05.04.2016 Views

Modern Engineering Thermodynamics

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

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.

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!