J1 PROPsychrometrics basics for HVAC engineers
The psychrometric properties every HVAC engineer uses, what they mean physically, and the standard formulas for sensible, latent and total air-side loads.
Psychrometrics is the study of moist air. Every cooling coil, fresh-air unit and humidity problem comes down to a few properties that you can read from a chart or calculate.
The properties
| Property | Meaning |
|---|---|
| Dry bulb (DB) | Ordinary air temperature. |
| Wet bulb (WB) | Temperature of a wetted thermometer in moving air. Lower than DB unless the air is saturated. |
| Dew point (DP) | Temperature at which moisture starts to condense. Any surface colder than this sweats. |
| Relative humidity (RH) | How close the air is to saturation at its current temperature, in %. |
| Humidity ratio (W) | Mass of water per mass of dry air, g/kg. Doesn't change with heating or cooling unless moisture is added or removed. |
| Enthalpy (h) | Total heat content of the air, kJ/kg dry air, sensible plus latent. |
Any two independent properties (for example DB and RH, or DB and WB) fix the state of the air. The rest can then be calculated.
Air-side load formulas (SI, standard air)
These are the ASHRAE standard-air factors. At high altitude or very high temperatures, correct them for air density.
Why it matters in practice
- Condensation: ductwork or diffusers colder than the surrounding dew point will drip. Insulate them, or keep the supply temperature above that dew point.
- Fresh-air load: hot, humid outdoor air has a far higher enthalpy than room air. The total formula shows why fresh air can be the largest part of the coil load.
- Coil selection: the coil's leaving condition must be cold enough in dew point, not just in dry bulb, to remove the latent load.
The J1 Pro Psychrometrics calculator gives every property from any two inputs and plots the point on a chart.
Guides are for preliminary design and learning. Always verify final designs against project specifications, local codes and manufacturer data.