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Objectives

Objectives. Continue with Psychometrics Psychometric chart Equations Plot processes in AHU and buildings on psychrometric chart. Ref: Tao and Janis (2001). Ref: Tao and Janis (2001). Ref: Tao and Janis (2001). Ref: Tao and Janis (2001). Psychrometric Chart.

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Objectives

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  1. Objectives Continue with Psychometrics • Psychometric chart • Equations Plot processes in AHU and buildings on psychrometric chart

  2. Ref: Tao and Janis (2001)

  3. Ref: Tao and Janis (2001)

  4. Ref: Tao and Janis (2001)

  5. Ref: Tao and Janis (2001)

  6. Psychrometric Chart • Need two quantities for a state point • Can get all other quantities from a state point • Can do all calculations without a chart • Often require iteration • Many “digital” psychrometric charts available • Can make your own • Best source is ASHRAE fundamentals (Chapter 6) • Also in your text (back cover fold-out)

  7. Examples • What is enthalpy of air in the classroom right now? • Condensation on windows when taking a shower • How cold does it have to be outside for condensation to form on windows? • Assumption is that windows are the same temperature as outside air • 80 °F, RH = 80%

  8. Alternate calculation for W • PV = mRT • What do we know about R ratio? • P = Pw + Pa R = gas constant P = pressure V = volume T = absolute temperature W = humidity ratio Subscripts: w is water vapor, a is dry air

  9. 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

  10. Adiabatic mixing • Governing equation External heat

  11. AHU

  12. Sensible heating

  13. Dehumidification by Cooling

  14. Real Dehumidification Process

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

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

  17. Evaporative Cooling (air washing)

  18. Condition Line for a Space

  19. 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

  20. Reading assignment • Chapters 7 & 8

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