Water-Air VLE and Psychrometric Chart: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • Dry bulb temperature: air temperature.
  • Wet bulb temperature: temperature of a thermometer that is wrapped in a wet cloth and cooled by flowing air. The temperature drops due to evaporative cooling. It is approximately the temperature where saturated air has the same enthalpy as the air with relative humidity less than one. At 100% relative humidity, it is equal to the dry bulb temperature.
  • Dew point temperature: temperature reached if cool air (without changing its moisture content) until reach saturation temperature. That is, this is the temperature where water starts to condense.
  • 100% relative humidity: water partial pressure water equals its saturation pressure.
  • At 1-bar pressure and 25°C, the mole fraction of air dissolved in water is approximately 10-5, so dissolved air has essentially no effect on water vapor-liquid equilibrium at these conditions.
  • When water and air are in equilibrium, \(P_{H_2O} \leq P_{H_2O}^{sat}\); when \(P_{H_2O} = P_{H_2O}^{sat}\) , liquid water forms.
  • The examples in this module are for air/water equilibrium, but the same concepts apply for other liquids in equilibrium with a non-condensable gas phase.
  • Absolute humidity is the mass of water vapor divided by the mass of dry air.
From studying this module, you should now be able to:
  • Use a psychometric chart to determine dry bulb temperature, wet bulb temperature, dew point temperature, relative humidity, and enthalpy.
  • Predict behavior of two-phase systems with two components where only one component is in the liquid phase (air/water systems).
  • Apply energy balances to vapor liquid equilibrium for air/water systems.
  • Apply mass balances two air/water systems equilibrium.
  • Use steam tables to determine equilibrium conditions for air/water systems.


Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder