Objectives

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 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
More details are in Chapter 7 of the textbook
13
Mold in a duct
Transport of saturated air
tsurface lt 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