Objectives
Learn about Psychometrics
• Psychometric chart
• Equations
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
Relative Humidity
• Φ = xw/xw,s = Pw/Pws
• Function of T

0.622  Ws
0.622  W
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
Dew-point temperature, td
• Temperature at which condensation will form
• Under appropriate surface conditions
• Vapor is saturated
• Φ=?
• Ws(P, td) = W
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
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
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)
Ref: Tao and Janis (2001)
Ref: Tao and Janis (2001)
Ref: Tao and Janis (2001)
Ref: Tao and Janis (2001)
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 (IGL)
mw
W
ma
 PwV 
RwT  Pw Ra



 PaV  Pa Rw
 Ra T 
• What do we know about R ratio?
R = gas constant
• P = Pw + Pa
P = pressure
Pw
W  0.622
P  Pw
V = volume
T = absolute temperature
W = 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 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
Adiabatic mixing
  m h

m
h

Q

• Governing equation 
in
External heat
out
Sensible heating
Q  m c p t
Dehumidification by Cooling
Real Dehumidification Process
Transport of saturated air
tsurface < tdp
Condensation
Mold in a duct
Humidification
hw Specific enthalpy
of water added to
system
hg Specific enthalpy
of saturated water
vapor
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 nonstandard conditions