Objectives
• Learn more about Psychrometrics
• Describe equations for psychometric processes
• Plot processes on psychrometric chart
• Use protractor to calculate SHR
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
Fog formation above rivers
Solution is in
Psychometric chart
Sensible heating
Q  m c p t
Dehumidification by Cooling
Real Dehumidification Process
Humidification
hw Specific enthalpy
of water added to
system
hg Specific enthalpy
of saturated water
vapor
Evaporative Cooling (air washing)
Condition Line for a Space
AHU
Single-Zone Systems
• Zone – any space with a thermostat
Air conditioning
Recirculation
Reheat
Evaporative Cooler
Air-to-air heat exchanger
Summary
• Show HVAC processes on a psychrometric
chart
• Draw condition lines
• Define sensible heat ratio
• Draw HVAC processes for real buildings
• Single zone
• Multiple zone
• Skipping Chapter 9