ARCH-432
Dew Point Saturation and
Psychometrics
Attendance
Who is typically
referred to as the
“Father of Cool”?
(hint: generally
credited with
developing the
science of
psychometrics)
A.
B.
C.
D.
E.
Willis Carrier
Karl Trane
Bill Lennox
Dr. Frederick Glock
Prof. Max Kirk
What You Need to Know
Understand the underlying
principles of psychrometrics
What You Need To Be Able To
Do
Read a Psychrometric chart
Be able to apply psychrometrics to
predict and solve simple HVAC
problems
Terms
Dew point
Wet bulb
temperature
Dry bulb
temperature
Enthalpy
Relative humidity
Sensible heat
Latent heat
Mixing line
Enthalpy
To understand in a simple way,
Enthalpy means change of heat.
Enthalpy can be positive or
negative. For water when
heated to a very high temp
changes its state into vapor.
Here heat change takes place.
Enthalpy
Enthalpy
Psychrometry
Psychrometry is the science dealing
with the physical laws of air – water
mixtures.

To obtain comfort, and select an
appropriate HVAC system, the
psychrometric conditions must be known.
Properties of Air on a
Psychrometric Chart
Dry-bulb temperature
Wet-bulb temperature
Dew-point temperature
Relative humidity
Humidity ratio
Humidity Ratio (grains/lb of dry air)
Air Conditions on the
Psychrometric Chart
22
0
20
0
18
0
16
0
14
0
12
0
10
0
80
dew point
humidity ratio
60
40
dry bulb
relative humidity
20
25
30
35
40
45
50
55
60
65
70
75
80
Dry-Bulb Temperature (°F)
85
90
wet bulb
95
100
105
110
Dew Point
Condensation
occurs at the Dew
Point Temperature
(oF)
Saturation
You get sheep?
No you get Fog!
 What happens when
the DP, WB, and DB are
the same?
Saturation
When the dry-bulb, wet-bulb, and dewpoint temperatures are the same, the
air is saturated. It can hold no more
moisture. When air is at a saturated
condition, moisture entering the air
displaces moisture within the air. The
displaced moisture leaves the air in the
form of fine droplets. When this
condition occurs in nature, it is called
fog.
Relative Humidity
Relative
Humidity
(percentage)
50% RH
Amount of moisture that a given amount of air
is holding
=
Amount of moisture that a given amount of
air can hold
100% RH Saturated
Relative Humidity
The fourth property RH, is therefore a
comparison of the amount of moisture
that a given amount of air is holding, to
the amount of moisture that the same
amount of air can hold, at the same
dry-bulb temperature.
Zones with ASHRAE Standard 55
Team Problem Solving

Based upon your estimates of
occupancy, met and clo value, what
DB/RH condition point will create
the most comfort in the first floor of
Carpenter Hall in summer?
A.
B.
C.
D.
73
75
75
78
F/50%
F/45%
F/55%
F/40%
RH
RH
RH
RH
Plot this point on
your Psychrometric
chart.
Team Problem Solving
Assume that the
gym is kept at 72° F
and 35% RH in
winter. At what
temperature does
the inside surface of
the glass have to
reach in order for
water to condense?
A. 24° F
B. 35 ° F
C. 42 ° F
D. 56 ° F
E. 60 ° F