1. Objectives
Learn about Psychometrics
Psychometric chart
Equations

2. Humidity Ratio, W W = mw/ma Degree of saturation, µ = W/Ws
Humidity ratio is hard to measure, but very useful in calculations
What are units?
Is W a function of temperature? What about Ws?
Ws = humidity ratio at saturation
ma = mass of dry air
mw = mass of water vapor

3. Relative Humidity Φ = xw/xw,s = Pw/Pws Function of T
Easy to measure and useful in some contexts, but often need to know temperature as well
x = mole fraction
P = pressure
μ = degree of saturation
W = humidity ratio

4. Dew-point temperature, td
Temperature at which condensation will form
Under appropriate surface conditions
Vapor is saturated
Φ = ?
Ws(P, td) = W

5. Wet-bulb temperature, VBT (t*)
Temperature of wet surface or
Temperature at which water, by evaporating into the air, will bring air to saturation adiabatically
* superscript is designation that variable is evaluated at the wet-bulb temperature
Note, distinct from that measured by a sling psychrometer
Section 9.5

6. Tables for Moist Air (P = 1 atm)
Tables A.4 in your text
Ability to get Ws for calculations
Subscripts:
a = dry air, s = saturated air
v = va+µvas
h = ha+µhas
s = sa+µsas

8. 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)

9. Ref: Tao and Janis (2001)

10. Ref: Tao and Janis (2001)

11. Ref: Tao and Janis (2001)

12. Ref: Tao and Janis (2001)

13. 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

15. Alternate 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

16. 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

17. Adiabatic mixing
Governing equation
External heat

18. Sensible heating

19. Dehumidification by Cooling

20. Real Dehumidification Process

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

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

23. 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