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Objectives

Objectives. Finish radiation and fluid dynamics review Learn more about psychrometrics Describe equations for psychometric processes Plot processes on psychrometric chart Use protractor to calculate SHR. Short-wave & long-wave radiation. Short-wave – solar radiation <3 m m

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Objectives

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  1. Objectives • Finish radiation and fluid dynamics review • Learn more about psychrometrics • Describe equations for psychometric processes • Plot processes on psychrometric chart • Use protractor to calculate SHR

  2. Short-wave & long-wave radiation • Short-wave – solar radiation • <3mm • Glass is transparent • Does not depend on surface temperature • Long-wave – surface or temperature radiation • >3mm • Glass is not transparent • Depends on surface temperature

  3. Figure 2.10 • α + ρ + τ = 1 α = ε for gray surfaces

  4. Radiation

  5. Combining Convection and Radiation • Both happen simultaneously on a surface • Slightly different temperatures • Often can use h = hc + hr

  6. Combining all modes of heat transfer

  7. Fluid Flow in Pipes and Ducts • Analogy to steady-flow energy equation • Consider incompressible, isothermal flow • What is friction loss? [Pa]

  8. Pitot Tubes

  9. Alternate calculation for W • PV = mRT (IGL) • 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

  10. 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

  11. Adiabatic mixing • Governing equation External heat

  12. Fog formation above rivers Solution is in Psychometric chart

  13. Sensible heating

  14. Dehumidification by Cooling

  15. Real Dehumidification Process

  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. AHU

  20. Single-Zone Systems • Zone – any space with a thermostat

  21. Air conditioning

  22. Recirculation

  23. Reheat

  24. Evaporative Cooler

  25. Air-to-air heat exchanger

  26. Summary • Show HVAC processes on a psychrometric chart • Draw condition lines • Define sensible heat ratio • Draw HVAC processes for real buildings • Single zone • Multiple zone • Skipping Chapter 9

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