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Objectives

Objectives. Plot processes in AHU and buildings on Psychrometric chart Discuss equations for calculation of moist air properties. Thermodynamics properties of most air are also in Tabel A4 E (textbook). Condition Line for a Space. AHU. Sensible heating. Dehumidification by Cooling.

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Objectives

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  1. Objectives Plot processes in AHU and buildings on Psychrometric chart Discuss equations for calculation of moist air properties

  2. Thermodynamics properties of most air are also in Tabel A4 E (textbook)

  3. Condition Line for a Space

  4. AHU

  5. Sensible heating

  6. Dehumidification by Cooling

  7. Real Dehumidification Process

  8. Adiabatic mixing • Governing equation External heat

  9. Humidification hw Specific enthalpy of water added to system hg Specific enthalpy of saturated water vapor

  10. Evaporative Cooling (air washing)

  11. Alternate calculation for W • PV = mRT • What do we know about R ratio? • P = Pw + Pa • Then you can connect RH and W: R = gas constant P = pressure V = volume T = absolute temperature W = humidity ratio Subscripts: w is water vapor, a is dry air, and s indicates saturated

  12. Calculation of psychometric quantities • For an ideal gas, • hda = ∫cpadT, hw = ∫cpwdT • So, hda = cp,dat which assumes a reference state of 0 °F or 0 °C – Tables A4 • Note different reference • hw = cpwt + hg0 • h = cp,dat + W(cpwt + hg0) Or you can use: • h = cpt + W∙hg0, cp = cp,da + Wcpw cp = specific heat h = enthalpy T = absolute temperature t = temperature W = humidity ratio Subscripts: w is water vapor, a is dry air, g is saturated water vapor More details are in Chapter 7 of the textbook

  13. Transport of saturated air Mold in a duct tsurface < tdp Condensation

  14. Summary • Describe psychrometric quantities • Given any two psychrometric quantities, calculate any other quantity • Use Tables A4 or psychrometric charts to look up psychrometric quantities • Calculate psychrometric quantities at non-standard conditions Reading assignment: Chapters 7 & 8

  15. Example: Single-Zone Air Systems Zone – any space with a thermostat

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