3. After completing this chapter, you will be able to:
Define psychrometry, the psychrometric chart and
psychrometrics
Describe the composition of air
Explain Boyle’s, Charles’ and Dalton’s laws
Read a psychrometric chart
Use the psychrometric chart to determine the
effects of latent and sensible
heat on air
Psychrometrics: The Properties of Air
4. • Psychrometry — the study of air,
temperature and water vapor relationships
• Psychrometric chart — graphical
representation of these relationships
• Psychrometrics — combination of the
concepts of psychrometry and the
psychrometric chart
Psychrometrics: The Properties of Air
5. 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
6. 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
7. 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
8. 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
9. Measuring Humidity
Wet bulb temperatures
Dry bulb temperatures
Sling psychrometer
Psychrometric table
Psychrometric chart
Psychrometrics: The Properties of Air
10. 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
11. 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
12. 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
13. 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
14. 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
15. 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.
16. 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
18. The combined affects of
environmental
conditions on
comfort
•Air temperature
•Relative humidity
•Air movement
•Surface radiation
19. 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
20. Breakdown of the Lines
Dry
Blub
Relative
Humidity
Dew
Point
100%
Saturation
Enthalpy
BTU / lb
Wet
Blub
Grains of
Moisture
Specific
Volume
21. 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
24. 50% RH
Air is 100% saturated
with moisture and
condensation begins
The saturation curve
25. 50% RH
Air is 100% saturated
with moisture and
continues to cool
Moisture removed
by condensation
What if cooling
continues ?
26. 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)
27. 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 = _________
28. 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 = _________
29. 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
