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

Psychrometric Chart

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

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

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

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

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

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

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

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

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

Enthalpy

Psychrometric Chart

ASHRAE Comfort Zone

New ASHRAE Comfort Zone

Sensible vs. Latent Heat

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