Psychrometrics Basics

Psychrometrics is the study of air and the water vapor mixed into it. Because air conditioning changes both temperature and moisture, you need more than a dry bulb thermometer to describe what the equipment is doing.

Total capacity depends on the heat content of the air, called enthalpy, not on dry bulb alone. Two coils with the same temperature split can be doing very different amounts of work if the entering wet bulb is different. Charging charts for fixed orifice systems are built around indoor wet bulb for exactly this reason.

• Warmer air can hold more moisture, so relative humidity falls as air is heated even though the moisture content has not changed.
• Condensation forms on any surface below the dew point, which is why uninsulated ducts sweat in humid basements.
• Comfort in summer usually means keeping indoor relative humidity in the range of about 40 to 55 percent.

On a psychrometric chart, dry bulb runs along the bottom, moisture content up the right side, and the curved left boundary is saturation, where dry bulb, wet bulb and dew point all meet. Plot entering and leaving air conditions and the line between them shows whether the coil is removing sensible heat only or sensible and latent together.

• Psychrometer or a hygrometer that reports wet bulb
• Digital thermometer
• Psychrometric chart or an app that does the plotting