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)

7. 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)

8. 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%
73 for example 1

10. 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

11. 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

12. Adiabatic mixing
Governing equation
External heat

13. AHU

14. Sensible heating

15. Dehumidification by Cooling

16. Real Dehumidification Process

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

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

19. Evaporative Cooling (air washing)

20. Condition Line for a Space

21. 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

22. Reading assignment
Chapters 7 & 8