Psychrometric Chart Theory
Introduction
Psychrometrics is the study of air-water vapor mixtures. The psychrometric chart graphically represents the thermodynamic properties of moist air, essential for HVAC design, drying processes, and comfort analysis. Any two independent properties can determine all others.
Temperature Properties
Dry Bulb Temperature (Tdb)
Air temperature measured by a standard thermometer. The "actual" temperature we feel. Primary x-axis of psychrometric chart.
Wet Bulb Temperature (Twb)
Temperature measured with a wetted wick. Lower than dry bulb due to evaporative cooling. At saturation: Twb = Tdb.
Dew Point Temperature (Tdp)
Temperature at which moisture begins to condense. Indicates absolute moisture content. Always ≤ Tdb. At saturation: Tdp = Tdb.
Humidity Properties
Relative Humidity (RH or φ)
Ratio of actual vapor pressure to saturation vapor pressure:
φ = Pv / Psat × 100%
Expressed as percentage. 100% = saturated air.
Humidity Ratio (W or ω)
Mass of water vapor per mass of dry air:
W = mv / ma = 0.622 × Pv / (P - Pv)
Units: kg water / kg dry air (or g/kg)
Degree of Saturation (μ)
Ratio of humidity ratio to saturation humidity ratio:
μ = W / Wsat
Similar to RH but based on mass ratio, not pressure.
Energy Properties
Specific Enthalpy (h)
Energy content per kg of dry air:
h = cpaT + W(hg + cpvT)
h ≈ 1.006T + W(2501 + 1.86T) kJ/kg
Used for energy balance in air conditioning.
Specific Volume (v)
Volume of moist air per kg of dry air:
v = RaT / (P - Pv)
Typically 0.8-0.9 m³/kg at standard conditions.
ASHRAE Comfort Zone
ASHRAE Standard 55 defines thermal comfort conditions for human occupancy:
Recommended Conditions
- Temperature: 20-26°C (68-79°F)
- Relative Humidity: 30-60%
- Air velocity: < 0.2 m/s for sedentary activity
Outside these ranges, occupants may feel too hot, cold, humid, or dry.
HVAC Processes
References
- ASHRAE Handbook - Fundamentals
- ASHRAE Standard 55 - Thermal Environmental Conditions
- Çengel, Y.A. "Heat and Mass Transfer"