Why and how does Dew point condensation temperature < WBT: A glance at thermodynamics

Summary

In summary the free energy change dG = 0 at wet-bulb temperature. When wet air is cooled below WBT The specific volume of wet air decreases reducing the entropy term TdS in Gibbs equation dG = dH - TdS to make dG positive. The system restores equilibrium by moving to dG = 0 by losing enthalpy as water.

Explanation

The concept is, that the dry bulb, wet bulb, and dew point temperatures are the same when the air is fully saturated, or when the relative humidity is 100%. The dew point condensation temperature is when moisture in the atmosphere starts to condense, or the temperature at which the first droplet of moisture appears.

Behind all psychrometric changes of air three factors play a big role, (1) enthalpy (2) entropy, and (3) Gibbs free energy.

Like any system, the moist air around us always seeks maximum stability (equilibrium) minimum free energy, and maximum entropy change. This is happening always in nature. The interactions of these three factors cause every change you see in your surrounding air.

On either side of equilibrium, zero free energy, there is instability in the surroundings and the system is seeking to reach equilibrium. To do so, either wet air condenses out water as liquid or makes the water evaporate to vapor to revert to equilibrium. This is exactly what happens at dew point condensation and wet bulb evaporation.

Point by point

Wet bulb temperature

Wet bulb temperature is the lowest temperature achieved by adiabatic evaporation (= without adding heat or with equal enthalpy) of water in the air until the air is saturated (absolute humidity rises).

Why?

Heat is required to evaporate water. If this evaporation is adiabatic, the heat is extracted from the environment (e.g. the air), causing the air to cool down. This is the lowest temperature wet air can cool at certain temperature and RH. At WBT air is saturated with water. The free energy change, dG = 0.

How and why?

Air becomes saturated at the wet-bulb temperature (WBT). Air can absorb water until its heat storage capacity allows.

?At the point of saturation, the wet air has no more heat storage capacity to absorb more enthalpy.

Phase change

Beyond the saturation stage, there is a phase change of wet air at a constant WBT and constant pressure. The amount of water entering the air from evaporation equals the amount of water condensing back onto the surface. The system reaches an equilibrium. Gibbs free energy mimeses at WBT.

Dew point condensation temperature

At dew point any cooling below WBT of wet air means going away from equilibrium which is opposed by wet air by losing water (enthalpy) by condensation to approach the equilibrium. This is how and why dew point condensation occurs below the wet-bulb temperature.

In summary the free energy change dG = 0 at wet-bulb temperature. When wet air is cooled below WBT The specific volume of wet air decreases reducing entropy term TdS in Gibbs equation dG = dH - TdS to make dG positive. The system restores equilibrium by moving to dG = 0 by losing enthalpy as water