1. Lecture Objectives Learn about Psychrometric Chart Quantities
How we use it for building systems design and analyses

2. Psychrometric Chart

3. Thermodynamics of Moist air or Psychrometrics
Variables
Temperature
Relative Humidity
Absolute Humidity
Enthalpy (total energy)
Dew Point Temperature
Wet Bulb Temperature ….

4. Temperatures Absolute Temperature (T) (K, R)
Dry-bulb temperature (t) [°F, °C]
Wet-bulb temperature (t*)
Dew-point temperature (td)

6. Which temperature do you expect to be higher?
Wet-bulb
Dry-bulb

7. Wet-bulb temperature (t*)
Temperature measured by a psychrometer
Lower than dry-bulb temperature
Evaporating moisture removes heat from thermometer bulb
The higher the humidity
Smaller difference between wet-bulb and dry-bulb temperature

9. Dew-Point Temperature, td
Define temperature at which condensation happen
td is defined as temperature of that air at saturation
i.e. RH = 100%
Surfaces below the dew point temperature will have condensation
Measured with a chilled-mirror apparatus

11. Absolute Humidity or Humidity Ratio
Humidity ratio (W) [lb/lb, g/kg, grains]
[grains/lb = 1/7000 lb/lb]

12. Humidity Ratio, W Mass of water vapor/divided by mass of dry air
Orthogonal to temperature
Not a function of temperature
Most convenient form for calculations involving airflow
Very hard to measure directly

14. Relative Humidity, RH or 
Ratio of partial pressure of water vapor to partial pressure of water vapor at same T and P at saturation
Strong function of temperature
For constant humidity ratio
Higher temperature, lower relative humidity
Saturation

16. Enthalpy or Total Energy
Enthalpy h or i [J/kg] or [Btu/lb]
Defines amount of energy contend in moist air

17. Enthalpy

18. Psychrometric Chart

19. ASHRAE Comfort Zone

20. New ASHRAE Comfort Zone

21. Sensible vs. Latent Heat

22. Process in HVAC systems
Heating
Cooling
Humidification
Dehumidification
All these processes ca be quantified in Psychrometric Chart