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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. - PowerPoint PPT Presentation
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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
• 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)
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%
Alternate calculation for W
• PV = mRT
• What do we know about R ratio?
• P = Pw + Pa
w
a
a
w
TRVP
TRVP
a
w
R
R
P
P
m
mW
a
a
w
w
w
w
PP
PW
622.0
R = gas constantP = pressureV = volumeT = absolute temperatureW = humidity ratio
Subscripts: w is water vapor, a is dry air
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 heath = enthalpyT = absolute temperaturet = temperature W = humidity ratio
Subscripts: w is water vapor, a is dry air, g is saturated water vapor
Adiabatic mixing
• Governing equation hmQhmoutin
External heat
AHU
Sensible heating
tcmQ p
Dehumidification by Cooling
Real Dehumidification Process
Mold in a duct
Transport of saturated air
tsurface < tdp Condensation
Humidification
hw Specific enthalpy of water added to system
hg Specific enthalpy of saturated water vapor
Evaporative Cooling (air washing)
Condition Line for a Space
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
