Dry bulb and Wet bulb temperature thermodynamics

Why does air not cool in a cooling tower? The answer is air receives potential energy in the form of latent heat and potential energy does not contribute to temperature. Latent heat is a form of potential energy which is held by the intermolecular bonds.

The?post?aims?to?delve?deeply?into?these?two?fundamental?temperatures?in?HVAC?and?seek?an?explanation?from?thermodynamics.

Definition

DBT

DBT is?the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature. It is directly proportional to the mean kinetic energy of the air molecules.?

WBT

Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin.?The adiabatic evaporation of water from the thermometer and the cooling effect is indicated by a "wet bulb temperature"

Thermodynamics

The dry bulb temperature is a measure of the actual temperature of the air, including the effects of air movement and humidity. It represents the kinetic energy of the air molecules, as it measures the speed at which they are moving.

The wet bulb temperature, on the other hand, is a measure of the lowest temperature that can be achieved by the evaporation of water into the air. This represents the potential energy of the air, as it measures the energy required to convert liquid water into water vapor by breaking intermolecular bonds..

DBT - WBT

The difference between the dry bulb and wet bulb temperatures, known as the wet bulb depression, reflects the potential for energy conversion within the air. When air expands, it can precipitate its moisture into water, releasing the stored potential energy as kinetic energy. This energy can then be used to do work, such as driving a wind turbine or powering a weather system.

In summary, the dry bulb temperature accounts for the kinetic energy of the air, the wet bulb temperature accounts for the potential energy of the air, and the difference between the two represents the conversion potential of energy within the air. The wet bulb temperature is a measure of the maximum cooling potential of saturated air through the process of evaporative cooling. When air comes into contact with a moist surface or water, the wet bulb temperature represents how low the air temperature can reach by evaporating water into it. This concept is important in areas such as HVAC and refrigeration, where understanding the wet bulb temperature is crucial for determining the efficiency of cooling systems.

In the process of evaporative cooling, potential energy in the form of heat is converted to kinetic energy as water changes from a liquid to a gas during the phase change. Then, when the water vapor condenses back into liquid form, the kinetic energy of air molecules is converted back to potential energy in the form of heat. This continuous conversion of energy helps in the cooling process.

Free energy and Entropy generation

Evaporative cooling in a cooling tower is a non-equilibrium process with negative free energy, and the expansion of air outside the cooling tower as a plume is a main source of entropy generation. The entropy generation in the form of a plume has a crucial role in keeping Gibbs's free energy dG = dH - TdS, negative and continuous cooling process.