1. Objectives
Psychometrics
Psychometrics of AHU

3. 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)

4. 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

6. 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

7. 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

8. Adiabatic mixing
Governing equation
External heat

9. AHU

10. Sensible heating

11. Dehumidification by Cooling

12. Real Dehumidification Process

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

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

15. Evaporative Cooling (air washing)

16. Condition Line for a Space

17. HVAC Systems Unlimited number
You can make your system
We will study several most common applied in practice

18. Air conditioning

19. Single-Zone Systems
Zone – any space with a thermostat