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1. Psychrometrics:
The Properties
of Air

2. The Composition of Air
 Dry air — 77% nitrogen, 22% oxygen, 0.04%
carbon dioxide, 0.96% other gases
 Water vapor — evaporation and transpiration
 Airborne impurities
• Smoke, dust, pollen, bacteria, noxious gases
• Particulate matter or gases
Psychrometrics: The Properties of Air

3. Predicting Air’s Behavior
 Boyle’s Law — Gas shrinks in volume when
pressurized at constant temperature
 Charles’ Law — Gas expands when heated at
constant pressure
 Dalton’s Law of Partial Pressures — Total pressure
of a gaseous mixture is the sum of the pressures
of the individual gases
Psychrometrics: The Properties of Air

4. Water Vapor in Air — Latent Heat of
Vaporization
 Equal to 970 Btus/lb of water vapor
 Heat added to water causes evaporation
 Heat removed from water vapor causes
condensation
Psychrometrics: The Properties of Air

5. Defining Humidity
 Humidity — airborne moisture
 Specific humidity — weight of water vapor in dry
air
 Relative humidity — percentage of moisture
saturation in air
 Dew point — point at which air has a relative
humidity of 100%
Psychrometrics: The Properties of Air

6. Measuring Humidity
 Wet bulb temperatures
 Dry bulb temperatures
 Sling psychrometer
 Psychrometric table
 Psychrometric chart
Psychrometrics: The Properties of Air

7. Example 5-1
 Dry bulb temperature = 92ºF
 Wet bulb temperature = 89ºF
 Use Percent Relative Humidity Table
 Difference between temperatures is 3ºF
 The point of intersection
is 89% RH
Psychrometrics: The Properties of Air

8. Other Psychrometric Properties of Air
 Specific Volume of air – cu ft/lb dry air
 Enthalpy – Btu/lb dry air
Introduction to the Psychrometric Chart
 Plotted graph of tables of moist air properties
 Standardized at sea level pressure
Psychrometrics: The Properties of Air

9. Constructing the Psychrometric Chart
 Dry bulb temperature scale along the bottom
• Constant dry bulb represented by vertical
lines
 Moisture measurements along the right side
• Constant moisture represented by
horizontal lines
 Dew point line
• Upward curved line at left of chart
• Also known as the 100%
humidity line
Psychrometrics: The Properties of Air

10. Examining the Psychrometric Chart
 When two properties are known, the others
can be read from the chart
• Dry and wet bulb temperatures
• Relative humidity and dew point
• Enthalpy
• Specific volume
Psychrometrics: The Properties of Air

11. Example 5-3
 Wet bulb temperature of 61 degrees
 Dry bulb temperature of 75 degrees
 Intersection of these two lines
• 42% relative humidity
Psychrometrics: The Properties of Air

12. Dew Point
• The temperature which moisture starts to
condense out of the air.
• Dew point is also known as saturation
temperature.
• Dew point temperature is determined by moving
from a state point horizontally to the left along
lines of constant humidity ratio until the upper,
curved, saturation temperature boundary is
reached.

13. Air temperature Relative humidity Air movement
Surface
temperatures
Air quality (IAQ)
Lighting
•quality
•quantity
Acoustics
Security
Building Envelop -The Third Skin
The 3rd. skin moderates the differences between the
natural environment and the built environment
75 degrees
65 degrees
+100
- 5

14. MOISTURE
CONTENT
(BTU/LB
AIR
)
dew point
Saturation
Line
30°
40°
50°
60°
70°
80°
Temperatures are read at the
Saturation line from a set
point to the left along
humidity ratio lines.

15. Dry Bulb Temperature Reading
Change
in
quantity
of
water
in
the
air
The psychrometric
chart
Charts the combined affects of air
temperature and moisture content

16. Breakdown of the Lines
Dry
Blub
Relative
Humidity
Dew
Point
100%
Saturation
Enthalpy
BTU / lb
Wet
Blub
Grains of
Moisture
Specific
Volume

17. 50% RH
Psychrometric Chart
Definition of Relative Humidity
The amount of moisture in the air
expressed as a percentage of the
maximum amount of moisture the air can
hold at the given temperature.
Example: at dry bulb temp. of 77 and wet
bulb temp. of 63, the air is holding 50% of
it’s maximum capacity at 77 degrees.
Enter dry bulb reading
Enter wet bulb reading

18. warming
cooling
drying
Adding
moisture

19. warming
cooling
drying
Adding
moisture
Warm &
Dry
Cool &
Humid
Winter
Summer
Energy efficient zones
Winter: higher humidity off-sets
lower furnace thermostat setting
Summer: lower humidity off-sets
higher AC thermostat setting

20. 50% RH
Air is 100% saturated
with moisture and
condensation begins
The saturation curve

21. 50% RH
Air is 100% saturated
with moisture and
continues to cool
Moisture removed
by condensation
What if cooling
continues ?

22. 50% RH
Air is 100% saturated
with moisture and
continues to cool
Moisture removed
by condensation
Same air is reheated to 70 degrees
and no moisture added – RH of the
room goes down (dryer)

23. Dry Bulb Temperature Reading
Change
in
quantity
of
water
in
the
air
Chart Exercises
What is the RH in the classroom?
Dry bulb ________
Wet bulb _________
RH = _________

24. Dry Bulb Temperature Reading
Change
in
quantity
of
water
in
the
air
Chart Exercises
Based on the current room RH, how
cool would the walls need to get to
start noticing condensation ?
Dry bulb ________
Wet bulb _________
RH = _________

25. Chart Exercises
Assume the room is at pt. “A” when class
begin and we are comfortable, which
direction would room condition tend to
move toward as a response to our
presents ?
Mechanically: (Summer)
we need to (add or remove heat)
we need to (add or remove moisture)
A - Room starts out
here
B
C
D
E