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10 TRG-TRC001-EN
notes
period one
The Psychrometric Chart
Another fact about saturated air should be discussed before we proceed.
Assume a volume of moist air has the initial conditions indicated in column D
of the table. The air has a 90°F dry-bulb temperature and a 60°F dew point. A
wet-bulb thermometer shows the wet-bulb temperature to be 70°F. From a
moisture-content table, the relative humidity of the air is approximately 37%.
With no change in the moisture content of this volume of air, the table shows
the progressive change that occurs as the air cools.
Point C: As the dry-bulb temperature drops from 90°F to 75°F, the wet-bulb
temperature drops from 70°F to 65.2°F, yet the dew point remains the same at
60°F. The relative humidity rises from 37% to 60%.
Point B: When the dry-bulb temperature reaches 62°F, the wet-bulb
temperature is about 60.8°F, the dew point remains constant at 60°F, and the
relative humidity is 92%.
Point A: Finally, when the dry-bulb temperature reaches the 60°F dew-point
temperature, the wet bulb cannot be reduced any lower because evaporation
can no longer occur: the air is saturated and contains all the moisture it can
hold. The relative humidity is now 100%.
At any point on a 100% relative-humidity curve, the three air
temperatures—dry bulb, wet bulb and dew point—are equal.
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Figure 18