Introduction to Psychrometrics, Part 2 of 3

Oct 5, 2010

In my first installment on psychrometrics, I covered the basics of air, humidity and evaporation. Here I’ll cover temperature, kinetic energy, attractive forces and condensation. If you haven’t already, I strongly suggest you read Part 1 first.

We’re all familiar with the general concept of temperature. Temperature ends up being quite complex if you dig deeper (and deeper we shall dig). Technically, temperature is related to the average energy of motion, known as kinetic energy. To use our illustration from the last blog post, the faster the billiard balls move on the table, the greater the temperature.

When there is enough kinetic energy in water, molecules can escape the attractive forces of a liquid surface. This is evaporation. Attractive forces include intermolecular forces known as “van der Waals interaction” and other concepts that are even too nerdy for the Indoor Air Nerd. Once evaporated, the water molecules bounce around in the air with all the gases. But those attractive forces never go away, always wanting to pull those water molecules back in. Think of the billiard table being slightly tilted toward one pocket.

When the temperature of the air cools down, the speed of the water molecules slows down (remember kinetic energy). The slower a ball is moving on a tilted billiard table, the more likely it will be pulled toward the pocket. When a water molecule doesn’t have enough kinetic energy to overcome the attractive forces, it will be pulled back into the liquid or other nearby molecules in a process called… you guessed it: condensation.

When a molecule condenses, it brings its kinetic energy along and thus warms the liquid. When evaporation occurs, the molecules escape the liquid and therefore have a cooling effect.

Dew point is the term used for the temperature below which condensation will occur. Or using our Indoor Air Nerd terminology, dew point is the amount of kinetic energy which is low enough to be overcome by the attractive forces. The air in my office right now is 72° F, with a dewpoint of 46°. If the air in my office cooled down to below 46° (say, around a soda can), there wouldn’t be enough kinetic energy to keep the water molecules in the air. They would be attracted to each other and condense on the surface.

Eventually, I’ll be pulling all of these concepts together and providing some practical applications for assessing the indoor environment.

In my third installment, I’ll introduce relative humidity (RH). It is one of the most misunderstood concepts in psychrometrics.