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Lecture Objectives

Learn about the psychrometric chart and its variables such as temperature, relative humidity, absolute humidity, enthalpy, dew point temperature, and wet bulb temperature. Discover how to use the chart for building systems design and analysis.

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Lecture Objectives

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  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 orPsychrometrics 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)

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

  6. 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

  7. 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

  8. Absolute Humidityor Humidity Ratio • Humidity ratio (W) [lb/lb, g/kg, grains] [grains/lb = 1/7000 lb/lb]

  9. 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

  10. Relative Humidity, RH or  • Relative humidity (RH, ) [%] • 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

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

  12. Enthalpy

  13. Psychrometric Chart

  14. ASHRAE Comfort Zone

  15. New ASHRAE Comfort Zone

  16. Sensible vs. Latent Heat latent sensible

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

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