamics

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12.6 The Sling Psychrometer 421<br />

Then, Eq. (12.30) becomes<br />

Adiabatic saturator inlet humidity ratio<br />

ω 1 = c paðT 2 − T 1 Þ + ω 3 ðh fg2 Þ<br />

h g1 − h f 2<br />

(12.31)<br />

Thus, by simply measuring the inlet temperature T 1 and the outlet adiabatic saturation temperature T 2 = T 3 ,we<br />

can calculate the inlet humidity ratio of the air–water vapor mixture, ω 1 ,directlyfromEq.(12.31).However,<br />

an adiabatic saturator must be extremely long to obtain 100% relative humidity at the outlet. This difficulty is<br />

overcome by the sling psychrometer discussed next.<br />

12.6 THE SLING PSYCHROMETER<br />

Figure 12.3 illustrates a simple device for determining air humidity, called a sling psychrometer. Itcontainstwo<br />

thermometers, one of which is covered with a wick saturated with ambient temperature liquid water. These two<br />

thermometers are called dry bulb and wet bulb. When the sling psychrometer is spun rapidly in the air, the evaporation<br />

of the water from the wick causes the wet bulb thermometer to read lower than the dry bulb thermometer.<br />

After the psychrometer has been spun long enough for the thermometers to reach equilibrium<br />

temperatures, the unit is stopped and the two thermometers are quickly read. A psychrometric chart (or table) is<br />

then used to convert the dry bulb temperature T DB and the wet bulb temperature T WB into humidity information.<br />

The wet bulb temperature is approximately equal to the adiabatic saturation temperature, so T WB ≈ T 2 = T 3<br />

in Eq. (12.31).<br />

Figure 12.4 illustrates the major characteristics of a psychrometric chart. Larger charts of professional engineering<br />

quality can be found in Charts D.5 and D.6 of Thermodynamic Tables to accompany <strong>Modern</strong> <strong>Engineering</strong><br />

Wet bulb thermometer<br />

0 10 20<br />

30 40 50 60 70 80 90 100<br />

0 10 20<br />

30 40 50 60 70 80 90 100<br />

Water soaked<br />

(wet) wick<br />

Dry bulb<br />

thermometer<br />

Swivel<br />

Handle<br />

FIGURE 12.3<br />

A sling psychrometer.<br />

Water vapor partial pressure p w<br />

p w<br />

Saturation curve<br />

T WB = T DP<br />

T WB<br />

φ = 100%<br />

State<br />

point<br />

φ = constant<br />

ω<br />

T WB = constant<br />

Humidity ratio ω<br />

Dry bulb temperature<br />

T DB<br />

FIGURE 12.4<br />

The elements of a psychrometric chart. The intersection of the dry bulb and wet bulb constant temperature lines determine the state of<br />

the water vapor in the system, from which T DP , p w , ϕ, and ω can then be found.

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