Objectives

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Objectives

• Psychometrics
• Psychometrics of AHU

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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)

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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

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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
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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 Whg0, 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
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Adiabatic mixing

• Governing equation

External heat
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AHU
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Sensible heating
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Dehumidification by Cooling
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Real Dehumidification Process
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Mold in a duct
Transport of saturated air
tsurface lt tdp
Condensation
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Humidification
hw Specific enthalpy of water added to
system hg Specific enthalpy of saturated water
vapor
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Evaporative Cooling (air washing)
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Condition Line for a Space
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HVAC Systems

• Unlimited number
• You can make your system
• We will study several most common applied in
  practice
• http//www.ce.utexas.edu/prof/Novoselac/classes/AR
  E389H/Handouts.html

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Air conditioning
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Single-Zone Systems

• Zone any space with a thermostat